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Atmospheric Environment 38 (2004) 7093–7100

www.elsevier.com/locate/atmosenv

The influence of air–fuel ratio on engine performanceand pollutant emission of an SI engine using

ethanol–gasoline-blended fuels

Chan-Wei Wua, Rong-Horng Chenb, Jen-Yung Pua, Ta-Hui Lina,�

aDepartment of Mechanical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROCbDepartment of Mechanical Engineering, Southern Taiwan University of Technology, Tainan 71043, Taiwan, ROC

Received 1 October 2003; received in revised form 5 January 2004; accepted 26 January 2004

Abstract

Ethanol–gasoline-blended fuel was tested in a conventional engine under various air–fuel equivalence ratios (l) for itsperformance and emissions. The amount of fuel injection was adjusted manually by an open-loop control system using

a CONSULT controller. It was found that without changing throttle opening and injection strategy, l could be

extended to a leaner condition as ethanol content increased. The results of engine performance tests showed that torque

output would increase slightly at small throttle valve opening when ethanol–gasoline-blended fuel was used. It was also

shown that CO and HC emissions were reduced with the increase of ethanol content in the blended fuel, which resulted

from oxygen enrichment. At an air–fuel equivalence ratio slightly larger than one, the smallest amounts of CO and HC

and the largest amounts of CO2 resulted. It was noted that under the lean combustion condition, CO2 emission was

controlled by air–fuel equivalence ratio; while under the rich combustion condition, CO2 emission is offset by CO

emission. It was also found that CO2 emission per unit horse power output for blended fuel was similar or less than that

for gasoline fuel. From the experimental data, the optimal ethanol content in the gasoline and air–fuel equivalence ratio

in terms of engine performance and air pollution was found.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Ethanol–gasoline-blended fuel; Air–fuel ratio; Pollutant emission; Engine performance

1. Introduction

With the rapid development of the industry and

society, the requirement of fossil fuels is growing higher

and higher, so there is great anxiety about the shortage

of energy because of finite reserves or other political

reasons (such as petroleum crisis and the Persian Gulf

War). Besides designing more efficient engines to save

e front matter r 2004 Elsevier Ltd. All rights reserve

mosenv.2004.01.058

ing author. Tel.: 886 6 2752525x62167; fax:

ess: thlin@mail.ncku.edu.tw (T.-H. Lin).

fuel, we need to look for other energy sources to

completely or partially substitute the fuels we are using

at present. Then the demand and dependence on fossil

fuels can be lowered. Furthermore, environmental

protection issues have been emphasized around the

world in recent years, so it is urgent to find some clean

and suitable alternative fuels to meet environmental

needs.

In alcohols, methanol and ethanol are used most often

as fuels and fuel additives. It is known that natural gas is

needed in manufacturing methanol. But natural gas

is not abundant in Taiwan, so the goal of energy

d.

ARTICLE IN PRESSC.-W. Wu et al. / Atmospheric Environment 38 (2004) 7093–71007094

independence cannot be reached. Since ethanol can be

fermented and distilled from crops, it can be considered

as a renewable energy. Based on economic and

environmental considerations for Taiwan, we have more

interest in using ethanol as fuel instead of methanol.

Moreover, for most unleaded gasoline, methyl tertiary

butyl ether (MTBE) is still a problem because it will

pollute groundwater and harm human health. It may be

possible for ethanol to replace MTBE in the future.

The properties of gasoline and ethanol are quite

different from each other because of their chemical and

physical differences. For the Reid’s vapor pressure

value, gasoline is higher than ethanol. The evaporative

emission of ethanol is thus smaller when it is stored, but

this will also cause cold-start problem. In theory, for an

un-throttled Otto-cycle engine, the efficiency � can be

written as � ¼ 1� ð1=gk�1Þ; where g is compression ratio

and k is specific heat ratio. The research octane number

of ethanol is higher than that of gasoline; this will

improve antiknock property of engine. Thus if we use

ethanol as the fuel, we can raise the compression ratio so

that the heat efficiency and engine power output can be

improved. In modern engines, however, if we use pure

ethanol as fuel, we will need to modify the engine

designs. To avoid changing engine design, we put

emphasis on ethanol–gasoline-blended fuel and then

some engine performance will be improved such as cold-

start and antiknock property.

