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Applied Thermal Engineering 27 (2007) 2919–2923

Short communication

Study on improvement of fuel economy and reduction in emissionsfor stoichiometric gasoline engines

Li Wei a,b, Wang Ying a,*, Zhou Longbao a, Su Ling a,c

a Institute of Internal Combustion Engines, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR Chinab Kong Jun Gong Cheng Da Xue, Xi’an, PR China

c Chang An Group, Chong Qing, PR China

Received 5 October 2005; accepted 10 April 2007Available online 25 April 2007

Abstract

This paper presents the experimental study results carried out on an electronically controlled fuel injection ‘stoichiometric gasolineengine’ by using cold EGR and increasing ‘compression ratio’ to improve fuel economy and reduce emissions. After the compressionratio of the engine is raised from 8 to 11.8, and EGR rate and air swirl ratio are optimized, the fuel economy is improved by 6.02%,and the NOx and (NOx + HC) emissions are decreased by 52.96% and 44.94%, respectively at full-load speed characteristics. The cal-culation results of heat release rates according to the measured indicator diagram show that the combustion process is remarkablyimproved.� 2007 Published by Elsevier Ltd.

Keywords: Spark-ignition-engine; Stoichiometric; Air–fuel ratio; Compression ratio; Exhaust gas recirculation

1. Introduction

Electronically controlled gasoline injection, stoichiome-tric air–fuel ratio adaptively controlled by oxygen sensorand three-way catalyst converter are widely used in gaso-line engine to control CO, HC and NOx emissions. How-ever, stoichiometric mixture is not the economicalmixture for gasoline engines, which may lead to the poorfuel economy and consequently the increase in CO2 emis-sion, a global green-house gas. So it is an important topicto improve the fuel economy of gasoline engines and makeit operate at stoichiometric air–fuel ratio without deterio-rating emissions from the engine [1–4]. An investigationis carried out to improve both fuel economy and emissionson gasoline engine at stoichiometric ratio by using cold

1359-4311/$ - see front matter � 2007 Published by Elsevier Ltd.

doi:10.1016/j.applthermaleng.2007.04.005

* Corresponding author. Tel.: +86 29 82663587; fax: +86 29 82668789.E-mail address: yingw@mail.xjtu.edu.cn (W. Ying).

EGR to suppress the knocking and increasing in compres-sion ratio of the engine.

2. Experimental apparatus

The experimental study is carried out on a single-cylin-der, water-cooled, four-stroke, electronically controlledgasoline injection engine. Test gasoline engine is modifiedfrom a single-cylinder, water-cooled, four-stroke, dieselengine, which is manufactured from Nan Yang diesel fac-tory. The specifications of original engine and modifiedengine are shown in Table 1. Electronic gasoline injectionsystem of the engine can accurately control the air–fuelratio as the stoichiometric ratio by using oxygen sensor.Ignition timing of the engine is also electrically controlledby open loop control system. A simple water-cooledexhaust gas recirculation device is designed and shown inFig. 1. The research octane number of gasoline is 90.

Gaseous emissions are measured by AVL exhaustgas analyzer, in which NOx is analyzed with a

Nomenclature

CO carbon monoxideHC hydrocarbonNOx nitrogen oxideEGR exhaust gas recirculationBMEP brake mean effective pressureX inlet swirl ratioXopt the optimum air swirl ratio

e compression ratioWOT wide opening of the throttleMP maximum pressureMRPR maximum rate of pressure riseMHRR maximum rate of heat releaseTDC top dead center

Table 1Engine specifications

Original engine Modified engine

Bore · stroke [mm · mm] 100 · 115Length of connecting rod [mm] 190Combustion chamber Bowl typeCompression ratio 8 11.8Rated speed [rpm] 2400Swirl ratio VariableIgnition system Electronic controlFuel system EFI

Fig. 1. EGR schematic diagram.

0 5 10 15 207

8

9

10

11

Wk/tuptuo re

woP

EGR ratio/%

swirl ratio =1.15 swirl ratio =1.5 swirl ratio =1.6

330

340

)h.W

1-

swirl ratio =1.15

2920 L. Wei et al. / Applied Thermal Engineering 27 (2007) 2919–2923

chemiluminescent detector(CLD), HC is analyzed with aflame ionization detector (FID) and CO and CO2 are ana-lyzed with a non-dispersive infrared (NDIR) analyzer. CO,HC, CO2 and NOx emissions are average values of theacquired data at each steady-state operating condition.All the tests are repeated.

