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Possibilistic fuel cycle assessment of retrofit technologiesfor two stroke powered tricycles

J.B.M. Biona a,b,*, A.B. Culaba b, R.R. Tan b, M.R.I. Purvis c

a Don Bosco Technical College, Center for Research and Training, 736 Gen. Kalentong Street, 1501 Mandaluyong City, Philippinesb Center for Engineering and Sustainable Development Research, De La Salle University, Manila, Philippinesc Department of Mechanical Design and Engineering, University of Portsmouth, UK

a r t i c l e i n f o

Keywords:Life cycle assessmentTwo stroke emissionsMotorcyclesLiquefied petroleum gas

a b s t r a c t

Two stroke powered tricycles are a major source of air pollution in the Philippines. A fuel-cycle assessment of the liquefied petroleum gas (LPG) and direct injection retrofit technol-ogies for these vehicles is conducted. The results when compared with carbureted two andfour strokes units indicate that retrofitting the units to direct injection provides lower fossilenergy depletion, global warming, human toxicity and photochemical ozone formationimpact potentials compared to LPG fueled carbureted two stroke tricycles while the latterexhibits lower acidification and nutrification impact values. The direct injection retrofittedunits show a lower aggregated impact score and dominance over four stroke units. Theconversion to LPG revealed minimal environmental benefits compared to the gasolinerun units.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Tricycles play an important role in the Philippine transport system serving feeder, residential and minor roads. Around70% of the 600,000 registered tricycles in the country are two stroke powered. While these engines provide features thatare essential to tricycles, such as high power to weight ratio, low cost and simple construction, they allow 15–40% of the fuelcharge to escape to the exhaust port unburned, making it a major environmental concern. Some local government units anddevelopment institutions have advocated the re-fleeting to four stroke engines by setting restrictions on franchise applica-tions of new units and franchise renewal of units beyond a certain cut-off age. Despite such policies, with an average life ofseven years and assuming a cut-of age of 15 years, two stroke tricycles will continue to pollute urban and rural communitiesfor at least the next eight years.

A number of intermediate interventions have been recommended to reduce emissions from these vehicles before they areeventually replaced. A direct injection retrofit technology for two stroke powered tricycles is being introduced in the marketand could possibly effect significant reduction in tricycles emissions (Willson, 2002; Bauer and Lorenz, 2006). In addition tothe elimination of fuel short-circuiting, it also has the ability to reduce oil consumption by eliminating the fuel ‘‘washing”effects of standard two stroke operation (Hirz et al., 2004; Archer and Bell, 2001). Another retrofit technology for tricyclesis the conversion to liquefied petroleum gas (LPG) fuel. Studies have shown that the use of this fuel could possibly lowercarbon monoxide, carbon dioxide and particulate matter (PM) emissions (Murillo et al., 2005; Ristovski et al., 2005) primar-ily due to its high combustibility and lower carbon content. In addition, its upstream energy requirement and emissions

1361-9209/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.trd.2008.03.010

* Corresponding author. Address: Don Bosco Technical College, Center for Research and Training, 736 Gen. Kalentong Street, 1501 Mandaluyong City,Philippines. Tel./fax: +63 2 5325812.

E-mail addresses: manuelbiona@yahoo.com, biona.manuel@dbtc.edu.ph, bionaj@dlsu‘.edu.ph (J.B.M. Biona).

Transportation Research Part D 13 (2008) 306–314

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production is lower compared to gasoline (Wang et al., 2007). When used in two stroke engines, HC emissions are expectedto remain high, however, as it does not directly address fuel short-circuiting (Briggs et al., 2007; Iyer and Shah, 2002).

The ability of these technologies to reduce environmental impact of two stroke powered tricycles is quantified and com-pared based on a fuel cycle assessment framework. Results were also compared with the environmental performance of fourstroke tricycles. Findings of the study would provide valuable inputs to policy makers in charting strategies to reduce envi-ronmental impact from the tricycle sector.

2. Methodology

A streamlined fuel cycle assessment approach was adopted limiting inventory analysis to:

� Energy Inventory in terms of energy and fossil fuels usage. Fossil fuels on the other hand are further broken down intopetroleum, natural gas and coal inputs.

