The influence of variations in the composition of natural gas on the ignition and combustion processes in engines is investigated. Particular attention is given to changes in the relatively small concentrations of high molar mass alkanes that may be present in the fuel. A detailed chemical kinetic scheme for the oxidation of the higher hydrocarbon components of up to n-heptane was used to investigate analytically the combustion reactions of different fuel mixtures under constant volume adiabatic conditions with initial states that are similar to those during the ignition delay of a typical internal combustion engine. These comprehensive simulation calculations require much computing capacity and time that would preclude their incorporation in full simulation models of engine processes. A simplification is introduced based on replacing artificially the small concentrations of any higher hydrocarbons that may be present in the natural gas by a kinetically equivalent amount of propane in the fuel mixture. This is done such that the resulting equivalent fuel has the same ignition delay as the original fuel under constant volume engine T.D.C. conditions. This “propane equivalent” concept was used in full engine simulation models while employing a relatively short scheme of 150 steps for the oxidation of fuel mixtures of propane, ethane, and methane in air.