Investigations on performance and emissions of a two-stroke SI engine fitted with a manifold injection system

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 13, April 2006, pp Investigations on performance and emissions of a two-stroke SI engine fitted with a manifold injection system M Loganathan, P V Manivannan & A Ramesh Internal Combustion Engines Laboratory, Department of Mechanical Engineering Indian Institute of Technology Madras, Chennai , India Received 28 February 2005; accepted 3 October 2005 Simple and cost effective electronically controlled injection systems have to be developed to combat the problem of urban pollution. In this work an electronically controlled fuel injection system developed in the Internal Combustion Engine Laboratory of Indian Institute of Technology Madras has been tested in detail on a two-stroke SI engine. The system is fitted on the intake manifold of a single cylinder, air cooled two-stroke scooter engine. Tests have been done at 3000 rpm and 4000 rpm at different throttle positions. The optimum injector pulse widths for thermal efficiency, lowest HC emissions and highest power are all different. The maximum brake thermal efficiency values are 22.6% and 23% at 3000 and 4000 rpm respectively. At a power output of 3 kw and 4000 rpm the brake thermal efficiency is about 21% for the carbureted engine. It increases to 23% with the fuel injection system. HC emissions are considerably lower than the carbureted version at all operating conditions and speeds. The engine can work with leaner mixtures with the injection system in general as compared to the carburetor. The maximum power increases with the injection system. The developed system can be used for mapping the engine for the development of software for injection system control. IPC Code: G01M15/00 In countries like India a large number of two and three wheelers are used. Many of them are powered by two-stroke spark ignition (SI) engines. These engines lead to considerable levels of pollution in urban areas. They are also relatively inefficient than their four-stroke counter parts. In India for example, the number of two-wheeled vehicles has gone up drastically. A majority of the two- and three-wheeled vehicles is powered by two-stroke engines. Though two-stroke engines lead to considerable levels of pollution they are simple in construction, cheap to produce and maintain and have a high power to weight ratio. They also lead to much lower friction losses than four-stroke engines of similar power output. Though four-stroke engines are gaining popularity there are some applications where the twostroke engine is the most viable source of power. The reason for the problems of two-stroke engines is poor scavenging which leads to considerable short circuiting of the fuel at high throttle conditions and dilution of the fresh fuel air mixture at part loads. There is a general trend towards the use of electronically controlled fuel injection systems in SI engines. Even small four stroke engines will use fuel injection systems in future to meet emission norms. Though injection of fuel into the manifold of twostroke engines will not reduce fuel short circuiting, it can improve performance due to better control over the air fuel ratio. In view of the above, studies were conducted on a two-stroke SI engine with a specially developed electronically controlled fuel injection system. Several researchers have investigated the effect of fuel injection in spark ignition engines. Giichi Yamagishi et al. 1 made one of the earliest attempts in They developed a mechanical fuel injection system for a 350 cc two-stroke SI engine wherein the injector was located in different places like cylinder bore, head and transfer port in sequence. The cylinder head injection led to the best results. Edmont Vieilldent 2 developed a low pressure electronic injection system for a 115 cc two-stroke SI engine. When the injector was located in the intake manifold the performance was similar to the carburetor. A capacitor discharge injection circuit enabled rapid opening and closing of the injector at high speeds. Douglas and Blair 3 reported that injection of fuel in a two-stroke engine from the cylinder bore can lead to about 30% reduction in fuel consumption and 60% reduction in exhaust emissions. An air assisted injection system was developed for small out board engines by Leighton et al. 4 This system used a small

