RIO 5 - World Climate & Energy Event, 15-17 February 5, Rio de Janeiro, Brazil Modifications on a Small Two Wheeler Two Stroke SI Engine for Reducing Fuel Consumption and Exhaust Emissions Kunam Anji Reddy, A.Ramesh Internal Combustion Engines Laboratory, Mechanical Engineering Department Indian Institute of Technology Madras, Chennai 3, India. email: aramesh@iitm.ac.in Abstract The performance of a two-stroke spark ignition engine was improved by partially insulating its combustion chamber and incorporating a stratified scavenging system. Initial experiments conducted at 3 and rpm under variable output conditions indicated that partially insulating the combustion chamber by coating a ceramic, namely, partially stabilized zirconia, improves the brake thermal efficiency and reduces HC and CO emissions. The engine was further modified to use the stratified scavenging system by connecting the top of the transfer ducts to a reed valve, which allowed air from the atmosphere to enter. This air entered the cylinder first and greatly reduced short circuiting of the incoming charge. With this system there was a significant improvement in brake thermal efficiency, heat release rate and reduction in HC & CO emissions. The airflow through the reed valve was optimized at each operating condition. A control valve mechanism was developed to vary this airflow in relation to the throttle opening of the carburetor. This was tested and found to be suitable. Key words: Two stroke engines, automobile emission control and environmental pollution. 1. Introduction Two stroke spark ignition (SI) engines are mainly used in developing countries for personal transportation. The principal advantages of these engines are high specific power output, mechanical simplicity and low production & maintenance costs. However, these engines have serious drawbacks, like poor brake thermal efficiency and emission levels of HC and CO. These are due to short-circuiting of the fresh fuel air mixture and poor combustion at light loads arising due to exhaust gas dilution of the charge. Attempts have been made in the past to reduce short-circuiting by adopting the stratified scavenging method by using reed valves at the top of the transfer ducts (Ramesh et al., 195), (Ramesh Babu et al., 1993) and (Magee et al., 1993). This system allows air to be trapped in the transfer ducts in the induction phase. Fuel air mixture is sucked into the crankcase in the normal manner. The air trapped in the transfer ducts participates in the initial scavenging phase and becomes the main component to be short-circuited. Thus the fresh fuel air mixture which follows is retained to a large extent. There is a significant improvement in the fuel economy and reduction in HC and CO emissions with this stratified scavenging system particularly at part & medium throttle conditions. The amount of air going to the reed valves at the transfer ducts has to be controlled for best performance. Attempts 131
Modifications on a Small Two Wheeler Two Stroke SI Engine for Reducing Fuel Consumption and... have been made to feed exhaust gas instead of air into the transfer ducts (Saxena and Mathur, 199). Partially insulating the combustion chamber walls can lead to an improvement in combustion and performance in SI engines (Dennis N.Assanis and Mather, 199). Partially Stabilized Zirconia (PSZ) has been used successfully for coating the combustion chamber for such purposes as it has very low thermal conductivity (Woods and Isao Oda, 19).. Present Work In this work the performance of a two-stroke SI engine was improved by partially insulating its combustion chamber and incorporating the stratified scavenging system. First the cylinder head and the crown of the piston were coated with Partially Stabilized Zirconia (PSZ) by the plasma spraying technique. Initial tests indicated that coating both the piston crown & cylinder head with PSZ did not lead to satisfactory results. Hence, tests were done with the PSZ coated cylinder head alone. Additionally the engine was modified to use the stratified scavenging system. The schematic layout of a stratified scavenging system is indicated in Fig. 1. The test engine had three transfer ducts. However, Fig. 1 indicates two transfer ducts only as a schematic. To achieve stratified scavenging, holes were drilled at the top of the transfer ducts and tubes interconnected them. A single reed valve was used to feed air into the transfer ducts from the atmosphere. Initially, the airflow through the reed valve (stratified scavenging system) was controlled manually for optimal performance. During the upward movement of the piston, fresh air enters the crank case through the carburetor in the normal way. Simultaneously air also enters the three transfer ducts through the reed valve and fills them. This air trapped in the transfer ducts first enters the cylinder during the scavenging phase and prevents short circuiting of the following fresh charge. A control valve mechanism was developed to vary the airflow through the stratified scavenging system in relation to the throttle opening. With this system the airflow rate through the read valves connected to the transfer ducts was automatically increased with the throttle opening. At idling conditions no airflow was allowed through the stratified scavenging system. 