Cycle to Cycle Variations: Their Influence on Cycle Resolved Gas Temperature and Unburned Hydrocarbons from a Camless Gasoline Compression Ignition Engine

Cycle to cycle variation in cylinder pressure is caused by the shifts in the auto-ignition timing of the air-fuel mixture. These variations during HCCI combustion were found to, be predictable to some extent, in the sense that an early phased combustion follows a later phased one and vice versa. When the engine was operated in spark ignition mode, a late combustion was correlated with a high gas temperature. No such correlation was found when the engine was operated in HCCI mode. Measurements of crank angle resolved unburned hydrocarbons showed that a much higher level of unburned HC was produced by a late burning HCCI cycle compared to an early one. No such correlation was found when the engine was operated in SI mode. Further investigation of the cylinder pressure revealed combustion in the gas exchange phase (compression of residuals) after a late combustion in HCCI mode, which increased the gas temperature, causing the mixture in the following compression stroke to auto-ignite earlier. This early combustion produced less unburned hydrocarbons, consequently no noticeable combustion during the gas exchange phase (no temperature increase) and a later combustion of the following engine cycle.