Statistical Analysis of Fuel Effects on Cylinder Conditions Leading to End-Gas Autoignition in SI Engines 2019-01-0630

Currently there is a significant research effort being made in gasoline spark/ignition (SI) engines to understand and reduce cycle-to-cycle variations. One of the phenomena that presents this cycle-to-cycle variation is combustion knock, which also happens to have a very stochastic behavior in modern SI engines. Conversely, the CFR octane rating engine presents much more repeatable combustion knock activity. The aim of this study is to assess the impact of fuel composition on the cycle to cycle variation of the pressure and timing of end gas autoignition. The variation of cylinder conditions at the timing of end-gas autoignition (knock point) for a wide selection of cycle ensembles have been analyzed for several constant RON 98 fuels on the CFR engine, as well as in a modern single-cylinder gasoline direct injection (GDI) SI engine operated at RON-like intake conditions. While the fuels investigated all had RON 98, they varied in RON-MON sensitivity and chemical composition by iso-paraffinic, aromatic and ethanol content. Additionally, the effects of intake pressure and temperature were explored on the CFR engine with primary reference fuel (PRF) 90. This test revealed that the iso-paraffinic fuels have a more deterministic behavior at knock point in both engines, suggesting that this could be a consequence of fuel properties. It was also shown that the intake boundary conditions do have a significant impact on the knock point behavior of the PRF90 fuel and its cycle-to-cycle variability.