2 96 INDIAN J. ENG. MATER. SCI., APRIL 2006 air compressor and a combination of air and fuel injectors mounted on the cylinder head. The highly stratified fuel mixture reduced the SFC by 40% and exhaust emissions by 60%. Duret et al. 5 developed an air assisted fuel injection device called the IAPAC system. Here, the crankcase compressed air is used to push the fuel injected in a cavity into the engine. The engine uses a mechanically operated valve located on the cylinder head to inject the fuel. Emmenthal et al. 6, Schechter and Levin 7 also developed air forced injection systems. Gentilli et al. 8 also developed a pumpless air assisted injection system that uses crank case compressed air and a rotary injection valve. Cobb 9 developed an injection system for a small twostroke engine that injects a mixture of air and fuel using the energy of a compression wave generated by cylinder gases. Yamato et al. 10 investigated the influence of injection timing on the performance of a manifold injection gas engine by firing tests. They found that the in-cylinder fuel distribution in the engine in the direction of the cylinder axis was changed by the variation in injection timing. In general, work has concentrated on injecting fuel after the ports close as it will reduce short-circuiting. Attempts have also been made to run the engine in the stratified and homogeneous modes. In this work, an electronically controlled injection system that was developed and presented earlier by the authors 11 was further tested in detail on a twostroke SI engine at two different speeds. This injection system with the electronic circuitry can be used to map any gasoline engine fitted with electronic injectors. The whole system will be useful while developing software for gasoline injected SI engines. The injector was located on the intake manifold of the two-stroke engine. Tests were conducted at constant speeds of 3000 and 4000 rpm. At each operating point, injection parameters were optimized. Performance and emission parameters were compared with those of the carbureted version under the condition that gave the best brake thermal efficiency. Experimental Procedure A single cylinder two-stroke SI engine whose specifications are shown in Table 1 was coupled to an eddy current dynamometer. The engine was instrumented for measuring performance, emission and combustion parameters. Fuel consumption was measured on the mass basis. Airflow rate was determined using a turbine flow meter coupled to a surge tank. An infrared gas analyzer (HORIBA, MEXA 554J) was used to measure the exhaust hydrocarbon (n-hexane equivalent), carbon monoxide and carbon dioxide concentrations. The schematic of the test set-up is shown in Fig. 1. Initial tests were done on the carbureted mode to establish base line conditions. For this the engine was tested with different fuel jets and the one that gave lowest HC levels was chosen as the optimum. Development of the manifold injection system The engine was modified to accommodate an electrically operated gasoline injector in the intake manifold in such a position that the contact of the spray with the wall is minimal. The injector was fed by a rail, which in turn was fed by a fuel feed pump. A pressure regulator was used to maintain the rail pressure at 3 bar. A throttle body with a potentiometric position sensor was fixed between the intake port of engine and the air supply surge tank. A specially developed monostable multivibrator based electronic circuit was used to generate pulses at the correct timing and for the correct duration to energize Table 1 Engine details Type Two-stroke SI Bore 57 mm Stroke 57 mm Power output rpm Compression ratio 8.8:1 Fig. 1 Experimental set-up (1.Engine, 2.Dynamometer, 3.Dynamometer controller, 4.Data acquisition system, 5.Pressure transducer, 6.TDC sensor, 7.Ice path, 8.Exhaust gas analyzer, 9.Engine speed indicator, 10.Temperature indicator, 11.Fuel tank, 12.Weighing machine, 13.Fuel pump, 14.Fuel filter, 15.Pressure gauge, 16.Flow control valve, 17.Fuel rail, 18.Pressure regulator, 19.Fuel injector, 20.Throttle body, 21.Throttle position sensor, 22.surge tank, 23.Air flow meter, 24.Electronic control system, and 25.12V Battery)

