A Study of LPG Lean Burn for a Small SI Engine.pdf

SAE TECHNICAL PAPER SERIES 2002-01-2844

A Study of LPG Lean Burn for a Small SI Engine

Liguang Li Shanghai Jiaotong University

Zhensuo Wang, Huiping Wang, Baoqing Deng and Zongcheng Xiao

Jilin University

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2002-01-2844

A Study of LPG Lean Burn for a Small SI Engine

Liguang Li Shanghai Jiaotong University

Zhensuo Wang, Huiping Wang, Baoqing Deng and Zongcheng Xiao Jilin University

Copyright 2002 S A E International

ABSTRACT

This paper presents a study of L P G lean burn in a motorcycle SI engine. The lean burn limits are compared by several ways. The relations of lean burn limit with the parameters, such as engine speed, compression ratio and advanced spark ignition etc. are tested. The experimental results show that larger throttle opening, lower engine speed, earlier spark ignition timing, larger electrode gap and higher compression ratio will extend the lean burn limit of L P G . The emission of a L P G engine, especially on N O x emission, can be significantly reduced by means of the lean burn technology.

INTRODUCTION

Low emission and high fuel efficiency are the development goals of modern engine technologies. Lean burn technology combined with the gasoline direct injection (GDI) and variable valve timing (VVT) has shown the advantage in improving fuel consumption and lowering the emission level. The lean burn technology is applied to gasoline engines in Europe and Japan [1~4]. Matsuki et al.[5] applied VVT technology to one lean burn engine. They used the engine crank angle speed fluctuation to control the lean burn limit. For the gaseous fuels, such as C N G , some studies are reported on lean burn technology. Uyehara [6] found that the lean burn limit of C N G could be enlarged by means of pre-chamber design. Klimstra [7] et al. proposed using the prechamber to ensure the combustion stability and reliability of spark ignition, based on their study on the heavy engine, focused on C N G lean burn to the engine performance and emission levels. Kubesh et al. [8] studied the effect of ambient humidity on C N G lean burn limits and emissions of the engine. Corbo et al. [9] reported their study of C N G lean burn in a SI engine and the emission characteristics when three way catalysts and E G R were used. However the study on L P G lean

burn is very limited. As the L P G is used widely in vehicles for emission reduction, especially in the public traffic fleet, the basic and applied research on L P G lean burn is necessary for L P G vehicles to meet increasingly strict legislation in the future.

There are more than 16 million vehicles and 50 million motorcycles in China now. Currently vehicle exhaust emissions are one main source of exhaust gases in the large cities in China. Reducing the emission levels of vehicles is one key project for the continuous development of most of the Chinese cities. The purpose of this study is to find the methods to optimize the combustion of L P G engines by lean burn technology applied to small SI engines to solve the emission problem of motorcycles in urban cities. This will significantly improve the air quality of urban cities and benefit both society and the economy [10~13].

ENGINE AND TESTING SYSTEM

EngineA water-cooled, four-stroke, 125cc motorcycle SI engine with carburetor fuel system was redesigned for L P G . The main specifications of the engine are listed in Table 1.

Testing SystemAn exhaust gas analyzer for 5 species is used to measure the emission levels. The engine torque is measured by an electrical eddy dynamometer. The engine speed is measured by a Honeywell sensor and recorded by a multi-channel data processing card. The air-fuel ratio is adjusted by the supply of L P G with the control of inlet valve of L P G . Data recording is carried out automatically by the computer system. The data recording and processing system realizes just in time, high accuracy and high data flow quantity. The schematic diagram of the testing system is shown in Fig. 1.


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Figure 1 The schematic diagram of testing system

L P G FuelThe L P G used in the test is purchased from the market, which is available for vehicles, such as taxi fleet. The composition of the L P G is shown in Table 2 .