0 5 10 15 20280

290

300

310

320

k(.g/noitpmusnoc leuf cificeps

EGR ratio/%

swirl ratio =1.5 swirl ratio =1.6

Fig. 2. Effect of air swirl ratio on power and fuel economy at differentEGR ratios (60% WOT, 1800 rpm).

3. Optimization of intake air swirl ratio

Cold EGR is usually adopted on the engine to reduceNOx emission. It can also suppress knock, so that the com-pression ratio of the engine could be increased. However,cold EGR could simultaneously cause the decrease of thecombustion speed rate and make combustion process ofthe engine unstable. It is necessary to organize the fast burnof the mixture in the cylinder to improve the fuel economy.

One kind of simple but effective fast-burn techniques isthe organization of appropriate inlet swirl. The test engineis modified, has a helical duct on the cylinder head and the

inlet swirl ratio (symbolized by X) reaches to 2.3, which istoo strong for gasoline engine. A valve type of air swirlratio control system is designed, which can adjust the airswirl ratio for the best power output and fuel economyof the engine under different EGR ratios. The test resultsare given in Fig. 2, from which it can be concluded thatunder the condition of e = 11.8, without EGR or EGRratio 67%, the optimum air swirl ratio (Xopt) is 1.15. IfEGR ratio is between 7% and 12%, Xopt is 1.5. If EGRratio is more than 12%, Xopt is 1.6. All these results demon-strate that the air swirl ratio should be increased with anincrease in EGR ratio.

L. Wei et al. / Applied Thermal Engineering 27 (2007) 2919–2923 2921

4. Engine performances and emissions

4.1. Speed characteristics at full load

Fig. 3 shows the performance curves of the modifiedengine with EGR compared with those of original engine

1200 1400 1600 1800 2000 2200 24007

8

9

10

11

pow

er o

utpu

t/kW

speed/r.min-1

original engine modified engine

1200 1400 1600 1800 2000 2200 24007

8

9

10

11

12

13

14

optim

imiz

ed E

GR

rat

io/%

speed/r.min-1

1200 1400 1600 1800 2000 2200 2400150

200

250

300

350

400

HC

em

issi

on/p

pm

speed/r.min-1

original engine modified engine

Fig. 3. Comparisons of performance and emission between origin

Table 2Comparisons of main performance between original engine and modified engi

Maximum effectivepower [kW]

Average specific fuelconsumption [g/(kW h)]

Nco

e = 8, EGRrate = 0 10.40 307.2 12e = 11.8, EGR rate

optimized10.45 288.7 47

Decreasing rate(%)

�0.96 6.02 52

without EGR at full load. Table 2 shows the comparisonsof main performance between original engine and modifiedengine at WOT speed characteristic. It can be seen that thepower output of modified engine with cold EGR is almostequal to that of original engine. The specific fuel con-sumption is reduced by 5.3%. NOx emission is decreased

1200 1400 1600 1800 2000 2200 2400260

270

280

290

300

310

320

spec

ific

fuel

con

sum

ptio

n/g.

(kW

.h)-1

speed/r.min-1

original engine modified engine

1200 1400 1600 1800 2000 2200 2400200

400

600

800

1000

1200

1400

1600N

Ox e

mis

sion

/ppm

speed/r.min-1

original engine modified engine

1200 1400 1600 1800 2000 2200 2400400

600

800

1000

1200

1400

1600

1800

NO

x+H

C e

mis

sion

/ppm

speed/r.min-1

original engine modified engine

al engine and modified engine (at WOT speed characteristics).

ne (at WOT speed characteristics)

Ox averagencentration [ppm]

HC averageconcentration [ppm]

(NOx + HC) averageconcentration [ppm]

23 230 14535 325 800

.96 �41.30 44.94

1200 1400 1600 1800 2000 2200 24006

7

8

9

10

11

pow

er o

utpu

t/kW

speed/r.min-1

original engine modified engine

1200 1400 1600 1800 2000 2200 2400280

290

300

310

320

330

spec

ific

fuel

con

sum

ptio

n/g.

(kW

.h)-1

speed/r.min-1

original engine modified engine

1200 1400 1600 1800 2000 2200 24007

8

9

10

11

optim

ized

EG

R r

atio

/%

speed/r.min-11200 1400 1600 1800 2000 2200 24000

400

800

1200

1600

NO

x em

issi

on/p

pm

speed/r.min-1

original engine modifeied engine

1200 1400 1600 1800 2000 2200 2400200

250

300

350

400

450

500

HC

em

issi

on/p

pm

speed/r.min-1

original engine modifeied engine

1200 1400 1600 1800 2000 2200 2400400

600

800

1000

1200

1400

1600

NO

x+H

C e

mis

sion

/ppm

speed/r.min-1

original engine modified engine

Fig. 4. Comparisons of performance and emission between original engine and modified engine (at 60%WOT speed characteristics).

