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Unregulated emissions of reformulated diesel fuels (sulfur < 50 ppm, aromatics < 20 vol-%) were compared to the European EN590 specification fuel (sulfur < 500 ppm, aromatics < 35 vol-%) in three IDI passenger cars and one DI van using FTP and/or ECE/EUDC emission test procedures. The effect of reformulated diesel fuels on the mutagenicity of particulate soluble organic fraction (SOF) was studied. Fuel reformulation reduced particulate emissions in IDI cars. Reformulating fuel by decreasing heavier aromatics - without decreasing final boiling point - reduced particulate mutagenicity on emission basis. At low ambient temperature (-7°C) particulate PAH and mutagenic emissions increased compared to the standard ambient temperature (+22°C) with all fuels.

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.

Diesel fuels must have good flow properties at low ambient temperatures. These fuels have lower densities than normal fuels. Fuel-injection pumps work on the basis of volume, and thus the fuel mass flow is reduced by density decrease. This leads to lower engine power. In Finland, for instance, the engine power reduction is about 10% when the fuel is changed from summer grade to arctic winter grade. The problem could be solved in future by using fuel density sensors in electronically governed injection pumps or by using fuel heaters so that vehicles could run on high density fuels also in winter.