Feasibility Study of Two-stage Hybrid Combustion in Gasoline Direct Injection Engines

Two-stage hybrid combustion for a 6-stroke gasoline direct injection SI engine is a new strategy to control the ignition of the HCCI combustion using hot-burned gas from the stratified lean SI combustion. This combustion is achieved by changing the camshafts, the cam-driven gear ratio and the engine control of a conventional 4-stroke gasoline direct injection engine without using a higher compression ratio, any fuel additives and induction air heating devices. The combustion processes are performed twice in one cycle. After the gas exchange process, the stratified ultra-lean SI combustion is performed. The hot-burned gas generated from this SI combustion is used as a trigger for the next HCCI combustion. After gasoline is injected in the burned gas, the hot and homogeneous lean mixture is recompressed without opening the exhaust valves. Thus the HCCI combustion occurs. Ignition timing of the HCCI depends on the gas temperature rather than the gas composition and it is effectively controlled by the ultra-lean fuel / air mixture and the large amount of EGR for the stratified SI combustion. The engine torque is mainly controlled by the fuel injection quantity for the HCCI combustion. The NOx is less than 1/10 that of the 4-stroke premixed SI combustion and the fuel consumption is almost equal to that of the 4-stroke stratified lean SI combustion under light load conditions. However, the load range is narrow due to pre-ignition of the HCCI.