Page 76 - Automotive Engineering

Emissions control CHAPTER 3.1

reasons it has to be confined to operation at moderate
loads. Electronic control of EGR is therefore desirable.
Fortunately, heavy commercial vehicles are driven most
of the time in the economical cruising range, maximum
power and torque being needed mostly for brief periods.
Reduction of the rate of swirl is another way of re-
ducing the output of NO x . It increases the time required
for the fuel to mix with the air, and therefore reduces the
concentration of oxygen around the fuel droplets. Con-
sequently, the temperature of combustion does not rise
to such a high peak. Again, however, it also reduces
thermal efficiency. Moreover, unless measures, such as
increasing the number of holes in the injector nozzle and
reducing their diameter, are taken to shorten the lengths
of the sprays, more fuel tends to be deposited on the
combustion chamber walls.
Delaying the start of injection has the effect of re-
ducing peak temperatures, and therefore NO x . This is
because the combustion process builds up to its peak
later in the cycle, when the piston is on its downward
Fig. 3.1-22 NO x emissions with direct and indirect injection. stroke and the gas is therefore being cooled by expansion.
However, to get a full charge of fuel into the cylinder in
converter, but this differential will be reduced as diesel the time remaining for it to be completely burnt, higher
combustion a control techniques improve. Efforts are injection pressures are needed. Therefore, to avoid in-
being made to develop catalysts suitable for diesel creasing the proportion of fuel sprayed on to the com-
application, but at the time of writing no satisfactory bustion chamber walls, the holes in the injector must
solution has been found. again be smaller in diameter and larger in number.
Unfortunately, most of the current conventional Turbocharging increases the temperature of combus-
methods of reducing NO x also impair efficiency and tion by increasing both the temperature and quantity of air
therefore increase fuel consumption and therefore the entering the cylinder. After-cooling, however, can help by
output of CO 2 . The relationship between NO x output removing the heat generated by both compression of the
and fuel consumption is illustrated in Fig. 3.1-23.In gas and conduction from the turbine. It also increases the
general, NO x tends to form most readily in fuel-lean density of the charge, and therefore thermal efficiency
zones around the injection spray. and power output. The net outcome of turbocharging
EGR displaces oxygen that otherwise would be avail- with charge cooling, therefore, is generally an increase or,
able for combustion and thus reduces the maximum at worst, no reduction in thermal efficiency.
temperature. However, it also heats the incoming charge,
reduces power output, causes both corrosion and wear, 3.1.21 Unburnt hydrocarbons
and leads to smoke emission at high loads. For these

Hydrocarbons in the exhaust are the principal cause of
the unpleasant smell of a diesel engine, though the lu-
bricating oil also makes a small contribution. There are
three main reasons for this. First, at low temperatures
and light loads, the mixture may be too lean for efficient
burning so the precombustion processes during the ig-
nition delay period are partially inhibited. This is why
some of the mixture subsequently fails to burn.
Secondly, because of the low volatility of diesel fuel
relative to petrol, and the short period of time available
for it to evaporate before combustion begins, HCs are
generated during starting and warming up from cold. In
these circumstances, fuel droplets, together with water
vapour produced by the burning of the hydrogen content
Fig. 3.1-23 Relationship between fuel consumption and NO x
emissions with (left) and without (right) charge cooling. of the remainder of the fuel, issue from the cold exhaust

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