Computer models of engine processes are valuable tools for predicting and analyzing engine performance and allow exploration of many engine design alternatives in an inexpensive fashion. At the present work, experimentally based burning rate sub-models for flame initiation and propagation periods were developed for a natural gas engine. The sub-models do not require any knowledge of the flame shape and propagation speed to calculate the burning rate. The flame initiation sub-model determines where the flame propagation period starts so that experimental spark timing can be used for the calculations. The flame initiation and propagation sub-models were used along with a Zero Dimensional (Zero-D) model to calculate cylinder pressure traces for various engine operating conditions. The Zero-D model is able to match the measured pressure data with less than 8% error in magnitudes if the computations are started at the experimental spark timing. If the flame initiation sub-model is tuned for a specific engine operating condition, then the model is able to match the measured pressure with less than 1% error in magnitudes. (C) 2003 Elsevier Ltd. All rights reserved.