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.