Table 1 Specifications of the test engine

Table 2 The Composition of LPG

RESULTS AND DISCUSSION

COMPARISON OF EVALUATION PARAMETERS OF L E A N B U R N LIMITBefore studying of the characteristics of L P G lean burn in SI engine, the evaluation parameter of lean burn limit should be selected first. The combustion in the engine would be unstable when it runs around the lean limit, which will bring s o m e problems such as combustion variation, and a sharp increase in H C emissions. The lean limit is usually defined based on the characteristics of those phenomena. There are two basic ways to define the lean burn limit; one w a y is to check directly the combustion variation by the pressure sensor installed in the combustion chamber, and the other is to check indirectly the combustion variation according to the test of engine's outputs. In this study, the later method is used, based on the engineering application possibility.

The following tests are carried out to check which criteria are suitable to define a lean burn limit. Figures 2 ~ 6 show the test results of relations between excess air ratio () and the evaluation parameters for the lean burn limit, such as torque fluctuation, H C emission variation and engine speed fluctuation etc. The engine load in the above figures is fixed at 3 0 % throttle position and engine speed at 4000rpm. The entire datum including the emission levels is collected by 10 cycles per second by the data recording system.

Torque FluctuationWhen the engine runs around the lean burn limit, the combustion is unstable and the pressure fluctuation in the combustion chamber increases. This will lead to the increase of the torque fluctuation of the engine. A s torque fluctuation is easily


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measured, this is why it is usually defined as one parameter to evaluate the lean burn limit. Figure 2 shows the torque output and its fluctuation versus excess air ratio. It shows that the coefficient of torque fluctuation can be controlled basically under 1% if the excess air ratio is less than 1.2. W h e n the excess air ratio is higher than 1.2, the coefficient of torque fluctuation increases obviously and it reaches higher than 8 % when the excess air ratio equals 1.5. According to the results in Fig.2, the variation of the coefficient of torque fluctuation versus the excess air ratio is marked.

Figure 2 Torque fluctuation versus excess air ratio (Coefficient of torque fluctuation =Torque standard

deviation/average of torque 100%)

Figure 3. Engine speed fluctuation versus excess air ratio (Coefficient of engine speed fluctuation =engine speed standard deviation / average of engine speed 100% )

Engine Speed FluctuationEngine speed fluctuation versus the excess air ratio is also tested in this study. The test results in Fig.3 show that when engine speed varies in the range from 50 r/min to 150 r/min and the coefficient of engine speed fluctuation varies from about 0 .4% to 0.9% versus the Lambda () in the range of 0.9 to 1.5. The coefficient of engine speed fluctuation is relatively small and its change rate is less than 1%. S o the engine speed fluctuation rate is not sensitive to the lean burn limit compared with the coefficient of the torque fluctuation.

H C Emiss ionWhen the engine runs around the lean burn limit, H C emission increases sharply due to

misfire or partial combustion. According to these phenomena, H C emission value or its variation m a y be suitable for being used for evaluating the lean burn limit. S o m e experiments are carried out and the results are showed in Figs. 4 and Fig. 5. Figures 4 & 5 show that H C emission is less than 150 p p m and H C variation ratio is very small when the excess air ratio is in the range of 1~1.4. Those test results show that L P G has a wider lean burn limit compared with that of gasoline. This is due to fact that, as one gaseous fuel, L P G can be mixed fully with air and its flame combustion is stable. S o the combustion range of L P G is wider and the mixture of fuel and air can be burned completely. W h e n the excess air ratio is higher than 1.4, partial combustion or misfire m a y occur and H C emission rises sharply due to unburned fuel.

Figure 4. The H C emission variation versus excess air ratio

Figure 5. The H C emission variation ratio versus excess air ratio ( H C emissions variation ratio = (HC2 - HC1 ) / ( 2 - 1 ) )

SummaryAccording to the comparisons of evaluation parameters for lean burn limit described above, it is easy to find that the torque fluctuation, H C emission and H C variation ratio varied obviously with the excess air ratio, while engine speed fluctuation w a s smaller. In the procedure, as the excess air ratio varies from 0.9 to 1.5, the coefficient of torque fluctuation rises from 0.5% to 8 % , increasing by more than eight times, while the coefficient of engine speed fluctuation goes up from 0.4% to 0.9%, increasing by only one times. Both the


coefficient of engine speed fluctuation and its variation range are small over the range of tested in this study.

