Effects of Spark Ignition and Stratified Charge on Gasoline HCCI Combustion With Direct Injection 2005-01-0137
HCCI combustion was studied in a 4-stroke gasoline engine with a direct injection system. The electronically controlled two-stage gasoline injection and spark ignition system were adopted to control the mixture formation, ignition timing and combustion rate in HCCI engine. The engine could be operated in HCCI combustion mode in a range of load from 1 to 5 bar IMEP and operated in SI combustion mode up to load of 8 bar IMEP. The HCCI combustion characteristics were investigated under different A/F ratios, engine speeds, starts of injection, as well as spark ignition enabled or not. The test results reveal the HCCI combustion features as a high-pressure gradient after ignition and has advantages in high thermal efficiency and low NOx emissions over SI combustion. At the part load of 1400rpm and IMEP of 3.5bar, ISFC in HCCI mode is 25% lower and NOx emissions is 95% lower than that in SI mode. By introducing the second fuel injection in 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 mixture concentration and temperature. At critical ignition temperature, especially in SI/HCCI combustion mode transition, assisted spark ignition improves the stability of HCCI combustion.
To better understand the physicochemical process within the cylinder, direct photographs on an optical engine using CCD camera had been used to analysis the intake, two-stage spray, compression and combustion process of the HCCI engine. The captured photographs show homogeneous lean charge could be realized by single fuel injection in intake stroke. Due to the intake swirl and squish flow during the compression stroke, the rich mixture formed by 2nd fuel spray are located near the spark. A great amount of spots are ignited simultaneously in fuel-rich zone, then it worked as an initiation to ignite the surrounding lean mixture zone. In this combustion system, a high intake swirl can be kept in the toroidal combustion chamber promoting fuel dispersion to reduce NOx emissions.