Many researches have focused on ethanol–gasoline-

blended fuels. Furey and King (1980) showed that

evaporative emissions were increased significantly by

adding 10% ethanol to gasoline, but were increased less

with 15% MTBE in gasoline. Exhaust HC, CO, and

NOx emissions from a car without closed-loop fuel

control were significantly lower with the ethanol and

MTBE fuel blends than with gasoline. Radwan (1985)

pointed out that by using 50% ethanol and 50%

gasoline the maximum antiknock limit was attained.

Ethanol has higher cetane number, so in early

researches, it was mainly used in diesel engine (Aulich

et al., 1994). Hamdan and Jubran (1986) in their study

found out that by using 5% ethanol, optimum engine

output can be obtained and the heat efficiency increases

by 4–21% under partial loading. Because of the lower

flash point and auto-ignition temperature of gasoline,

ethanol can be more safely transported or preserved

than gasoline. It is noteworthy, however, that ethanol

will corrode mechanical components, especially those

made of copper, brass and aluminum materials owing to

water solubility of ethanol (Coelho et al., 1996). In order

to reduce this problem on fuel delivery system, it is

advisable to use fluorocarbon rubber as a replacement

for the above-mentioned materials.

The environmental protection issue has gradually

become important in many industrial countries in which

a major part of air pollution is caused by vehicles. Thus,

another goal of using ethanol–gasoline fuel is to lower

exhaust emissions. Because ethanol contains an oxygen

atom, it can provide additional oxygen and reduce

engine exhaust emission such as CO. In the national

vehicle and fuel laboratory of American EPA, Guerrieri

et al. (1995) made a test and discovered that as ethanol

content was increased up to 40%, the emission of

organic matter hydrocarbon equivalent and total

hydrocarbon (including methane and ethane) increased

a little at first and then decreased along with CO

emission, and the CO2 emission decreased with ethanol

content over 25%. In Washington, DC and Chicago

laboratories, Kelly et al. (1999) used three fuels: base

gasoline, E50 and E85 in 21 cars and concluded that

using E85 would decrease non-methane hydrocarbon

emission and CO emission in comparison with base

gasoline. Palmer (1986) showed that the gasoline

containing 10% ethanol could lower CO emission about

30%. Moreira and Goldemberg (1999) pointed out that

the aromatic hydrocarbons (such as benzene), which are

toxic, were eliminated and the sulfur content was

reduced as well by using ethanol–gasoline-blended fuel

because the ethanol has the same antiknock improving

quality as the aromatics have and the sulfur content in

gasoline was more than that in ethanol; the aromatics

and sulfur can be replaced and reduced by using ethanol.

Some studies (Chao et al., 2000; Rideout et al., 1994)

showed that using ethanol–gasoline-blended fuels would

increase the emission of formaldehyde, acetaldehyde and

acetone. Although the emission of aldehyde will increase

when ethanol is used as a fuel, the damage to the

environment by the emitted aldehyde is far less than that

by the poly nuclear aromatics emitted from burning

gasoline. Alexandrian and Schwalm (1992) indicated

that different air–fuel ratios would clearly change CO

emission. Using ethanol–gasoline-blended fuel would

produce less CO and NOx than using gasoline, especially

in rich condition. Taylor et al. (1996) used four alcohol

fuels to blend with gasoline and concluded that adding

ethanol can reduce CO, HC and NO emissions. Hsieh et

al. (2002) has recently studied different ethanol–gaso-

line-blended fuel in an engine with a closed-loop system,

and found that adding ethanol would cause leaning

effect and decreased CO and HC about 10–90% and

20–80%, respectively. However, NOx emission de-

pended on the engine operating condition rather than

the ethanol content and the result was not conclusive.

Al-Hasan (2003) investigated the effect of using

unleaded gasoline–ethanol blends on SI engine perfor-

mance and exhaust emission. The results showed that

blending unleaded gasoline with ethanol increased the

brake power, torque, volumetric and brake thermal

efficiencies and fuel consumption, while it decreased the

brake specific fuel consumption and equivalence air–fuel

ratio. The CO and HC emission concentrations in the

engine exhaust decreased, while the CO2 concentration

ARTICLE IN PRESSC.-W. Wu et al. / Atmospheric Environment 38 (2004) 7093–7100 7095

increased. From the above literature review, we realize

that alcohol–gasoline-blended fuels can effectively re-

duce the pollutant emission and slightly enhance engine

performance. But the influence of air–fuel ratio on

engine performance and pollutant emissions by using

different ethanol–gasoline-blended fuels was not yet

clearly studied. Therefore, we used an open-loop

control system to work on the overall understanding

on the correlations of ethanol–gasoline-blended fuels,

air–fuel equivalences, engine performance and exhaust

emissions.

