Development of a reduced chemical mechanism targeted for a 5-component gasoline surrogate: A case study on the heat release nature in a GCI engine

Gasoline Compression Ignition (GCI) is a promising engine operating mode that can reduce maximum pressure rise rate (MPRR) without knock tendency and better control the combustion phasing compared to the Homogeneous Charge Compression Ignition (HCCI) by using a late direct-injection (DI). In this study, a 107-species reduced mechanism and a 207-species skeletal mechanism were developed using the Computer Assisted Reduction Mechanism (CARM) and validated under engine conditions for a newly developed 5-component surrogate for a Haltermann 437 certification gasoline (AKI=93). Then, 3D computational fluid dynamics (CFD) simulations with an optimized grid size determined by a grid convergence study were performed with the 107-species reduced mechanism and the 5-component certification gasoline surrogate. Two experimental boosted GCI cases with similar, moderate MPRR and heat release parameters, but different second DI timings (-52 degrees aTDC and -5 degrees aTDC), were validated and analyzed. For the -52 degrees aTDC DI case, the combustion can be interpreted as a partially sequential auto-ignition due to the competition between the charge cooling effect and the equivalence ratio (phi)-sensitive effect of the stratified mixture, which is responsible for mitigating the MPRR. For the -5 degrees aTDC DI case, the combustion can be decoupled into a partially sequential auto-ignition and a subsequent non-premixed combustion by the DI fuel near top dead center in the compression stroke. The MPRR is relaxed through the slow, mixing-limited combustion between the injected fuel and the premixed mixture. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.