Page 79 - Automotive Engineering

CH AP TER 3 .1 Emissions control

disadvantage of a limited life. Better results have been Oel, is carded under pressure in a special container. An
obtained with alloys of copper with silver, vanadium and electronic control releases additive automatically, as
titanium. needed, into the fuel supply line, so that regeneration

It is important to regenerate the filters reliably at the proceeds at temperatures as low as 200 C.
appropriate intervals, otherwise they will become over- Of the systems currently available for commercial
loaded with soot, which can then ignite and burn un- vehicles, those requiring the engine to be stopped during
controllably, developing excessive heat and destroying the burn-off are unsuitable for any automotive applications
filter. Even with normal burn off, however, local temper- other than city buses and large local delivery vehicles

atures near the centre of the beds can be as high as 1200 C. operated on regular schedules. The continuous and cyclic
Some particulate filters rely solely on excess air in the burn-off filters are generally even more bulky. On the
exhaust for burning off the particulates. Others have other hand, many of these systems are claimed to serve
extra air injected into them, generally at timed intervals, also as a silencer. There is not enough space here to give
either from the brake system or by a compressor or details of proprietary filter systems, but most are de-
blower. With air injection, burn-off can continue when scribed in Automotive Fuels and Fuel Systems, Vol. 2, by
the engine is operating at or near maximum power and T.K. Garrett, Wiley.
therefore with little excess oxygen in the exhaust. At
lighter engine loads and therefore with excess air present, 3.1.25 Influence of fuel quality
once ignition has been initiated, combustion can continue
without the burner. on diesel exhaust emissions
With some systems the engine must be stopped
while regeneration is in progress but with others, which How individual emissions are influenced by different fuel
are regulated by electronic control, either the paticu- properties have been summarised by the UK Petroleum
lates are burnt off intermittently while the vehicle is Industry Association as follows:
running or there are two filters in parallel, with
deflector valves directing the exhaust gases first NO x Increases slightly with cetane number.
through one filter, while the other is being regenerated, Decreases as aromatic content is
and then vice versa. It is also possible to program the lowered.
electronic controller to bring both filters into operation CO No significant effects.
simultaneously as maximum power and torque output HC Decreases slightly as cetane number
is approached, to cater for the increased exhaust flow increases.
under these conditions. This enables smaller filters to Decreases with density.
be used. Relationship with volatility inconsistent.
Claimed efficiencies of particulate removal range from Black smoke Increases with fuel density and decreases
about 70–95%, but some of these claims are suspect. with aromatic content.
However, since the carbon content is easiest to burn off, Is not significantly affected by volatility.
up to 99% of it can be removed. Currently, the useful Increases with injection retard (e.g. for
lives of the catalytically coated filters vary enormously reducing NO x ).
from type to type. Particulates Reduced as volatility is lowered.
Since we are in the early stages of development, the Reduced as cetane number is lowered,
situation is in a continuous state of flux; some systems no though inconsistently.
doubt will soon fall by the wayside, while others will be Unaffected by aromatic content.
developed further. Clearly, however, if particulates can Reduced as sulphur content is lowered.
be removed in the combustion chamber by using better
quality fuel or taking measures to improve combustion, it A good-quality fuel is generally regarded as one having
will be to the advantage of all concerned, except of a cetane number of 50 and a sulphur content of no greater
course the manufacturers of particulate filters! than 0.05%.
All filter systems are bulky and present burn-off In Sweden, Volvo have shown that by bringing the
problems, including high thermal loading. None pro- sulphur content down from 0.2 to 0.05% particulate
duced so far can be regarded as entirely satisfactory and, emissions can be reduced by up to 20% and NO x is also
at the time of writing, none has been used on cars. In- reduced. Furthermore, ignoring the effect of the tax on
terestingly, VW have been investigating the possibility of fuel prices, the cost of such a reduction is only about 2
using an iron-based additive to reduce the oxidation pence per gallon whereas to obtain a commensurate im-
temperature of the deposits down to that of the exhaust provement by reducing the aromatic content and in-
gas. A Coming monolithic ceramic filter is used, and the creasing the cetane number would cost about 22 pence
additive, developed jointly with Pluto GmbH and Veba per gallon.

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