2. Experimental setup

Our experimental apparatus include four major sys-

tems namely, the engine system, power measurement

system, exhaust system and injection system. The first

three systems can be referred to Hsieh’s (Hsieh et al.,

2002) experimental equipments. The engine system used

in the experiment is a 4-cylinder 8-valve commercial

engine New Sentra GA16DE, which is a 1600 cm3 multi-

point injection gasoline engine with cylinder bore and

stroke 76.0 and 88.0mm, respectively. The ports arrange-

ments are Dual Overhead Camshaft (D.O.H.C.) and the

compression ratio is 9.5. The engine output power was

measured by the eddy-current dynamometer made by

BORGHI & SAVERI (FE60-100-150 series).

Table 1

Properties of different ethanol/gasoline-blended fuels (E0, E5, E10, E

Property item Test fuel

E0 E5 E

Density (kg/l at 15.5 1C) 0.7575 0.7591

RON (octane number) 95.4 96.7

RVP (kPa at 37.8 1C) 53.7 59.3

Sulfur (wt%) 0.0061 0.0059

Washed gum (mg/100ml) 0.2 0.2

Unwashed gum (mg/100ml) 18.8 18.6

Lead content (g/l) o0.0025 o0.0025

Corrosivity (3 h at 50 1C) 1a 1a

Distillation temperature (1C)

IBP 35.5 36.5

10 vol% 54.5 49.7

50 vol% 94.4 88.0

90 vol% 167.3 167.7

End point 197.0 202.5

Heating value (cal/g) 10176 9692 9

Carbon (wt%) 86.60 87.70

Hydrogen (wt%) 13.30 12.20

H/C ratio 1.84 1.67

Residue (vol%) 1.7 1.5

Color Yellow Yellow Y

In the experiment, the concentrations of exhaust gas

species, including CO, CO2 and HC, were measured on-

line by the gas analyzer UREX-5000-4T with pre-

calibration. The air–fuel equivalence ratio l(l ¼ ð _mair= _mfuelÞ=ð _mair= _mfuelÞs the subscript s refers to

the stoichiometric condition) can be calculated simulta-

neously by the gas analyzer according to the composi-

tion of the exhaust. The location of on-line sampling of

exhaust gas was described in Hsieh’s apparatus (Hsieh et

al., 2002). Exhaust-gas recirculation (EGR) and cata-

lysts are not used in the engine. In the injection system,

we used CONSULT controller to adjust the amount of

injection manually to change the air–fuel equivalence

ratio. For a fixed throttle opening (at 20–100%), the

system original injection strategy was to adjust at laround 0.9. We manually changed injection rate from

�25% to 25% of the original. Then l can be calculated

according to the injected volume percentage of ethanol

and gasoline along with air flow rate (obtained from

oxygen sensor in the exhaust stream). l is used as our

basis of comparison for different ethanol–gasoline-

blended fuels.

The experimental conditions were as follows: two

engine speeds, 3000 and 4000 rpm and six throttle valve

openings, 0%, 20%, 40%, 60%, 80% and 100% (wide

open throttle) were used. The fuels were E0, E5, E10,

E20 and E30, indicating the content of ethanol in

different volume ratios (e.g., E5 contains 5% ethanol

and 95% gasoline in volume) (Table 1). The controllable

20, E30)

Method

10 E20 E30

0.7608 0.7645 0.7682 ASTM D4052

98.1 100.7 102.4 ASTM D2699

59.6 58.3 56.8 ASTM D5191

0.0055 0.0049 0.0045 ASTM D5453

0.2 0.6 0.2 ASTM D381

17.4 15 14.4

o0.0025 o0.0025 o0.0025 ASTM D3237

1a 1a 1a ASTM D130

ASTM D86

37.8 36.7 39.5

50.8 52.8 54.8

71.1 70.3 72.4

166.4 163.0 159.3

197.5 198.6 198.3

511 9316 8680

86.70 87.60 86.00

13.20 12.30 13.90

1.83 1.68 1.94

1.5 1.5 1.5

ellow Yellow Yellow Visual

ARTICLE IN PRESSC.-W. Wu et al. / Atmospheric Environment 38 (2004) 7093–71007096

range of injection was between+25% and �25% of

original injection strategy and an increment of 5% was

used for a step.

