Control of noise and exhaust emissions from dual fuel engines

Control of noise and exhaust emissions from dualfuel engines

Salah B. Al-Omari, Mohamed Y. E. Selim,a) and A. A. J. Al-AseeryMechanical Engineering Department, UAE University, Al-Ain, United Arab Emirates

(Received 19 October 2010; accepted 29 April 2011; published online 12 July 2011)

An experimental investigation has been carried out to examine the effects of

adding water vapor to the intake air of a dual fuel engine on the performance,

combustion noise, and emission of CO, HC gases, and smoke opacity. Dual fuel

engine is a diesel engine using a small amount (5%) of liquid diesel fuel as a pilot

fuel and burns a liquefied petroleum gas as the main fuel. The addition of water

vapor to the combustion chamber of a diesel engine has known effects on reducing

the dangerous emission of nitrogen oxides. Yet it is not currently clear for the dual

fuel engine how much the sacrifice is of engine performance, noise, and other

exhaust emissions. Experimental procedures conducted using a Ricardo diesel

version variable compression research engine are discussed. Results from testing

dual fuel engine with varying the added water vapor to fuel ratios and other design

and operating parameters are presented and discussed. The addition of water vapor

to the intake air affected the performance, combustion noise, and exhaust

emissions. VC 2011 American Institute of Physics. [doi:10.1063/1.3596170]

I. INTRODUCTION

The use of gaseous fuels, e.g., natural gas, liquefied petroleum gas (LPG), etc. is a promis-

ing approach for lowering the dependence on petroleum liquid fuels and to reduce the emis-

sions of CO2 and other pollutants from the diesel engines. For countries having gaseous fuels

resources, it is advantageous to utilize gaseous fuels in diesel engines. The advantages of using

gaseous fuels in such engines stem from the economical and environmental benefits. Economi-

cal benefits are to reduce the dependence on liquid fuels and utilizing alternative gaseous fuels.

Environmental benefits include the reduction of CO2 green house gas, the hydrocarbons, partic-

ulate matter, and diesel odor. However, the NOx emission is still very high and needs to be

reduced if such engines are to be commercialized.

There have been many published works on the use of gaseous fuels in dual fuel engines.

Natural gas use in dual fuel engines has been studied from the combustion duration and ignition

delay point of view as indicated in Ref. 1 and from performance and emissions point of view

as presented in Ref. 2. Combustion, thermal loading and temperature distribution have also

been studied for dual fuel engines.3 Pure methane has also been studied in dual fuel engines

from flame spread limits point of view4 and performance and emissions point of view.5 LPG

has been studied from the point of view of performance and emissions.5

There have been many methods to control the air pollution problem from diesel engines.

The use of exhaust gas recirculation (EGR), adding water to the combustion chamber with any

method (in form of liquid or vapor) and adjusting the engine design parameters helped to

reduce the problem of NOx.

It has been concluded from diesel engines combustion6,7 that the addition of properly

mixed water (or water vapor) in the combustion chamber effectively reduces the NOx emission;

however, there was no measurements of ignition delay period of diesel combustion that may

lead to increased noise level from such engines.

a)Author to whom correspondence should be addressed. Electronic mail: [email protected].

1941-7012/2011/3(4)/043103/26/$30.00 VC 2011 American Institute of Physics3, 043103-1

JOURNAL OF RENEWABLE SUSTAINABLE ENERGY 3, 043103 (2011)

Downloaded 23 Jul 2011 to 195.229.235.46. Redistribution subject to AIP license or copyright; see http://jrse.aip.org/about/rights_and_permissions

http://dx.doi.org/10.1063/1.3596170

http://dx.doi.org/10.1063/1.3596170

http://dx.doi.org/10.1063/1.3596170

Four major approaches for introducing water into the combustion zone have been reported.

These include fumigating the water into the engine intake air, direct injection into the engine

through separate injectors, in-line mixing of water and fuel prior to injection (unstabilized emul-

sion), and mixtures of stabilized emulsions treatable as a single-phase drop-in replacement fuel.

The Urbach et al.8 demonstrated that water mist injection into the bell housing of diesel-

fueled turbine engines can lead to promising results. Water mist introduced to the intake of

reciprocating compression ignition engines, particularly two-stroke engines with the intake air

passing through the crankcase, poses significant corrosion potential. Separate water injecting

valves in the engine avoids intake system contact with the mist.9 Several works have evaluated

all or most of the options for introducing water into the combustion process and have primarily

determined that water-in-fuel emulsions, stabilized or unstabilized, are most effective in reduc-

ing NOx, brake specific fuel consumption (BSFC), and result in lower increases of CO and

unburned hydrocarbon (UHC) emissions.10

An emulsion is defined as a mixture of two or more generally insoluble liquids. A permanent

emulsion exists when sufficient droplet sizes have been reached to prevent the separation of the in-

soluble materials. Unstabilized emulsions are generated through the high-speed, high-shearing of

particles, and solids in a liquid. A limiting concern with emulsions is the high capital costs of emul-

sification mixers and pumps, which are used extensively in the food and agriculture industries.

Unstabilized emulsions require high shear to suspend small droplets of water in the fuel.11

An advantage of unstabilized emulsions are reduced fuel costs, due to lack of additives needed,

and reduced emissions from not combusting surfactants or other emulsifying agents12 propose a

complex fueling system including a vortex chamber to provide in-line mixing of water and die-

sel fuel without requiring the addition of an emulsifying agent.

