A Study of Efficiency and Emissions for a 4-Stroke SI and a CAI Engine with EEGR and Light Boost 2006-32-0042
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 ISNOx emissions and there was slight reduction in ISFC. However, when the engine was tested in CAI mode, both ISNOx and ISFC were lower than the SI engine. To reduce the high Pressure Rise Rates (Prise) 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 Prise, ISFC and ISNOx. However, when the mixture was leaner, the IMEP reduced as more EEGR was applied. At lean mixtures with EEGR, the ISFC and ISNOx 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 ISNOx 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 Prise to a minimum. λ values between 1.1 and 1.3 were tested. At leaner mixtures and early exhaust valve closing, the ISNOx emissions were almost eliminated. The leaner mixture ensured that the ISFC was minimised and the ISCO and ISCO2 emissions were substantially decreased.