Variable valve timing is one of the attractive ways to control HCCT (homogeneous charge compression ignition) engine. Hot internal residual gas which can be controlled by VVT device, makes fuel is evaporated easily, and ignition timing is advanced. Regular gasoline was used as main fuel and DME (di-methyl ether) was used as ignition promoter in this research. HCCT engine operating range is limited by high combustion peak pressure and engine noise. High combustion pressure can damage the engine during operation. To avoid engine damage, the rich Iimits have to define using various methods. Peak combustion pressure, rate of cylinder pressure rise was considered to determine rich limit of engine operating range. Knock probability was correlated with the rate of cylinder pressure rise as well as the peak combustion pressure.