Stabilized emulsions use an emulsifying agent to suspend the water in the fuel and reduce

the energy required for a permanent emulsion.13 Authors of Ref. 13 used a surfactant to create

a permanent, stabilized emulsion that can be treated as a single-phase fuel. A drawback to

water-fuel emulsions is the amount of air bubbles reportedly contained in the emulsion mixture.

They evaluated the effect of water-fuel emulsions on diesel engine performance and emissions

under transient conditions. It has been concluded that air bubbles in the fuel and its variability

contribute to fluctuation in the injection timing and a poorer performance under transient condi-

tions. They13 recommend the removal of bubbles from the water-fuel emulsion.

Research has also been conducted extensively on the use of additives to improve the lubri-

cation, to reduce the corrosive effect of water in the fuel, and to improve the emulsion stability.

Nitrate-containing ignition improvers are recommended to reduce exhaust emissions.14 Lubricity

additives composed of dimer or trimer acides, phosphate esters, and sulfurized castor oils are

recommended by Ref. 15 and catalysts can also be used in situ in the fuel to reduce NOx.15 In

addition to providing lubrication improvement, additives to water–fuel emulsions can be

employed for antifreeze characteristics16 obviously important when significant volume percents

of water are present in a fueling system in freezing climates. The Montagne et al.17 demon-

strated that the surfactants added to diesel fuels can clean up fuel injectors as well as prevent

further injector deposits. They also reported a slight increase in NOx from the combustion of

the surfactants, as the case without adding water to the fuel.

Crookes et al.18 attributed the improved combustion and the reduction in particulate and NOx

emissions to the secondary atomization of the water, often designated as micro-explosions. Micro-

explosions of emulsified fuels has been also extensively examined by Ref. 19 and determined that

there is a minimum percent water content in the emulsion required for micro-explosions to occur,

and that the percent increases with the kinematic viscosity of the fuel. There has been an improve-

ment in brake thermal efficiency of engines that use diesel-water emulsion reported.20

It has been shown above that the NOx and noise can be reduced from diesel engines that

uses single fuel; however, there was no attempt to reduce such problems from dual fuel

engines.21 Dual fuel engine is the diesel engine that uses diesel fuel as a pilot fuel but the main

fuel is normally compressed the natural gas or liquefied petroleum gas.

Therefore, it is the main objective of the present work to examine the effects of adding

water vapor to the intake air of a dual fuel engine on the performance, combustion noise, and

043103-2 Al-Omari, Selim, and Al-Aseery J. Renewable Sustainable Energy 3, 043103 (2011)


exhaust emissions of a dual fuel engine running on diesel—LPG. The water has been added in

the present work to the intake air of the engine in the form of vaporized water produced by a

special test rig explained in Sec. II.

II. EXPERIMENTAL TEST RIG AND PROCEDURE

The research engine used in the present study is the research engine Ricardo E6 single cyl-

inder variable compression indirect injection (IDI) diesel engine. Full details of the engine are

given in Table I.

Measurements of gaseous flow rate has been carried out by a Platon Gapmeter type rotame-

ter, while the exhaust gas analysis has been carried out by Heshbon HG520 as analyzer and the

smoke in the exhaust gases has been measured by a Shady X2000 Opacimetro smoke meter.

A. Measurement of combustion pressure and crank angle

A data acquisition system is used to collect the cylinder combustion pressure and crank angle

data. The pressure signal is collected by a high pressure water cooled piezo-electric pressure

transducer made by AVL and its output is fed into a charge amplifier then to a data acquisition

card linked to the personal computer. The crank angle signal is fed into a degree marker shaper

channel and the output is fed into the acquisition card. The acquisition card could collect data at

the rate of 250 kHz. A LabView program has been written to collect the data from the two chan-

nels at a sampling rate of 10 000 points per second and store the pressure and crank angle data

in the computer disk for offline analysis. A computer program is written in MS Excel to find the

maximum combustion pressure and the pressure rise rate data at all cycle points from mid com-

pression stroke to mid expansion stroke. The maximum value of pressure rise rate is then

obtained and recorded. This value will be used to represent the noise level at that operating con-

dition. Experiments have been carried out after running the engine for some time until it reaches

steady state, and oil temperature is at 60 oC 6 5 and cooling water temperature is at 70 oC 6 5.

The addition of water vapor in the intake of the engine has been carried out by vaporizing

liquid water from a water tank that contains three electric heaters (of 2, 2 and 1 kW, respec-

tively). When the amount of water vapor is required to increase, another heater is turned on

and more vapor is generated. The dry bulb and wet bulb temperatures of the intake air have

been measured to get the amount of water vapor added in the form of the air specific humidity.

The variation of specific humidity is accomplished by injecting more water vapor at atmos-

pheric pressure in the intake stream. Saturated water vapor at 1 atm with an enthalpy of

2675 kJ/kg has been generated in the hot water tank. Injecting this vapor in the intake air is

approximated by an adiabatic humidification process in which the steam is injected without

extra heating. Psychrometric chart analysis shows that the process is almost vertical line (fol-

lowing hw¼ 2675 kJ/kg) until air is fully saturated (relative humidity of 100%). So three cases

were considered here: firstly atmospheric condition of 27 �C dry bulb temperature and 27% rel-

ative humidity (specific humidity of 6 g w/kg air), secondly saturated air with 28 �C dry bulb

temperature and 100% relative humidity (specific humidity of 24 g w/kg air). Thirdly a midway

point of about 27.5 �C and 61% relative humidity has been also used (gives specfic humidity of

TABLE I. Ricardo engine specifications.

Engine type and name Ricardo E6 on hydra panel

Number of cylinders 1

Bore 76.2 mm (3 in.)

Stroke 111.1 mm (4.375 in.)

