Impact of low temperature combustion attaining strategies on diesel engine emissions for diesel and...

Energy Conversion and Management 80 (2014) 329–356

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Energy Conversion and Management

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Impact of low temperature combustion attaining strategies on dieselengine emissions for diesel and biodiesels: A review

http://dx.doi.org/10.1016/j.enconman.2014.01.0200196-8904/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: Department of Mechanical Engineering, Uni-versity of Malaya, 50603 Kuala Lumpur, Malaysia. Tel.: +60 146985294; fax: +60 379675317.

E-mail address: [email protected] (S. Imtenan).

S. Imtenan ⇑, M. Varman, H.H. Masjuki, M.A. Kalam, H. Sajjad, M.I. Arbab, I.M. Rizwanul FattahCentre for Energy Sciences, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 12 September 2013Accepted 13 January 2014Available online 17 February 2014

Keywords:Low temperature combustionBiodieselHCCIPCCIRCCI

a b s t r a c t

Simultaneous reduction of particulate matter (PM) and nitrogen oxides (NOx) emissions from dieselexhaust is the key to current research activities. Although various technologies have been introducedto reduce emissions from diesel engines, the in-cylinder reduction techniques of PM and NOx like lowtemperature combustion (LTC) will continue to be an important field in research and development ofmodern diesel engines. Furthermore, increasing prices and question over the availability of diesel fuelderived from crude oil have introduced a growing interest. Hence it is most likely that future dieselengines will be operated on pure biodiesel and/or blends of biodiesel and crude oil-based diesel. Beinga significant technology to reduce emissions, LTC deserves a critical analysis of emission characteristicsfor both diesel and biodiesel.This paper critically investigates both petroleum diesel and biodiesel emissions from the view point ofLTC attaining strategies. Due to a number of differences of physical and chemical properties, petroleumdiesel and biodiesel emission characteristics differ a bit under LTC strategies. LTC strategies decrease NOx

and PM simultaneously but increase HC and CO emissions. Recent attempts to attain LTC by biodieselhave created a hope for reduced HC and CO emissions. Decreased performance issue during LTC is alsobeing taken care of by latest ideas. However, this paper highlights the emissions separately and analyzesthe effects of significant factors thoroughly under LTC regime.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The diesel engine is the most efficient type of internal combus-tion engine, offering good fuel economy and low carbon dioxide(CO2) emission [1]. Unfortunately, it is also a source of particulatematter (PM) and nitrogen oxides (NOx), both of which are nowsubjected to legislative limits because of their adverse effects onthe environment and human health [2]. In the last few years, dieselengines have been subjected to progressively stringent emissioncontrol standards; especially as far as NOx and PM emissions areconcerned. Fig. 1 shows this trend for Europe (Euro 2, 1996–Euro5, 2008), the United States (US04–US10) and Japan. In order to meetthe requirements of future emission standards, emission of thesesubstances, as well as carbon monoxide (CO) and hydrocarbon(HC) emissions must be reduced significantly. Three general meth-ods can be applied to the engines to meet lower regulated emissionlimits, viz. alternation of fuels [3,4], alternation of combustion

processes and after-treatment of the exhaust [5]. Considerable pro-gress has been made on both combustion and catalyst control path-ways to reduce emission. Diesel particulate filters (DPF) for PMfiltration and selective catalytic reduction (SCR) of NOx are nowavailable for after-treatment of engine out emissions. Nevertheless,to minimize the cost and complexity of exhaust after-treatment sys-tems as well as for potential fuel economy penalties—considerableresearch efforts have also focused on the in-cylinder control of emis-sions through the application of low-temperature combustion (LTC)techniques.

LTC is now widely demonstrated covering light-duty [7–11] toheavy-duty [12–14] engines. It is the concept at the heart of ad-vanced diesel combustion. LTC is a general term for HomogeneousCharge Compression Ignition (HCCI) combustion, and PremixedCharge Compression Ignition (PCCI) combustion [5]. To explain thetheory of LTC, Akihama et al. [15] simulated combustion by a com-pression ignition (CI) 3D-CFD KIVA2 model and plotted local equiv-alence ratio (U) vs. flame temperature (T) for the stratifiedcombustion process. This particular figure showed the NOx–PMtrade-off related to conventional diesel combustion, where at theedge of spray flame, fuel lean zones produce abundant NOx and fuelrich zones inside the spray flame produce abundant soot (an

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Fig. 1. Decreasing limit of NOx and PM [6].

330 S. Imtenan et al. / Energy Conversion and Management 80 (2014) 329–356

element of PM). With their model and U–T map they explained thatLTC takes place at temperatures below the formation regime of NOx

and at local equivalence ratios below the formation regime of dieselsoot. As mentioned earlier, these systems can be divided into twocategories [16]. Those in which the combustion phasing isdecoupled from the injection timing and the kinetics of the chemicalreactions dominate the combustion, are in the first category which isknown as HCCI mode. In the second category, combustion phasing isclosely coupled to the fuel injection event which is termed as PCCImode. In the former category, air and fuel are thoroughly premixedin such a way that at the start of the combustion, the mixture isnearly homogeneous and characterized by an equivalence ratio,which is lower than 1 everywhere. For the second category, pre-mix-ing occurs between the fuel injection and start of combustion event,but significant regions exist where the equivalence ratio is greaterthan unity at the start of the combustion. Fig. 2 shows the plot of lo-cal equivalence ratio (U) vs. flame temperature (T) with differentcombustion mechanisms. It can be seen that, NOx forms in the leanmixture zone where flame temperature is above 2200 K, whereassoot forms in the rich mixture zone above 1800 K. Conventionalcombustion overleaps the formation zones of NOx and soot, butLTC techniques like HCCI and PCCI avoid these zones and reduceNOx and soot simultaneously. Recently, a new approach of LTC,Reactivity Controlled Compression Ignition (RCCI) has been pro-posed by several authors [17–19]. This technology has the potentialto overcome some of the limitations of HCCI and PCCI.

The objective of this article is to present the state of the art ofthe effects of different LTC mode (HCCI, PCCI, RCCI) attaining

Fig. 2. Plot of local equivalence ratio vs. flame temperature with differentcombustion mechanisms [17].

strategies on particular diesel emissions (NOx, PM, CO, UHC) usingboth petroleum diesel and biodiesel. The attainment of these strat-egies primarily depends on some factors like, application of ex-haust gas recirculation (EGR), change in injection timing (IT) &injection pressure (IP), variation in compression ratio (CR) henceoperating load, changes in fuel blends, etc. Therefore the analysishas been governed by these significant factors surely. To providea complete overview of the whole scenario, more than 150 techni-cal articles have been reviewed to collect significant informationrelated to this article’s objective. At first, the article briefly intro-duces the LTC strategies and then analyzes how the attainmentof these strategies may affect the emissions for petroleum dieseland biodiesel respectively. Though LTC mode has a positive impacton NOx and PM emissions but many of the researchers have re-ported reduced performance during LTC modes [20,21] due tohigher rates of EGR and incomplete combustion. Impact of LTCmodes on engine performance is also briefly presented here in thisarticle.

2. LTC strategies

2.1. Homogeneous Charge Compression Ignition (HCCI)

HCCI engine is a combination of SI (homogeneous charge sparkignition) and CI (stratified charge compression ignition) engineswith a sense that it uses premixed charge like SI engine but dependson autoignition like CI engine [22]. In HCCI, fuel is injected wellbefore the combustion event which allows the homogeneous mix-ture of air–fuel. This homogeneous mixture initiates combustionsimultaneously at different sites of the combustion chamber unlikeSI (flame propagation) or CI (locally rich flame front) engines. Withdiesel fuel, HCCI combustion shows two-stage heat release. The firststage is low temperature kinetic reactions and the second stage ismain heat release regime [23]. HCCI autoignition is controlled bylow temperature chemistry and the main heat release is dominatedby CO oxidation [24]. The main advantage of the HCCI combustionover conventional combustion mode is the reduction of NOx and sootin the exhaust. Though the concept gives higher indicated thermalefficiency, inability to control the combustion phasing has led theresearchers to try different combustion control strategies e.g. portfuel injection [25,26], early direct injection [27,28], multiple fuelinjection [29,30], compound combustion technology [31,32], nar-row angle injection [33–35], late direct injection [36,37], variable in-let temperature, variable valve timing, internal or external EGR, etc.[22]. In addition, use of alternative fuels and fuel blends according tocompression ratios and operating conditions have much potential tocontrol the combustion phasing [22,38,39]. Actually, fuels withdifferent autoignition points can be blended at varying ratios to con-trol the ignition point at various load–speed regions [40]. This hasyield alternative fuels to be tested in HCCI engines [41–51]. In die-sel–fueled HCCI engines, these combustion control technologiesare not often used alone. The combination of several strategies helpsin achieving better effects on the combustion mechanism.

2.2. Premixed Charge Compression Ignition (PCCI)

Premixed charge compression ignition or the partially premixedcharge compression ignition (PPCI) evolved from the HCCI combus-tion mode for the sake of better control over the start of combustion(SOC). In-cylinder homogeneity causes rapid combustion by simul-taneous ignition throughout the cylinder space and produces greatcombustion noise in the HCCI mode. It is also very tough to controlthe combustion phases in HCCI mode. PCCI process is introducedto solve these problems. It is not fully homogeneous like HCCI. Itachieves desired ignition delay through enhanced charge motion,

S. Imtenan et al. / Energy Conversion and Management 80 (2014) 329–356 331

reduced compression ratio, higher injection pressure and extensiveuse of EGR. In the PCCI combustion process, fuel can be injected intothe combustion chamber in three ways, they are, advanced directinjection, port fuel injection and late direct injection. Advanceddirect injection and port fuel injection suffer from fuel sprayimpingement on the cylinder walls and incomplete fuel evaporation.Consequently HC and CO emissions increased [52,53]. However, nar-row spray angle injectors and EGR reduce the wall impingement[35,54,55]. Late direct injection avoids the fuel-wall impingementand gives a way to switch the combustion style to the conventionalat higher loads. Researchers have tried to increase the high load lim-its and reduce the emissions of PCCI by applying additives and tun-ing fuel properties [56,57], variable valve timing, multiple injections[58], and fuel–air mixing enhancement [10,59]. A newer approach inPCCI introduces air–fuel premixing by early injection followed by alate injection of fuel pulse in the compression stroke, which governsthe onset of ignition. Early injected fuel stratifies in the cylinder withthe air and as the compression stroke reaches near the TDC (top deadcenter) it creates HCCI like condition. When the late direct injectionoccurs, the fuel-rich area of the late injection burns before the fuel-lean homogeneous mixture. This variable fuel–air mixture preventsthe entire charge from igniting instantaneously which gives a bettercontrol over the combustion phase and rate. Moreover adoption ofhigher EGR permits longer ignition delay. It permits better premix-ing of air–fuel, results in less fuel-rich pockets followed by a lowtemperature combustion, which simultaneously reduces NOx andsoot level [60].

2.3. Reactivity Controlled Compression Ignition (RCCI)

Reactivity controlled compression ignition is the newest ap-proach where multiple fuels of different reactivity are injected atscheduled intervals which governs the reactivity of the charge inthe cylinder for the desired combustion duration and magnitude.Mainly, in this approach, relatively low reactive fuel is injected(port injection) very early in the engine cycle which mixes withthe air homogeneously. Later on, a higher reactive fuel is injecteddirectly into the cylinder; it creates pockets of different air–fuel ra-tios and reactivity, which govern the onset of combustion at differ-ent times and rates.

This process of combustion originated from the effort of theresearchers to reduce the EGR at higher loads while working onthe PCCI regime. Inagaki et al. [61] investigated PCCI with two dif-ferent reactive fuels and they succeeded to run the engine at higherloads (up to 12 bar) with minimal EGR. They reported, stratifica-tion of fuel reactivity made it possible to reduce the heat releaserate and they achieved control over the combustion phasing be-yond PCCI combustion. In RCCI combustion process, the combus-tion is staged [62] and proceeds from locally high reactivity fuelareas to low reactivity fuel areas. Such staging results in significantexpansion of the premixed combustion duration and consequentlyproduces high thermal efficiency, low pressure rise rate, low emis-sion for higher loads up to 16 bar IMEP [63,64]. Therefore, as thecombustion parameters are governed by the degree of reactivityof the charge in RCCI process, it is likely that, different operatingconditions will need different fuel blends. For this reason, capabil-ity to operate with fuel blends covering the spectrum from neatgasoline to neat diesel fuel (low reactive to high reactive) is man-datory to get the best output from this kind of strategy.