3. Results and discussions

The difference between open- and closed-loop control

is that in the closed-loop control, the on-board central

unit controls the fuel injection strategy with feedback

signal from the oxygen sensor placed in the exhaust pipe,

while in open-loop control, injection is adjusted manually

to change the air–fuel equivalence ratio. In this study, we

use open-loop control to investigate the influence of the

air–fuel ratio on engine performance and pollutant

emission of ethanol–gasoline-blended fuels.

3.1. Torque output

Fig. 1 shows the influence of air–fuel ratio on torque

output using ethanol–gasoline-blended fuels at two

engine speeds and various throttle valve openings. At

the same engine speed, the torque output rises with the

3000 rpm

0.6 0.8 1 1.2 1.4

4000 rpm

0.6 0.8 1 1.2 1.4 1.6

Tor

que

(Nm

)

Test Fuel E0 E05 E10 E20 E30

20%

40%

60%

80%

100%

1401301201101009080

1301201101009080

1301201101009080

1301201101009080

80

60

40

20

0

λ

Fig. 1. The influence of air–fuel ratio on torque output using

ethanol–gasoline-blended fuels under different rotational

speeds and throttle valve openings.

increase of throttle valve opening because the amount of

fuel injection becomes larger and more heat is produced.

For all throttle valve openings, when air–fuel equiva-

lence ratio is slightly smaller than one, maximum torque

output is available. With the increase of l, torque outputis getting smaller, because the amount of combustible

vapor in the cylinder reduces. With the decrease of l,although the amount of combustible vapor increases,

torque is still getting smaller because of incomplete

combustion.

For the influence of different ethanol–gasoline-

blended fuels on engine output, it deserved to be noted

that with increasing ethanol content, torque output

slightly increases, especially in small throttle opening

(20% and 40%) at 3000 and 4000 rpm. It can be seen

from Fig. 1 that at 20% throttle opening and 3000 rpm,

using E5 leads to higher torque output in lean condition.

At 40% throttle opening and 4000 rpm, using E20 and

E30 results in higher torque output and the increment is

about 5–8%. Between 60% and 100% throttle openings

at 3000 and 4000 rpm, the trend of torque output

variation due to ethanol addition is not clear. From the

figure, we can find that at l � 0:95 and 4000 rpm, using

E30 can produce the highest torque output.

3.2. Brake-specific heat consumption (bshc)

From the experimental data, we can calculate the bshc

value to investigate the variation of heat consumption

when using different volume ratios of ethanol–gasoline-

blended fuels. bshc can be defined as follows:

bhsc ¼ ð _mGHG þ _mEHEÞ=brake power;

where HG is the heating value of gasoline, HE the

heating value of ethanol, _mG the mass consumption rate

of gasoline and _mE the mass consumption rate of

ethanol.

Fig. 2 focused on bshc variations of different

ethanol–gasoline-blended fuels and different air–fuel

equivalence ratios at 3000 and 4000 rpm. The calories

used in Fig. 2 are based on raising the temperature of 1 g

of water by 1 1C. From the result we find that when l is

around unity, the value of bshc is the smallest. The

reason was that power output in this situation is the

largest owing to complete combustion. On the other

hand, bshc will be higher in richer or leaner condition

because of lower power output due to incomplete

combustion. Thus the engine needs more heat consump-

tion to maintain the same power output. It can also be

found that bshc increases when engine speed rises.

At a fixed throttle opening and a fixed engine speed,

the amount of air intake is a constant. To obtain the

same l, we need more volume flowrate of ethanol–gaso-

line-blended fuel than base gasoline (E0) because the

air–fuel ratio of gasoline is 1.6 times of ethanol. The

added volume flowrate compensate for the 1.6 times

ARTICLE IN PRESS

600

900

1200

15003000 rpm

600

900

1200

600

900

1200

600

900

1200

0.6 0.8 1 1.2 1.4300

600

900

1200

1500

4000 rpm

0.6 0.8 1 1.2 1.4 0.6

bshc

(ca

l/kW

)

Test FuelE0E05E10E20E30

20%

40%

60%

80%

100%

λ

Fig. 2. The influence of air–fuel ratio on bshc value using

ethanol–gasoline-blended fuels under different rotational

speeds and throttle valve openings.02468

1012

3000 rpm

02468

10

02468

10

02468

10

0.6 0.8 1 1.2 1.402468

10

4000 rpm

0.6 0.8 1 1.2 1.4 1.6

CO

Em

issi

on (

%) 80%

60%

100%

40%

20%

Test FuelE0E05E10E20E30

λ

Fig. 3. The influence of air–fuel ratio on CO emission using

ethanol–gasoline-blended fuels under different rotational

speeds and throttle valve openings.

