Homogeneous charge compression ignition (HCCI) combustion mode has some benefits compared to the most popular conventional combustion forms used in the internal combustion (IC) engines: spark ignition (SI) and compression ignition (CI). This combustion mode provides low oxides of nitrogen (NOx) emissions and high thermal efficiency. However, it can produce higher unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions than those of conventional engines due to the lower combustion temperatures. In the naturally aspirated HCCI engines, the low engine output power limits its use in the current engine technologies. Intake air pressure boosting is a common way to improve the engine output power which is widely used in the high performance SI and CI engine applications. Therefore, in this study, the effect of inlet air pressure on the combustion characteristics and exhaust emissions of a direct injection homogeneous charge compression ignition (DI-HCCI) gasoline engine was investigated. For this purpose, a heavy-duty diesel engine was converted to a HCCI direct-injection gasoline engine. The experiments were performed at three different inlet air pressures while operating the engine at the same equivalence ratio and intake air temperature as in normally aspirated HCCI engine condition at different engine speeds. The start of injection (SOI) timing was set dependently to achieve the maximum engine torque at each test condition. The effects of inlet air pressure both on the combustion characteristics (such as cylinder pressure, heat release rate, engine efficiencies, and mean effective pressure) and on the exhaust emissions (such as CO, UHC and NOx) were discussed. The coefficients of variation (COV) of the indicated mean effective pressure (IMEP) were also provided. (C) 2012 Elsevier Ltd. All rights reserved.