3. Emission analysis under LTC modes

This section investigates emission characteristics for diesel andbiodiesels under LTC modes. Results are summarized in Tables 1and 2.

3.1. NOx emission analysis

3.1.1. Formation of NOx

NO (nitric oxide) and NO2 (nitrogen dioxide) are generallygrouped under the NOx emission. But among the nitrogen oxides,NO is the predominant oxide produced inside the engine cylinder.Oxidation of the atmospheric nitrogen (molecular) is the mainsource of NO and this is called thermal NOx. Strong triple bond ofnitrogen molecules breaks down by high combustion temperatureand disassociated atomic state of nitrogen takes part in series ofreactions with oxygen which results in thermal NOx. This mecha-nism is also known as Zeldovich mechanism [61,65]. However ifthe nitrogen content of the fuel is higher, then the nitrogen con-taining compounds get oxidized and become a potential sourceof NOx, which is also called fuel NOx. Formation of fuel NOx is quitecomplex because numerous intermediate species are there. Severalhundred reversible reactions take place and still the true rate con-stant values are unknown. Another process of NOx formation isprompt mechanism. By this mechanism, the amount of NOx is quitelower than fuel and thermal NOx [66]. Mainly, free radicals formedin the flame front of the hydrocarbon flame generate this rapidproduction of NOx.

Formation of NOx generally depends on oxygen concentration,in-cylinder temperature, air surplus coefficient and residence time.NOx forms both in the flame front as well as in the post flame gases[67]. In engines, flame reaction zone remains extremely thin, as thecombustion pressure is very high. In addition, residence time isshort within this zone. On the other hand, the burned gases, whichare produced early in the combustion process, are compressed to ahigher temperature than they reached just after the combustion.That is why NO formation on the post flame gases usually domi-nates over the flame-front-produced NO.

3.1.2. NOx emission under LTC modes for dieselIn LTC modes, the combustion temperature is reduced by pre-

mixed or leaner mixture with moderated use of EGR, consequentlyNOx emission reduces [68]. EGR hinders the O2 flow rate into theengine and results in reduced local flame temperature, which helpsto reduce thermal NOx. Again EGR extends the ignition delay whichindicates delayed start of combustion. It results in lower pressureand temperature rise during the combustion. The effect of lateinjection strategy on NOx emission is just like as ignition delay[69]. Many researchers have attained LTC modes like PCCI, HCCIor RCCI, optimizing various parameters such as fuel reactivity (Ce-tane number, CN of fuel), injection timing and pressure, dilution ofcharge by EGR, controlling the operating load. Effects of theseparameters for attaining the LTC modes are discussed below con-cerning the literature review.

Valentino et al. [68] tested blends of fuels having lower cetanenumber, higher resistance to autoignition and higher volatilitythan diesel fuels to reach partially premixed LTC mode. The fuelswere neat diesel, 20% and 40% blend of n-butanol with diesel.Along with EGR, late injection and higher injection pressure gavethem LTC mode for neat diesel. They reported that higher injectionpressure allowed better mixing before the combustion and suffi-cient ignition delay provided by the use of EGR gave them a par-tially premixed LTC mode, which resulted in lower NOx. Again,blends of n-butanol with diesel gave them premixed LTC modeby elongating the ignition delay which can be attributed to thelower CN of n-butanol. Longer ignition delay permitted earlierinjection as well as lower injection pressure with lower EGR rateto achieve the LTC mode and obviously lower NOx. Zhang et al.[21] also attempted lower cetane numbered gasoline and dieselfuel mixture (50:50) to reach premixed LTC mode. They used singleadvanced injection (up to 28� BTDC, before top dead center) with

Table 1Emission for diesel at LTC.

Engine setup Operatingcondition

Fuel Injection timing Percentage ofEGR/O2

concentration

NOx CO UHC PM/Soot Refs.

4S,4-cylinder, DI, TCRS: 1500 rpmDV: 1.7 LIP: 1000 barInjection system:common rail

PCI D 8 to 25� BTDC Up to 50% E ; As EGR ", Increased Increased E ; As EGR ", [72]Load: 3.75 bar Very low

(0.3 ppm) at 50%EGR

Very low (0.03FSN)at 50% EGR and25� BTDC

4S,4-cylinder, DI, TCDV: 4.5 LCR: 16.57:1RS: 2400 rpmRP: 115 kW@2400 rpmInjection system:common rail

Retardedinjection LTC

D Sweep of injectiontiming

56% E ; as ITretarded,

Increased Increased E ; as IT retarded,91% ; as IT sweptfrom �8 to �2�ATDC

[73]

�8� to �2� ATDC 53%Speed:1400 rpm ; as IT swept

from �8 to �2�ATDC

Variabletorque:54–80 N m

4S,1-cylinder, DI, supercharged,DV: 2.022 LRS: 1500 rpmCR: 14:1Injection system:electronically controlledinjector

LTC 25% load,charge airpressure:1.3 bar (abs.)

D Sweep of IT 9 to20� BTDC

Up to 60% 0 g/kW h E ; as IT advanced, 28%; as IT swept from 9� to20� BTDC

E ; as IT advanced,71% ; as IT swept from9� to 15� BTDC

E ; as IT advanced,92% ; as IT sweptfrom 9� to 20� BTDC

[20]

50% load,charge airpressure:2.4 bar (abs.)

Sweep of IT 9 to20� BTDC

Up to 65% 0 g/kW h Very high for such EGRlevel 56 g/kW h at 20�BTDC

Very high for suchEGR level, 28.126 g/kW h at 20� BTDC

E ; as IT advanced,93% ; as IT sweptfrom 9o to 15� BTDC

4S,1-cylinder, supercharged, DI, WCDV: 781.7 cm3

CR: 13RS: 1000 rpmInjection system: commonrail

PCCI Low sulfurdiesel.

Sweep of IT 15 to25� BTDC

0% and 40% Became 0 g/kW h as40% EGR applied

E " with advancementof IT and increment ofEGR

E " with advancementof IT and increment ofEGR

At constant EGRremained almostsame with the sweepof IT

[74]Injectionpressure:140 MPa

4S, 1-cylinder, DIDV: 2.34 LCR: 11.2RS: 1200 rpmInjection system: commonrail

Late injectionLTC

71% n-heptane,29% iso-octane, 1%toluene

[124]

152� injectionangle

Near TDC 12.7% O2

concentrationN/A Higher emission Higher emission Higher soot emission


Near TDC 12.7% O2

concentrationN/A Comparatively Lower

emission than 152�injection angle

Comparatively Loweremission than 152�injection angle

Lower soot than 152�injection angle


Near TDC 12.7% O2

concentrationN/A Higher emission than

124� injection angleHigher emissionthan124� injectionangle

Lower soot than 152�injection angle

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Table 1 (continued)



concentration


4S,1-cylinder, DIDV: 373 cm3

CR: 15:1RS: 1500 rpmIP: 100 MPaInjection system: Boschcommon rail

HCCI D Dual injection. N/A 47% decrementas the 1st IT sweepfrom 50� to 60� BTDCwhile 2nd injectionat 20� ATDC

360% increment as the1st IT sweep from 50� to60� BTDC while 2ndinjection at 20� ATDC

40% increment as the1st IT sweep from 50�to 60� BTDC while 2ndinjection at 20� ATDC

N/A [35]Injectionangle: 60�

Sweep of 1stinjection: 50 to 70�BTDCSweep of 2ndinjection: TDC�20� ATDC

4S,1-cylinder, DI,DV: 416 cm3

CR: 15:1Injection system: Boschcommon rail 2ndgeneration

HCCI [77]Spray coneangle: 148�1500–2500 rpm

Fischer–Tropschfuel, FAME, N/A

About 50% On average 0.06 g/kW h E ; as speed and load ",Almost 67% decrement.

E ; as speed and load", Almost 58%decrement.

E ; as speed and load", Almost 47%decrement.rpm 20% ethanol

IMEP: 8.6 bar to11 bar Fossil-diesel,1500 to2500 rpm

FAME, 20% ethanolUp to 47% On average 0.055 g/kW h E " as speed and load ",

Almost 40% increment.E " as speed and load", Almost 70%increment

Not affected by thechange of speed andload

IMEP: 9.8 bar to12.2 bar

4S,1-cylinder, DI, WCDV: 1.08 LCR: 16:1RS: 1400 rpmInjection system: Commonrail

EGR andtoluene in thefuel paved LTC

70% n-heptane + 30%toluene,

�10� ATDC 12% O2

concentrationQuite similar for all thefuels. On average 26 ppm.

E " as the% of n-heptane", Commercial dieselgave highest emission

E " as the% of n-heptane ",Commercial dieselgave highest emission

E " As the% of n-heptane ",Commercial dieselgave highestemission, On average5 FSN

[166]

80% n-heptane + 20%toluene,Commercial dieselfuel

4S,1-cylinder,DV: 2.44 LCR: 11.6:1RS: 1200 rpmIMEP: 4.75 barDirect IP: 600 barPort IP: 4.14 bar

RCCI Port injected fuel:iso-octane

Single directinjection: Sweepfrom 150� to 10�BTDC

N/A Remained lower than 0.1 g/kW h, advancement after 60�BTDC caused rapid increment

Remained lower than17.7 g/kW h,advancement after 70�BTDC caused rapidincrement

Remained lower than5 g/kW h,advancement after50� BTDC caused rapidincrement.

N/A [79]

Direct injected fuel:n-heptane

Dual directinjection: 25�CAdwell in between,Sweep from 150�to 10� BTDC

Remained lower than 0.1 g/kW h, advancement after 50�BTDC caused rapid increment.

Remained lower than20 g/kW h,advancement after 60�BTDC caused rapidincrement.

Remained lower than5 g/kW h,advancement after40� BTDC caused rapidincrement

N/A

(continued on next page)

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Table 1 (continued)



concentration


4S,1-cylinderDV: 2.44 LCR: 16.1:1IMEP: 9 barDirect IP: 400 barPort IP: 5.17ar

RCCI Port injected fuel:gasoline

N/A 43% Remained quite lower than0.1 g/kW h,

Lowest E was 4.5 g/kW h, E " with thefraction of port fuel

No significant impactof% of DTBP,

0.75% DTBP gave thelowest E, Average Elevel was below0.005 g/kW h.

[78]

On average 3 g/kW h.Direct injected fuel:gasoline + variablepercentage of DTBP(di-tert-butylperoxide)

No significant impact of DTBPpercentage.

4S,1-cylinder, DIDV: 1.9 LCR: 16.7:1RS: 1500 rpmRated IMEP: 4.5 barFuel rail pressure: 860 bar

PPCI withadvanced IT

Commercial diesel 26.6� BTDC Sweep of O2

concentrationfrom 15 to 9%

N/A E " 288% as the load ; to1.5 bar

E " 430% as the load ;to 1.5 bar

N/A [151]

E " 140% as the%O2 ; to9% from 15%

E " 260% as the%O2 ;to 9% from 15%

4S, 4-cylinderDV: 1.7 LRS: 1500 rpmIMEP: 2.6 bar

PCCI Low sulfur diesel Advancedinjection

Up to 50% PCCI gave almost 86% ; than CC PCCI gave almost 90% "than CC

PCCI gave almost 46%" than CC

PCCI strategy gavealmost 51% ; than CC

[75]High injectionrail pressure.

4S,4-cylinder, DIDV: 1.7 LCR: 16:1RS: 1500 rpmBMEP: 4 barInjection system: commonrail

PCI Ultra low sulfurSwedish diesel

Sweep from 8.5� to4.5� BTDC

48% "EGR and retarded IT gave;NOx, 48% EGR gave lower than1 g/kg-fuel all over the runningcondition.

"EGR and retarded ITgave " E, About 28% "forIT sweep from 6.5� to4.5� BTDC at 48%EGR

"EGR and retarded ITgave " E, About 80%"for IT sweep from6.5� to 4.5� BTDC at48% EGR

Retarded IT gave ; E,About 53% ; as ITsweep from 6.5� to4.5� BTDC at 48% EGR

[53]

Sweep from 8.5� to2.5� BTDC

43%

4S,1-cylinder, DIDV: 1.7 LCR: 15:1RS: 1500 rpmIMEP: 7.5 barIP: 1000 barInjection system: commonrail

PCI 0% to 40% mixture ofgasoline with diesel

Start of ignition isat 6� ATDC onaverage

35–50% for1.5 bar intakepressure

N/A Primarily dominated bythe mixture equivalenceratio. Not affectedsignificantly by ignitiondelay.

