Density is a measure of a fuel’s mass per unit volume. It is temperature dependent and for diesel fuel is normally deter- mined at 15 1C. As diesel consists of a mixture of many different hydrocarbon compounds of various densities and molecular weights, the overall density depends on the composition of the fuel. Density is strongly correlated with other fuel parameters, particularly cetane number, aromatics content, viscosity and distillation (boiling range or volatility). In diesel engines, the fuel is injected directly into the combustion chamber using a volume based metering system (in most cases). The energy content of the fuel is approximately proportional to the mass of the fuel injected. Thus, for a constant volume injection system, variations in the fuel density can result in variations in the energy content of the fuel injected. Consequently, engine power, emissions and fuel consumption may be affected by the fuel density. In order to optimize the engine performance and exhaust emissions, the fuel density must be controlled within a fairly narrow range. Sulphur