INFLUENCE OF FUEL TYPE AND INTAKE AIR PROPERTIES ON COMBUSTION CHARACTERISTICS OF HCCI ENGINE

The paper presents the analysis of the combustion characteristics of homogeneous charge compression ignition (HCCI) combustion using diesel fuel, jet fuel, gasoline and ethanol. The experiments were conducted on a modified four-stroke, four-cylinder engine at constant engine speed of 1400 rpm. The in-cylinder pressure, heat release analysis and exhaust emission measurements were employed for combustion diagnostics. The effect of the intake air temperature, relative air/ fuel ratio and exhaust gas recirculation (EGR) rate on the combustion parameters and emission were analyzed. The experimental results indicate that the intake-air temperature, airfuel ratio and EGR rate have significant effect on the maximum in-cylinder pressure, heat release rate, start of combustion and emissions. The engine in HCCI mode operates more stable when fuels with a lower cetane number (gasoline and ethanol) were used. When working at lower intake air temperature homogeneities of the air fuel mixture decreased especially when heavier fuels were used. The combustion of ethanol-air mixtures at lambda <= 3.33 started 3 CAD (Crank Angle Degree) before TDC when the air temperature was 120 degrees C. When the intake air temperature was increased to 130 degrees C the engine ran stably until lambda = 4.4. The exhaust gas recirculation allows the extension of air-fuel mixture composition limits towards higher fat mixtures without knocking combustion. The results show that for all stable operation points NOx emissions were lower than 12 ppm, however HC and CO emissions are higher.