Swept volume 0.507 l (31 in.3)

Maximum speed 50 rev/s (3000 rpm)

Maximum power, diesel (CR¼ 20.93)—naturally aspirated) 9 kW

Compression ratio Maximum CR 22

Injection timing Varied 20�-45� BTDC

043103-3 Pollution reduction from diesel engines J. Renewable Sustainable Energy 3, 043103 (2011)


14 g w/kg air). Enthalpy of intake air has been increased from 43 kJ/kg at first point, increased

to 64 kJ/kg at mid-point, and 89 kJ/kg at final point. Although the enthalpy of intake air is

increased, but it is expected that, as water vapor mass is increased, during the compression

stroke and as the pressure is increased, more water droplets may be formed. Hence, the liquid

water available in the compressed air will absorb some heat from air resulting in a reduction in

the final compression temperature. Another possibility of reducing the final compression tem-

perature for the air-vapor mixture is the increased specific heats for the water vapor, which

leads to increased specific heats of the air-vapor mixture. This eventually leads to a drop in the

maximum combustion temperature.

B. Experimental procedure

Table II illustrates the operation and design parameters changed during the experiments

and the associated level.

It may be worth mentioning that the air to fuel ratio will be changed when the mass of liq-

uid pilot fuel or gaseous fuel change. The inlet conditions of the engine air were atmospheric

pressure and temperature (1 atm and 27 �C, respectively). The test conditions will be illustrated

with each data figure given below. The vapor conditions have been varied as mentioned above

to achieve the three levels of specific humidity. The range of water/fuel ratio was from 12% to

48%, which was calculated from the equation

m�wm�fuel

¼ m�wm�air

� m�air

m�fuel

The common values of the experimental parameters were as follows: engine speed¼ 20 rps,

injection timing (IT)¼ 35� before top dead center (BTDC), and compression ratio (CR)¼ 22.

C. Uncertainties in some important parameters

The uncertainties calculated in Table III are based on the maximum scattering percentage

around a mean reading (for example, a typical mean reading of the fuel pressure drop across

the orifice meter is 50 mV (this is the reading on the digital pressure meter). It is found that

the reading fluctuates between 46 and 55 mV, so the maximum scatter is 6 4.5 mV from which

we can find the maximum scattering percentage of 9%.

A test of the experimental scattering has been carried out for the torque, maximum pres-

sure, and maximum pressure rise rate in which the same experiment to measure them was

repeated three times in three different days. It was noticed that the repeatability of the experi-

mental results was acceptable and the maximum uncertainties were as given in Table III.

III. RESULTS AND DISCUSSION

A. Effect of water vapor addition on engine performance

Figure 1 shows the experimental results of the brake power output of the engine as a func-

tion of the engine speed and water addition. The compression ratio is kept constant at 22, and

TABLE II. Experimental parameters and associated level.

Engine parameter Level of change Units

Air specific humidity 6, 14, and 24 gm water/kg air

Engine speed 18–30 rev/s

Pilot diesel fuel mass injected 0.14–0.72 kg/h

Engine injection timing for the pilot fuel 20–45 degrees BTDC

Engine compression ratio 14–22 —

Mass of gaseous fuel 1–4.5 kg/h



the injection timing of the pilot diesel fuel is kept at 35� BTDC changing the engine speed

(done here through the generator dynamometer) actually increase the consumption of diesel

fuel and gaseous fuel per unit time.

It can be seen from the figure that increasing the engine speed in this range generally

increases the volumetric efficiency of the engine which increases the mass flow rate of the

engine followed by an increase in the liquid fuel flow rate as well as the gaseous fuel flow rate

which increase the brake power output of the engine. As the rack position of the pilot diesel

fuel pump is kept fixed here, the mass of pilot fuel injected per cycle is constant but the mass

flow rate of diesel fuel is increased per unit time when the engine speed is allowed to increase.

Similarly, the needle valve that allows the gaseous fuel to enter to the engine is kept fixed here

as well, so as the engine speed is increased, the engine would consume more fuel per unit time.

It may be seen also from the same figure that increasing the water addition level from 6 g/

kg to 14 g/kg then to 24 g/kg generally reduce the brake power output of the engine; however,

this reduction in the brake power output is not much compared to the benefit of using the water

in the engine as the NOx is proved to reduce by other researchers. For example, the brake

power output form the engine at speed of 30 rps (1800 rpm) and at 6 g/kg water (atmospheric

conditions) is 1775 W, while for the highest specific humidity of 24 g/kg is 1650 W. The per-

centage drop for 14 g/kg is 4.2% and for 24 g/kg is 7%. The drop in the brake power output as

the water addition increases may be attributed to the reduction in the maximum combustion

temperature and pressure which reduces the brake mean effective pressure.

TABLE III. Uncertainties in some independent variables.

Independent variable Uncertainty (%)

The water specific humidity (g) 0.83

The engine speed (rpm) 2.5

The engine output torque (nm) 4

The pilot diesel fuel volume injected (kg/s) 6

The engine injection timing for the pilot fuel (BTDC) 5

The engine compression ratio 1

The mass of gaseous fuel (kg/s) 1.25

FIG. 1. Effect of water addition and engine speed on brake power output, CR¼ 22, IT¼ 35� BTDC.



