A pressure sensor was installed in the combustion chamber of a single-cylinder diesel engine that works in HCCI mode (homogeneous charge compression ignition) using gasoline (75%) and ethanol (25%) as fuel. The experiments evaluate the pressure and the thermal efficiency of the engine for different temperatures of the air/fuel mixture and for different rotations. The results indicate that the temperature of the mixture had the greatest influence on the start of auto-ignition while the lambda factor had it on the advance of ignition and the release of thermal energy. The engine speed has an influence on the operating range in which the engine can work in HCCI mode, always considering minimum values of the cycle-to-cycle variation (COVPmax). The pressure variation from cycle to cycle decreases with the increase in the equivalence ratio until the detonation zone is reached and an increase in COV values is observed. The maximum pressure peaks increase with lambda and with the mixture temperature since any variation in the fuel injection will have a direct effect on the energy release and therefore on the pressure levels. Finally, rising the temperature of the mixture increases as well the pressure at which the HCCI operation starts.