C.-W. Wu et al. / Atmospheric Environment 38 (2004) 7093–7100 7097

lower heating value of ethanol than that of gasoline.

According to these factors, there is no appreciable

difference on the bshc between using pure gasoline and

ethanol–gasoline-blended fuels, as observed in Fig. 2.

But at 20% throttle valve opening, in general, the bshc

increases with the increase of ethanol content, instead.

From the figure, we can find that at l � 1:03 and

throttle opening larger than 40%, using E30 can obtain

the lowest bshc, and that at 20% throttle opening, using

E0 or E10 can obtain the lowest bshc.

3.3. Engine exhausts emission analysis

In this study, we use exhaust analyzer to measure the

exhaust species concentration to recognize the influence

of different ethanol–gasoline-blended fuels at different

air–fuel equivalence ratios on exhaust emissions. The

following sections discussed separately the regulated

emissions: CO, CO2 and HC.

3.3.1. CO emission

Generally speaking, as the engine is running in rich

condition, the exhaust will contain large amount of CO

emission, because there is not sufficient oxygen to

convert all carbon atoms of fuel into CO2. Thus, the

most important parameter that affects CO emission is

the air–fuel equivalence ratio.

Fig. 3 shows the variations of CO emission concen-

tration for different fuels under different air–fuel

equivalence ratios. It can be seen obviously from the

figure that at fixed engine speed, as throttle opening

become larger, l can be extended to richer condition and

CO gets higher accordingly. With air–fuel equivalence

ratio approaching unity, CO emission diminishes, and

even becomes zero in lean condition.

As to the effect of different ethanol contents on CO

emission, CO emission is likely to be reduced due to

oxygen enrichment coming from ethanol. This result can

be regarded as a ‘‘pre-mixed oxygen effect’’ to make the

reaction go to a more complete state. From Fig. 3, it can

be found that E10 always produces the smallest amount

of CO at l � 1:04:

3.3.2. CO2 emission

Fig. 4 indicates the variation of CO2 emission for

different ethanol–gasoline-blended fuels under different

ARTICLE IN PRESS

4

6

8

10

12

143000 rpm

4

6

8

10

12

4

6

8

10

12

4

6

8

10

12

0.6 0.8 1 1.2 1.44

6

8

10

12

4000 rpm

0.6 0.8 1 1.2 1.4 1.6

CO

2 Em

issi

on (

%)

100%

40%

20%

60%

80%

Test Fuel E0 E05 E10 E20 E30

λ

Fig. 4. The influence of air–fuel ratio on CO2 emission using

ethanol–gasoline-blended fuels under different rotational

speeds and throttle valve openings.

C.-W. Wu et al. / Atmospheric Environment 38 (2004) 7093–71007098

air–fuel equivalence ratios. The result shows that when lis slightly larger than one, the amount of CO2 emission

is the largest, owing to complete combustion. When l is

far away from unity, for l41, the total amount of CO2

emission stays a constant, but with the air–fuel

equivalence ratio increases, the inhaled air increases.

Thus, the concentration of CO2 emission would decrease

due to dilution. For lo1, the concentration of CO2

emission is offset by CO emission. CO and CO2 have

complementary correlation, that is, with increasing CO

emission, the amount of CO2 decreases.

With regard to the effect of different ethanol contents

on CO2 emission, it can be clearly discovered that

adding ethanol can decrease CO2 emission. The reason

can be given from the following chemical formulae:

C2H5OHþ 3ðO2 þ 3:76N2Þ

! 2CO2 þ 3H2Oþ 11:28N2; ð1Þ

) CO2ð%Þethanol ¼2

2þ 11:28 100% ¼ 15:06ð%Þ

C8H15 þ 11:75ðO2 þ 3:76N2Þ

! 8CO2 þ 7:5H2Oþ 44:18N2; ð2Þ

) CO2ð%Þgasoline ¼8

8þ 44:18 100% ¼ 15:33ð%Þ:

Formulae (1) and (2) are the complete combustion

processes of pure gasoline and ethanol. Since the

exhaust analyzer adopts dry analysis, the result shows

that as the ethanol content increases, CO2 emission will

slightly decrease. From the figure, we can find that for

all the tested fuels, the maximum CO2 emission appears

at l � 1:01; but using E30 can produce the minimum

CO2 emission.

