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This paper presents an analysis of combustion phasing in a controlled auto-ignition (CAI) engine fuelled with gasoline. Auto-ignition was achieved using an exhaust gas trapping method via negative valve overlap (NVO). Under slightly lean mixture conditions variable intake and exhaust valves timings were applied in order to analyze influence of amount of retained exhaust on auto-ignition timing and combustion duration. Combustion on-set was independent of exhaust valve closing event, which was responsible for amount of trapped residuals. However, it was found that auto-ignition timing was determined by intake valve timing. Combustion duration was affected by both exhaust and intake valve timings. Direct injection allowed for application of different mixture formation strategies including in-cylinder fuel reforming during the NVO phase. When fuel was injected in the late stage of NVO increase of air-fuel ratio (AFR) caused a retard of auto-ignition and reduction of heat release rate.

A single cylinder research engine with negative valve overlap (NVO) and direct gasoline injection was run in a homogeneous charge compression ignition (HCCI) mode. The split fuel injection technique was used, where the first injection was applied during exhaust re-compression, while the second injection was applied at the beginning of main compression. The quantities of the fuel injected at the two timings were varied from the whole fuel injection during NVO to the whole fuel injection during the main compression event. These split fuel ratio sweeps were repeated both for a stoichiometric mixture and for a slightly lean mixture. In the study, NVO reactions were assessed via analysis of the exhaust-fuel mixture composition after the NVO period and referred to the main event combustion. The results showed that fuel injection during NVO resulted in the production of substantial quantities of auto-ignition promoting species, such as acetylene and formaldehyde.

The paper presents results of research of a gasoline engine capable of controlled auto-ignition (CAI) mode of combustion. The experiments were conducted on a single cylinder research engine equipped with direct fuel injection and a fully variable valvetrain. CAI operation was obtained with the use of the negative valve overlap (NVO) technique. The engine was operated at variable rotational speed and load. Comparative analysis of performance, fuel consumption and exhaust emissions was made for both SI and CAI combustion modes. Combination of variable valvetrain settings and variable excess air ratio allowed loads between 0.2 and 0.42 MPa IMEP in CAI combustion mode. Direct fuel injection provided the possibility to control the heat release rate via injection angle.