The knock characteristics in an engine were investigated under homogeneous charge compression ignition (HCCI) operation. Liquefied petroleum gas (LPG) and gasoline were used as fuels and injected at the intake port using port fuel injection equipment. Dimethyl ether (DME) was used as an ignition promoter and was injected directly into the cylinder during the intake stroke. Different intake valve timings and fuel injection amounts were tested to verify the knock characteristics of the HCCI engine. The LPG is more suitable for high-load operation of HCCI than gasoline because of its high latent heat of vaporization and octane number. The knock intensity of LPG-DME combination was lower than that of gasoline-DME, because of the lower self-ignitability and the higher latent heat of vaporization of the total injected fuel. The ringing intensity (RI) increased as the intake valve open (IVO) timing was advanced. This is attributed to higher volumetric efficiency and residual gas, which promotes combustion. The knock intensity can be predicted by measuring and calculating the crank angle degree of 50% mass fraction burned. Carbon monoxide and hydrocarbon emissions were minimized at high ringing intensity conditions. The shortest burn duration under 0.5 MW/m(2) of RI was effective in achieving low hydrocarbon and carbon monoxide emissions.