Table 3Comparisons of performances between original engine and modified engine (at 60% WOT speed characteristics)

Maximum effectivepower [kW]

Average specific fuelconsumption [g/(kW h)]

NOx averageconcentration [ppm]

HC averageconcentration [ppm]

(NOx + HC) averageconcentration [ppm]

e = 8, EGRrate = 0 9.99 304.7 1053 290 1343e = 11.8, EGR rate

optimized10.21 293.8 390 395 785

Decreasing rate(%)

�2.22 3.58 62.96 �36.21 41.54

2922 L. Wei et al. / Applied Thermal Engineering 27 (2007) 2919–2923

obviously by 54.84%. HC emission is slightly increased by11.51%, and the (NOx + HC) emissions are reducedobservably by 43.24%.

4.2. Speed characteristics at part-load

Fig. 4 shows the results of speed characteristics at part-load (60%WOT). The trends are similar to those at full

load condition. In comparison with performance at WOTspeed characteristics, the optimum EGR ratios at differentspeeds are smaller for the same NOx reduction level. Itcould be contributed to the fact that larger residual gasin cylinder at part-load could suppress the formation ofNOx. In addition, the HC emission is increased at part-load. The reduction level of (NOx + HC) at part-load issmaller than that at full load. The comparisons of main

Fig. 5. Comparisons of combustion characteristics between originalengine and modified engine (WOT, 2200 rpm).

Table 4Main combustion characteristics parameters at WOT speed characteristics

Speed(rpm)

Compressionratio

EGRrate(%)

Poweroutput(kW)

MP/Positionof MP(MPa/�CA)

MR/Positionof MR(MPa/�CA/�CA)

MH/Positionof MH(J/�CA/�CA)

Flamedevelopmentperiod (�CA)

Fast-burnperiod(�CA)

Total-burnperiod(�CA)

Cyclevariationcoefficient (%)

2200 8 0 10.4 3.684 /14 0.205 /3 2004.1 /6 22 24 46 3.922200 11.8 11.6 10.45 5.681 /15 0.218 /4 2200.5 /4 21 21 42 6.32

L. Wei et al. / Applied Thermal Engineering 27 (2007) 2919–2923 2923

performance and emission between original engine andmodified engine are shown in Table 3.

5. Combustion characteristics

Fig. 5 shows the comparisons of indicator diagrams andheat-release-rates between original gasoline engine and the

modified engine at WOT speed characteristics. It can beseen that the modified engine with cold EGR has higherMP (maximum pressure), MRPR (maximum rate of pres-sure rise) and MHRR (maximum rate of heat release).The fast-burn period becomes shorter. The location of areacenter in the curve of heat release rate is closer to the topdead center (TDC) and the combustion process is remark-ably improved for modified engine. Table 4 shows the com-parisons of main combustion characteristic parameters fortwo engines.

6. Conclusion

A valve type of air swirl ratio control system is designedand tested in the experiments.The results show that for thedefinite EGR ratios, there exists an optimum intake airswirl ratio. The optimum intake air swirl ratio value is1.50 for the test engine under moderate EGR ratio.

When the compression ratio is increased from 8 to 11.8and EGR ratio as well as air swirl ratio of the engine areoptimized, the fuel economy is improved by 5.3% and theNOx and (NOx + HC) emission is decreased by 54.8%and 43.2%, respectively at WOT speed characteristics.

Based on the measured indicator diagrams, the rate ofheat release is calculated and the combustion process isanalyzed. The results show that the combustion process isremarkably improved.

Acknowledgements

The authors wish to express their deep thanks to Projectof National Science Foundation of Xi’an Jiaotong Univer-sity (Approval No: xjj2004009).

References

[1] Thomas J. Labus. Reduction of NOx emission in medium-speedengines. SAE Paper 891917, 1989.

[2] M.Gambino et al., Low Emission stoichiometric CNG engine withEGR, Natural Gas and Alternative Fuels for Engines. I.C.E., vol. 24,ASME, 1995.

[3] Borje Grandin et al., Knock suppression in turbocharged si engine byusing cooled EGR. SAE Paper 982476, 1998.

[4] H. Kurbo, The fast Burn with heavy EGR, new Approach for low NOx

emission and improved fuel economy. SAE Paper 780006, 1978.