An experimental study of negative valve overlap injection effects and their impact on combustion in a gasoline HCCI engine

This study pertains to negative valve overlap (NVO) phenomena and subsequent combustion of gasoline in a direct injection homogeneous charge compression ignition (HCCI) engine. Experiments were performed at variable valvetrain settings, which resulted in NVO crank angle (CA) values ranging from 157° to 182°. Fuel was injected directly into the cylinder in a single dose applied during the NVO period, and injection timings were varied from the early stage of exhaust compression to the later stage of exhaust expansion. During the experiments, air excess ratio values were varied from a stoichiometric mixture to lean mixtures, limited by misfires occurrence. A Fourier transform infrared analytical system and simple gas sampling method from the intake port were applied in order to analyze changes in fuel composition resulting from the reforming process during the NVO period. As a result of reforming, which took place after fuel injection during exhaust compression, up to 10.2% of fuel carbon was converted into CO carbon. Auto-ignition promoting species were also identified in considerable quantities, where fuel conversion into acetylene was 26.5 g, ethylene - 133.7 g and formaldehyde - 13.5 g per kilogram of fuel. Certain quantities of other light unsaturated hydrocarbons and methane were identified in the fuel as well. Furthermore, reforming products were present in the exhaust gases, where the fractional composition of unburned hydrocarbons was correlated with the composition of reformed fuel after NVO. Also, the influence of both NVO injection timing and air excess ratio on combustion was studied. It was found that retard of fuel injection during exhaust compression advanced combustion timing, while retard of fuel injection during exhaust expansion retarded combustion due to thermal NVO effects. The examination of combustion timings at a variable air excess ratio and two injection strategies: early and late NVO injections, showed the effect of mixture strength and NVO thermochemistry on the main event. At late NVO injection, applied 20 °CA after top dead center (TDC), the combustion timing was retarded both for the stoichiometric and lean mixtures, due to the mixture strength itself, while for early NVO injection, applied 40 °CA before TDC, the thermal effects of NVO played a dominant role in combustion control. However, the presence of reforming products in the mixture increased the heat release rate.