Cold performance is a very important issue for diesel engines. Customers expect their engines to start reliably under all ambient conditions, and to quickly deliver useful power without unacceptable noise or exhaust emissions.In this programme the low ambient temperature cold start performance of two generations of Mercedes-Benz Heavy-Duty diesel engines has been explored. Both are typical of the smaller European Heavy-Duty engine design. The OM364 LA meets Euro 2 emissions legislation using mechanical controls; the OM904 LA is the first evolution of an all-new design to replace the OM364 LA, and features a three-valve cylinder head and high pressure unit-pump injection, with fully electronic controls.Engines were tested in a temperature controlled chamber, using a procedure that studied the first few minutes of operation from cold. In production engines intended for use in cold climates a cold starting aid is used which improves cold start performance, and reduces sensitivity to fuel properties. In order to more clearly discriminate fuel effects, starting aids were not used in this programme. Three production fuels were tested in both engines, including European specification fuels from the UK and Germany, and an ultra-low sulphur fuel from Finland. Work was focussed on the OM904 LA which was tested at ambient temperatures ranging from -5°C to - 20°C, whilst the OM364 LA was tested only at -15°C.The results showed that the new OM904LA gave major improvements over the OM364 LA in all aspects of low temperature cold start performance, including starting time and exhaust emissions of white smoke and unburned hydrocarbons. Performance was strongly affected by ambient temperature, with substantial increases in starting time and emissions as the temperature was reduced from -5 °C to -20 °C. The Finnish fuel gave the best overall performance in both engines. The German fuel tested had lower density / higher volatility than the UK test fuel and gave reduced smoke, but increased HC emissions. Fuel effects on starting time were significant in the OM364 LA, but not in the OM904 LA.Additional work was carried out to provide a better understanding of the important fuel variables, using a special fuel matrix designed to allow the effects of cetane number, front-end and back-end volatility to be separated. This showed that cetane number was the dominant fuel parameter affecting cold start performance. No effects of back-end volatility were detected, and effects of front-end volatility were relatively small and varied between engines. These results helped explain the performance of the production fuels, and suggested that other fuel properties had only a minor effect. The performance of fuels with and without cetane improver was tested at 50 cetane number and found to be comparable at the same measured cetane number. This confirmed the effectiveness of the selected cetane improver at low ambient temperatures and at the treat rates investigated. Use of cetane improver was tested up to 55 cetane number, and gave significant reductions in hydrocarbon emissions.