� Air Emissions Inventory in terms of CO2, CH4, N2O, VOC, CO, NOx, PM10, PM2.5 and SOx. Non-methane VOCs during the in-use phase were also quantified.

In-use emissions were generated using an instantaneous emissions model developed specifically for tricycles. Fuel up-stream inventories were determined using the greenhouse gases, regulated emissions and energy use in transportation mod-el (GREET 1.7) of the Argonne National Laboratory (Wang et al., 2007). The impact assessment process was based on theenvironmental design of industrial products (EDIP) framework (Hauschild and Wenzel, 1997; Wenzel et al., 1997). The im-pact areas covered include global warming, acidification, photo chemical ozone formation, nitrification, human toxicity andfossil energy depletion. Data uncertainty was quantified using possibilistic uncertainty propagation while aggregated scoreswere generated and evaluated using possibilistic compromise programming (Tan et al., 2004). A brief description of each ofthese approaches is provided in the succeeding sub-sections.

2.1. Possibilistic uncertainty propagation

Fuzzy set theory describes imprecision associated with vagueness or ambiguity, such as that encountered in the evalu-ation of prospective or unproven technologies. Possibility theory is an interpretation of fuzziness that assigns a possibilityvalue, P, within the interval [0,1] that is proportionate to the degree of plausibility of a statement or argument. Quantitativepossibilistic uncertainty can be expressed through fuzzy numbers. These numbers are ‘‘spread out” over a range of values, incontrast to crisp (ordinary) numbers which have unique values. For simplicity, triangular distribution was employed in thisstudy wherein fuzzy quantities ð~eÞwere expressed as a triad of the mode (eM) and upper (eR) and lower (eL) bounds as shownin the following equation:

~e ¼ ðeL; eM; eRÞ ð1Þ

The lower and upper bounds take a 0 possibility value while the mode is assigned a possibility of 1. Vehicle operationalparameters, in-use and upstream emission factors and energy inventories were expressed as fuzzy quantities and computedusing interval arithmetic rules (Kaufmann and Gupta, 1991).

2.2. Emission and fuel factor module

The fuzzy in-use fuel rate ðeFRiÞ and emission factors ðeEðxÞiÞ for gas specie (x) of tricycle technology (i) were determinedusing the following equation:

eFRi ¼Pn

j¼mþ1fðeFRbaseÞðeTFiÞðeDFjÞðeCFjÞgðn�mÞ ð2Þ

eEðxÞi ¼Pnj¼mþ1fðeEðxÞbaseÞðeTFðxÞiÞðeDFðxÞjÞðeCFðxÞjÞg

ðn�mÞ ð3Þ

x ¼ CO;HC;NOx;PM

An average age (m) of seven years and mean life span (n) of 15 years were assumed for the two stroke retrofit technologies.Emissions and fuel consumption of the four stroke units were thus based on their first eight years of operation.

The fuel use and emission factors were computed as a function of the fuzzy fuel and emissions rates of a zero mile car-bureted two stroke powered unit (eFRbase and eEðxÞbase), technology factors (eTFi and eTFðxÞi), age deterioration factor (eDFj andeDFðxÞj) and condition factors ðeCFj and eCFðxÞj). CO2 emissions were computed by carbon balance while SOx emissions weredetermined based on the fuel’s sulphur content. Volatile organic compound (VOC) and methane emissions were estimatedusing factors provided by the US Environmental Protection Agency (2005a). PM emission projections on the other hand wereobtained using the PM/HC factor derived from the data provided by Kojima et al. (2002) for two stroke powered three wheel-ers in Dhaka.

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The fuzzy fuel consumption and emission factor of carbureted two stroke units were generated using an instantaneousemissions model specifically developed for tricycles in the Philippines.1 Five driving patterns were randomly selected froma depository of tricycle drive cycles gathered from various routes in Metro Manila and used in the simulation.

