Gasoline HCCI Engine with DME as an Ignition Promoter

Abstract

Homogeneous charge compression ignition (HCCI) combustion is an attractive way to lower carbon dioxide (CO2), nitrogen oxides (NOx) emission and to allow higher fuel conversion efficiency. However, HCCI engine has inherent problem of narrow operating range at high load due to high in-cylinder peak pressure and consequent noise. To overcome this problem, the control of combustion start and heat release rate is required. It is difficult to control the start of combustion because HCCI combustion phase is closely linked to chemical reaction during a compression stroke. The combination of variable valve timing (VVT) and gas fuel injection of high cetane number was chosen as the most promising strategy to control the HCCI combustion phase in this study. Regular gasoline was injected at intake port as main fuel, while small amount of di-methyl ether (DME) was also injected directly into the cylinder as an ignition promoter for the control of ignition timing. Different intake valve timings were tested for combustion phase control. Regular gasoline was tested for HCCI operation and emission characteristics with various engine conditions. This paper investigates the steady-state combustion characteristics of the HCCI engine with VVT, to find out its benefits in exhaust gas emissions. With HCCI operation, higher internal exhaust gas recirculation (EGR) rate and the longer combustion duration reduced NOx emissions. However, hydrocarbon (HC) emission is relatively higher than that under spark-ignition combustion.