3 LOGANATHAN et al.: TWO-STROKE SI ENGINE 97 the injector. The block diagram of this circuit is seen in Fig. 2. The output of the crankshaft position sensor was conditioned and used to trigger the first monostable multivibrator to fix the injection timing. The end of this pulse marked the start on injection. This was used to trigger another monostable multivibrator that determined the duration of injection. The timing and duration were set using potentiometers. This circuit was used to study the performance of the engine under different injection timings and durations. Injector characteristics Initially, the flow rate of the injector at different pulse widths was measured. This was done when the engine was run at speeds of 3000 and 4000 rpm at different throttle positions. The injection pressure was maintained at 3 bar for 3000 rpm and it could only be maintained at 2.6 bar for 4000 rpm. The pressure regulator based on the manifold pressure automatically set these values. The injector was always fed with a 12 V supply. It is seen in Fig. 3 that there is an almost linear relation ship between the pulse width and mass of fuel injected per cycle. Extrapolation of the curve will indicate that below a certain pulse width, the injector stops injecting. After an electric driving signal is given to an injector, it takes a certain length of time to inject the fuel. This period is termed the injector dead time (t d ). During the injector dead time, there is no flow through the injector. At very small injection pulse widths the injection becomes irregular as the dead time starts to dominate. By design, one has to avoid this region of operation. In this work however this could not be avoided as a commercially available injector had to be used. The injector that was used is the one normally employed on a passenger car engine with a displacement of 266 cc per cylinder. It has 4 holes and when measured under a microscope the diameter of each hole was found to be 0.32 mm. The static continuous flow rate of the injector was measured by Iyaraja 12 in a separate work in the laboratory and was reported as 2166 mg/s at a pressure difference of 3 bar. Results and Discussion Tests were done at 3000 and 4000 rpm at different throttle positions to determine the effect of injection timing and injection duration. These results were later compared with those of the carbureted engine. Fig. 4 indicates the effect of injection pulse duration or Fig. 3 Effect of pulse width on fuel injected Fig. 2 Injection timing circuit Fig. 4 Effect of pulse width on brake thermal efficiency

4 98 INDIAN J. ENG. MATER. SCI., APRIL 2006 width (i.e., injected quantity) on the brake thermal efficiency. By varying the injection timing it was noted that its influence was not significant. This is because the injection is done at the intake manifold and the mixture resides in the crankcase till the transfer port opens, i.e., for about 75 CA after intake port closure or about 4.2 ms at 3000 rpm irrespective of the injection timing. Tests were done at six different throttle positions namely, 10%, 15%, 25%, 40%, 50% and 100%. However, the results at 10 %, 25% and 100% alone are indicated at both the speeds. The pulse width where the brake thermal efficiency is maximum increases with throttle position as expected. The peak brake thermal efficiency is highest at 25% throttle at both the engine speeds. This is probably because at this throttle the effects of charge dilution and short circuiting are not very significant. At low throttles the peak brake thermal efficiency is limited by charge dilution. At high throttle positions it is limited by charge short circuiting. The maximum brake thermal efficiency values are 22.6% and 23% at 3000 and 4000 rpm, respectively. At full load the injector flow was mg/cycle at 3000 rpm and the pulse width was 7.36 ms with an injection pressure of 3 bar. The flow rate was 9.55 mg/cycle and pulse width was 7 ms at 4000 rpm with an injection pressure of 2.6 bar. The variation of HC emissions is shown in Fig. 5. HC values increase with throttle position due to short circuiting. As the pulse width increases from low values at any throttle, we found that the HC level falls to a minimum and then rises. This is because at very low pulse widths the mixture is too lean which leads to low flame speeds and flame quenching. At very high pulse widths the HC emissions rise because the mixture became rich and hence the short circuited charge carries more unburned hydrocarbons with it. By comparing Fig. 5 and the air fuel ratio shown in Fig. 11 later, we find that as the throttle is opened wider the air fuel ratio at which the HC level is a minimum increases. This is because the effect of charge dilution reduces as we open the throttle and leaner mixtures can be tolerated. The optimal pulse widths can be recognized from Fig. 5. The effect of pulse width on the power output of the engine at constant speeds of 3000 and 4000 rpm under different throttle openings is seen in Fig. 6. The maximum power generally occurs in an SI engine with slightly richer than stoichiometric