Single-zone Wiebe-function HCCI combustion models tend to over-predict the peak cylinder pressure. The over-prediction arises because it is not possible for the standard Wiebe function to fully match both the slower combustion (i.e. the large spread of autoignition times) that occurs in the cooler boundary regions adjacent to the walls and the faster combustion (small spread of autoignition times) in the hot core. The slower combustion by the wall is commonly modeled with a multi-zone approach. The aim of this work was to improve the ability of a single-zone model to predict cylinder pressure without introducing a separate wall zone. This was accomplished by using, within a single zone, a double-Wiebe function combustion model in which most of the fuel burns as usual but a minor fraction (typically 10-20%) bums at a reduced rate. In the present article, cylinder pressure traces predicted by using both standard and double-Wiebe functions are compared to experimental pressure traces obtained from a Ricardo Hydra HCCI engine. The best agreement with the experiments was obtained by using double-Wiebe function approach. (C) 2007 Elsevier Ltd. All rights reserved.