3. Experimental-Setup and Experiments A single cylinder, loop scavenged two-stroke spark ignition engine (with a displacement volume of 15cc and 5.kW @ 55rpm) was used for the tests. The schematic diagram of the complete experimental set-up is shown in Fig.. A Non Destructive Infrared (NDIR) gas analyser was used for the measurement of HC/CO concentration in the tail pipe. Pressure-crank angle data was acquired on a personal computer using a flush mounted piezo electric transducer. Experiments were conducted at constant speeds of 3 and rpm under variable output conditions with gasoline as the fuel. Initial tests were conducted with the normal engine and the engine with the partially insulated combustion chamber. Subsequently tests were conducted with the combination of the partially insulated combustion chamber and the stratified scavenging system. 13
RIO 5 - World Climate & Energy Event, 15-17 February 5, Rio de Janeiro, Brazil 1 1 3 1 Reed Valves at the transfer Ducts Exhaust port 3 Transfer Duct - Crank case Fig.1 Stratified Scavenging System Fig. Experimental setup Airflow through the reed valve was optimized for minimum HC emissions. It was found that this operating condition also led to the highest brake thermal efficiency. Finally experiments were conducted with the control valve mechanism developed in this work to vary the airflow through the stratified scavenging system in relation to the throttle position.. Results & Discussion The variation of brake thermal efficiency with brake power at 3rpm and rpm is shown in Figs. 3 &. The figures indicate the base performance of the engine at two carburetor jet sizes namely.5mm and.9mm. The Jet size of.5 mm leads to higher efficiency on account of the leaning of the mixture. It is seen that there is a small improvement in brake thermal efficiency particularly at rpm with the partially insulated combustion chamber at all load ranges (.5mm jet). This is due to improved combustion as a result of increase in the gas temperature. With the combination of the stratified scavenging system and partially insulated combustion chamber, there is significant improvement in brake thermal efficiency at all conditions. Brake thermal efficiency improves with the stratified scavenging system because of a reduction in short-circuiting of the fresh mixture and lower exhaust gas dilution (due to better scavenging). The improvement in thermal efficiency is quite significant at low outputs. At a power output of.5kw and rpm and jet size of.5mm, the brake thermal efficiency increases from.3% to 17.7% with the partially insulated combustion chamber and to % with the combination of the partially insulated combustion chamber and the stratified scavenging system. The brake thermal efficiency with the.9mm jet size is the lowest as the mixture is very rich. 133
Modifications on a Small Two Wheeler Two Stroke SI Engine for Reducing Fuel Consumption and... Brake Thermal Efficiency (%) 1 1 1 3rpm STANDARD (.9mm) STANDARD (.5mm) (.5mm) (.5mm) Brake Thermal Efficiency (%) 1 1 1 rpm STANDARD (.9mm) STANDARD (.5mm) D (.5mm) STARTIFIED SCAVENGING (.5mm) 1 3 Fig.3 Brake thermal Efficiency at 3 1 3 5 Fig. Brake thermal Efficiency at 1 1 1 Hydrocarbon Emissions (gm/kw-hr) 3rpm STANDARD (.9mm) STANDARD (.5mm) (.5mm) (.5mm) 1 1 1 Hydrocarbon Emissions (gm/kw-hr) STANDARD (.9mm) STANDARD (.5mm) (.5mm) (.5mm) rpm 1 3 Fig. 5 Variation of HC level at 3 1 3 5 Fig. Variation of HC level at rpm As seen in Figs. 5& at 3rpm and rpm, there is a reduction in HC emissions with the partially insulated combustion chamber. With the combination of stratified scavenging and partial insulation of the combustion chamber, there is a significant reduction in HC levels at rpm due to reduction in short circuiting. At rpm and.5 kw, HC reduces from gm/kw-hr with the jet size of.9 mm to about gm/kw-hr with the jet size of.5mm. With insulation it drops to gm/kw-hr. With the combination of partially insulated combustion chamber and the stratified scavenging system it drops to 3 gm/kw-hr. 13
RIO 5 - World Climate & Energy Event, 15-17 February 5, Rio de Janeiro, Brazil There is a small reduction in CO levels with the partially insulated combustion chamber at both speeds as seen in Figs. 7 & due to improved combustion when the jet size is kept at.5mm. The CO levels are very high with the jet size of.9 mm as the mixture is rich. There is a significant reduction in CO emission with the stratified scavenging system mainly due to leaning of the mixture on an overall basis. The reduction is significant at rpm. Comparison of the cylinder pressure versus crank angle variation (average of 1 cycles) which is shown in Figs. 9 & 1 indicates a significant increase in cylinder pressure with the stratified scavenging system. Occurrence of the pressure peak is also closer to TDC. This indicates faster combustion due to reduced dilution by exhaust gas with the stratified scavenging system. Figures 11 & show higher heat release rates with the stratified scavenging system compared to the standard engine at both speeds. Combustion starts early and occurrence of peak heat release rate is closer to TDC position. Results with the Control Valve: It was observed that the control area needed for airflow through the reed valve connected to the transfer ducts has to be increased almost linearly with the throttle opening. A mechanism was developed to vary the area in this fashion automatically when the throttle opening is altered. This device was fitted on the engine along with the partially insulated combustion chamber and tested. The variation of brake thermal efficiency with the control valve Carbon Monoxide Emissions (gm/kw-hr) 7 5 3 1 STANDARD (.9mm) STANDARD (.5mm) (.5mm) (.5mm) 3rpm Carbon Monoxide Emisssions (gm/kw-hr) 1 9 7 5 3 1 STANDARD (.9mm) STANDARD (.5mm) (.5mm) JET 5 rpm 1 3 Fig. 7 Variation of CO level at 3 1 3 5 Fig. Variation of CO level at 135