mixtures. However, very high pulse widths (i.e., too rich mixtures) lead to poor combustion and reduced power output. We found that the optimum pulse widths for thermal efficiency, lowest HC emissions and highest power are all different for both the speeds. Always the pulse width for highest power is also the largest. At medium and high throttle positions of 25% and 100% the pulse width for minimum HC is the lowest indicating that lean mixtures are needed. At the lowest throttle position of 10%, minimum HC needs a richer mixture than the maximum brake thermal efficiency condition as charge dilution dominates in this case. In general the pulse width for best brake thermal efficiency is in Fig. 5 Effect of pulse width on HC emissions Fig. 6 Effect of pulse width on power

5 LOGANATHAN et al.: TWO-STROKE SI ENGINE 99 between those for power and minimum HC at medium and high throttles. One has to switch between the different pulse widths depending on the mode of operation desired. CO level reduces with a decrease in the pulse width as seen in Fig. 7 due to leaning of the mixture. NO emission reaches a peak as the mixture becomes slightly leaner than stoichiometric and then reduces as seen in Fig. 8. NO levels depend on both availability of oxygen and temperature. We find that at very low throttle positions the dilution by retained exhaust gas reduces the oxygen concentration in the charge and also reduces the peak gas temperature after Fig. 7 Effect of pulse width on CO emissions combustion and thus leads to low NO levels. As the throttle is opened wider, the NO level rises as expected. We find that the exhaust gas temperature rises and falls with pulse width at any given throttle position as seen in Fig. 9. This is because the exhaust gas temperature predominantly follows the combustion gas temperature, which depends on the air fuel ratio to a large extent and on other effects like charge dilution. We also find that the exhaust gas temperature increases as we move from 10% to 25% throttle and then it falls when 100% throttle is reached. At 10% throttle a very rich mixture is used to combat the problem of short-circuiting. At 25% throttle the combustion is good and peak gas temperature is higher. Hence, the exhaust gas temperature is also high. At 100% throttle the shortcircuiting of fuel air mixtures lower the temperature of the exhaust gas. The spark plug seat temperature depends on the power output and too lean or rich mixtures lower it at any given throttle as seen in Fig. 10. It may be needed to use wider that the optimal pulse widths at high throttle positions to lower spark plug seat temperature. The air fuel ratio at different throttle positions and pulse widths is shown in Fig. 11. As the pulse width increases the amount of fuel-injected increases and the mixture becomes richer. The engine was operated at different throttle positions at 3000 and 4000 rpm. At each of these throttle positions, the pulse widths of the injector were varied to obtain the value that gives the highest brake thermal efficiency. Fig. 8 Effect of pulse width on NO emissions Fig. 9 Effect of pulse width on exhaust gas temperature

6 100 INDIAN J. ENG. MATER. SCI., APRIL 2006 Fig. 10 Effect of pulse width on spark plug temperature Fig. 12 Comparison of brake thermal efficiency Fig. 11 Effect of pulse width on air fuel ratio The set of graphs indicated in Figs indicate the performance and emissions at the condition of best brake thermal efficiency at each throttle position with the injection system. The results of the carbureted engine are also shown for comparison. The fuelinjected engine shows a considerable improvement in the brake thermal efficiency at medium and high outputs as seen in Fig. 12. This is because the mixture is lean with the fuel-injected engine at these conditions as seen later in Fig. 17. At a power output of 3 kw and 4000 rpm the brake thermal efficiency is about 21% for the carbureted engine. It increases to Fig. 13 Comparison of HC emissions 23% with the fuel injection system. At low loads however the brake thermal efficiency with the fuelinjected engine is inferior to the carbureted version. This is because, at low pulse widths the injection system does not produce consistent injection patterns. A smaller injector probably would have resulted in better performance. HC emissions seen in Fig. 13 are considerably lower than the carbureted version at all operating conditions and speeds. This is because of better mixture preparation, better control over the air fuel ratio at medium and high outputs and also use of