Figure 2 shows the effect of the mass of pilot fuel injected on the brake power output for

the different levels of water addition. The following parameters are kept constant: CR¼ 22,

IT¼ 35� BTDC, and mgas¼ 4 kg/h while the mass of pilot fuel is increased. It may be seen

from the figure that increasing the mass of pilot fuel generally increases the brake power output

of the engine for all ratios of water level. This increase is due to the increase of the mass pilot

fuel per unit time which increases the heat released from the injected fuel. However, any

increase in the water addition ratio causes a drop in the brake power output and this drop is

also expected as the mean effective pressure is reduced fuel to the drop in the maximum com-

bustion temperature and pressure. However, it may be seen that the drop in the brake power

output is less for small amounts of pilot fuel and bigger for higher amounts of pilot fuel. It

may be mentioned here that the dual fuel engine generally uses small amount of pilot fuel

which means the drop in the brake power output will be very small and there is no need to go

to higher amount of pilot fuel.

Figure 3 shows the effect of water addition and injection timing on brake power output.

The following parameters are kept constant: CR¼ 22, N¼ 20 rps, mgas¼ 4 kg/h, and mdiesel

¼ 0.28 kg/h while the injection timing for pilot diesel fuel is varied. It may be seen from the

figure that increasing the injection timing advance from 20� to 45� before top dead center gen-

erally reduces the brake power output. Increasing the injection advance causes the combustion

to start earlier, which leads to an increase in the combustion temperature and pressure. From

the pressure–time history, it has been shown that this increase occurs in the compression stroke

which decreases the maximum pressure during the expansion stroke and hence decreases the

mean effective pressure and reduces the net work of the engine and the brake power output. It

may be also seen from the figure that the water addition decreases the brake power output

slightly.

Figure 4 shows the effect of compression ratio and water addition on brake power output

of the dual fuel engine. The following parameters are kept constant: N¼ 20 rps, mgas¼ 4 kg/h,

and mdiesel¼ 0.28 kg/h while the compression ratio is varied. The maximum compression ratio

of 21.07 has been extended to 22 to test the highest possible compression ratios and this

occurred by the extension in the calibration scale of the engine. It can be noticed from the fig-

ure that the increase in the compression ratio from 14 up to 22 generally increases the brake

power output of the engine. Increasing the compression ratio of the engine increases the thermal

efficiency of the ideal cycle which in turn increases the brake power output of the engine.

FIG. 2. Effect of water addition and mass of pilot fuel on brake power output, CR¼ 22, IT¼ 35� BTDC, mgas¼ 4 kg/h.



It may be noticed that the water addition decreases the brake power output of the engine at

most of the compression ratios except for compression ratio 22. It may be noticed also that for

compression ratio of 22, the brake power output is slightly decreased. This may be due to the

fact of running the dual fuel engine at a high compression ratio of 22 and because of the engine

knocking which tends to reduce the brake power output. It is not recommended, therefore, that

the dual fuel engine uses compression ratio of 22—to avoid the engine knocking. The use of

slightly lower compression ratios, e.g., 20 or 21 would produce a higher brake power output,

and even in the case of using high amount of water in the admitted air (24 g/kg), the engine

FIG. 4. Effect of water addition and compression ratio on brake power output, N¼ 20 rps, mgas¼ 4 kg/h, mdiesel¼ 0.28 kg/h.

FIG. 3. Effect of water addition and injection timing on brake power out, CR¼ 22, N¼ 20 rps, mgas¼ 4 kg/h, mdiesel¼ 0.28 kg/h.



would give the same power as of compression ratio of 22. This is favorable in addition also to

the knocking-free run of the dual fuel engine.

Figure 5 shows the effect of the engine speed and the water addition on bsfc. It may be

seen from the figure that increasing the engine speed generally increases the bsfc for all water

addition ratios. The brake specific fuel consumption is calculated from the mass of the pilot die-

sel fuel and the gaseous fuel divided by the brake power output of the engine. Increasing the

engine speed increases the mass of pilot fuel and mass of gaseous fuel as well as the brake

power output of the engine.

It seems that the mass of the fuel increases more than the increase in the brake power out-

put that is why the brake specific fuel consumption tends to increase. It may be also noticed

from the figure that the increase in the water addition form 6 g/kg to 14 g/kg increases the

brake specific fuel consumption. This increase is due to one parameter. This parameter is the

drop of the engine brake power output with the water addition as may be seen from Figure 1.

Figure 6 shows the effect of the water addition and the mass of pilot fuel on the brake spe-

cific fuel consumption. The dual fuel engine is running on both fuels at the same time: pilot

and LPG. It may be seen from this figure that the increase in the mass pilot fuel generally

decreases the brake specific fuel consumption. The decrease in the specific fuel consumption is

due to the increase in the brake power output of the engine, as may be seen in Figure 2. How-

ever, the mass of the gaseous fuel (LPG) is constant. It may be also seen from the figure that

adding more water will increase the specific fuel consumption at all masses of pilot fuel.

Figure 7 depicts the effect of the water addition and injection timing on brake specific fuel

consumption for the three water levels tested. It may be seen that advancing the injection tim-

ing from 20� to 45� before the top dead center generally increases the brake specific fuel con-

sumption. The increase in the bsfc is due to the drop in the brake power output of the engine

as a result of increasing the pressure and temperature earlier in the compression stroke which

decreases the mean effective pressure. Also increasing the water addition level from 6 to 14 g/kg

then to 24 g/kg increases the brake specific fuel consumption slightly for the same reasons men-

tioned above.

The increase in the brake specific fuel consumption is almost the same for all water addi-

tion conditions under the whole range of injection timing tested. For the brake power output

and brake specific fuel consumption, it may be concluded to run the engine at early injection

FIG. 5. Effect of water addition and engine speed on brake specific fuel consumption, CR¼ 22, IT¼ 35� BTDC.



timing for higher brake power output and less brake specific fuel consumption at all water addi-

tion ratios while increasing the injection timing to 45, for example, will reduce the brake power

output of the engine and increases the specific fuel consumption.

