1%左右。The effects of methanol and ethanol additives on homogeneous charge compression ignition (HCCI) engine knock were investigated by add small quantity of methanol and ethanol to the base fuel (PRF90). Six test fuels were confected with the volume fractions of 1%, 3% and 5% containing methanol and ethanol, respectively. HCCI combustion tests were conducted on the fourth cylinder of a modified 4-cylinder diesel engine. The experimental results showed that, under steady engine speed of 1400r/min and at the same fuel injection quantity per cycle, with the increase of the volume fractions of methanol and ethanol added, the peak values of HCCI combustion cylinder pressures and heat release rates decreased gradually, causing combustion duration increased, knock effectively controlled, and load range enlarged. At the small volume fraction, methanol is better than ethanol in controlling engine knock, and methanol is much better than ethanol with the increase of the volume fraction. However, the misfire limits for the fuels containing methanol and ethanol are narrower than that for the base fuel, resulting in more possibility of engine misfire. For the tradeoff of knock and misfire, the volume fraction of methanol or ethanol in the base fuel needs to be about 1%. 