E " as the% of thegasoline "

N/A [146]Intake pressure1.5 bar to 2 bar

40–60% for2 bar intakepressure

4S,1-cylinder, DIDV: 1.806 LCR: 14.4:1RS: 1200 rpmIMEP: 7 barIP: 1450 barInjection cone angle: 120�

PCI CommercialEuropean diesel.

�24� ATDC to�33� ATDC

O2

concentration12.1%

N/A N/A 66% " as IT advancedfrom �30� ATDC to�33� ATDC

185% " as ITadvanced from �30�ATDC to �33� ATDC

[125]

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Table 1 (continued)



concentration


4S,4-cylinder, DI, WC, TCDV: 1910 cm3

RS: 2500 rpmCR: 17.5:1BMEP: 0.8 MPaVariable IP: 100–160 MPaInjection cone angle: 148�

PPCI Low sulfur diesel, Retarded IT O2

concentration19.5%

Retarded IT, lower IP and EGRgave ; NOx for D Blendspermitted slight advancementand reduction of IT and EGRrespectively.

N/A " IP and " EGRincreased E Blendsgave higher E thanpure D

Higher% of n-butanol,retarded IT, higher IPand EGR gave ; PM

[68]20% and 40% blendof n-butanol withdiesel

Sweep from �10�to 2� ATDC

4S,4-cylinder, DIDV: 2198 ccRS: 3500 rpmCR: 16.6:1Injection system:common railInjection cone angle: 153�

PPCI 50% blend ofgasoline with D

Advancedinjection at 28�BTDC

50% Very low like 0.06 g/kW h Higher value like 10.5 g/kW h

Higher value like2.5 g/kW h

Very low like0.008FSN

[21]1800 rpmBMEP: 2.95

4S,1-cylinderDV: 500 ccCR: 18.4:1/16:1RS: 1500 rpmIMEP: 3/7.7/10.8 barInjection system: highpressure pump injection

PCCI Ultra-low sulfurdiesel

Sweep from �6� to3� ATDC

Up to 45.4% E ; about 20% as CR ; N/A N/A Very high E forhigher load. Forlower load, E ;irrespective of CR

[76]Variable CR andIMEP

E" significantly as load "

4S,1-cylinder, DI, WCRS: 1800 rpmCR: 17.8:1IP: 120 MPaInjection system: commonrail

PCCI 15–20% blend ofethanol with diesel

First injection: 60�BTDC

25% E ; up to 71% as 2nd IT retardedfrom TDC to 15� ATDC at afixed U

E " up to 100% as the2nd IT retarded fromTDC to 15� ATDC at afixed U

E ; up to 64% as 2nd ITretarded from TDC to15� ATDC at a fixed U

E " about 160% as the2nd IT retarded fromTDC to 15� ATDC at afixed U

[71]Dual stageinjection Second injection:

sweep from TDC to15� ATDC


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concentration


4S,1-cylinder, DIDV: 638 ccCR: 17.7:1RS: 1150 rpmIP: 140/180 MPaInjection system: commonrail

PCI 20% blend ofethanol with diesel

Sweep from 20� to40� BTDC

N/A As IT advanced eventually gotzero NOx

N/A Very advanced andretarded IT gave " E

N/A [70]

4S,1-cylinder, WCDV: 422 cm3

CR: 18.7:1RS: 1500 rpmIP: up to 1350 barInjection system: commonrail

Late injectionpremixed LTC

Diesel Sweep from�30.25� to 7.75�ATDC

Up to 65% As IT retarded and EGR ", E ;drastically. EGR higher than60% gave quite zero level E

As IT retarded and EGR", E "

N/A As IT retarded andEGR ", E ; drastically.

[16]

EGR higher than 60%gave quite zero levelE

4S,1-cylinder, DIDV: 2.44 LCR: 16.1:1Injection system: commonrailDirect IP: 800 barPort IP: 4.14 bar

RCCI operatingloads: 9.6–16.5 bar

Port injected fuel:15% gasoline + 85%ethanol

1st directinjection: 55�BTDC

Up to 47% Remained lower than 0.15 g/kW h all through the runningconditions.

At higher loadsincreased value.

Decreased as loadincreased

Constant low Ethroughout theoperating conditions.

2nd directinjection: 36�BTDC

[19]

Direct injected fuel:Diesel

4S,1-cylinder, DIDV: 300 ccCR: 19.5:1RS: 1500 rpmIP: 600–1000 barIMEP: 3 barInjectionsystem: common railIntake pressure: 1.4 bar

Retardedinjectionassistedpremixedhomogeneouscombustion.

ULSD Retarded singleinjection close toTDC

25% " IP caused " E, As the ITretarded emission ;

N/A N/A " IP caused ; E, Atretarded injectionsimultaneous ; ofsoot and NOx

[59]

E = Emission, CR = Compression ratio, RP = Rated power, RT = Rated torque, IP = Injection pressure, IT = Injection timing, BD = Biodiesel, D = Diesel, DV = Displacement volume,4S = 4 stroke, DI = Direct injection, WC = Water cooled, TC = Turbo charged, NA = Naturally aspirated, CC = Conventional combustion, ELTC = Early LTC, LLTC = Late LTC.

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Table 2Emission for Biodiesels at LTC.


Fuel Injectiontiming

Percentage ofEGR/O2

concentration

NOx CO HC PM/Soot Refs.

4S,1-cylinder, DI HCCI Diesel, 10� BTDC Up to 32% E " as BD content ", E ;as EGR "

E " as BD content andEGR ",

E as BD content andE R "

E ; as BD content", And E " as EGR "

[110]DV: 708 cm3

100%, 65% and30% blend of colzabiodiesel.

Up to 32% increment forBD than D

Average 16%increment for BDthan D

U to 52% increment forB than D

Up to 61%decrement for BDthan D

CR: 18.4:1RP: 11 kW@3000 rpmRT: 45 N m@2100 rpmIP: 650 barInjection system: pump

injection

4S,1-cylinder, DI High EGRenabled HCCI

100% soybiodiesel

17� BTDC Up to 60% E ; as EGR " Increased I reased E ; as EGR " [111]DV: 857 cm3

Variable loadingSlight higher E than D

CR: 17.8:1RP: 12.5 kW@ 2400 rpmIP: 4 barInjection system: injection pump

100% canolabiodiesel

Same trend like soy butrelatively loweremission

Increased I reased Same trend likesoy but relativelyhigher emission

4S,4-cylinder, DI Late injectionand EGR enabledLTC

Various blends ofsoybean oilderived biodiesel.

Sweep from�20� ATDC to5� ATDC

30% E " as IP and BD content", E ; as IT retarded.

E ; as IP and BDcontent ", E " as ITretarded.

E as IP and BDc ntent ",

Pure BD andhigher IP gives ;emission,

[103]DV: 4.5 L

Up to 12% increment forhigher IP for same blend Up to 22% decrement

for higher IP for sameblend

H ge " at IT beyond� � ATDC Up to 33%

decrement forhigher IP forsame blend.

CR: 17:1RS: 1400 rpmIP: 150/180 MPa

4S,4-cylinder, DI Variable loading(IMEP 5 to10 bar).

Biodiesel blend ofSoy, Canola,Yellow grease andTallow biodiesel.

Single andmultipleinjectionswith widerange sweepof IT.

Up to 70%according tothe condition.

For low load retarded ITup to 368�CA and higherEGR gave low emission.

At low load up to IT368�CA remained verylow and no effect ofEGR

A low load Almost zero atlow loadconditions

[105]DV: 1998 cm3

Variable boostpressure.

u to IT 368�CACR: 18.2:1

Single shot EGRassisted LTC forlow loads.

r ained very low andE light " as EGR "RS: 1500 rpm

Multi-pulse EGRassisted HCCI forheavy loads.

IP: 950 barInjection system:

common rail

At higher loads" EGRand " boost pressuregave; E,

At higher loads" EGRand; boost pressuregave" E,

A higher loads ;boostp essure gave" E,

At higher loads "EGR gave " E forsingle injection,Two earlyinjections gavegood results

Lowest E for two earlyinjections at 340�CA

Lowest E for two earlyinjections

L west E for two earlyi ections

4S,4-cylinder, DI EGR and lateinjectionassisted LTCmode

Biodiesel blend ofSoy, Canola,Yellow grease andTallow biodiesel.

Single shotinjection(IMEP 8 bar)

Up to 70% For single shot injectionE got zero value at 50%EGR. For multi-pulse,retarded IT and reducednumber of injection ; E.

Multiple injections (4shots) gave reduced Eat lower EGR.Otherwise increased.

V ry " for singlei ection, Multiple( hots) injectionsr uce the E

Very reducedvalue both forsingle andmultipleinjections.

[108]DV: 1998 cm3

Variable intakepressure (1.2/1.5 bar)

Multi-pulseinjection(IMEP 6 bar)

CR: 18.2:1

Wide range(347–367�CA)sweep of IT

RS: 1500 rpmIP: 950 barInjection system:

common rail



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Table 2 (continued)



Percentage ofEGR/O2

concentration


4S,4-cylinder, DI Late injectionpremixed LTCmode.

20, 50 and 100%Soy-based methylester

5�, 7� and 9�BTDC

50% E " as IT advanced andBD content "

Higher than 1500 ppmfor all the cases

E " for retarded IT and ;for " BD portion

Very lowespecially for100% BD

[109]DV: 1.7 L

Up to 50% increment forBD than ULSD atretarded IT

Up to 42% decrementfor BD than ULSD atretarded IT

CR: 16:1RS: 1500 rpmLoad: 400 kPaIP: 800, 1000, 1200 barInjection system:

common rail

4S,4-cylinder, DI, TC Late and earlyinjectionpartiallypremixed LTC

100% soy basedmethyl ester and50% blend of soybased methylester with ULSD

5.9–7.1� BTDC(for LLTC)

45%(LLTC) N/A N/A E ; as BD content ",ELTC, LLTC and CC gave64%,25% and 66% ;respectively for B100than ULSD

ELTC gave thehighest E, 94% "when used B100than ULSD, CCgave lowestemission.

[143]DV: 1.7 L

17.3–24.1�BTDC (forELTC)

55%(ELTC)CR: 16:1RS: 1500 rpmRP: 75 kW@4400 rpmIP: on average 870 barInjection system:

common rail

4S,1-cylinder, DI, NA EGR assistedsingle injectionLTC and pilotignited HCCIcombustion

100% yellowgrease basedbiodiesel.

17� BTDC(conventionalsingle shot)

Up to 32% As load ;, EGR and BDcontent " emission ; forsingle injection, Pilot-ignited HCCI ; emission.

Comparatively low Efor pilot injection

Same trend as CO E " as load andEGR " for singleinjection,

[112]DV: 857 cm3

Variable BMEP:3.3–8 bar

Pilot (4–8)injectionsstarting at 17�BTDC

Pilot-ignitedHCCI gave verylow E

CR: 17.8:1RP: 12.5 kW@2400 rpmInjection system: pump

injection

4S,1-cylinder, DI Late injectionLTC

20%, 50% and100% soybiodiesel

Sweep ofinjectiontiming from�25� ATDC to3� ATDC

N/A Retarded IT gave ; Ethan early IT; exceptearly IT, E " as BDcontent "

Very decreasedemission forretardedinjection

[107]

DV: 300 cc

Up to 68% decrementthan CC

CR: 19.5:1RS: 1500 rpmIP: 600 barInjection system:common rail

4S,4-cylinder, DI, TC Late injectionEGR assisted LTC

40% biodieselblended withultralow sulfurdiesel.

Singleinjection (6�BTDC)

Up to 38% As EGR " E ;, BD blendshowed slight " E thanULSD Single injectionwith EGR showed betterresults than doubleinjections.

N/A N/A N/A [167]DV: 2.5 L

1600 rpm and25% loading. Double

injection(pilot: 25�BTDC, main:2� ATDC)

CR: 17.5:1RP: 103 kW@4000 rpmIP: up to 1600 barInjection system:

common rail

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Table 2 (continued)



Percentage ofEGR/O2

concentration


4S, 1-cylinder, DI PPCI 20, 50 and 100%palm oil methylester andsunflower methylester blendedwith diesel fuel.

Sweep from32� BTDC to 4�BTDC

O2

concentration9–10%

N/A At 19� BTDC E waslowest for all the fuels.PME gave less E thanSME. At " loadMinimum E shifted toadvanced CAo

At 21� BTDC E waslowest for all thefuels.100% SME gavelowest E. At " loadMinimum emissionshifted to advanced CA�

N/A [163]DV: 0.477 L (1) IMEP: 3 bar,

1500 rpm, intakepressure: 1.5 bar

CR: 14: 1

(2) IMEP: 6 bar,2000 rpm, intakepressure:1.85 bar

RS: 1500prmIP: 860 barInjection system:

common rail

4S,4-cylinder, DI Late injectionpremixed LTC

ULSD, 100%soybean methylester, biodiesel–ethanol (80–20%)blend.

