As an alternative combustion mode, the HCCI combustion has some benefits compared to conventional SI and CI engines, such as low NO(x) emission and high thermal efficiency. However, this combustion mode can produce higher UHC and CO emissions than those of conventional engines. 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 high performance SI and CI engine applications. Therefore, in this study, the effect of inlet air pressure on the performance and exhaust emissions of a DI-HCCI gasoline engine has been investigated after converting a heavy-duty diesel engine 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 SOI timing was set dependently to achieve the maximum engine torque at each test condition. The effects of inlet air pressure both on the emissions such as CO, UHC and NO, and on the performance parameters such as BSFC, torque, thermal and combustion efficiencies have been discussed. The relationships between the emissions are also provided. (C) 2007 Elsevier Ltd. All rights reserved.