Diesel and spark ignition engine produce the same emissions. On the other hand, owing to the low volatility of diesel fuel relative to that of gasoline and the fact that carburettors are not employed, evaporative emissions are not so significant. Crankcase emissions, too, are of less importance, since only pure air is compressed in the cylinder and blow-by constitutes only a minute proportion of the total combustion gases produced during the expansion stroke.
Sulphur, which plays a major part in the production of particulates and smoke emissions is present in larger proportions in diesel fuel than in petrol. This is one of the reasons why the combustion of diesel fuel produces between 5 and 10 times more solid particles than that of
petrol.
Because diesel power output is governed by regulating the supply of fuel without throttling the air supply, there is excess air and therefore virtually zero CO in the exhaust under normal cruising conditions. Reduction of NOx on the other hand can be done only in an oxygen-free atmosphere, so a three-way catalytic converter is impracticable for diesel engine emissions.
Measures taken to reduce NOx tend to increase the quantity of particulates and HC in the exhaust . This is primarily because, while NOx is reduced by lowering the combustion temperature, both soot and HC are burned off by increasing it. In consequence, some of the regulations introduced in Europe have placed limits on the total output of both NOx and HC, instead of on each separately, leaving manufacturers free to obtain the best compromise between the two.
The problem of emission control, however, is not so severe as might be inferred from the last paragraph. Both NOx output and heat to exhaust become significant only as maximum torque and power are approached. At lighter loads, the gases tend to be cooled because of both their excess air content and the large expansion ratio of the diesel engine. Since the proportion of excess air falls as the load increases, oxidising catalysts can be used without risk of overheating, even at maximum power output.
Fuel blending and quality have a profound effect on emissions. Since fuel properties and qualities are interrelated, it is generally unsatisfactory to vary one property unilaterally. Indeed, efforts to reduce one exhaust pollutant can increase others and adversely affect other properties.