Page 75 - Automotive Engineering

CH AP TER 3 .1 Emissions control

have placed limits on the total output of both NO x and but also on the rate of rise and fall to and from it. Sec-
HC, instead of on each separately, leaving manufacturers ondly, the combustion temperatures depend on primarily
free to obtain the best compromise between the two. the quantity and, to a lesser degree, the cetane number of
The problem of emission control, however, is not so the fuel injected.
severe as might be inferred from the last paragraph. Both Increasing the cetane number reduces the delay
NO x output and heat toexhaust become significantonly as period, so the fuel starts to bum earlier, so higher tem-
maximum torque and power are approached. At lighter peratures and therefore more NO x are generated while
loads, the gases tend to be cooled because of both their the burning gas is still being compressed (Fig. 3.1-21).
excess air content and the large expansion ratio of the However, a smaller quantity of fuel is injected before
diesel engine. Since the proportion of excess air falls as the combustion begins, and this, by reducing the amount of
load increases, oxidising catalysts can be used without risk fuel burning at around TDC, reduces the peak combus-
of overheating, even at maximum power output. tion temperature. The net result of the two effects is
Fuel blending and quality have a profound effect on relatively little or even no change in NO x output. An
emissions. Since fuel properties and qualities are interesting feature in Fig. 3.1-22 is the enormous dif-
interrelated, it is generally unsatisfactory to vary one ference between the NO x outputs from direct and in-
property unilaterally. Indeed, efforts to reduce one ex- direct injection systems.
haust pollutant can increase others and adversely affect The popular concept that increasing fuel volatility
other properties. reduces NO x is an illusion: what happens in reality is that
the weight of fuel injected is reduced, and the engine is
therefore de-rated. Consequently, combustion tempera-
3.1.20 Oxides of nitrogen, NO x tures are lowered. This is explained in more detail in
Section 3.1.26, in connection with black smoke.
To understand the effects of fuel properties on NO x In the early 1990s, the overall output of NO x from the
output certain basic facts must be borne in mind. First, it diesel engine was, on average, between 5 and 10 times
depends not only on the peak temperature of combustion that of an equivalent gasoline power unit with a catalytic

Fig. 3.1-21 (Left) Tests by BP showing how injection timing influences combustion and therefore NO x output; (right) the influence of
cetane number on the principal emissions.

68