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Different qualities of diesel fuels have been tested in naturally aspirated IDI engines and turbocharged DI engines to determine the effect on exhaust emissions (NOx, CO, HC, PAH, particulates, sulfates and smoke) and combustion characteristics (heat release, ignition delay) under cold ambient conditions. Measurements were made during engine warm-up and under steady state operating conditions at ambient temperatures between -20°C and +20°C. The startability was tested down to -30°C. The results showed that the behaviour of the diesel engine is dependent on engine design, the lubricant used and the fuel properties. Low viscosity and good flow properties of the fuel are important for starting - more important than cetane number, for example. Fuel consumption and exhaust emissions at the beginning of the warm-up period increased considerably as the starting temperature was reduced.