Diesel engines are having to face the prospect of running with lower quality fuels as Refineries restructure to meet changing demands. Conversion of the heavier products will cause a lowering of the cetane number which is a measure of the combustion property of the fuel. A range of diesel fuels were investigated to study: Cetane improvers Ignition delay correlation between the CFR engine and a single cylinder version of a production type D.I. engine. Cold starting using D.I. and prechamber engines as used in production vehicles. Smoke and emission characteristics with the changing fuel properties. Injector fouling due to deposit build up. The investigation covers a wide range of tests giving an insight into diesel engine operation with varying fuel compositions. The drive towards reduced emissions and noise makes it imperative we look at diesel fuel quality, and it is hoped that this paper will contribute towards the work that is in progress.