Sweep from8.3� to 7.5�BTDC (for100%biodiesel)

40% for highload Up to 50%for low loads.

E less than 1 g/kg-fuel ateach loading for BD-ethanol

N/A N/A For BD and BD-ethanol blendless than 0.25FSNfound almost allover engine load.

[106]DV: 425 cm3 (1-cylinder)

Variable loading(IMEP: 0.25–0.65 MPa)

13� to 10.5�BTDC (forbiodiesel–ethanol)

for other fuels at 0.4–0.65IMEP

CR: 15:1RS: 1500 rpmIP: 100 MPaIntake pressure: 120/150 kPaInjection system:

common rail

4S,1-cylinder, DI HCCI Rapeseed methylester

Advancedinjection tocreate HCCI

N/A E ; as IP ", E ; as IP " up to 43% decrementthan D at higher IP

Tends to zerolevel of emission.

[144]DV: 522 cm3 Variable

injectionpressure (400–700 bar)

Up to 33% decrementthan D.CR: 17.5:1

RS: 1500 rpmLoad: BMEP1.6 barInjection style:

common rail

4S,1-cylinder HCCI 20% blend ofpalm, coconut,rape, soy andmustard oilmethyl ester withULSD

N/A N/A E " with " intake airtemperature

E " with ; intake airtemperature

E " with ; intake airtemperature

N/A [165]Constant speed Variable intake

air temperature(30–300 �C)

IMEP: 3 bar

50% burn pointconstant to365�CA

CR: 10.5:1Injection system:

Heated port atomizationfuel system.

4S,1-cylinder, AC HCCI Soy biodieselblended withULSD up to 50%

N/A N/A E ; as the intaketemperature ;,

E ; as intaketemperature ", E quitesame for all the blends.

N/A N/A [51]DV: 517 cc Variable intake

air temperatureusing 6 kWheater.

At 160–170 �C intaketemperature Ewas 6 1 ppm

CR: 10.5:1RP: 7.9 kW@3000 rpmInjection system: air-assisted

partial-vaporizationport fuel injection.


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Table 2 (continued)



Percentage ofEGR/O2

concentration


4S,1-cylinder, DI, AC Fuel vaporizerwith port fuelinjectionassisted HCCI.

100% biodiesel 23� BTDC (fordirectinjection)

N/A BD vapor induction gavevery low E,

As load ", E ;, up to20% decrement for BDvapor induction

As load ", E ;, BD vaporinduction emittedlowest

As load ", E ", BDvapor inductionemitted almost1/3 of the DIsystem

[113]DV: 662 cm3

Variable loading

Up to 87% decrement at2–4 bars BMEP than DIsystem

CR: 17.5:1RP: 4.4 kWRS: 1500 rpmIP: 2 barInjection system: port

fuel injection with fuelvaporizer, direct injection

4S,1-cylinder, DI Late injectionHCCI

Neat soybeanbiodiesel and 20–50% blend ofbiodiesel withlow sulfur diesel.

�25�ATDC,�10�ATDC and 3�ATDC

N/A At IT �25� ATDC very "E, At 3� ATDC the lowestE for all fuel blends, E "as BD content "

N/A N/A At 3� ATDCsimultaneousreduction of sootand NOx

[142]DV: 300 ccCR: 19.5:1RS: 1500 rpmIP: 600 barInjection system: common railInjection cone angle: 150�

4S,4-cylinder, DI Single lateinjectionPremixed LTC

Neat soy-basedmethyl ester.

Late injection 45% Very low E, Within therange of 26–35 ppm

About 17% lessemission than ULSD

About 30% lessemission than ULSD

Engine out PMwas over anorder ofmagnitudehigher than ULSD

[115]DV: 1.7 L

BMEP: 400 kPaCR: 16:1

Intake pressure:100 kPa

RS: 1500 rpmRP: 11.3 bhpIP: 1007 barInjection system:

common rail

4S,4-cylinder, DI, TC Early injectionLTC and lateinjection NissanMK typecombustion

50% and 90%blend ofrapeseed methylester withEuropean diesel

�30� to �10�ATDC for lowload,

Up to 60% For early IT, E " as IP andBD content ",

For early IT E " as IP ;, For early IT E ; as IP andBD content ",

At higher load, "EGR and retardedIT gave ; E

[104]DV: 2 L

BMEP: 2 and5 bar

At TDC forhigher load.

Two stageEGR coolerconfiguration.

At " loads retarded ITgave low E like 30 ppmat high EGR

For late IT E " as EGRand BD content ".

For late IT, E " as EGRand BD content "

CR: 14.4:1

Variableinjectionpressure: 700–900 bar for earlyinjection,1600 bar for lateinjection.

RS: 1600 rpmIP: 1600 barInjection system: common rail

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extensive EGR (50%) to gain PPCI mode and reported very lowamount of NOx (0.06 g/kW h). Masuda and Chen [70] also got sametype of results by the use of ethanol. Mohammadi et al. [71] triedtwo stage injection with 15–20% blend of ethanol with diesel fuelto reach PCCI condition. First injection was at 60� BTDC and theyreported that retarding the second injection along with 25% EGRimproved the NOx emission. Low cetane number of ethanol permit-ted very early injection giving a long ignition delay and moderateduse of EGR suppressed the possible NOx emission from the secondinjection.

Not only the lower CN fuel blend but also late injection can elon-gate the ignition delay to reach premixed LTC mode. Jacobs andAssanis [72] experimented with retarded injection and high EGRrate which gave them increased ignition delay and combustionduration. Such experimental data confirmed the achievement ofPCI (premixed compression ignition) which showed reduced NOx.As the EGR rate increased and injection time retarded, emission ofNOx decreased. Bittle et al. [73] reported application of immenseEGR gave them about 94% decrement of NOx while they were tryingto get a universal determination of LTC mode attainment criteria.Retardation of injection timing with EGR gave them even better re-sults of emission. Han et al. [53] also reported same results by theuse of cooled EGR. However, Kiplimo et al. [74] reported lowerNOx even in early injection (20� BTDC) during PCCI combustionstrategy. With EGR, they got about 75% decrement of NOx emissionand negligible difference of IMEP. They worked out an optimumspray-targeting zone where they got simultaneous reduction ofCO, HC and soot but could not manage to reduce the NOx withoutEGR. Without EGR, lower injection-pressure (80 MPa) with lateinjection (2–15� BTDC) gave reduced amount of NOx while for high-er injection pressure (140 MPa) advanced injection (20–40� BTDC)resulted in reduced NOx. This lower NOx for higher injection pres-sure at advanced injection timing can be attributed to the achieve-ment of PCCI regime. PCCI regime ensured lower in-cylindertemperature and longer premixing time, which resulted in lowerNOx [75]. On the contrary, Alriksson and Denbratt [20] reportedhigher NOx when they tried advanced injection timing to keep theBSFC (brake specific fuel consumption) low. However, Kook et al.[16] investigated the effect of dilution and injection timing veryprecisely on low temperature combustion emission. In a fixed SOI(start of injection) they observed that NOx emission decreased asthe dilution increased. NOx emission was actually correlated withthe adiabatic flame temperature. Again, in a fixed level of dilution,retardation of injection timing gave lower NOx. They commentedthat earlier injection timing assisted by high level of dilution, gener-ated greater adiabatic flame temperature than the late injectioneven with less amount of dilution. From NOx formation point ofview, we can infer that, as late injection LTC mode generates lowertemperature than early injection LTC mode, the former one is betterin this regard.

Laguitton et al. [76] observed the effect of compression ratio onNOx emission under a wide range of PCCI like combustion styles.Lowering the compression ratio gave them lowered NOx. This effectwas more pronounced at higher loads. They also observed that athigher loads, combustion style proceeded to premixed-charge fromthe combination of premixed and diffusion type combustion as theinjection timing was swept from very early to retarded. Hence, NOx

emission was converged as the diffusion combustion suppressed.They also concluded that below a certain combustion temperature,as in fully premixed charge combustion, NOx emission was domi-nated by the air fuel ratio and the local oxygen concentrationrather than in-cylinder pressure and temperature.

However, HCCI combustion mode has also been cited for reducedNOx emission. Pidol et al. [77] evaluated that ethanol–diesel–biodie-sel blend could be used to keep NOx under the HCCIacceptance criteria (<0.1 g/kW h) even at higher loads. They used

two types of ethanol–diesel–biodiesel blends. In one type of blendfossil diesel was used and to another synthetic Fischer–Tropsch die-sel was used. In both cases 20% ethanol was used. Use of ethanol ex-panded the ignition delay which helped to reach HCCI mode. Theysucceeded to increase the IMEP to 12.2 bar and 11 bar for fossil die-sel and Fischer–Tropsch diesel blend respectively while for both ofthe cases NOx emission was below 0.10 g/kW h. Reason for sustain-ing higher load with lower NOx of fossil diesel–biodiesel–ethanolblend can be attributed to the lower cetane number of the blendwhile Fischer–Tropsch diesel–biodiesel–ethanol blend sustainedat bit lower load for the reason of higher CN of Fischer–Tropsch die-sel. Again higher volatility contributed by the ethanol permittedlower injection pressure to reach homogeneity which reduced thecombustion rate hence lowered NOx. Kim and Lee [35] also cited verylow NOx at HCCI combustion style.

While researchers were trying to increase the operating load un-der LTC mode keeping the NOx lower, RCCI combustion mechanismgave them very good results. Splitter et al. [19] got very reducedNOx (below than 0.1 g/kW h) while they tried gasoline as the lowreactive fuel and diesel as the high reactive fuel at higher loads like14.5 bar. Though use of ethanol–gasoline blend (85% ethanol) as thelow reactive fuel gave a bit higher NOx emission, but due to its lowercetane number it sustained even higher load (16.5 bar) than the gas-oline. This dual-fuel mechanism needed comparatively lower EGR tokeep the NOx lower, which increased the thermal efficiency as well.Splitter et al. [78] also tried single fuel stock as the basis for both highand low reactive fuels. They reported lower NOx but the load level waslower than the former experiments. They reported that at about 9 barload the emission characteristics were just similar to the dual fuel ap-proach of RCCI. Again Splitter et al. [79] tried to reveal the impacts ofinjection timing on emission within RCCI combustion process. Theyinjected iso-octane by port injection and n-heptane by direct injec-tion. They reported, regardless of the single or double direct injection,injection timing had minimal effect on reduced NOx emission exceptbeyond �60� ATDC (after top dead center). NOx increased beyond�60� ATDC for both injection style and that can be attributed to theless available mixing time. Rapid ramp on the heat release rate at thatinjection timing clarified the scenario. For double direct injection,they succeeded to retard the injection timing 10 more crank angledegrees as for such injection, fuel mass was more mixed.

However, after such discussion we can come to some salientpoints which are following:

� Regardless of PCCI, HCCI or RCCI combustion modes, lower NOx

depends on higher ignition delay and lower combustion ratewhich result in lower in-cylinder temperature and pressure riserate.� Below a certain combustion temperature, NOx emission is con-

trolled by air–fuel ratio and local oxygen concentration morethan in-cylinder temperature.� Late injection premixed LTC is better than early injection pre-

mixed LTC mode regarding NOx emission.� Advanced injection assisted HCCI combustion mode needs

higher EGR to control NOx emission as advanced injectioncauses higher in-cylinder temperature.� RCCI combustion mode has succeeded to keep the NOx level

lower at higher loads even with low EGR rate by the help of fuelreactivity gradient inside cylinder.

3.1.3. NOx emission under LTC modes for biodieselsIncreased NOx is an established phenomenon while using bio-

diesel in internal combustion engines [80–82]. This increment isnot solely controlled by the change of a single fuel property, rathersome coupled mechanisms which may strengthen or cancel oneanother in various circumstances depending on combustion andfuel properties [83]. Potentially contributing factors to make


differences in NOx emission for biodiesel can be summarized asinjection timing, injection pressure-spray-mixing, ignition delay,combustion stages and heat release, heat radiation from soot,combustion temperature, fuel unsaturation and system responseissues [84].

