A study of pressure-rise ignition delay was conducted using fuels varying in their aromatic, paraffin and naphthene content. A Ricardo E6 compression swirl diesel engine was fully instrumented for the measurement of ignition delay and operating parameters. The data from the electronic indicating equipment used were fed into a digital computer for the purpose of drawing the cylinder pressure-crank angle diagrams under firing and compression conditions and to calculate the delay. The test variables included the fuel structure, equivalence ratio, rotational speed, compression ratio, injection timing and air inlet temperature. A correlation was developed for the data viz: It was found that the ignition delay is a function of fuel structure and that the engine speed and injection timing were the most influential factors affecting delay followed by the equivalence ratio, compression ratio and air inlet temperature.