Modifications on a Small Two Wheeler Two Stroke SI Engine for Reducing Fuel Consumption and... Cylinder Pressure (bar) 1/ Throttle 3rpm JET 5 Cylinder Pressure (bar) 1/ Throttle rpm JET 5 1 3 3 Crank Angle (deg) Fig.9 Pressure crank angle diagrams at 3 rpm 1 3 3 Crank Angle (deg) Fig.1 Pressure crank angle diagrams at rpm Rate of Heat Realease (J/deg.CA) 1 1/ Throttle 3rpm JET 5 Rate of Heat Realease (J/deg.CA) 1 1 1 1/ Throttle rpm JET 5 17 1 19 1 Crank Angle(deg) 17 1 19 Crank Angle(deg) Fig.11 Heat Release Rate at 3 rpm Fig. Heat liberate Rate at r pm mechanism is indicated in Figs. 13 & 1. The results seen here are with the jet of size.9 mm. We find that the brake thermal efficiency with the developed control valve is as good as the values obtained when the airflow through the reed valve was manually adjusted for optimum performance. The system developed is quite easy to use as it automatically works when the throttle position is varied. HC emission with the developed control mechanism is very low as compared to the normal engine as seen in Figs. 15 &. Again the values are close to that obtained with manual control of the airflow for optimum performance. Similar trends are seen in the case of CO emissions also at 3rpm & rpm in Figs. 17 & 1. Thus it is concluded that the control valve mechanism performs satisfactorily. 13
RIO 5 - World Climate & Energy Event, 15-17 February 5, Rio de Janeiro, Brazil Brake Thermal Efficiency(%) 1 1 1 3rpm JET 9 Brake Thermal Efficiency(%) 1 1 1 rpm JET 9 PARTILA INSULATION 1 3 1 3 5 Fig.13 Comparison of Brake thermal efficiencies at 3 rpm Fig.1 Comparison of Brake thermal efficiencies at rpm Hydrocarbon Emissions (gm/kw-hr) 1 1 1 3rpm JET 9 1 1 Hydrocarbon Emissions (gm/kw-hr)1 STRATIFIED SCAEVENGING rpm JET 9 1 3 Brake power(kw) 1 3 5 Brake power(kw) Fig.15 Comparison of HC emission at 3 rpm Fig. Comparison of HC emission at rpm 137
Modifications on a Small Two Wheeler Two Stroke SI Engine for Reducing Fuel Consumption and... Carbon Monoxide Emissions (gm/kw-hr) 7 5 3 1 3rpm JET 9 Carbon Monoxide Emissions (gm/kw-hr) 1 9 7 5 3 1 rpm JET 9 1 3 Brake power(kw) Fig.17 Comparison of CO levels at 3 rpm 1 3 5 Brake power(kw) Fig.1 Comparison of CO emission at rpm 5. Conclusions The following conclusions are drawn based on this experimental work on improving the performance of a small two-stroke SI engine. Partially insulating the combustion chamber leads to an improvement in brake thermal efficiency and reduction in HC and CO emissions of the two stroke engine. There is a significant improvement in brake thermal efficiency at all load ranges and speeds with the combination of the stratified scavenging system and partially insulted combustion chamber. There is also a significant reduction in the HC and CO levels with the stratified scavenging system. There is an optimum air flow rate through the stratified scavenging system at which simultaneously the brake thermal efficiency is maximum and HC & CO emissions are minimum Rate of heat release rate and peak pressure are higher due to improved combustion with the stratified scavenging system and partially insulated combustion chamber. A mechanism, which controls the amount of air going to the stratified scavenging system in relation to the throttle opening, has been developed and successfully tested. Performance and emission characteristics with the developed valve are close to the results obtained when the airflow through the stratified scavenging was adjusted manually for best performance 13
RIO 5 - World Climate & Energy Event, 15-17 February 5, Rio de Janeiro, Brazil References 1. Dennis., N Assanis and T. Mather, (199), The effect of thin ceramic coatings on SI engine performance, SAE paper No: 93, 91-99.. Magee., S.J., R.Douglus, and G.P. Blair, (1993), Reduction of fuel consumption and emissions for a small capacity two-stroke cycle engine, SAE paper No: 93393. 3. Ramesh., A., B. Nagalingam and K. V. Gopalakrishnan, (195), Improvement in the performance of a two-stroke spark-ignition through extra reed induction valves fitted at the transfer ducts, SAE paper No: 5937.. Ramesh Babu., P., B. Nagalingam, and K. V. Gopalakrishnan, (1993), Study of a new scavenging system to reduce the losses of fresh charge in three different types of two stroke SI engines, SAE paper No: 93155. 5. Saxena., M. and H.B Mathur, (199), Reduction of fresh charge losses by selective exhaust gas recalculation (SEGR) in two-stroke engines, SAE paper No: 9.. Woods., M..E and Isao Oda, (19), PSZ ceramics for adiabatic engine components, SAE paper No:. 139
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