7 LOGANATHAN et al.: TWO-STROKE SI ENGINE 101 Fig. 14 Comparison of CO emissions Fig. 16 Comparison of exhaust gas temperature Fig. 15 Comparison of NO emissions leaner mixtures. The HC level falls from 2310 to1200 ppm at 4000 rpm and 3 kw output with the injection system. This is the most significant advantage with the injection system. Figure 16 indicates the variation of CO emission. CO levels are considerably reduced with the injection system as leaner mixtures are used and combustion is complete. NO levels are higher with the injection system due to the use of leaner mixtures, i.e., oxygen availability and higher combustion temperatures. Figure 16 shows that the Fig. 17 Comparison of air fuel ratio exhaust gas temperature is higher with the injection system. This is due to the use of leaner mixtures and better combustion. The air fuel ratio indicated in Fig. 17 shows that the engine can work with leaner mixtures with the injection system in general. Another important feature is that the injection system leads to a higher power output than the carbureted version at 4000 rpm. The maximum power increases from 3.6 kw to 4.2 kw with no adverse effect on brake thermal efficiency or emissions. Lean mixtures can be used particularly at high outputs because the injection

8 102 INDIAN J. ENG. MATER. SCI., APRIL 2006 system can lead to better control over the injected fuel quantity at this condition. Conclusions The following conclusions are made based on this experimental work on electronically controlled gasoline injection into the manifold of a two-stroke SI engine: 1. Optimum injection pulse widths for best thermal efficiency, lowest HC emissions and best power are all different. In general, the pulse width for best efficiency is in between those for power and minimum HC at medium and high throttles. At the lowest throttle position of 10%, minimum HC needs a richer mixture than maximum brake thermal efficiency condition as charge dilution dominates in this case. 2. Generally, injection timing does not have a significant effect on performance and emissions with manifold injection. 3. At a power output of 3 kw and 4000 rpm the brake thermal efficiency is about 21% for the carbureted engine. It increases to 23% with the fuel injection system. At low loads however the brake thermal efficiency with the fuel injected engine is inferior to the carbureted version. This is because good control over the injected quantity could not be achieved at low pulse widths. 4. NO levels are higher with the injection system due to the use of leaner mixtures, i.e., oxygen availability and higher combustion temperature, due to improved combustion. 5. HC emissions are considerably lower than the carbureted version at all operating conditions and speeds. The HC level falls from 2310 to1200 ppm at 4000 rpm and 3 kw output with the injection system. This is the most significant advantage with the injection system. 6. The engine can work with leaner mixtures with the injection system in general as compared to the carburetor. Another important feature is that the injection system can lead to a higher power output than the carbureted version. The maximum power increases from 3.6 kw to 4.2 kw with no adverse effect on brake thermal efficiency or emissions at 4000 rpm. 7. At high outputs the injection duration may have to be increased beyond that for best efficiency in order to control component temperatures. Acknowledgement The authors are thankful to Ministry of Human Resource Development, Government of India, for the financial support provided for this work. References 1 Giichi Yamagishi, Tadanori Sato & Hiroyoshi Iwasa, Trans SAE Paper No , 29 (1972) Edmont Vieilledent, Trans SAE Paper No , 87 (1978) Douglas R & Blair G P, Trans SAE Paper No , 91 (1982) Leighton S, Cabis M & Southern M, Trans SAE Paper No , 97 (1988) Pierre Duret & Stephane Venturi, Trans SAE Paper No , 105 (1996) Emmenthal K D, Muller C & Schafer O, Trans SAE Paper No , 94 (1985) Schechter Michael M & Levin Michael B, Trans SAE Paper No , 100 (1991) Gentili R, Frigo S & Tognotti, Trans SAE Paper No , 103 (1994) Cobb Jr William T, Trans SAE Paper No /4237, 110 (2001) Tadao Yamato, Masara Hayashida & Hirofumi Sekino, Trans SAE Paper No , P-348 (1999). 11 Loganathan M, Manivannan P V & Ramesh A, Development of a manifold injection system for a small SI engine, 2 nd Natl Conf Automotive Infotronics, SAE (India), Indian Institute of Technology Madras, Chennai, India, Dec 10-12, Iyahraja S, Study of gasoline fuel injector characteristics, M Tech Thesis, Indian Institute of Technology Madras, Chennai, India, 2005.