Figure 8 gives the effect of the water addition and compression ratio on the brake specific

fuel consumption for the three levels of water addition. It may be seen that increasing the com-

pression ratio from 14 to 22 generally decreases the brake specific fuel consumption. This

decrease is due to the increase in the brake power output of the engine. As the compression ra-

tio is increased beyond 20 (up to 22), a slight increase in the brake specific fuel consumption is

FIG. 6. Effect of water addition and mass of pilot fuel on brake specific fuel consumption, CR¼ 22, IT¼ 35� BTDC,

mgas¼ 4 kg/h.

FIG. 7. Effect of water addition and injection timing on brake specific fuel consumption, CR¼ 22, N¼ 20 rps, mgas¼ 4 kg/h,

mdiesel¼ 0.28 kg/h.



observed. Increasing the water addition from 6 g/kg to 14 g/kg then to 24 g/kg, the specific hu-

midity generally increases the brake specific fuel consumption and this is due to the drop in the

brake power output.

It is recommended that the engine uses compression ratio of 20 or 21 to prevent the engine

knocking, where at that compression ratio higher levels of water vapor addition can be intro-

duced without significant increase in the bsfc, while at the same time enjoying the advantage of

reduced NOx emissions.

B. Effect of water vapor addition on maximum pressure rise rate

Figure 9 shows the effect of the engine speed and the water addition on the maximum pres-

sure rise rate for the three water addition levels studied. It may be seen from this figure that the

increase in the engine speed from 16 to 32 rps generally decreases the maximum pressure rise

rate ðdP=dhÞmax. The maximum pressure rise rate is a measure of the combustion noise and it is

directly related to it. Higher pressure rise rate produces higher combustion noise and vice versa.

Increasing the engine speed increases the mixing between air and fuel which makes the com-

bustion to start smoother and hence decreases the ignition delay period. This reduction in the

ignition delay period causes the maximum pressure rise rate to decrease, so the engine will be

running more smoothly especially at higher engine speeds.

It may be seen also from the same figure that increasing the water addition increases the

maximum pressure rise rate in the engine. Adding more water to the engine decreases the maxi-

mum combustion temperature and increases the ignition delay period and this again increases

the maximum pressure rise rate, so the engine will be nosier at a higher water level. However,

at the highest speed of (30 rps), the maximum pressure rise rate is about 4 bars/degrees which

is much lower than the value at low engine speed. Therefore, running the engine at high speed

with high water level would decrease the NOx emission, because of the decrease in tempera-

tures, and at the same time decrease the combustion noise.

Figure 10 shows the effect of water addition and mass of pilot fuel on maximum pressure

rise rate ðdP=dhÞmax. It may be seen from the figure that increasing the mass of pilot fuel

increases the maximum pressure rise rate. Increasing the mass of pilot fuel means that it will

stay longer time for this amount to be vaporized, mixed with air and ignited, and this increases

FIG. 8. Effect of water addition and compression ratio on brake specific fuel consumption, N¼ 20 rps, mgas¼ 4 kg/h,




the ignition delay period. The increase in the ignition delay period causes the maximum pres-

sure rise to increase, and hence the engine will run noisier at high amount of mass of pilot fuel

used.

For the dual fuel engine, it should be noted that the use of mass of pilot fuel should be as

low as possible as the main fuel here is the gaseous fuel and the function of the pilot fuel is

just to ignite it. Therefore, running the engine at small pilot fuel will reduce the maximum pres-

sure rise rate. It may be seen also from the same figure that increasing the water addition level

resulted in an increase in maximum pressure rise rate. The increase in the pressure rise rate is

due to the increase in the ignition delay period as adding more water to the mixture lead to

FIG. 9. Effect of water addition and engine speed on maximum pressure rise rate, CR¼ 22, IT¼ 35� BTDC.

FIG. 10. Effect of water addition and mass of pilot fuel on maximum pressure rise rate, CR¼ 22, IT¼ 35� BTDC, mgas¼ 4 kg/h.



lower temperatures which in turn increases the ignition delay and produces more pressure rise

rate. However, at very small amount of pilot fuel mass, this increase in pressure rise rate is

very small, i.e., the engine will not be noisy even if we add more water.

Figure 11 shows the effect of water addition and pilot fuel injection timing on maximum

pressure rise rate. It may be seen from the figure that advancing the injection timing leads to

higher pressure rise rate. Increasing the injection timing advance means injecting the pilot fuel

earlier and earlier at low pressures temperatures. Reducing the pressure and temperature at the

injection increases the ignition delay period which in turn increases the pressure rise rate. It

may be seen also from the figure that increasing the water addition from 6 g/kg to 14 g/kg to

24 g/kg leads to an increase in pressure rise rate as stated above in the previous figure.

The effect of water addition and compression ratio on the maximum pressure rise rate may

be depicted in Fig. 12. It may be noticed from this figure that increasing the compression ratio

from 14 to 22 generally increases the maximum pressure rise rate or combustion noise. The

increase in the compression ratio causes all pressures and temperatures to increase during the

engine cycles. This increase may lead a more probability of self ignition of LPG before the pilot

flame ignites it. The self ignition of the gaseous fuel (low ignition temperature for LPG) causes

the pressure rise rate to increase. It is advised that for the dual fuel engine, when it runs on

LPG, to reduce the compression ratio to avoid having the engine running at high combustion

noise or even knocking. Similar trend for the water addition may be noticed to the previous fig-

ures and the advantage of adding water may be greatly noticed when the engine runs at lower

compression ratios as the maximum pressure rise rate is small.

