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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 quantities of species in the NVO mixture were affected by both the quantity of fuel injected during NVO and main event air excess ratio. It was found that at stoichiometric mixture and at the whole fuel injected during the NVO period production of auto-ignition promoting species was reduced when compared to reduced amounts of fuel. However, auto-ignition timing was neither correlated with content of promoting species nor end of compression temperature. In the case of the lean mixture, a decrease in the quantity of fuel injected during NVO led both to reduced active species production and to a drop in end-of-compression temperature. Consequently, delay in auto-ignition was noted.
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