A Study of Efficiency and Emissions for a 4-Stroke SI and a CAI Engine with EEGR and Light Boost

A single-cylinder, 500cc research engine was tested under Spark-Ignition (SI) and Controlled Auto-Ignition (CAI) operation with similar load and speed conditions. Camshafts with low-lift and short duration, run with a negative valve overlap, were used to obtain CAI at wide open throttle. Two different camshaft profiles were tested in order to get a wide span of loads at 1200 and 2000rpm. The SI engine was Port Fuel-Injected (PFI) while the CAI engine was tested with both PFI and an Orbital Air-Assist Direct-Injection (DI) system.

To reduce the high Indicated Specific Nitrogen Oxide (ISNOx) emissions at λ=1, 10% External Exhaust Gas Residuals (EEGR) was applied to the SI engine. EEGR reduced ISNO_x emissions and there was slight reduction in ISFC. However, when the engine was tested in CAI mode, both ISNO_x and ISFC were lower than the SI engine. To reduce the high Pressure Rise Rates (P_rise) associated with higher loads, 5 and 10% EEGR was applied to the DI CAI engine at λ values between 1 and 1.3 with fixed valve timing. At λ=1, the decrease in Indicated Mean Effective Pressure (IMEP) was minimal as the EEGR level was increased. The EEGR reduced the P_rise, ISFC and ISNO_x. However, when the mixture was leaner, the IMEP reduced as more EEGR was applied. At lean mixtures with EEGR, the ISFC and ISNO_x were both very low. The application of EEGR also reduced the Indicated Specific Carbon Monoxide (I SC O) and Indicated Specific Carbon Dioxide (ISCO₂) emissions.

To reduce ISFC and ISNO_x at low load, a 0.18bar boost was applied to the engine intake. The boost allowed the exhaust valve to be closed earlier, which meant a larger amount of Internal Exhaust Gas Residuals (IEGR) was trapped. As the airflow had increased from the boost, leaner mixtures were used to keep the P_rise to a minimum. λ values between 1.1 and 1.3 were tested. At leaner mixtures and early exhaust valve closing, the ISNO_x emissions were almost eliminated. The leaner mixture ensured that the ISFC was minimised and the ISCO and ISCO₂ emissions were substantially decreased.