The autoignition of natural gas injected into a combustion bomb at pressures and temperatures typical of top-dead-center conditions in compression ignition engines is studied by combining a detailed chemical kinetic mechanism, consisting of 22 species and 104 elementary reactions, with a multi-dimensional reactive flow code. The effect of natural gas composition. ambient density and temperature on the ignition process is studied by performing, calculations for three different blends of natural gas on a three-dimensional computational grid. The predictions of ignition delay compare very well with measurements in a combustion bomb. Based on this work, it is established that a particular mass of fuel burned is a much better criterion to define the ignition delay period than a specified pressure rise. The effect of additives like ethane and hydrogen peroxide in increasing the fuel consumption rate as well as the influence of physical parameters like fuel injection rate and intake temperature is studied. It is thus shown that apart from accurate predictions of ignition delay. the coupling between multi-dimensional flow and multi-step chemistry is essential to reveal detailed features of the ignition process.