As an alternative combustion mode HCCI seems as one of the most effective choice to increase the thermal efficiency and reduce the soot and NOx emissions among the other conventional combustion modes. HCCI combustion has common properties which gasoline and diesel engines have. Although the spark ignition engines have lower thermal efficiency compared to compression ignition engines, they have been commonly used due to their better starting properties in cold conditions, combustibility and controlling the combustion. For both conventional combustion modes, it is needed to use particle filters, catalytic converters and EGR mechanisms. However, these systems are too complex, expensive and not sufficient for future emission restrictions. HCCI combustion has advantages like increasing the thermal efficiency and reducing exhaust emissions. In order to obtain HCCI combustion, using variable valve mechanism is the most effective and practical method in spark ignition engines. In this study, four different valve mechanisms were used in order to extend HCCI operating range in a four stroke, single cylinder gasoline engine. The experiments were performed between 800 and 1900 rpm engine speeds. The test engine was operated at full HCCI combustion mode at different air/fuel ratios (lambda = 0.5 - 2) and inlet air temperatures (T-in = 20 - 120 degrees C). The effects of air/fuel ratio and inlet air temperature were investigated on HCCI combustion, cylinder pressure, heat release rate, engine performance and exhaust emissions. The test results showed that HCCI operating range can be extended using low lift cams on knocking and misfiring operating zones. It was also found that the test engine was run on HCCI combustion mode at leaner air/fuel ratio as the inlet air temperature increased. (C) 2015 Elsevier Ltd. All rights reserved.