3.3.3. HC emission

In general, the unburned hydrocarbon in exhaust is

mainly caused by three mechanisms: (a) misfiring or

incomplete combustion, which occurs in highly rich or

lean situation, or when the air–fuel mixture contains

large amount of burned exhaust or nitrogen to

make flame propagate incompletely in combustion

chamber; (b) flame quenching effect, which takes

place near combustion chamber surface area or

clearance and (c) deposits or oil membrane, which

absorbs fuel.

Fig. 5 shows the variation of HC emission for

different ethanol–gasoline-blended fuels under different

air–fuel equivalence ratios. It can be found that the

minimum HC emission occurs in the condition of

stoichiometric to slightly lean combustion, under which

conditions there is sufficient air to make unburned HC

participate in oxidation reaction. However, if combus-

tion condition is too lean or too rich, HC emission will

rise as observed from Fig. 5. For example, when l is

larger than 1.4 at 20% throttle opening, the amount of

HC emission increases because combustion becomes

highly incomplete.

Concerning the effect of different fuels on HC

emission, it can be clearly found in Fig. 5 that as

the ethanol content increases, HC emission will

decrease for all throttle openings. Ethanol does not

contain lead, so formation of porous deposits can be

avoided. In addition, ethanol molecules are polar, which

cannot be absorbed easily by un-polar molecule in

lubricating oil layer; and therefore ethanol can lower the

possibility of producing HC emission. From the figure, it

can be found that the reduction of HC is about 5–30%.

Above 40%, for both 3000 and 4000 rpm, there are

strange peaks between l=0.9 and 1 for E0 and E5

blended fuels. For a fixed throttle opening, the

concentration of unburned hydrocarbons at 4000 rpm

is less than that at 3000 rpm. The optimal operation

condition and fuel to reduce HC emission is to use E10

at l � 1:02:

ARTICLE IN PRESS

0

60

120

180

2403000 rpm

0

60

120

180

0

60

120

180

0

60

120

180

0.6 0.8 1 1.2 1.40

60

120

180

4000 rpm

0.6 0.8 1 1.2 1.4 1.6

HC

Em

issi

on (

ppm

)

20%

100%

40%

60%

80%

Test Fuel E0 E05 E10 E20 E30

λ

Fig. 5. The influence of air–fuel ratio on HC emission using

ethanol–gasoline-blended fuels under different rotational

speeds and throttle valve openings.

C.-W. Wu et al. / Atmospheric Environment 38 (2004) 7093–7100 7099

4. Conclusion

In this study, engine performance and pollutant

emission were measured on the utilization of the

ethanol–gasoline-blended fuel under different air–fuel

equivalence ratios. The results showed that the air–fuel

equivalence ratio and ethanol content play an important

role in combustion process. We made the conclusions as

follows:

1.

When air–fuel ratio is slightly smaller than one,

maximum torque output and minimum bshc are

available. Using ethanol–gasoline-blended fuels im-

proves torque output. However, bshc does not

change noticeably.

2.

CO emission depends on air–fuel equivalence ratio.

With the increase of ethanol content, CO emission is

reduced due to oxygen enrichment coming from

ethanol.

3.

CO2 emission depends on air–fuel equivalence ratio

and CO emission concentration. The maximum CO2

concentration appears at l � 1:

4.

Unburned HC is the product of incomplete combus-

tion. It is related to air–fuel equivalence ratios. When

l is slightly larger than one, HC emission is the

lowest; but if l is far from one, HC emission will rise

again. It is noted that adding ethanol can reduce HC

emission because of oxygen enhancement.

According to these results, we conclude that using E10

blended fuel at an air–fuel equivalence ratio slightly

larger than one can reduce pollutant emission efficiently.

Acknowledgements

This study was financially supported by the Commis-

sion on Sustainable Development Research, National

Science Council, and the Bureau of Air Quality

Protection and Noise Control, Environmental Protec-

tion Administration, Taiwan, ROC, under the contract

of NSC88-EPA-Z006-008.

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