Due to higher densities, bulk modulus of compressibility andspeed of sound, start of fuel injection is advanced for biodiesel rel-ative to petroleum diesel in rotary/distributor-style fuel injectionsystems [85,86]. An advance in injection timing is considered asa main reason for observed increases in NOx emissions with biodie-sel as it helps to elevate diffusion reaction temperatures and ulti-mately post flame gas temperature. Of course, this incident is notpresent in common rail fuel injection system [87]. Szybist et al.[88] investigated NOx emission characteristics of different fuelsincluding biodiesel, altering injection timings at high and low loadconditions. They observed at higher loads, relation between NOx

emission and injection timing was independent of fuel types butat low loads emission characteristics were unique for each fueltypes. Therefore, it can be said that, for higher loads increase inNOx emission is due to the advancement of injection timing butthis is not true for low loads. This confirms the existence of otherfactors for increased NOx for biodiesels along with the advancedinjection timing. Such as, biodiesels have higher cetane numberwhich depicts shorter ignition delay [89–92]. A short ignition delayreduces the premixed burn, consequently increases the fraction ofdiffusion burn [93]. In the diffusion stage, the equivalence ratio atthe flame front is essentially always at a stoichiometric value [94].Therefore, once the fuel is largely being consumed in a diffusionflame, it is more relevant to consider the oxygen fraction withinit. It is well-known that higher oxygen fractions yield higher dieselcombustion temperatures and NOx formation rates for diffusionflame [95–97]. Ullman et al. [98] has confirmed that, because of in-creased oxygen content and decreased sulfur content, PM forma-tion is comparatively low in the biodiesel combustion thanpetroleum fuel. Less PM depicts less radiation heat transfer whichincreases post-flame gas temperature therefore increased NOx

emission [84]. Again biodiesels have got higher degree of unsatura-tion [99,100], and Graboski et al. [101] reported increase in NOx

emission, with the increase in unsaturation and decrease in meancarbon chain length. Finally, the changes in NOx emission forbiodiesel are largely dependent on pre-combustion chemistryof hydrocarbon free radicals [102]. It incorporates promptmechanism of NOx formation more in consideration, because it ismore sensitive to radical concentration within the reaction zonewhereas thermal mechanism remains quite unaffected by fuelchemistry.

Low temperature combustion is a promising technique for NOx

reduction not only for petroleum diesel but also for biodiesels,though they produce much higher NOx than petroleum diesel asdiscussed earlier. Veltman et al. [103] experimented sweep ofSOI from �20� ATDC to TDC with a common rail injection withmoderated EGR to gain premixed LTC. Electronically controlledinjection system ensured same injection timing for all the fuels.Still higher biodiesel content showed higher NOx emission whichcontradicts the so called general clarification (advanced injectionfor higher density) of the higher NOx. They got reduced NOx (lessthan 0.5 g/kW h) at 30% EGR at very retarded SOI due to lowercombustion temperature. Though higher injection pressure causedhigher emission for increased combustion temperature, it wasinsignificant at higher EGR as the emission was already low. Wealland Collings [104] also reported higher NOx emission for higherinjection pressure at premixed LTC. Along with EGR and injectionpressure, intake pressure has also been cited for having commandon NOx emission in premixed LTC. NOx Emission decreases as

intake pressure increases for biodiesels [105,106]. Better premixof charge was responsible for such results.

Fang et al. [107] claimed that even in premixed LTC mode, oxy-gen content in biodiesel dominated the NOx emission more thanignition delay while they tried a sweep of SOI from �25� ATDCto 3� ATDC. They observed higher ignition delay of biodiesel thanEuropean low-sulfur diesel, which attributed to lower cetane num-ber and higher boiling point of biodiesel that slowed down thedroplet evaporation rate hence preparation of the ignitable air–fuelmixture. In spite of higher ignition delay, increasing portion of bio-diesel showed increasing NOx at the conventional and late SOI.They attributed this phenomenon to the higher oxygen contentof the biodiesel. They suggested a trade-off between ignition delayand oxygen concentration was responsible for this incident andconcluded commenting that late SOI was better to reduce theNOx emission than early SOI. Similarly, Zheng et al. [108] observedlower NOx for late injection but unlike Fang et al. [107], theyobserved higher cetane number of biodiesels and commentedthat for this reason biodiesels sustained late SOI as well asEGR-incurred LTC better.

Along with the oxygen concentration, injection timing and igni-tion delay, combustion phasing has influence on NOx emission inthe case of premixed low temperature combustion. From a com-mon baseline condition of combustion, created by keeping the50% mass fraction of the fuel burned point constant, Northropet al. [109] got the NOx emission curves more or less same for allthe fuels they tested. It proves the command of combustionphasing on NOx emission in premixed LTC modes. They also ob-served the combustion location as a dominant factor of NOx

emission.To reduce NOx emission from biodiesel combustion, HCCI has

also been tried by the researchers. Jiménez-Espadafor et al. [110]showed the effect of EGR on NOx emission at late injection HCCI.They got the same story of EGR and biodiesel content like premixedLTC. Interestingly they observed that even on higher EGR,higher biodiesel content showed lowest ignition delay and theysuggested that ignition delay relied more on chemical kineticsmechanism than the temperature reduction made by EGR.However, shorter ignition delay may also produce less NOx if theignition delay is short enough to make a weak mixture [111]. Ifthe mixture gets close to the stoichiometry then again the NOx willbe higher.

EGR assisted single injection LTC and pilot ignited HCCI com-bustion were investigated by Zheng et al. [112]. They reportedthat EGR was the instrumental factor to reduce the NOx at singleshot injection by reducing in-cylinder flame temperature anddiluting the oxygen concentration. Pilot ignited HCCI with im-mense EGR reduced the NOx emission even more by helping toovercome mixing problem which led to homogeneity of the mix-ture. Later, the same authors [111] experimented with variousbiodiesels with the same setup and got the same trend of resultsincluding lower emission for lower loads and higher emission forthe higher loads. This can be attributed to the higher flame tem-perature for higher loads and vice versa. However, they [105] gotan improvement at higher loads while they tried two early injec-tions with higher boost pressure. They mentioned better combus-tion process and improved combustion phasing due to enhancedfuel–air mixture responsible for such improvement. Recently,Ganesan et al. [113] tried a unique technique to reach HCCI likecombustion and they succeeded to keep the NOx substantiallylow. They used a fuel vaporizer with port fuel injection to achievethe mixture homogeneity as well as to attain HCCI like combus-tion process which gifted low level of NOx. Pidol et al. [114] usedethanol–diesel blend, stabilized by biodiesel, and they got quite


low NOx all over the engine loading range. Low soot tendency ofethanol permitted higher EGR which helped to reduce NOx drasti-cally. Again low ignitability let the mixture to be homogeneouswhich was conducive to the LTC.

However, some points can draw a summary to this discussion:

� Increment of NOx for biodiesel is a matter of aggregated factors.Only advancement of injection timing or lower ignition delay isnot responsible for this.� Higher percentage of EGR or late injection timing increases igni-

tion delay which can reduce the NOx.� Ignition delay relies more on chemical kinetics mechanism of

biodiesel than temperature reduction made by EGR.� Keeping combustion phasing constant for different biodiesels

by tuning concerning parameters gives similar trend of NOx

emission. It establishes that combustion phasing is one of thedominant parameters regarding NOx emission of premixed LTC.� Late SOI is better than early SOI for lower NOx emission in bio-

diesel combustion. Higher percentage of oxygen and highercetane number of biodiesel can sustain higher EGR and lateSOI incurred LTC better. Therefore, better NOx reduction.

3.2. PM emission analysis

3.2.1. PM formationDiesel particulates are principally combustion generated carbo-

naceous material (soot) where some organic compounds remainabsorbed as well [67] and grow via gas to particle conversion pro-cess [115]. Diesel engines significantly emit particulate matter. Tobe precise, particulate matter is a highly complex mixture of fineparticles and liquid droplets including soot, ash, hydrocarbon solu-ble organic fraction (SOF) and water SOF [99]. PM varies in size,shape, number, surface area, solubility, chemical composition andorigin [99,116,117]. Size distribution of the PM has three modesconsisting coarse particles, fine particles, and ultrafine particles[118,119]. These particles exist in various shape and densities inthe air thus aerodynamic diameter is used to define the size ofthe particle [120]. Soot particle size can be as small as 1–2 nm atinitial state [66]. Collision of rings causes coagulation and cluster-ing together similar to a chain, making the soot grow to agglomer-ates with size ranging 100–1000 nm. Soot content in the exhaustgas is indicated by the smoke opacity; hence, this parameter canbe correlated with fuels tendency to form PM during combustion.

Incomplete combustion of fuel hydrocarbons produces most ofthe particulate matter with little contribution of lubricating oil. Itsources from the rich combustion zones where the equivalence ra-tio is higher than 1. This is the reason for the highest particulateconcentrations in the core region of each fuel spray in direct injec-tion diesel engines [67]. Generally, soot formation takes place athigher than 1800 K temperature in diesel combustion environ-ment. Net soot release is commonly defined as the difference be-tween formation and oxidation of soot. Formation and oxidationof soot are strongly coupled with the combustion temperature justlike the NOx formation. So, conventionally soot and NOx formationhave got an inverse relation known as soot–NOx tradeoff [73].

3.2.2. PM emission under LTC modes for dieselIn LTC, simultaneous reduction of soot and NOx are achieved by

reducing the combustion temperature lower than soot formationlevel. Once LTC is attained, soot formation loses its strong depen-dence even on equivalence ratio [72]. In LTC mode soot formationoccurs primarily downstream in the head of the jet [121]. This is incontrast to the upstream soot-producing core, in conventional die-sel jets [122]. This shift is due to the charge dilution employed in

LTC and mixing between the end of injection and second-stageignition [123].

Many researchers have reported successful reduction of sootwhen they attained LTC. Actually more complex relationship existsin soot reduction than that of NOx. Soot oxidation process is moresensitive to temperature than the soot formation process [73].Therefore, when application of EGR reduces the combustion tem-perature, oxidation rate falls dramatically and emission of sootincreases. Further reduction of temperature by higher level ofEGR or retarding the SOI, below the soot formation level gives verylow amount of soot. For example, PCI combustion condition at-tained by late injection have been reported to produce very lowamount of PM at about 48% EGR [53]. Alriksson and Denbratt[20] also reported that they needed almost 50% EGR even on 25%loading when the peak combustion temperature was compara-tively low. They observed an increment of soot up to 50% EGRand then it suddenly decreased, which supports the soot oxidationand formation relationship discussed previously. Genzale et al.[124] studied the effect of spray targeting on engine out soot emis-sion of a heavy-duty optical diesel engine in LTC regime achievedby late injection. They measured fuel-vapor concentration byfuel-tracer (toluene) fluorescence and measured OH, PAH fluores-cence and combined formaldehyde to evaluate combustion, mixingand formation of pollutants. They observed that spray pattern withincluded angle of 152�, injected fuel towards the vertical center ofthe piston bowl and wall impingement occurred. Also, mergingwith neighboring jets happened before the peak heat release. Nearthe piston bowl floor, jet–jet interaction created fuel-rich regionswhich were the main sources of PAH (poly aromatic hydrocarbon,precursor of soot) formation. They concluded that narrow injectionangle (124�) or wider injection angle (160�) could avoid thisproblem.

It is reported that PCI or PCCI achieved by advanced injectionand extensive use of EGR also produced lower soot emission.Jacobs and Assanis [72] experimented with an AF (air–fuel) ratiowhich is lower than stoichiometric (14.7) and the injection timingwas 25� BTDC, confirming the low peak combustion temperature inthe combustion. They reported tremendous low soot (0.03 FSN)emission. Though, lower global AF ratio indicated rich mixture,soot decreased. Therefore, within the PCI combustion regime netsoot release has very little sensitivity to local equivalence ratio.Parks li et al. [75] also reported the same result when they attainedPCCI by advanced injection timing and extensive EGR. Although PMemission decreased by about 51% than conventional combustion,PM size was smaller and the soluble organic fraction was in-creased. But opposite results are also been reported by Benajeset al. [125]. They studied the PM emission with advanced injectiontiming within the premixed LTC regime. Advancing the injectiontiming empowered them to control ignition delay and local equiv-alence ratio of the injected fuel. Higher value of the ignition delayand maximum local equivalence ratio for advanced injection tim-ing were supposed to give less PM formation. Surprisingly theyobserved increased PM with injection advancement. Most drasticincrement of PM (185%) happened when they advanced theinjection from �30� ATDC to �33� ATDC. Though here the end ofinjection occurred before the start of combustion as in premixedLTC mode, PM formed because of fuel deposition on the surfaceof piston bowl. As the injection was advanced, the fuel spray trajec-tory crossed the piston bowl surface at a higher point. It caused in-creased liquid fuel spray and combustion chamber surfaceinteraction, hence more deposition. This was the reason behind in-creased PM formation, though it was a premixed LTC mode.