Figures 2 ~ 5 show that parameters used to evaluate the lean burn limit show good consistent behavior. A s the excess air ratio reaches 1.2, the coefficient of torque fluctuation starts to rise, but it is so negligible little and around 1%, H C emission is at the lowest and will start to rise, and H C variation ratio is near zero. W h e n the excess air ratio reaches 1.4, H C emission increases sharply and the coefficient of torque fluctuation is more than 4 % and the H C variation ratio rises markedly too, which implies that the engine begins to run unstably from this excess air ratio point.

Based on the test results above, it can be concluded that torque fluctuation, engine speed fluctuation, H C emission, and H C variation ratio vary differently with the excess air ratio. S o m e of them, such as engine speed fluctuation and the value of H C emission, are not suitable for being as the evaluation parameters for lean burn limit. The coefficient of torque fluctuation and H C variation ratio are suitable for being as the evaluation parameter for the lean burn limit. The 4 % of the coefficient of torque fluctuation and 2000 p p m of the H C variation ratio m a y be regarded as good evaluation parameters for the lean burn limit of L P G . The above values are also used as the evaluation parameters for lean burn limits in the following studies.

EFFECT OF KEY PARAMETERS ON LEAN BURN LIMITThe application of lean burn will require s o m e modifications on key parameters of the testing engine, such as compression ratio, the ignition timing and the electrode gap of spark plug. The relation of those factors with the lean burn ability is studied in the following tests.

Engine SpeedFigure 6 shows the effect of engine speed on the lean burn limit at the condition of 2 0 % and 3 0 % throttle positions. It shows that the lean burn limit will decrease with the increase in engine speed. This result can be explained as follows: if the throttle position is constant, the turbulent intensity in the cylinder will increase with the increase in engine speed. That will enhance the flame speed and combustion in cylinder. O n the other hand, the higher turbulence in the cylinder will increase the tendency of blowing away for the spark. That will lead to the ignition delay and instability for the combustion. Under the condition of lean burn environment, it can be concluded from Fig.6 that the effect of ignition blowing away plays an important role. This test result also shows that spark ignition timing and stability will be the key combustion factors, when the mixture becomes increasingly lean and the engine speed is increased.

Throttle PositionFigure 7 shows the effect of throttle position on the lean burn ability under different engine speeds. Adjusting the load of L P G SI engine is similar to the gasoline engine, in which both the control by m e a n s of a throttle that regulates the quantity of mixture inducted each cycle. At a constant engine speed, decrease throttle opening will increase the throttling loss and the charge efficiency will decrease. Under this condition the residual of exhaust gases will increase and it leads to the spark lag and the tendency of unstable combustion. W h e n the engine speed is higher, the throttling loss is larger at a fixed throttle opening. This is why the lean burn limits decrease as the throttle opening becomes smaller and a decrease with increasing engine speed, as shown in Fig.7.

Figure 6. Effects of engine speed on lean burn limit = 11.05, Ignition timing advanced 8 C A to the baseline

Figure 7. Effects of throttle position on lean burn limit =11.41, Ignition timing advanced 8 C A to the baseline

Compression RatioThe effect of compression ratio () on lean burn ability under different throttle openings and engine speeds is shown in Figs.8~10. Those figures show that the lean burn limit extends as the compression ratio increases. This is because that when the compression ratio is higher, the temperature of mixture in cylinder will be higher and the compression pressure


will increase. At the s a m e time, the residual of exhaust gases will decrease. Those will benefit the mixture ignition and enhance the flame speed of combustion. A s a result, the lean burn range is increased.