After the fuel consumption and HC, CO and NOx emission factors for each of the drive cycle were generated, the corre-sponding possibilistic values were computed using the following equations:

eL ¼ l� 0:5r ð4ÞeM ¼ l ð5ÞeR ¼ lþ 0:5r ð6Þ

Table 1 provides the summary of the baseline fuel consumption and emission factors obtained and used in the study.Technology factors (eTFi and eTFðxÞi) are the ratio between the emissions or fuel use factor of a given technology with that

of a carbureted two stroke engine. The possibilistic technology factors summarized in Table 2 were generated using Eqs. (4)–(6) based on data from related technology comparison studies. Direct injection retrofit technology factors were derived fromLeigthon and Ahern (2003), Archer and Bell (2001) and Bauer and Lorenz (2006). Technology factors for LPG powered car-bureted two strokes on the other hand were obtained from Iyer and Shah (2002) and Briggs et al. (2007) while those forcarbureted four strokes were computed from figures provided by Priest et al. (2000), Tzeng and Chen (1998), Mittal and Shar-ma (2006), Tsai et al. (2000) and Archer and Bell (2001).

The term ‘‘deterioration” refers to the degradation of an engine’s exhaust emissions and fuel economy performance overits lifetime due to normal use excluding decrease in efficiency from malfunctioning parts and tampering. The deteriorationfactors (eDFj and eDFðxÞj) were obtained based on the approach provided by US Environmental Protection Agency (2005b)assuming that 95% of deterioration occurs before the median life of the vehicle

eDFj ¼ 1þ ðDRÞð1� e�3ðAeF jÞÞ ð7ÞeDFðxÞj ¼ 1þ ðDRðxÞÞð1� e�3ðAeF jÞÞ ð8Þ

Deterioration rate (eDR and eDRðxÞ) refers to the increase in emissions and fuel consumption of a vehicle over its lifetime. Withthe absence of actual data and considering the rugged operation of tricycles, deterioration rates for recreational and off-roadmotorcycles were adapted. AeF j refers to the fuzzy age factor of an age j unit and is the ratio between the cumulative distancetraveled by the vehicle at age j and the mileage traveled at the median design life of the vehicle

AeF j ¼Mileagej

MileageML¼ ðAn eMÞðjÞ

80;000ð9Þ

The 160,000 km lifetime mileage for off-road vehicles in the US Environmental Protection Agency (2005b). Tricycle mileagemay exceed this value thus for cumulative mileage (Mileagej) beyond 160,000 km, AFj was assigned the value of 1. The fuzzyannual mileage ðAn eMÞ traveled by a vehicle was computed based on the daily fuel consumed ðDeFLÞ, hot soaked evaporativefuel losses ðEeFLÞ, service factor ðeSFÞ and the simulated fuel consumed per km traveled ðeFMÞ

An eM ¼ ðDeFL� EeFLÞðeFMÞðeSFÞ ð365Þ ð10Þ

The resulting lower, mode and upper bound annual mileage values are 19,012, 24,197 and 31,551 km/year, respectively.The condition factors (eCFj and eCFðxÞj) account for the increase in fuel consumption or emissions from poor maintenance

and minor components malfunctions. Values are aggregated with vehicle age (j) and computed using the followingequations:

Table 1Baseline fuel consumption and emission factor values in g/km

Fuel HC CO NOx

(29.82, 31.34, 33.14) (11.60, 12.63, 13.75) (9.18, 10.29, 10.94) (0.068, 0.072, 0.076)

Table 2Technology factor summary

Technology Fuel HC CO NOx

DI 2S Retrofit (0.83, 0.78, 0.73) (0.15, 0.12, 0.09) (0.32, 0.28, 0.25) (1.15, 1.10, 1.05)Carb 4S (0.88, 0.82, 0.76) (0.20, 0.18, 0.16) (0.87, 0.80, 0.73) (7.09, 5.56, 4.04)Carb 2S Retrofit LPG (1.22, 1.16, 1.10) (1.01, 0.98, 0.95) (1.19, 1.16, 1.14) (0.66, 0.62, 0.58)

1 A detailed discussion of the model is provided in Biona et al. (2007).

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eCFj ¼ 1þ ð% failjÞI eM

ð1� I eMÞ !