C. Effect of water addition on maximum combustion pressure

The effect of the engine speed and water addition on the maximum combustion pressure

may be seen in Fig. 13. As may be noticed from the figure that increasing the engine speed

from 18 to 30 rps generally decreased the maximum combustion pressure for the same amount

of water exists in the inlet air. The increase in the speed has shown a decrease in the maximum

pressure rise rate (Fig. 9) above. The decrease in the maximum pressure rise rate means a slow

increase of the combustion pressure during the compression stroke while the piston is going

inward. This slow increase in the pressure causes most of the combustion to occur during the

FIG. 11. Effect of water addition and injection timing on maximum pressure rise rate, CR¼ 22, N¼ 20 rps, mgas¼ 4 kg/h,




expansion stroke and reduce the maximum pressure of the cycle as shown in Fig. 13. On the

contrary, a decrease in the engine speed causes the pressure rise rate to increase or the pressure

goes up fast during the compression stroke which causes the maximum pressure to increase.

Similar to previous effects, the increase in the water added in the air from 6 to 14 to 24 g/kg

caused the maximum combustion pressure to increase. This increase is due to the fact that add-

ing more water increased the pressure rise rate (above) and it then increased the maximum

pressure.

FIG. 12. Effect of water addition and compression ratio on maximum pressure rise rate, N¼ 20 rps, mgas¼ 4 kg/h,


FIG. 13. Effect of water addition and engine speed on maximum combustion pressure.



Figure 14 shows the effect of the mass of pilot fuel and the amount of water on the maxi-

mum pressure. It may be noticed from this figure that increasing the mass of pilot fuel increases

the maximum pressure of the cycle. This increase is a result of the increase of the maximum

pressure rise rate as shown in Fig. 10. The maximum pressure of the cycle increased from

about 60 bars at the lowest amount of pilot to about 71 bars at the highest amount. Similar to

previous effects, when the water added is increased the maximum pressure has been also

increased as a result of the increase of the pressure rise rate.

Figure 15 depicts the effects of increasing the injection timing advance and the water

added on the maximum pressure of the cycle. It may concluded from this figure that advancing

FIG. 14. Effect of water addition and mass of pilot fuel on maximum combustion pressure.

FIG. 15. Effect of water addition and injection timing on maximum combustion pressure.



the injection of the pilot diesel fuel generally increases the maximum pressure of the cycle as

the pressure rise rate becomes higher (Fig. 11). The dual fuel engine becomes noisy when the

injection is advanced and it may be recommended to retard the injection to limit its noise and

prevent high knocking of the engine which may reduce the life of it as well as deteriorating the

performance of the engine. Adding more water in the inlet air slightly increases the maximum

pressure of the cycle as may be seen from the figure.

The effects of increasing the compression ratio and adding more water on the maximum

pressure of the cycle may be shown in Fig. 16. It may be concluded from the figure that

increasing the compression ratio from 14 to 22 generally increases the maximum pressure of

the cycle. This increase is a result if two parameters which lead to the sharp increase from

about 35 to about 70 bars. The first is the increase in the maximum pressure rise rate which

causes the combustion to accelerate and increase the maximum pressure of the cycle. The sec-

ond is the increase in the whole cycle pressures as a result of the increasing the compression ra-

tio. This includes the pressure during the compression stroke before the injection of the pilot

fuel which leads to an increase in the maximum pressure of the cycle. The increase in the water

added also increased the maximum pressure of the cycle as seen in the figure. It is also recom-

mended to reduce the compression ratio of such dual fuel engine to reduce the maximum pres-

sure rise rate or the combustion noise. There has been like sharp increase in the pressure rise

rate and maximum pressure when the engine compression ratio increased from 20 to 22.

D. Effect of water addition on CO emissions

Figure 17 shows the effect of water addition and the engine speed on CO emissions. It

may be seen from this figure that increasing the engine speed increases the CO emission up to

a speed of 28 rps, then starts to fall down and this occurs at all water addition levels. Increasing

the engine speed generally increases the mass of fuel used and this tends to increase the COemission. It may be seen also from the same figure that increasing the water addition level

from 6 to 14 g/kg to 24 g/kg increases the CO on the exhaust gasses as injecting more water in

the inlet air tends to dilute the mixture more with water and this makes the mixture more

diluted and it may give higher level of CO emissions in the exhaust.

Figure 18 shows the effect of water addition and mass of pilot fuel on CO emissions. It

may be seen from the figure that increasing the mass of pilot fuel generally decreases the CO

FIG. 16. Effect of water addition and compression ratio on maximum combustion pressure.



emissions. This is due to the more complete combustion that occurs at higher mass of pilot fuel

injected, as higher mass of pilot fuel injected gives bigger and bigger flames, and this tends to

burn completely the gaseous fuel that occur in the combustion chamber. However, it may be

also seen that at very high level of mass of pilot fuel, there is an increase in CO emissions and

this may be due to some unburned fuel that comes out of the diffusion flame at higher mass of

pilot fuel. It may be also seen from the same figure that increasing the water level in the inlet

air increases in the CO emissions in the exhaust gasses. This is due to the increased dilution of

the inlet air fuel mixture which gives more amounts of CO emissions in the exhaust gasses.

FIG. 17. Effect of water addition and engine speed on CO emission.

FIG. 18. Effect of water addition and mass of pilot fuel on CO emission.