Figure 8. Effects of Compression ratio on lean burn limit at the condition of 20% throttle opening Ignition timing advanced 8 C A to the baseline



At the s a m e time, it is also found that the lean burn limit appears bigger w h e n the engine speed is lower and the throttle opening is larger. Those results are the s a m e as the effect of the engine speed and the throttling opening on the lean burn limit. It shows that the residual of the exhaust gases in the cylinder on the combustion is bigger than other factors. This also implies that the effect of compression ratio on lean burn limit is not as big as throttle opening and engine speed in the test conditions.

Advanced IgnitionSome tests of the ignition timing advanced from 0~15 C A on power output and emission characteristics show that the 8 C A advanced ignition to the baseline ignition timing for gasoline is the best for L P G fuel. S o in the study of this paper, only 0~8 C A ignition timing advanced were carried out. Figure 11 shows the effect of advanced ignition on the lean burn limit. It is easy to find that the lean burn limit is extended when the ignition timing is advanced. It is known that when the mixture is lean, it is difficult for spark ignition and the combustion delay period will be longer. At the s a m e time, the flame speed decreases and the combustion period is longer. Those require the ignition timing to be advanced for ensuring the reliability of spark ignition and combustion. It concludes that the spark

Figure 11 Effects of ignition timing advanced on lean burn limit

(4000 r/min, =11.41, 30% throttle opening )

Figure 12 Effects of electrode gap on lean burn limit Operating condition as in Fig. 11


Figure 13. Comparison of torque and C O emission in different at the condition of 3 0 % throttle opening 4000 r/min, =11.41, Ignition timing advanced 8 C A to the baseline.

Figure 15. Comparison of torque and C O emission in different at the condition of 4 0 % throttle opening Operating condition as in Fig. 13

advance will extend the lean burn limit.

Electrode G a p of Spark PlugFigure 12 shows the effect of the electrode gap of spark plug on the lean burn ability. Bigger electrode gap appears to extend the lean burn range, especially under the condition of ignition timing being advanced. This can be explained that when the electrode gap is increased, the core of spark will m o v e away from the surface of chamber, this will help to avoid the effects of residual of exhaust gases near the chamber wall. O n the other hand, as the quantity of mixture increases in the electrode gap, the ignition probability will be increased. During the formation of the spark core, the electrode will absorb the energy. This phenomenon is called the "fade flame" action of electrode. It is apparent that as the electrode gap becomes wider, the action of fade flame will become weaker. But the electrode should not be too wide. Otherwise, the higher ignition energy will be required. It m a y also cause the difficulty for ignition, higher misfire rate and unstable combustion m a y occur.

Figure 14 Comparison of H C and N O x emission in different at the condition of 3 0 % throttle opening Operating condition as in Fig. 13

Figure 16. Comparison of H C and N O x emission in different at the condition of 4 0 % throttle opening Operating condition as in Fig. 13

ANALYSIS OF LEAN BURN TECHNOLOGY FOR A P P L I C A T I O N T h e m a x i m u m compression ratio in this study is 11.41. Under this compression ratio, there is not any other change in the structure of chamber. It implies that the characteristics of the airflow in the cylinder and chamber are the s a m e as that of the baseline. But it m a y also limit the potential of compression ratio on the lean burn ability.

The above test results show that, the 0.9 m m of electrode gap is the best under the testing condition. The best of advanced angle of the ignition timing is 8 C A based on baseline of the gasoline.

Figures 13~16 give the engine torque output and emission characteristics under the optimal conditions at the selected Lambda values for comparisons. The typical excess air ratios selected are listed in Table 3.

The lean burn limit for application listed in Table 3 m e a n s that it might be suitable to the practical application. W h e n an internal combustion engine runs


around the lean burn limit for practical applications, the power loss can be compensated partially by increasing the throttle opening, exhaust emission can be reduced, and fuel efficiency can be improved. In these tests, the lean burn limit for stable combustion was defined by torque fluctuation of 4 % and a H C emission variation ratio of 2000 ppm.