ð11Þ

eCFðxÞj ¼ 1þ ð% failjÞI eMðxÞ

ð1� I eMðxÞÞ !

ð12Þ

Derivation details of Eqs. (11) and (12) are provided in Biona (2007). I eM and I eMðxÞ refer to the fuel economy and emissionsimprovement effected by proper vehicle maintenance and minor repair. These parameters were quantified based on relatedAsian two- and three-wheeled vehicle maintenance studies (Das et al., 2001; Kojima et al., 2002; Sekar and Sardesai, 1998).% failj on the other hand represents the percentage of the tricycle fleet that fail to satisfy in-use emissions standards aggre-gated according to age (j). In the absence of reliable local data, % failj was based on emissions testing data of two strokemotorcycles in India. Considering the high variability of this parameter, an uncertainty range of plus and minus 10% wereassumed.

2.3. Fuel cycle assessment

The upstream inventories were obtain by multiplying the in-use fuel factor by their corresponding fuel production spe-cific inventories provided by GREET 1.7. The lower, mode and upper bound specific inventory values were based on the soft-wares’ energy and fuel production efficiencies for 1990, 2010 and 2015, respectively. Energy mix was set to the 2009projected energy mix of the Philippines.

As mentioned, the possibilistic compromise programming approach was adapted in the impact category weighing andenvironmental performance score aggregation. It uses the concept of ranking alternatives based on a modified weighted aver-age calculated as a mathematical distance from a best case–case positive ideal solution (PIS) that corresponds to an environ-mental benchmark. The technique effectively allows a single, composite environmental index to be calculated based on scoresfrom different environmental impact categories, coupled with information on the relative weights of these categories. At thesame time, final comparison and ranking of alternatives based on the composite environmental indices are modified to ac-count for fuzzy or possibilistic uncertainties in the data. Further details of this methodology are provided in Tan et al. (2004).

The evaluation was implemented in two phases. The first run assumed equal weights for all impact categories. The Phil-ippines imports all of its petroleum and is thus is very vulnerable countries to rising fuel prices. A second run was thusimplemented giving priority importance to fossil fuel depletion. Higher weights were also assigned to global warmingand human health impact (Table 3).

Possibilistic compromise programming generates a pairwise degree of dominance comparison of the technologies eval-uated. The linguistic equivalents of these numerical values are provided in Table 4.

3. Results

Figs. 1 and 2 summarize the mean in-use and upstream energy use and emission production for the technologies evalu-ated. Figs. 3–8 provide the possibilistic comparison of the fossil energy resource depletion, global warming, acidification,photochemical ozone formation, nutrification and human toxicity comparison of the technologies compared

Table 3Impact category weights adapted

Impact category Weights

GWP 0.20AP 0.05POF 0.05NP 0.05HTP 0.20DP 0.05Energy 0.40

Table 4Linguistic equivalents of degrees of dominance

Numerical value Interpretation

0 No demonstrable dominance0.25 Weak dominance0.5 Moderate dominance0.75 Strong dominance1 Conclusive dominance

Source: Tan et al. (2004).

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Ene

rgy

(MJ/

km)

VO

C x

102

(g/k

m)

CO

x 1

02 (g

/km

)

NO

x (g

/km

)

PM

10 (

g/km

)

SO

x (g

/km

)

CH

4 (g

/km

)

N2O

(g/

km)

CO

2 (k

g/km

)

0.E+00

2.E-01

4.E-01

6.E-01

8.E-01

1.E+00

1.E+00

1.E+00

2.E+00

2.E+00

Carb 2S

DI2S

LPG 2S

Carb 4S

Fig. 1. Mean in-use energy and emission comparison.

Fossil Fuel Resource Impact

0.0E+00

5.0E-12

1.0E-11

1.5E-11

2.0E-11

2.5E-11

3.0E-11

Carb 2S DI 2S LPG 2S Carb 4S

Eq

. kg

Sb

/ km

Fig. 3. Possibilistic fossil energy resource impact.

