Page 74 - Automotive Engineering

Emissions control CHAPTER 3.1

proportions in diesel fuel than in petrol. This is one of the
reasons why the combustion of diesel fuel produces be-
tween 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 NO x on
the other hand can be done only in an oxygen-free atmo-
sphere, so a three-way catalytic converter is impracticable.
As a diesel engine is opened up towards maximum
Fig. 3.1-19 Fuel tank pressure control valve. When the engine is power and torque, NO x output increases because of the
not running, the valve is closed by the spring and the tank is then higher combustion temperatures and pressures. At the
vented only through the restriction.
same time HC, CO and sooty particulates also increase.
the engine is started, the lower diaphragm is lifted by the However, because of the relatively low volatility of diesel
purge control system depression and this closes the valve fuel and the extremely short time available for evapora-
serving the float chamber venting system. As the throttle is tion, problems arise also when the engine is very cold. In
opened further, the depression signal, again timed by the the latter circumstances, under idling or light load con-
passage of the throttle valve over a hole in the wall of the ditions, the situation is aggravated by the fact that the
manifold, lifts the upper diaphragm valve to open it and minute volumes of fuel injected per stroke are not so well
purge the canister. atomised as when larger volumes are delivered through
When a car is parked for long periods in hot sunshine the injector holes.
there could be a tendency for large volumes of vapour to
pass from the fuel tank into the canister. This would call 3.1.19 Reduction of emissions:
for the use of an otherwise unnecessarily large capacity
canister so, to avoid this, a fuel tank pressure control conflicting requirements
valve (Fig. 3.1-19) may be fitted. This is a diaphragm
valve the port of which is normally closed by a spring. To Measures taken to reduce NO x tend to increase
allow for variations in volume of air and vapour in the the quantity of particulates and HC in the exhaust
tank with changes in ambient temperature, the tank is (Fig. 3.1-20). This is primarily because, while NO x is
then vented through the restricted orifice alongside the reduced by lowering the combustion temperature, both
port. Thus, when the engine is not running the major part soot and HC are burned off by increasing it. In conse-
of the fumes tends to be kept in the tank. When the quence, some of the regulations introduced in Europe
engine is started the manifold depression opens the valve
port, to vent the fuel tank to the canister, which of course
will be cleared when the purge valve opens.
Yet another method of controlling the purge is
available if an electronic control module is installed. A
solenoid-controlled valve can be used and its opening and
closing regulated by the computer.

3.1.18 Diesel engine emissions

Diesel and spark ignition engines produce the same
emission. 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 pro-
portion of the total combustion gases produced during
the expansion stroke.
Fig. 3.1-20 Tests done in the late 1980s to demonstrate how NO x
Sulphur, which plays a major part in the production of increases when measures are taken to reduce particulates in
particulates and smoke emissions is present in larger different types of engine.

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