Visualization of the Impact of Diesel Injection Pressure on Combustion Characteristics of Homogenous Charge Induced Ignition

Homogenous charge induced ignition(HCII)is a promising combustion mode due to its potential advantages combined from both the compression ignition combustion and the spark ignition combustion, thus simultaneously improves the engine thermal efficiency and emission characteristics. An investigation of the impact of diesel injection pressure on the HCII combustion characteristics was carried out on an optical engine in comparison with the traditional diesel combustion mode. High speed in-cylinder ignition images were analyzed in conjunction with the combustion information derived from the in-cylinder pressure data. The results suggest that increasing diesel injection pressure results in shorter ignition delay and earlier ignition timing. An enlarged flame area and faster combustion rate were also observed. As a result, the peak cylinder pressure and the heat release rate are increased and the phase of the peak pressure is advanced. Under the same diesel injection pressure, the HCII combustion mode gives more ignition points scattered and more ignition timing advanced, but a slower flame development rate at the early stage than the diesel combustion mode. Furthermore, the spray based flame development of diesel combustion mode appears more obviously under medium and high injection pressures. While in the HCII combustion mode, it suggests the coexistence of premix combustion and diffusion combustion. At medium injection pressure, a large range of blue flame was observed, which suggests a typical premixed combustion. At high injection pressure, the combustion is mainly the premixed combustion of homogenous gasoline charge, while the diffusion combustion becomes predominant after the peak heat release rate.