Researchers have also reported decreased PM for HCCI combus-tion strategy [14,77,126]. Cracknell et al. [126] investigated the ef-


fect of a broad range of fuel properties on HCCI combustion strat-egy. They observed several fuels, at certain speeds and loads, brokethe NOx–PM trade-off curve and produced simultaneous reductionof NOx and soot. But as the load increased, all fuels tended revert-ing to classic diesel NOx–PM trade-off curve. Singh et al. [14] stud-ied the effect of dual injection strategy in the HCCI combustionprocess. They started the first injection early (22� BTDC) withlow load condition which helped the fuel to be premixed well. Ear-lier injection caused cool-flame heat release followed by secondstage combustion. As the fuel was premixed well, soot formationwas low because there were less fuel-rich soot pockets. At the sec-ond injection (15� ATDC), the combustion was much like conven-tional diesel combustion process. Significant mixing-controlledcombustion occurred and soot was formed in less well-mixedzones.

Therefore, concerning the above discussion, some prominent is-sues can be noted regarding soot/PM emission for LTC conditions.They are following:

� PM/soot emission increases with the increase of EGR rate,advancement or retardation of injection timing to achieve LTCmode, until the combustion temperature is above the formationtemperature of soot.� Under soot formation temperature, soot emission does not

depend on local equivalence ratio.� Narrow angle of injection is conducive to lower soot emission

during late injection premixed LTC.� Although very advanced injection can create premixed charge

in HCCI mode, deposition of fuel can increase PM/soot emission.� Very late injection during premixed LTC creates fuel rich pock-

ets which can increase soot emission.

3.2.3. PM emission under LTC modes for biodieselsThough some of the researchers have reported increased PM

emissions for biodiesel time to time [127–129], it is almost unani-mous scenario that biodiesel reduces PM emission significantly[130–134]. Researchers claiming increased PM emission, have acommon explanation that, reduction of the insoluble fraction (ISF)of the PM is compensated by the increase of soluble organic fraction(SOF) which increases with the use of biodiesel [101,135–137]. Mainreasons for reduced PM emission for biodiesel than diesel can besummarized as, increased oxygen content, lower stoichiometricneed of air, absence of aromatics and sulfur, combustion advanceand soot structure formed while using biodiesel [94]. Higher oxygencontent of biodiesel molecule ensures complete combustion even onthe fuel-rich zones and reduces PM emission [138]. Possibility offuel-rich zones reduces as the stoichiometric need of air is less forbiodiesel. On the other hand advanced combustion elongates theresidence time which confirms better oxidation of soot particleshence reduces the emissions [139,140]. Structure of soot particlesof biodiesel also have been reported as a reducing factor of PM asit helps in oxidation of soot [141]. Aromatics, considered as theprecursors of soot, reduce PM emission by its absence in biodiesel[85].

Though biodiesel reduces PM emission, LTC with biodiesel re-duces PM emission even in a better form as it reduces simulta-neously NO x and PM. Veltman et al. [103] reported higherinjection pressure reduced PM emission when they achieved lateinjection premixed LTC. They commented that as higher injectionpressure had less significance on NOx emission at higher EGR, itcould be used to achieve simultaneous reduction of PM and NOx.Zheng et al. [112] also reported that higher injection pressure, boostpressure, multi-pulse injection with higher EGR reduced PM andNOx simultaneously. To explain higher emission of soot with EGR,they emphasized on not achieving the threshold temperature ofsoot formation as EGR was not increased to that level. Later,

the same authors [108] experimentally proved the previousexplanation by increasing EGR, to much higher level, and got re-duced soot emission. They also attempted retarded multiple injec-tion with higher EGR and got much reduced emission. In aseparate experiment, Zheng et al. [105] showed multiple injectionreduced soot emission even in higher loads while other authors[113] claimed higher PM emission for higher loads in premixedLTC. Fang et al. [107] reported that retarding injection timing untilafter the TDC was a potential way for simultaneous reduction of sootand NOx. In a separate experiment [142] they again claimed post-TDC injection provided ultra-low soot. They attributed this to thecombination of low soot formation characteristics of biodiesel andlow temperature combustion feature of the retarded post-TDCinjection strategy. Weall and Collings [104] also reported very de-creased amount of smoke emission as they retarded injection tothe TDC. However, opposite findings are also there about PM emis-sion during premixed LTC. Northrop et al. [115] reported over an or-der of higher magnitude of PM emission than diesel fuel while theyexperimented premixed LTC with biodiesel. Though the engine outsoot was lower for biodiesel, they mentioned that PM emission in-creased due to the conversion of unreacted biodiesel to PM throughcondensation. In a separate experiment of partially premixed LTCwith early and late injection, Northrop et al. [143] again found veryhigh emission of PM due to the same factor.

Along with premixed LTC, HCCI combustion can reduce PM emis-sion to a satisfactory level. Mancaruso and Vaglieco [144] reportedvery low amount of PM when they achieved HCCI combustion withrapeseed methyl ester. Homogeneous lean charge in the combus-tion chamber helped to reduce PM. Moreover, higher injection pres-sure facilitated the atomization of fuel and higher oxygen content ofbiodiesel ensured complete oxidation of soot. They attributed suchreduction to less soot formation and enhanced rate of oxidation.Jiménez-Espadafor et al. [110] corroborated this finding in theirarticle. However, overall emission was quite low and pilot-ignitedHCCI gave them very low amount of soot. Zhu et al. [106] exploredthe potential of very low soot emission by using ethanol-biodieselblend with late injection HCCI strategy. However, advancedinjection timing can also reduce PM emission but it depends onachieving HCCI like combustion mode [144]. Ganesan et al. [113] re-ported, along with early and late fuel injection, port fuel injectionwith a fuel vaporizer reduced PM emission because of absence ofdiffusion combustion and localized fuel rich mixture.

Nevertheless, salient points of PM emission during LTC of bio-diesel are as follows:

� Inherent properties of biodiesel, like increased oxygen content,lower stoichiometric need of air, absence of aromatics and sul-fur, combustion advance and soot structure spontaneouslyreduces PM emission irrespective of conventional or low tem-perature combustion.� Higher injection pressure, retarding injection timing, multi-

pulse injection with higher boost pressure along with higherlevel of EGR are instrumental factors of PM reduction in pre-mixed LTC for biodiesel.� Due to conversion of unreacted biodiesel into PM through con-

densation, premixed LTC can increase PM emission time totime.� HCCI combustion is very efficient in PM reduction for biodiesels

as it permits higher injection pressure and EGR. Consequently,ignites homogeneous mixture.

3.3. UHC and CO emissions analysis

3.3.1. UHC and CO formationIn conventional diesel combustion the main reasons for HC

emission are trapping of fuel in the crevice volumes of the


combustion chamber, low temperature bulk quenching of theoxidation reactions, locally over-lean or over-rich mixture, liquidwall films for excessive spray impingement and incomplete evap-oration of the fuel [67]. Turns [65] described two ways to form COand UHC, viz. overly lean mixture and overly rich mixture. In thecase of overly lean mixture, flame cannot propagate through themixture and fuel pyrolysis with partial oxidation causes CO andUHC. For overly rich mixture, fuel cannot mix with sufficientamount of air, or even mix, does not get sufficient amount oftime to get oxidized. This results in significant amount of COand UHC.

3.3.2. UHC and CO emissions under LTC modes for dieselIn LTC regime, to reduce the locally fuel-rich region, intake

charge and fuel are mixed more thoroughly before combustion.With reduced combustion temperature, though this mode helpsto provide simultaneous reduction of NOx and soot [16,20,145]on contrary due to the reduction of in-cylinder combustion tem-perature and oxygen concentration, incomplete combustion prod-ucts like HC and CO increase. Actually, this is one of the primarychallenges of applying HCCI, PCCI or PPCI. Homogeneous or par-tially homogeneous mixtures are formed in these combustion pro-cesses and a significant amount of fuel is stored in the crevices atthe time of compression and escape combustion. Since, the burnedgas temperature is not that much high in these processes to con-sume the fuel, when they are back into the cylinder during expan-sion, CO and UHC formation becomes inevitable.

Han et al. [53] experimented with increased EGR and lateinjection timing assisted premixed LTC mode. Long ignition delayover mixed the air–fuel and in some regions the charge becametoo lean to burn which caused unburned fuel. Actually, whenthe injection occurs just before the TDC, lower peak bulk temper-ature hinders the oxidation process of the fuel. Because in thatcase, combustion occurs after TDC, when with every crank angle,the contents of cylinder are getting cooler for expansion. There-fore, for late injection LTC, bulk quenching mechanism and overmixing of the charge are responsible for HC and CO emissions.Another experiment was conducted by Han et al. [146] confirmingthe effect of ignition delay on HC emission. They experimentedwith diesel and gasoline blends. Greater percentage of gasolinein the fuel blend extended the ignition delay. Therefore, they ob-served that on same equivalence ratio, HC emission increasedwith the higher percentage of gasoline in the blend. This one isa clear indication of the effect of ignition delay on HC emission.However, they also observed that unlike HC, CO emission was pri-marily dependent on equivalence ratio. As the equivalence ratiowas higher, they got higher CO emission even for the same fuel.Kook et al. [147] reported that CO emission showed a rapid de-crease from the maximum as SOI was advanced, particularly atthe highest swirl ratios. The numerical simulations showed, at afixed swirl ratio, earlier injection timing enhanced pre-combus-tion mixing hence lower peak in-cylinder CO mass. The enhancedmixing was not only for increased ignition delay, but also for in-creased mixing rates under high-swirl conditions. A reduction inCO emission with increased pre-mixing implies that, CO emissionstems predominantly from under-mixed fuel (rich mixtures), afinding which is supported experimentally by a strong tendencytowards reduced CO emission with increased ignition delay andinjection pressure.

Opat et al. [148] reported that mixing time effect or tempera-ture effect does not control the HC and CO emissions entirely. Theymentioned about fuel impingement in the piston bowl. Numerousresearchers have accused liquid fuel film on piston bowl, generatedby spray impingement as the primary reason for higher HC emis-sion in early injection premixed LTC [53,149–153]. As the injectionadvances, the fuel spray trajectory crosses the piston bowl surface

at a higher point, causes more deposition consequently higher COand UHC. But wall impingement can be avoided by some changeswith the injection angle. Genzale et al. [124] showed the solutionwhich is mentioned earlier. However, Kim and Lee [35] investi-gated the impact of injection with narrow fuel spray angle and adual injection idea on the emission characteristics. They tunedthe first and second injection timing with a narrow fuel spray angle(injection angle 60�) and compared the results with conventionalinjection parameters. Dual fuel injection strategy, consisting anearly injection (50� BTDC) and a late second injection (20� ATDC)with narrow angle injection gave them reduced CO and HC emis-sion. Again under the LTC mode, higher load emits lower HC andCO. Because higher load means higher peak bulk temperatureand it helps in oxidation and hence complete combustion of thefuel. Alriksson and Denbratt [20] got an experimental proof regard-ing this.

Besides experimenting about the reasons of HC and CO emis-sions, some researchers have worked out the regions of HC and COgeneration during premixed LTC. Ekoto et al. [151] worked out thesources of UHC and CO applying PPCI in a light duty diesel enginewith low load and engine speed. UHC and CO emissions are mostsignificant under low load and speed while the engine is runningin the LTC condition. They spotted out three regions where UHCand CO originate primarily in the PPCI strategy. At centerline andsquish-volume, the got experimental evidence but at bowl and cen-tral clearance volume they only predicted by simulation. Kim et al.[152] tried to reveal the UHC and CO emissions regions in the cylin-der under premixed LTC. They came to a decision that during expan-sion stroke the spatial distribution of UHC in the clearance volumewas dominated by a region near the cylinder centerline and by a re-gion near the cylinder wall, the latter likely due to UHC releasedfrom the top ring-land crevice. When injection timing is advancedor retarded, UHC extends throughout the squish volume, due to for-mation of over-rich or over-lean mixtures, respectively. At lightload, UHC mainly generates at the centerline from lean mixture. In-creased dilution slows down oxidation throughout the cylinder, butmost noticeably within the squish volume. On the other hand, CO isgenerally observed near the cylinder centerline and broadly distrib-uted within the squish volume. Advanced injection timing increasessquish volume CO; retarded injection also does so but later in the cy-cle. Increased dilution generally increases CO throughout the cylin-der, but especially within the squish region, just like UHC. Musculuset al. [154] observed that when end of injection (EOI) was shorterthan ignition delay, UHC and CO were increased significantly. Fuelvapor measurement analysis revealed that maximum portion ofHC emission came from the incomplete combustion of lean mixturenear the injector after EOI. Ekoto et al. [151] claimed that late-cycleaddition of partially oxidized liquid and vapor phase fuel throughnozzle dribble or ejected fuel droplets were found to be a major con-tributor of UHC and CO emissions in the PPCI regime. Singh et al.[14] also observed, near the injector the mixture did not undergocomplete combustion which resulted in higher UHC emission dur-ing HCCI like combustion mode. It is not enough to mention that,HCCI combustion strategies are also have been reported for higherHC and CO emissions [35,77]. Basic principles of higher emissionof HC and CO are more or less similar for PCCI, HCCI or RCCI combus-tion modes which are already discussed here. However, some strik-ing points about UHC and CO emissions under LTC modes can benoted here.