Table 3 The typical excess air ratio for comparisons

Figures 13 and 14 show the engine performance of Lambda changing from stoichiometric air-fuel ratio to lean burn limit for application and the lean burn limit for stable combustion under the condition of 30% throttle opening. With the change in Lambda, the torque output decreased from 5.65 Nm to 2.7 Nm and 1.45 Nm, the lower rates of the output are 52% and 74%. At the same time, the H C emission are decreased by 56% and 14%, the N O x emission are reduced by 9 4 % and 98% and the C O level is lowered by 9 2 % and 8 7 % respectively. It shows that the exhaust emission can be reduced significantly under the lean burn limit. Especially the N O x emission can be lowered by more than 90%, which is usually difficult to achieve by other means. The low emission characteristics under lean burn condition at 40% throttle opening are also shown in Figs. 15 and 16.

The torque output is decreased about 52% from stoichiometric air-fuel ratio to the lean burn limit for application at the condition of 30% throttle position as shown in Fig. 13. This loss of the torque output is marked. But under the condition of 4 0 % throttle position; the torque output of the lean burn limit for application is decreased by about 43% as compared with the torque at the stoichiometric air-fuel ratio. This implies that the torque loss under the lean burn condition can be compensated partly by increasing the throttle opening. This implies that how to increase the torque output under lean burn condition is a challenge for its application in engineering.

CONCLUSION

The comparisons for lean burn limits show that the torque fluctuation 4 % and H C variation ratios greater than 2000 ppm can be used as evaluation parameters for the lean burn limit. Their relations with excess air ratio are similar and accurate for lean burn limit evaluation.

With the increase of throttle opening and decrease of engine speed, the lean burn limit tends to increase. High compression ratio and advanced ignition timing will extend the range of lean burn. Under the condition of enough ignition energy, the wide electrode gap is effective in extending the lean burn limit.

The emission level of an L P G engine can be improved significantly by means of lean burn technology, especially for reducing N O x emissions. By increasing the throttle opening, the power output loss due to lean mixture at part load conditions can be compensated partly.

REFERENCES

1. Shuliang Liu et al., Twice Electronic Fuel Injection (TEFI) A N e w Idea for Realizing Lean Combustion in Port Injection SI Engine, Internal Combustion Engine for Small Volume, No .1 , 1999

2. Shuliang Liu et al., Experimental Research into Lean Combustion on a EFI4 Valve SI Engine, Transactions of CSICE, No .2 , 1999

3. Maruhara Sheishi et al., Investigation of High Compression Lean Burn Engine, Foreign Internal Combustion Engine (Chinese), No.5 , 1989

4. Inoue T et al., Toyota Lean Combustion System The Third Generation System, Foreign Internal Combustion Engine (Chinese), No.2 , 1995

5. Masato Matsuki et al., Development of a Lean Burn Engine with a Variable Valve Timing Mechanism, S A E Paper, 960583

6. Otto A . Uyehara., Prechamber for Lean Burn for Low N O x for Natural Gas , S A E Paper, 951937

7. Jacob Klimstra., Performance of Lean-Burn Natural-Gas-Fueled EnginesOn Specific Fuel Consumption, Power Capacity and Emissions, S A E Paper, 901495

8. John T . Kubesh et al., Humidity Effects and Compensation in a Lean Burn Natural Gas Engine, S A E Paper, 971706

9. P. Corbo et al., Comparison Between Lean-Burn and Stoichiometric Technologies for C N G Heavy-Duty Engines, S A E Paper, 950057

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11. Shao Qianjun et al., L P G as the N e w Energy of the Vehicle Engine, Vehicle Engine, No.6, 1999

12. Liguang Li, Zhensuo W a n g , Huiping W a n g , Baoqing Deng and Zongcheng Xiao, A Study of L P G Lean Burn in SI Engine, the Proceedings of the 11th International Pacific Conference on Automotive Engineering, Nov. 6-9th, 2001, Shanghai, China

13. Zhensuo W a n g , A Study of L P G Lean Burn Engine, Master Dissertation, College of Automotive Engineering, Jilin Univ. Feb. 2001


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