A quasi-dimensional model based on the concepts of fractal geometry has been developed for an SI engine fuelled with natural gas/hydrogen blends. The fundamentals of the thermodynamic model, the fractal combustion model and related equations are introduced. This paper investigates the influence of manifold absolute pressure, equivalence ratio and hydrogen fraction on fractal dimension and improves the fractal dimension expression. Comparisons are conducted between the improved and original models by the prediction outcomes. After the determination of model constants by calibration, the model predictions of cylinder pressure histories and mass fraction burned of an HCNG engine are then compared with experimental data over a wide range of loads, equivalence ratios, engine speeds and hydrogen blending ratios. The pressure profiles show that predictions of the improved model match quite well with the experimental results except for the early combustion stage. The improved model is proved to be more suitable for predicting HCNG engine performance. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.