An Evaluation of Different Combustion Strategies for SI Engines in a Multi-Mode Combustion Engine

Future pressures to reduce the fuel consumption of passenger cars may require the exploitation of alternative combustion strategies for gasoline engines to replace, or use in combination with the conventional stoichiometric spark ignition (SSI) strategy. Possible options include homogeneous lean charge spark ignition (HLCSI), stratified charge spark ignition (SCSI) and homogeneous charge compression ignition (HCCI), all of which are intended to reduce pumping and thermal losses. In the work presented here four different combustion strategies were evaluated using the same engine: SSI, HLCSI, SCSI and HCCI. HLCSI was achieved by early injection and operating the engine lean, close to its stability limits. SCSI was achieved using the spray-guided technique with a centrally placed multi-hole injector and spark-plug. HCCI was achieved using a negative valve overlap to trap hot residuals and thus generate auto-ignition temperatures at the end of the compression stroke. The experiments were performed using a 6 cylinder, 3.2 liter Volvo engine equipped with cam profile switching (CPS), variable cam timing (VCT) for both intake and exhaust valves, and a spray guided direct injection (SGDI) system. In conjunction with a fully programmable control unit these features allowed the engine to be run in all the tested modes without any hardware modifications. Five operating points in the low load/speed zone of the engine map were optimized for fuel consumption using full factorial, 2-D and 3-D experimental designs (with centre points). The same operating points were used for each combustion strategy and their effects on exhaust emissions, combustion and fuel-consumption were evaluated.