The Nature of Heat Release in Gasoline PPCI Engines

The heat release characteristics in terms of the maximum pressure rise rate (MPRR) and combustion phasing in a partially premixed compression ignition (PPCI) engine are studied using a calibration gasoline. Early port fuel injection provides a nearly homogeneous charge, into which a secondary fuel pulse is added via direct injection (DI) to provide stratification which is affected by the timing of the start of injection (SOI). As the SOI the DI fuel is retarded from early compression, MPRR first decreases, then increases substantially, and decreases again. The MPRR correlates mostly with the combustion phasing. The SOI timing plays an indirect role. The observation is explained by a bulk heat release process of which the rate increases with temperature rather than by a sequential ignition process. Observations from compression ignition of representative homogeneous charges in a Rapid Compression Machine support this explanation.