The present paper describes a method of controlling the time of ignition in homogeneous-charge compression ignition (HCCI) combustion. In the described experiments some control of ignition timing in HCCI combustion is achieved through alteration of the fuel molecular structure using a chemical reaction of the fuel with ozone, prior to introduction of the fuel into the combustion chamber. Controlling ignition timing is essential, in achieving high thermal efficiency and low pollutant emission in HCCI engine operation. To this end, ignition should occur in the vicinity of piston top-dead-centre (TDC), the point of maximum compression of the fuel-air charge. The present paper proposes a method of controlling the time of ignition of the fuel-air charge by adapting the ignitability of the fuel through prior chemical reaction of the fuel with ozone. Ozone can be readily produced using air in conjunction with a corona discharge ozoniser and may be brought into contact with the fuel in a reaction chamber before its injection into the engine. It was shown through experiments that an acetal fuel which has undergone treatment with ozone, ignites earlier during the engine cycle in HCCI combustion, than fuel which has not undergone treatment with ozone, as a result of changes in its molecular structure prior to combustion. The observed changes in molecular structure consisted primarily in the formation of peroxides within the fuel. This method can be used to operate an engine in HCCI combustion mode with some control over the point of ignition of the fuel-air charge by varying the proportions of fuel previously treated with ozone and fuel not treated with ozone. The experiments showed that the time of ignition could be controlled, whilst keeping other parameters such as the load and speed of the engine, and pressure and temperature of the intake air and fuel, constant. (C) 2010 Elsevier Ltd. All rights reserved.