Controlling autoignition timing over a wide range of speeds and loads is challenging. Overcoming this challenge in practical HCCI engines requires an improved understanding of the in-cylinder processes and how these processes can be favorably altered by various control techniques. In the current study, a zero-dimensional thermodynamic model that contains a simple heat release sub-model and an autoignition model was used in a predictive fashion to better understand the in-cylinder processes and the efficiency potential of a natural gas engine in the HCCI mode. The model was also used for parametric studies to evaluate HCCI control strategies that can be tested on the research engine. The results indicated that if the initial conditions of the mixture are known precisely at intake valve closing, the autoignition timing is controllable. A thermal efficiency close to 0.45 is possible with an IMEP range from 4 to 5 bar for the described engine, also. (C) 2004 Elsevier Ltd. All rights reserved.