The Homogeneous Charge Compression Ignition (HCCI) engine combustion uses heat energy from trapped exhaust gases enhanced by the piston compression heating to auto ignite a premixed air/gasoline mixture. As the HCCI combustion is controlled by the charge temperature, composition and pressure, it therefore, prevents the use of a direct control mechanism such as in the spark and diesel combustion. Using a large amount of trapped residual gas (TRG), is seen as one of the ways to achieve and control HCCI in a certain operating range. By varying the amount of TRG in the fresh air/fuel mixture (inside the cylinder), the charge mixture temperature, composition and pressure can be controlled and hence, the auto ignition timing and heat release rate. The controlled auto ignition (HCCI) engine concept has the potential to be highly efficient and to produce low NOx, carbon dioxide and particulate matter emissions. It has however been found that the TRG promoted HCCI combustion mainly depends on the quantity and quality of TRG, that on the other hand depend on the combustion quality of the previous cycle, valve timing, engine load and speed. In that way, the operating range in terms of engine load and speed, for a naturally aspirated HCCI engine, is restricted by a misfire at low load and by fierce (knocking) combustion at high load. One possible approach to extend the operating range of the HCCI combustion is to influence quality of the TRG by adjusting the coolant temperature. The engine coolant temperature influences the in-cylinder heat transfer process, which in turn influences the charge mixture temperature and therefore the HCCI combustion process itself. The aim of this paper is to present tests and results obtained on the single cylinder research engine, equipped with a Fully Variable Valve Train (FVVT) run over a range of coolant temperature in the HCCI combustion mode and fuelled with gasoline fuel. The results obtained suggest that with reducing the coolant temperature, the high load limit can be extended up to 14%, while with increasing the coolant temperature the low load limit can be extended up to 28%. Copyright © 2005 SAE International.