Ion Current Features of HCCI Combustion in a GDI Engine

Homogeneous charge compression ignition (HCCI) gasoline engines have the merit of good fuel economy and very low NOx emissions. The ion current signal in a gasoline direct injection-HCCI engine is the main focus of this paper. Experiments showed that the ion signal is significant correlated with the combustion status. Fuel spray and air-fuel mixture motions significant decrease the signal-to-noise ratio of the ion current signal. However, the current waveforms still vary regularly as the boundary conditions change, and their phases have significant linear relationships with the combustion phases. By combining the analysis with cylinder pressure data, the current can be used to effectively detect the combustion phase when the air fuel mixture is not lean. When the mixture gets leaner, the signal amplitude diminishes dramatically, and the linear correlation decreases substantially. The ratio of two-stage fuel injection has a strong effect on signal amplitude and combustion stability, and the linear relationship between the signal and combustion characteristics becomes insignificant as the pre-injection fuel amount decreases. A reaction kinetics analysis of the mechanism for the ion current signal in the HCCI engine explains the experimental observations.