Ene

rgy

(MJ/

km)

VO

C (

g/km

)

CO

(g/

km)

NO

x (g

/km

)

PM

10 (

g/km

)

Sox

(g/

km)

N2O

x 1

0-2 (g

/km

)

0.E+00

5.E-02

1.E-01

2.E-01

2.E-01

3.E-01

3.E-01

4.E-01

Carb 2S

DI 2S

LPG 2S

Carb 4S CO

2 (k

g/km

)

CH

4 (g

/km

)

Fig. 2. Mean upstream energy and emission comparison.

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Retrofitting carbureted two stroke tricycles to direct injection provided fuel consumption reduction benefits comparableto four stroke units (Fig. 3). Shifting these vehicles to LPG fuel on the other hand increased fossil fuel depletion. The increase

Global Warming Impact

0.0E+00

5.0E-02

1.0E-01

1.5E-01

2.0E-01

2.5E-01

Carb 2S DI 2S LPG 2S Carb 4S

Eq

. kg

CO

2 / k

m

Fig. 4. Possibilistic global warming impact.

Acidification Impact

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

6.0E-04

Carb 2S DI 2S LPG 2S Carb 4S

Eq

. kg

SO

x / k

m

Fig. 5. Possibilistic acidification impact.

Photo Chemical Ozone Formation Impact

0.0E+001.0E-032.0E-033.0E-034.0E-035.0E-036.0E-037.0E-038.0E-039.0E-031.0E-02

Carb 2S DI 2S LPG 2S Carb 4S

Eq

. kg

C2H

4 / k

m

Fig. 6. Possibilistic photo chemical ozone formation impact.

Nutrification Impact

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

6.0E-04

7.0E-04

8.0E-04

9.0E-04

Carb 2S DI 2S LPG 2S Carb 4S

Eq

. kg

NO

3 / k

m

Fig. 7. Possibilistic nutrification impact.

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in in-use fuel consumption in LPG system (Fig. 2) effected by its lower volumetric efficiency (Bayraktar and Durgun, 2005)outweighed its lower upstream energy use (Fig. 3) hence the increase in fuel depletion impact.

Despite its lower carbon content and upstream specific emissions, the global warming potential of the LPG system is high-est (Fig. 4) due to its higher fuel consumption. Its higher methane concentration of its in-use HC emissions further exacer-bated this impact. Retrofitting the two stroke units to direct injection and the adoption of new four stroke units providedlower global warming emissions basically due to their significantly lower HC emissions.

Conversion of gasoline run carbureted two strokes to LPG fuel provided the least acidification impact as shown in Fig. 5.This is primarily due to its zero sulphur content and lower in-use NOx. These points are significant enough to negate thehigher upstream NOx and SOx emissions effected by its higher in-use fuel consumption. Four stroke tricycles provide thegreatest acidification potential due to their significantly higher in-use NOx emissions.

The three technologies evaluated exhibited the capacity to significantly reduce photochemical ozone formation impact oftricycles (Fig. 6). While there is not much difference between the amount gasoline of VOC emissions from gasoline and LPGrun two stroke units, the reactivity of VOC emissions from the latter is much weaker (Chang et al., 2001) hence the lowersmog formation potential. This mechanism, however, is not enough to provide smog reduction benefits comparable to theother two systems where VOC emissions are a lot lower.

With the highest and lowest in-use NOx emissions, four strokes and LPG two strokes generated the greatest and leastnutrification impacts, respectively. The lower upstream NOx and N2O and in-use NOx emissions of the direct injection sys-tems provided its lower nutrification impact compared to the four stroke units. Its nutrification impact variation with car-bureted two stroke units, however, is minimal due to the lower in-use NOx emissions of the reference vehicles.

The higher health impact of LPG two strokes compared to the direct injection and four stroke units could be attributed toits greater in-use HC and upstream methane and NOx emissions. The lower toxicity of its VOC emissions (Christensen andHoffmann, 2005), however, rendered the LPG units more desirable compared to carbureted two strokes in terms of healtheffects. The health effects of the direct injection units are lower compared to the four stroke systems despite producing moreparticulate matter emissions due to their lower in-use VOC and NOx emissions.