Figure 19 shows the effect of water addition and the injection timing on CO emissions for

the three tested water addition level of 6, 14, and 24 g/kg. It may be seen from the figure that

the CO emissions in the exhaust gasses is maximum at injection timing of 25�-30� BTDC and

it is minimum at very late or very early injection timing. At very late injection timing, the COemission is very low as the injection occurs very early during the compression stroke. Injecting

the fuel during the compression stroke give more time for the combustion to occur and burn all

the CO emissions that will result in the exhaust gasses; hence, the CO emissions will decrease.

Increasing the water addition from 6 to 14 to 24 g/kg generally increases the CO emission in

the exhaust gasses because of the dilution effect as more water exists in the fuel air mixture

and this tends to increase the CO emissions in the exhaust gasses. However, the level of the

CO emission is far below the allowable limits with exhaust gasses.

Figure 20 shows the effect of water addition and the compression ratio on the CO emis-

sion. It may be seen from the figure that increasing the compression ratio from 14 to 22

increases generally the CO emission in the exhaust gasses; however, the CO emission level is

far below the allowable limits for the current exhaust gasses limits. Increasing the water addi-

tion level from 6 to 14 then to 24 g/kg generally increases the CO emission because of the dilu-

tion effect as more water is injected in the exhaust gasses.

E. Effect of water addition on HC emissions

Figure 21 shows the effect of water addition and the engine speed on CO emission. It may

be noticed from the figure that increasing the engine speed generally decreases the HC emission

for all water addition level tested. Increasing the engine speed increases the swirl and turbu-

lence level in the combustion chamber and this improves the mixing between the liquid fuel

injected, the gaseous fuel, and the air. More mixing enhances the combustion and should reduce

the HC emission.

Increasing the water additions from 6 to 14 to 24 g/kg increases the HC emission in the

exhaust gasses as more dilution occurs and more incomplete combustion and this leads to higher

levels of HC emission in the exhaust gasses. However, all are below the allowable limits for the

HC emissions in the exhaust gasses. Moreover, the technology of catalytic converter is very well

advanced and it should be relatively easy to reduce the HC emission lower by using it.

FIG. 19. Effect of water addition and injection timing on CO emission.



The effect of water addition and the mass of pilot fuel on HC emission may be seen in Fig-

ure 22. It may be noticed from the figure that increasing the mass of pilot fuel generally

decreases the HC emission in the exhaust gasses. The reason for this is that increasing the mass

of pilot fuel tends to make the flame bigger and bigger and this tends to completely burn the

gaseous fuel that exist in combustions chamber. This reduces the HC emission in the exhaust

gasses. Increasing the water addition level generally increases the HC emission because of the

dilution effect. Nonetheless, as mentioned above that using a catalytic converter may help to

reduce the HC emission even when the water is increased.

FIG. 20. Effect of water addition and compression ratio on CO emission.

FIG. 21. Effect of water addition and engine speed on HC emission.



The effect of water addition and the injection timing on HC emission may be depicted in

Figure 23. It may be noticed from the figure that increasing the injection timing advance from

20� to 45� before the top dead centre generally decreases the HC emission in the exhaust gasses

for the three tested water addition level. Starting the combustion earlier and earlier during the

compression stroke gives higher temperature and pressure in the combustion chamber. Higher

temperature and pressure gives more chance for more complete combustion and gives less HC.

Similar effect of adding the water to the combustion chamber has shown, that increased the HC

emission because of the dilution effect. More water is injected in the combustion chamber gives

higher level of emissions of HC.

FIG. 22. Effect of water addition and mass off pilot fuel on HC emission.

FIG. 23. Effect of water addition and injection timing on HC emission.



Figure 24 shows the effect of water addition and compression ratio on HC emission. It

may be seen from the figure that HC emission becomes higher and higher with increasing the

compression ratio of the engine. The reason for this may be due to the higher knocking possi-

bility of the engine at higher compression ratio which tends to increase the unburned hydrocar-

bon in exhaust gases. Similar trends of adding the water to the HC emission is noticed that

resulted in increasing the HC emission in the exhaust gasses because of the more dilution

effect.

F. Effect of water addition on smoke emissions

Figure 25 shows the effects of adding the water to the inlet air and the engine speed on

opacity or the smoke level of the exhaust gases. It may noticed from the figure that increasing

the engine speed generally increases the opacity level of the exhaust gases for the three water

addition levels tested. Increasing the engine speed means more fuel is injected in the combus-

tion chamber and more fuel is burned. At the same time, there is a less time for combustion of

this high amount of fuel which tends to increase the opacity or the smoke level in the exhaust

gases. Increasing the water addition level in the inlet air has resulted in increasing the opacity

level due to more water exists in the combustion chamber that leads to more dilution effect and

more unburned carbons atoms that lead to higher level of smoke in the exhaust gases.

The effect of the water addition and the mass of pilot fuel on opacity level is given in Fig-

ure 26. It may be noticed from the figure that increasing the mass of pilot fuel injected

increases generally the opacity level of the exhaust gasses. More pilot fuel injected means the

higher amount of diesel fuel injected to ignite the gaseous fuel. This increases the diffusion

combustion for the diesel fuel injected which leads to bigger possibility of having unburned

carbon atoms and higher level of opacity. The increase in the water addition level from 6 to 14

to 24 g/kg gives similar effect as before of an increase in the opacity level at all values of mass

of pilot fuel tested.

The good advantages of the dual fuel engine would be then to utilize as small amount of

pilot fuel as possible to reduce the smoke emissions in the exhaust gases. Adding more water

to the inlet air at small amount of pilot fuel did not increase much the opacity level.