� Reduction in in-cylinder combustion temperature and oxygenconcentration are the main reasons for higher HC and CO emis-sions for LTC modes.� Higher ignition delay of LTC modes over mixes the charge. Thus

it becomes too lean to burn, consequently HC and CO emissionsincrease.


� Advanced injection for both PCCI and HCCI causes sprayimpingement on piston bowl which results in higher emissionof HC and CO.� HC emission primarily depends on ignition delay whereas CO

emission mostly depends on equivalence ratio.

3.3.3. UHC and CO emissions under LTC modes for biodieselsBiodiesel with conventional diesel combustion process reduces

UHC and CO emissions significantly [99,155–161]. The mostimportant reason behind is the oxygen content and the cetanenumber. As the oxygen content is higher in biodiesel, it helpsattaining complete oxidation and comparatively complete andefficient combustion than diesel thus reduces UHC and CO[162]. Higher cetane number reduces the ignition delay thatmeans advances the combustion and also decreases the possibil-ity of fuel-rich zones [99] which reduces the UHC and CO emis-sions. Therefore, using biodiesel in LTC mode can be anattractive way to mitigate the obvious increment of UHC andCO [163] which are general effect of LTC described earlier. Inspite of this, some researchers have mentioned higher UHC andCO emissions for biodiesel than diesel fuel under LTC mode[110]. They attributed it to poor air–fuel mixture formation ofbiofuels.

However, during premixed LTC, Tormos et al. [164] reported27% and 45% decrement of CO and UHC respectively for biodieselthan diesel. It establishes, during premixed LTC, biodiesel is betterthan diesel fuel regarding UHC and CO emissions. But within thebiodiesels, at LTC mode UHC and CO emissions depend on severalfactors like injection pressure and timing [104], operating load,injection style [112], intake air temperature [165], in-cylinder tem-perature and combustion phasing [163], etc. Besides these factors,Petersen et al. [163] observed short premixed combustion durationof biodiesel resulted in low UHC and CO emissions which was sta-ted earlier by Northrop et al. [109]. They mentioned addition ofbiofuels resulted in shorter combustion duration, consequent ofhigher ignition quality, which reduced UHC and CO emissions. Ina separate experiment of premixed combustion of biodiesel, Nor-throp et al. [115] again mentioned shorter ignition delay for re-duced UHC and CO emissions.

However, in premixed LTC mode Northrop et al. [109] also re-ported higher emission of UHC and CO at late injection. Theyattributed this to incomplete combustion and fuel passing unre-acted through the cycle. Later on, same authors [143] reportedthat in the premixed mode, both early and late injection emittedhigher UHC and CO than conventional combustion of biodiesel.Extended ignition delay of LTC mode created over-lean regionsand thus increased the quantity of injected fuel species outsidelean flammability limits hence increased UHC. It implies that,though biodiesel reduces UHC and CO at premixed LTC than die-sel fuel, still its higher than conventional combustion of biodie-sel. Opposite results are also there. Zheng et al. [105] reportedvery low UHC and CO emissions at late injection premixed LTCfor low load.

Researchers have also tried multiple injections and got compar-atively reduced UHC and CO emissions than conventional singleshot injection [105,108,112]. However, at multiple injections, veryadvanced SOI showed increased UHC and CO emission due to poormixing of fuel–air [105]. Same result was achieved by Veltmanet al. [103] at advanced pilot injection. Along with SOI, injectionpressure has a significant effect on UHC and CO emissions[103,104]. Veltman et al. [103] got reduced UHC and CO emissionsas they increased the injection pressure to 180 MPa from 150 MPaon premixed LTC.

However, regarding UHC and CO emissions, premixed LTC andHCCI give more or less same trends for biodiesel. Still there aresome evidences where researchers have got lower UHC and CO

for HCCI. Mancaruso et al. [144] observed quite low UHC for bio-diesel than diesel fuel while they experimented HCCI mode andattributed this reduction to the increased oxygen content. Gane-san et al. [113] got reduced emission for higher loads duringHCCI. Bunting et al. [165] reported increased intake airtemperature helped to reduce the UHC and CO emissions as itadvanced the combustion phasing of HCCI combustion modewith biodiesel.

Some salient points regarding this discussion are as follows:

� LTC of biodiesel reduces UHC and CO than LTC of diesel, but stillgives higher emission than conventional combustion ofbiodiesel.� Both late and early injection timing of biodiesel give

higher emission in premixed LTC than conventionalcombustion.� Comparative reduction of UHC and CO emissions during pre-

mixed LTC of biodiesel than diesel can be attributed to the shortpremixed combustion duration and higher oxygen content ofbiodiesel.� Higher injection pressure, intake pressure or multiple injection

methods are conducive to reduce UHC and CO emissions duringbiodiesel combustion both for PCCI and HCCI.� HCCI mode emits higher UHC and CO when the mixture is over

mixed and beyond the lean flammability zone.

4. Performance analysis under LTC

Though LTC modes reduce NOx and PM simultaneously, unfor-tunately it is accompanied by higher fuel consumption andlower thermal efficiency for both diesel and biodiesels[20,21,73,109,110]. Tables 3 and 4 summarize the results respec-tively for diesel and biodiesels. Considerable increase of unburnedfuel (in the form of unburned HC and CO in the exhaust) at higherEGR percentage or retarded injection timing to achieve LTC areresponsible for this. Not only in the case of late injection for PCCIlike combustion but also at advanced injection timing for HCCI likecombustion, fuel consumption increases drastically. Poor evapora-tion, formation of the air–fuel mixture outside the combustionchamber or too much lean charge to burn due to over mixing arethe issues responsible for such increment of fuel consumption[35]. Though biodiesels have got higher oxygen content which per-mit a higher percentage of EGR keeping the CO and HC lower, manyresearchers have reported higher fuel consumption and lowerthermal efficiency when they used biodiesel in LTC modes[105,108,109,111,113]. However, some exceptions have also beenreported [107,167].

Researchers have tried many ideas to reduce the fuel consump-tion during LTC. Kim and Lee [35] reported lower fuel consumptionapplying the narrow fuel injection system in HCCI combustion.Fang et al. [142] reported lower BSFC for all the tested fuels in aHCCI like combustion process when they tried injection afterTDC. However, during PCCI combustion process, most of theresearchers have reported increased fuel consumption due to lateinjection timing (primary requirement for premixed combustionprocess) [68,74,108,109] whereas, a few researchers have got abit different result. Lilik and Boehman [5] reported lower fuel con-sumption even in late injection when they tried a synthetic fuelproduced in a low temperature Fischer–Tropsch process. They ex-plained this reduction by the combustion phasing of this high reac-tive fuel (CN: 81), which was maintained near the TDC. Kook et al.[16] reported maximum fuel conversion efficiency at moderateEGR and slight late injection timing. Fang et al. [107] reported low-er fuel consumption for biodiesel blends as they retarded injectiontiming. Zhang and Boehman [167] reported low fuel consumptioneven at higher EGR level at 6� BTDC injection timing. Hence, proper

Table 3Performance for diesel at LTC.



concentration

Power/torque/IMEP BSFC/ISFC BTE/ITE Refs.

4S,4-cylinder, DI, TCDV: 4.5 LCR: 16.57:1RS: 2400 rpmRP: 115 kW@2400 rpmInjection system:common rail

Retarded injectionLTC

D Sweep ofinjection timing

56% Torque ; as the IT retarded " as the IT retarded N/A [73]

Speed: 1400 rpm �8� to �2� ATDCVariable torque:54–80 N m

4S,1-cylinder, DI, supercharged,DV: 2.022 LRS: 1500 rpmCR: 14:1Injection system:electronicallycontrolled injector

LTC D Sweep of IT 9� to20� BTDC

Up to 60% N/A Value " as the IT retarded after 50%EGR

N/A [20]

25% load, charge airpressure: 1.3 bar(abs.)

Sweep of IT 9� to20� BTDC

Up to 65% Advanced IT like 20� BTDCgave notable ; value

50% load, charge airpressure: 2.4 bar(abs.)

4S,1-cylinder, supercharged, DI, WCDV: 781.7 cm3

CR: 13RS: 1000 rpmInjection system:common rail

PCCI Low sulfur diesel Sweep of IT 15 to25� BTDC

0% and 40% 40% EGR gave higher IMEP than0% EGR

N/A 40% EGR gave " value than0% EGR

[74]Injection pressure:140 MPa


CR: 15:1RS: 1500 rpmIP: 100 MPaInjection system:Bosch common rail

HCCI D Sweep of injectionfrom 40� to 70�BTDC

N/A Narrow angle injection gavehigher IMEP than conventionalinjection

Narrow angle injection gave lowerISFC than conventional injection allthrough

N/A [35]Injection angle: 60�

4S,4-cylinder, DI, TCDV: 2.5 LCR: 17.5RS: 4000 rpmRP: 103 kW@4000 rpmInjection system:Bosch common rail

Advanced PCCI D, Fuel processed by highand low temperatureFischer–Tropsch process(HTFT and LTFT)

Sweep of injection�8� to 0� ATDC

40% N/A " as IT retarded, LTFT gave " value at late IT [5]LTFT gave lowest BSFC

4S,4-cylinder, DI,WC, TCDV: 1910 cm3

RS: 2500 rpmCR: 17.5:1BMEP: 0.8 MPaVariable IP:100–160 MPaInjection cone angle:148�

PPCI Low sulfur diesel, Retarded IT O2

concentration19.5%

N/A For each condition 5–7% highervalue

Lower value when LTCachieved.

[68]20% and 40% blend ofn-butanol with diesel

Sweep from �10�to 2� ATDC


S.Imtenan

etal./Energy

Conversionand

Managem

ent80

(2014)329–

356347

Table 3 (continued)



concentration

Power/torque/IMEP BSFC/ISFC BTE/ITE Refs.

4S,4-cylinder, DIDV: 2198 ccRS: 3500 rpmRP: 96 kWCR: 16.6:1Injection system:common railInjection cone angle:153�

Optimized PPCI 50% blend of gasolinewith D

Advancedinjection at 28�BTDC

50% N/A N/A 2.5% lower value than CC [21]1800 rpmBMEP: 2.95

4S,1-cylinder, WCDV: 422 cm3

CR: 18.7:1RS: 1500 rpmIP: up to 1350 barInjection system:common rail


Diesel Sweep from�30.25� to 7.75�ATDC

Up to 65% Power ; as EGR " N/A Maximum at moderateEGR and slight late IT (�5�ATDC)

[16]

4S,1-cylinderDV: 2.44 LCR: 16.1:1IMEP: 9 barDirect IP: 400 barPort IP: 5.17ar

RCCI Port injected fuel:gasoline

N/A 43% IMEP: 6–9 bar N/A 57% at 3.5% DTBP ingasoline

[78]

Direct injected fuel:gasoline + variablepercentage of DTBP(di-tert-butyl peroxide)

4S,1-cylinder, DIDV: 2.44 LCR: 16.1:1Injection system:common railDirect IP: 800 barPort IP: 4.14 bar

RCCI Port injected fuel: 15%gasoline + 85% ethanol

1st directinjection: 55�BTDC

Up to 47% IMEP: 9.6–16.5 bar N/A 59% at lower EGR [19]Operating loads:9.6–16.5 bar Direct injected fuel:

Diesel 2nd directinjection: 36�BTDC

348S.Im

tenanet


andM

anagement

80(2014)

329–356

Table 4Performance for biodiesels at LTC.

Engine setup Operating condition Fuel Injection timing Percentage ofEGR/O2

concentration

Power/torque BSFC/ISFC BTE/ITE Refs.