The possibilistic impact comparison for the equal impact weight scenario is provided in Fig. 9.Under this scenario, the direct injection retrofit technology provided the least aggregated environmental impact. This is

primarily due to its ability to reduce global warming, nutrification and human toxicity potential impacts. The pairwise dom-inance comparison in Table 5 indicated its conclusive environmental superiority over the LPG retrofitted system, carburetedtwo and four stroke units. These results account for overlaps in the environmental scores. The signs of these numbers indi-cate the direction of preference, while the magnitudes indicate the strength of the preference in view of the data uncertainty.For example, the entry of ‘‘�1” in Row 1, Column 1 indicates that carbureted two stroke units have decidedly worse

Human Toxicity Impact

0.0E+00

1.0E+04

2.0E+04

3.0E+04

4.0E+04

5.0E+04

6.0E+04

7.0E+04

Carb 2S DI 2S LPG 2S Carb 4S

M^

3 ai

r /

km

Fig. 8. Possibilistic human toxicity impact.

Possibilistic Normalized Aggregated Impact

0.0E+00

1.0E-01

2.0E-01

3.0E-01

4.0E-01

5.0E-01

6.0E-01

7.0E-01

8.0E-01

Carb 2S DI 2S LPG 2S Carb 4S

Fig. 9. Possibilistic aggregated impact for equal weights.

312 J.B.M. Biona et al. / Transportation Research Part D 13 (2008) 306–314

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environmental performance than direct injection two stroke vehicles. Both the LPG run two strokes and four stroke unitsprovided moderate dominance over carbureted two strokes. The incremental benefits of four strokes over LPG two strokesis too small to even warrant weak dominance.

While it could be noted that the direct injection two stroke retrofit system remained as the dominant technology, the newset of impact weights weakened its dominance over carbureted four strokes as shown in Fig. 10. It will be recalled earlier thatdirect injection two stroke systems have significantly lower acidification, nutrification and ozone formation potential com-pared to carbureted four stroke units while its fossil fuel depletion, global warming and human toxicity potential advantageare a lot lower, thus resulting in the weakened aggregated dominance (see Table 6).

It could be observed also that the inferiority of the LPG two stroke units relative to carbureted four stroke ones have wid-ened. This could be traced to the lower weights provided to acidification and nutrification impact areas. It could be recalledthat these are strong points of the LPG system. For the same reasons, its dominance over carbureted two stroke systems wasdowngraded from moderately dominant to weak dominance. The dominance level of direct injection retrofitted units overthe LPG retrofitted units remained unchanged.

4. Conclusions

Retrofitting carbureted two stroke tricycles to direct injection systems will provide greater benefits compared to convert-ing them to LPG systems in terms of fossil fuel depletion, global warming, photo-chemical ozone formation and human tox-icity impacts. The latter option, on the other hand, produces lower acidification and nitrification impacts. Generally, thedirect injection systems provide greater environmental benefits compared to LPG two stroke units. The direct injection ret-rofit system was also found to be more environment-friendly compared to carbureted four stroke units. Its degree of dom-inance over these systems depends on the weights assigned to the different impact areas. The same is true with regards tothe dominance of four strokes and LPG two strokes over carbureted two strokes.

References

Archer, M., Bell, G., 2001. Advanced Electronic Fuel Injection Systems – An Emissions Solution for Both 2 and 4 Stroke Small Vehicle Engines. SAE TechnicalPaper 2001-01-0010.

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Table 5Pairwise degree of dominance table for equal impact weights

DI 2S LPG 2S Carb 4S

Carb 2S �1.00 �0.55 �0.58DI 2S 1.00 1.00LPG 2S �0.12

Possibilistic Normalized Aggregated Impact

0.0E+00

1.0E-01

2.0E-01

3.0E-01

4.0E-01

5.0E-01

6.0E-01

7.0E-01

8.0E-01

Carb 2S DI 2S LPG 2S Carb 4S

Fig. 10. Possibilistic aggregated impact for unequal weights.

Table 6Pairwise degree of dominance table for unequal impact weights

DI 2S LPG 2S Carb 4S

Carb 2S �1.00 �0.26 �0.92DI 2S 1.00 0.71LPG 2S �0.55

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