Effects of Charge Motion Control During Cold Start of SI Engines 2006-01-3399

An experimental study was performed to investigate the effects of various intake charge motion control valves (CMCVs) on mixture preparation, combustion, and hydrocarbon (HC) emissions during the cold start-up process of a port fuel injected spark ignition (SI) engine. Different charge motions were produced by three differently shaped plates in the CMCV device, each of which blocked off 75% of the engine's intake ports. Time-resolved HC, CO and CO₂ concentrations were measured at the exhaust port exit in order to achieve cycle-by-cycle engine-out HC mass and in-cylinder air/fuel ratio. Combustion characteristics were examined through a thermodynamic burn rate analysis.

Cold-fluid steady state experiments were carried out with the CMCV open and closed. Enhanced charge motion with the CMCV closed was found to shorten the combustion duration, which caused the location of 50% mass fraction burned (MFB) to occur up to 5° CA earlier for the same spark timing. By the use of the CMCV, significant improvements in combustion stability and fuel efficiency were also achieved with increased levels of spark retard mainly due to enhanced fuel-air mixing and relatively faster burning.

Engine start-up experiments were conducted with three different geometric CMCVs using various fuel injection and spark timing strategies. The CMCV improved mixture preparation due to increased swirl and tumble intensities, which enhanced fuel transport, distribution, and evaporation. Flow bench and engine test results indicate that more fuel was evaporated with the CMCV geometry that produced moderate swirl and tumble intensities rather than high swirl or tumble alone. The effects of enhanced charge motion on combustion characteristics were decoupled from those on mixture preparation and were found to increase combustion stability and burning rate. Those combined effects of greater fuel evaporation and faster burning rate with the CMCV closed allowed strategies with reduced fuel injection and additional spark retard during cold start, resulting in a significant reduction in HC emissions. For the same in-cylinder air/fuel ratio profile, engine-out HC emissions following cold start-up were reduced by 18% during 0-3 seconds and by 7% during 3-20 seconds with the CMCV closed using reduced fuel injection and retarded spark timing strategy.