CR: 18.4:1RP: 11 kW@3000 rpmRT: 45 N m@2100 rpmIP: 650 barInjection system: pumpinjection

HCCI Diesel, 10� BTDC Up to 32% N/A BSFC " with the percentage ofEGR

N/A [110]100%, 65% and 30% blend ofcolza biodiesel


CR: 17.8:1RP: 12.5 kW@2400 rpmIP: 4 barInjection system: injectionpump

High EGR enabledHCCI

100% soy biodiesel 17� BTDC Up to 60% Notable power loss withthe increment of EGR

N/A ; BTE with increasedEGR

[111]

Variable loading



Variable loading(IMEP 5–10 bar).

Biodiesel blend of Soy, Canola,Yellow grease and Tallowbiodiesel.

Single and multipleinjections with widerange sweep of IT.

Up to 70%according tothe condition.

Power ; as% of EGR " N/A N/A [105]

Variable boostpressure.

Single shot injectiongave better power thanmultiple injection.

Single shot, EGRassisted LTC for lowloads.Multi-pulse EGRassisted HCCI forheavy loads.



EGR and lateinjection assistedLTC mode

Biodiesel blend of Soy, Canola,Yellow grease and Tallowbiodiesel.

Single shot injection(IMEP 8 bar)

Up to 70% N/A Above 50% EGR and at late ITvery high fuel consumption.

N/A [108]

Variable intakepressure (1.2/1.5 bar)

Multi-pulse injection(IMEP 6 bar)

Multiple injection gave " valuethan single shot injection.

Wide range (347–367�CA) sweep of IT


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Table 4 (continued)


concentration


4S,4-cylinder, DIDV: 1.7 LCR: 16:1RS: 1500 rpmLoad: 400 kPaIP: 800, 1000, 1200 barInjection system: commonrail

Late injectionpremixed LTCmode.

20%, 50% and 100% Soy-basedmethyl ester

5�, 7� and 9� BTDC 50% Retarded IT gave " BSFC ; BTE at retarded IT [109]

4S,4-cylinder, DI, TCDV: 1.7 LCR: 16:1RS: 1500 rpmRP: 75 kW@4400 rpmIP: on average 870 barInjection system: commonrail

Late and earlyinjection partiallypremixed LTC

100% soy based methyl esterand 50% blend of soy basedmethyl ester with ULSD

5.9� to 7.1� BTDC (forLLTC)

45%(LLTC) Lower power for LLTCand ELTC than CC.

N/A N/A [143]

17.3� to 24.1� BTDC(for ELTC)

55%(ELTC)ELTC gave lowest.

4S,1-cylinder, DIDV: 300 ccCR: 19.5:1RS: 1500 rpmIP: 600 barInjection system: commonrail

Late injection LTC 20%, 50% and 100%soy biodiesel

Sweep of injectiontiming from �25�ATDC to 3� ATDC

N/A N/A ISFC ; up to 11.1% as the IT wasretarded.

N/A [107]

4S,4-cylinder, DI, TCDV: 2.5 LCR: 17.5:1RP: 103 kW@4000 rpmIP: up to 1600 barInjection system: commonrail

Late injection EGRassisted LTC

40% biodiesel blended withultralow sulfur diesel.

Single injection (6�BTDC)

Up to 38% N/A Reduced BSFC at 60 BTDC ITeven at higher EGR.

60 BTDC IT gave highthermal efficiencyeven at higher EGR.

[167]

1600 rpm and 25%loading.

Double injection(pilot: 25� BTDC,main: 2� ATDC)

350S.Im

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Table 4 (continued)


concentration


4S,4-cylinder, DIDV: 425 cm3 (1-cylinder)CR: 15:1RS: 1500 rpmIP: 100 MPaIntake pressure: 120/150 kPaInjection system: commonrail


Biodiesel–ethanol (80–20%)blend.

13 to 10.5� BTDC 40% for highload

IMEP range was 0.35–0.82 MPa

N/A Maintained at least96% combustionefficiency

[106]

Variable loading Up to 50% forlow loads.

4S,1-cylinder, DI, ACDV: 662 cm3

CR: 17.5:1RP: 4.4 kWRS: 1500 rpmIP: 2 barInjection system: port fuelinjection with fuelvaporizer, direct injection

Fuel vaporizer withport fuel injectionassisted HCCI.

100% biodiesel 23� BTDC (for directinjection)

N/A N/A 3–5% higher value than CCsystem

About 5.5% ; valuethan CC system

[113]

Variable loading

4S,1-cylinder, DIDV: 300 ccCR: 19.5:1RS: 1500 rpmIP: 600 barInjection system:common railInjection cone angle: 150�

Late injection HCCI Neat soybean biodiesel and20–50% blend of biodiesel withlow sulfur diesel.

�25� ATDC, �10�ATDC and 3� ATDC

N/A N/A For all the fuels late injection(3� ATDC) gave lower valuethan even CC condition.

N/A [142]

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optimization of the operating conditions and fuel chemistry cankeep the fuel consumption low even in the LTC modes.

However, regarding load level, LTC modes were always confinedto the low to mid region as higher load means higher in-cylindertemperature which drives NOx higher. Fortunately, experimentsregarding fuel reactivity have unwrapped higher load region forLTC strategies with higher efficiency. Bessonette et al. [168] re-ported 60% increment of the operating load during HCCI combus-tion while they experimented with a fuel having autoignitionquality in between diesel and gasoline. Inagaki et al. [61] used dualfuel of different reactivity in PCCI combustion and succeeded to in-crease the IMEP up to 12 bar. Being inspired by these works Splitteret al. [62] worked regarding reactivity gradient of fuel insidecylinder during PCCI combustion and experimentally demon-strated that combustion proceeded from areas of locally high fuelreactivity to areas of locally low fuel reactivity. Such staged com-bustion process extended the duration of the premixed combus-tion event which resulted in high thermal efficiency, lowpressure rise rate, for loads as high as 16 bar IMEP [63] keepingthe emissions low. Later on, the same authors tried RCCI combus-tion using gasoline as the low reactive fuel and gasoline + di-tert-butyl peroxide as high reactive fuel. They got up to 57% indicatedthermal efficiency [78]. Application of ethanol–gasoline blend aslow reactive fuel gave even higher efficiency like 59% [19]. Niemanet al. [17] also reported higher efficiency with higher load levelwhen they used natural gas as the low reactive fuel. Therefore,regarding performance, though lower power, higher fuel consump-tion with lower thermal efficiency are the inevitable effects of LTCmodes, we have some potential methods as well to avoid thesedrawbacks. Low emission level with permeable fuel consumptionand moderate load level are achievable for HCCI, PCCI and obvi-ously for RCCI combustion processes.

Some highlighting points of the above discussion are asfollows:

� Unburned fuel (in the form of HC and CO in the exhaust) due tohigher EGR rate or late injection timing of premixed LTC mode,is responsible for higher fuel consumption.� In the case of advanced injection during HCCI like combustion,

over mixing or poor evaporation and sometimes formation ofcharge outside the combustion chamber are responsible forhigher fuel consumption.� Proper optimization of the combustion conditions and fuel

chemistry can reduce the fuel consumption of LTC modes.� Fuel reactivity stratification method used in RCCI combustion

has unfastened a new era of LTC by reducing BSFC aswell as increasing the operating load with high thermalefficiency.

5. Uncovered gaps and probable solution of the drawbacks ofLTC

It is not enough to mention that, to meet the future emissionregulations, along with the after-treatment methods, in-cylinderemission reduction technologies like LTC modes are of immensenecessity. Though researchers are trying to improve LTC modesday by day, still there are some drawbacks or some problems ofLTC modes which need to be solved. New ideas or methods shouldcome to light regarding the improvement of this marvelous idea oflow emitting combustion system. Some points are noted below tohighlight the uncovered gaps and probable solutions regarding thissystem:

� Higher emission of UHC and CO is a huge drawback of this sys-tem. Use of oxygenated fuels like ethanol, n-butanol, diethylether or biodiesel can be a good solution to this problem. There

is a huge gap of research using such fuels during LTC modes.Compositional differences between biofuels are responsiblefor a change in CO oxidation behavior especially at late injec-tion. These issues should be explored visibly to make oxygen-ated fuels more suitable for LTC modes.� In the case of late injected PCCI like combustion, degree of

atomization and mixture quality are vital issues for CO andHC emissions. High injection pressure with in-cylinder swirlformation can be used to mitigate these emissions.� Use of fuel vaporizer has made a new development for using

heavy fuel like biodiesel in HCCI like combustion. However,research should be conducted thoroughly to improve its effi-ciency level for different biodiesels.� New concepts should come to increase the operating load level

of PCCI and HCCI.� RCCI combustion mode has not been tested with biodiesel as

the higher reactive fuel yet. Different fuels should be tested cov-ering a wide range of reactivity. It may bring some new con-cepts for better emission characteristics.

6. Conclusion

Diesel engines are significant power sources for numerousapplications because of stability, fuel flexibility, and higher ther-modynamic efficiency. Over the past two decades, diesel enginepollutant emission regulations have progressively become morestringent. To cope with the order-of-magnitude reductions of bothPM and NOx, required by upcoming regulations, engine developerswill have to rely on alternative in-cylinder strategies that use someform of LTC. Compared to conventional diesel combustion, LTCstrategies generally employ enhanced pre-combustion mixing,which helps to avoid locally rich regions. Less locally fuel rich re-gion means low PM formation. Similarly, using EGR results in dilu-tion by pre-combustion mixing which reduces the peakcombustion temperature. The peak combustion temperature atLTC can be reduced to about 2000 K or even lower, which drasti-cally reduces the thermal NOx formation. Engine researchers anddevelopers have proposed several practical strategies of LTC likeHCCI, PCCI, PCI, and RCCI. Researchers have attained thesestrategies using both diesel and biodiesels. As the objective of thisarticle was to analyze the effects of LTC strategies on emissions fordiesel and biodiesels, from review, the following points aresummarized.

� LTC mode visibly reduces NOx emission. Irrespective of combus-tion strategies, low NOx emission basically depends on higherignition delay and lower combustion rate, which result in lowerin-cylinder temperature. Higher use of EGR and optimized SOIcontrol these parameters on LTC to keep the NOx emissionlow during diesel combustion. LTC modes have also succeededto reduce the usual higher emission of NOx for biodiesels.Higher percentage of oxygen and cetane number of biodieselsenable them to sustain higher EGR and late SOI to attain betterLTC. Therefore, better NOx reduction. Regarding combustionmodes, irrespective of fuels, premixed LTC reduces NOx betterthan HCCI. However, considering efficiency, RCCI is the best one.� LTC mode simultaneously reduces NOx and PM emissions. Gen-

erally LTC modes take the combustion temperature below theformation temperature of PM. Below the formation temperatureeven the equivalence ratio has no command on PM emissions.Consequently, PM emissions decrease for diesel fuel combus-tion during LTC modes. For biodiesels, condensation ofunburned fuel sometimes increases PM emissions during LTCmodes. However, generally, inherent properties of biodieselslike increased oxygen content, lower stoichiometric need of


air, absence of aromatics and sulfur, combustion advance andsoot structure spontaneously reduce PM emissions during allthe LTC strategies.� UHC and CO emissions increase during LTC of diesel for the rea-

son of reduction in in-cylinder combustion temperature andoxygen concentration. Biodiesels reduce UHC and CO emissionsthan diesel during LTC modes because of their short premixedcombustion duration and higher oxygen content. Still, the emis-sion level remains higher than the conventional combustionsystem.� Generally, LTC strategies show higher fuel consumption.

Unburned fuel (in the form of HC and CO in the exhaust) dueto higher EGR rate or late injection timing is responsible forhigher fuel consumption. Proper optimization of injection tim-ing and fuel chemistry can reduce it to a satisfactory level.

Since, LTC method is an emerging idea of modern combustionscience, new ideas or strategies should come to light to eliminateor decrease its inherent drawbacks. Using oxygenated fuels oradditives, high injection pressure with in-cylinder swirl formation,optimized use of fuel vaporizer and finally, use of different fuelshaving a wide range of reactivity for the RCCI combustion systemcan be potential fields for future studies. However, taking stringentemission policies into concern and continuous improvementregarding the performance of LTC, it is sure that LTC is on the vergeof being the most suitable and acceptable technology of futurecombustion.

Acknowledgement

The authors would like to appreciate University of Malaya forfinancial support through High Impact Research grant titled:‘‘Clean Diesel Technology for Military and Civilian Transport Vehi-cles’’ having Grant number UM.C/HIR/MOHE/ENG/07.

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