New gasoline homogeneous charge compression ignition combustion system using two-state direct injection and assisted spark ignition

Homogeneous charge compression ignition (HCCI) combustion was studied in a four-stroke gasoline engine with a direct injection system. The spark ignition and electronically controlled two-stage gasoline injection system are adopted to control the mixture formation, ignition timing, and combustion rate in the HCCI engine. The engine could be operated in HCCI combustion mode in a range of loads from I to 5 bar indicated mean effective pressure (IMEP) and operated in SI combustion mode up to loads of 8 bar IMEP. The HCCI combustion characteristics were investigated under different air-fuel ratios, engine speeds, starts of injection, as well as spark ignition enabled or not. By introducing the second fuel injection in the compression stroke, the stratified concentration is formed and the mixture is cooled, which means that HCCI ignition timing can be controlled and the load range can be extended by tuning the mixture concentration and temperature. At a critical ignition temperature, especially in SI-HCCI combustion mode transition, assisted SI improves the stability of HCCI combustion. In order to understand better the physicochemical process in the cylinder, an improved three-dimensional computational fluid dynamics code has been utilized to simulate the intake, two-stage spray, compression, and combustion processes of the HCCI engine. The calculated results show that a homogeneous lean charge could be realized by single fuel injection in the intake stroke. With the help of the intake swirl and squish flow in the compression stroke, a rich mixture resulting from the second fuel injection can be formed near the spark. Hydroxyl ions (OH) reach to a maximum at the periphery of the fuel-rich zone; a great number of spots are ignited simultaneously. It works as an initiation to ignite the surrounding lean mixture zone.