The effect of injection timing of the pilot diesel fuel on the opacity for the three tested

water addition level may be seen in Fig. 27. It may be noticed from this figure that for injection

FIG. 24. Effect of water addition and compression ratio on HC emission.



timing of around 30� BTDC, the opacity level is minimum and increases both sides for the late

and very early injection. At this injection timing, it seems that the combustion is most complete

and produces the least smoke in the exhaust. It seems that the 30� BTDC is optimum for this

engine.

The effect of compression ratio and the water addition level on the opacity level of the

exhaust gasses may be seen in Figure 28. It may be seen from this figure that increasing the

compression ratio of the engine generally decreases the opacity level of the exhaust gasses or

the smoke level. Increasing the compression ratio tends to increase all the cycle temperatures

and pressure which gives more complete combustion for all the hydrocarbon fuel and the less

FIG. 25. Effect of water addition and engine speed on opacity.

FIG. 26. Effect of water addition and mass off pilot fuel on opacity.



opacity level or smoke level of the exhaust gasses. The same effect for the water addition to

increase the opacity level of the exhaust gasses occurs.

G. Knocking limits of dual fuel engine

The effect of load and water addition level on the dual fuel knocking limit is illustrated in

Figures 29–32. Figure 29 shows an increase in the maximum pressure rise rate or the combus-

tion noise when the load output is increased. The load (or output torque) is increased from

FIG. 27. Effect of water addition and injection timing on opacity.

FIG. 28. Effect of water addition and compression ratio on opacity.



minimum value close to zero until the engine starts to knock at very high amount of gaseous

fuel. The torque output has been increased by increasing the amount of LPG gaseous fuel while

maintaining the pilot fuel constant. The end of the experiment is when there is heavy knocking

(appears from the combustion pressure oscillation and or the pressure rise rate is greater than

10 bar/degrees) and any further increase to the gaseous fuel amount would decrease the output

power (or torque) of the engine. That point is recognized as the onset of ignition failure.

Figure 29 shows the knocking limit (or the point of highest torque) to be around 10 nm,

while it is around 15 nm for compression ratio of 20 and it is around 16 nm for compression

ratio of 18 (not presented). It may be concluded that the decrease in the compression ratio from

22 to 20 and then to 18 enabled the engine to run at a higher torque output without heavy

FIG. 29. Effect of water addition and torque on maximum pressure rise rate for compression ratio of 22.

FIG. 30. Effect of water addition and torque on maximum pressure for compression ratio of 22.



knocking or ignition failure. The reduction in the compression ratios reduces the cycle pressure

and temperature which reduces the possibility of self-ignition of the LPG gaseous fuel and hav-

ing knocking in the dual fuel engine. It is recommended therefore to reduce the compression ra-

tio of the dual fuel engine, to reduce the knocking possibility, and to reduce the combustion

noise and enables the engine to run at higher output torques.

It may be also noticed that increasing the output torque leads to an increase of the maxi-

mum pressure rise rate. This is a result of the increase in the amount of LPG gaseous fuel

admitted in the intake air which—when burned—produces high pressure rise rate.

Similar trend may be found for the maximum combustion pressure as shown in Fig. 30.

The increase in the maximum pressure with increasing the torque is a result of the increase in

FIG. 31. Effect of water addition and torque on brake specific fuel consumption for compression ratio of 22.

FIG. 32. Effect of water addition and torque on opacity for compression ratio of 22.



the pressure rise rate shown above in Fig. 29. The same conclusion for the knock limit is also

seen here for the maximum pressure.

Figure 31 shows the effect of output torque on the bsfc for the three tested water addition

level tested. It may be shown in this figure that increasing the output torque results in a

decrease in the bsfc. The increase in the output torque is a result of the increase on the mass of

LPG gaseous fuel which also produces more power output; therefore, it seems that the increase

in the power is more than the increase in the fuel mass and also due to an improvement of the

engine efficiency at high loads.

Figure 32 depicts the effect of output torque, compression ratio, and the water addition

level on the opacity level of the exhaust gases. It could be seen from this figure that the

increase in the output torque has resulted in the increase in the smoke level or opacity level of

the exhaust gases. Even at higher amount of water addition levels, the same trend is noticed.

This increase in the opacity may be due to the increase in the amount of gaseous fuel admitted

with the inlet air. The increase in the gaseous fuel may have resulted in the increase of fuel to

air ratio and this reduces the amount of oxygen available for the complete combustion of the

fuel. This may have resulted in the increase in the opacity level of exhaust gases.

IV. CONCLUSIONS

1. Increasing the mass of pilot fuel generally increases the brake power output of the engine for

all ratios of water level.

2. Increasing the water addition level from 6 g/kg air to 14 g/kg then to 24 g/kg air, what generally

reduce the brake power output of the engine; however, this reduction in the brake power output

is not much compared to the benefit of using the water in the engine to reduce the NOx.

3. The use of slightly lower compression ratios, e.g., 20 would produce higher brake power out-

put, and even in the case of using high amount of water in the admitted air (24 g/kg air), the

engine would give the same power as of compression ratio of 22.

4. Adding more water increases the specific fuel consumption at all masses of pilot fuel.

5. Adding more water increases the maximum pressure rise rate and maximum pressure for all

design and operating conditions tested. However, this can be decreased if the engine speed is

increased, pilot fuel injection timing is retarded, mass of pilot is reduced, and compression ratio

is reduced.

6. Adding more water increases the level of CO, HC, and smoke opacity in the exhaust gases;

however, the level of CO is far below the allowable limits and HC can be reduced with a cata-

lytic converter. The smoke opacity may be reduced by decreasing the mass of pilot fuel.

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Control of noise and exhaust emissions from dual fuel engines

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