Page 77 - Automotive Engineering

CH AP TER 3 .1 Emissions control

pipe in the form of what is generally termed white smoke, sweep the oil out of the pools that collect in the hollows
but which is in fact largely a mixture of fuel and water formed by distortion of the bores, thus reducing the ef-
vapours. At about 10% load and rated speed, both HC fectiveness of oil control. Other means of reducing con-
and CO output are especially sensitive to fuel quality tamination by lubricating oil include improving the
and, in particular, cetane number. sealing around the inlet valve stems, the use of piston
Thirdly, after cold starting and during warm-up, rings designed to exercise better control over the thick-
a higher than normal proportion of the injected fuel, ness of the oil film on the cylinder walls and, if the engine
failing to evaporate, is deposited on the combustion is turbocharged, reduction of leakage of oil from the
chamber walls. This further reduces the rate of evapo- turbocharger bearings into the incoming air.
ration of the fuel, so that it fails to be ignited before the
contents of the chamber have been cooled, by expansion
of the gases, to a level such that ignition can no longer 3.1.22 Carbon monoxide
occur. Similarly, the cooling effect of the expansion
stroke when the engine is operating at or near full load Even at maximum power output, there is as much as 38%
can quench combustion in fuel-rich zones of the mixture. of excess air in the combustion chamber. However, al-
This is the fourth potential cause of HC emissions. though carbon monoxide (CO) should not be formed, it
Unburnt HCs tend to become a problem also at maxi- may in fact be found in small quantities in the exhaust.
mum power output, owing to the difficulty under these The reason is partly that, in local areas of the combustion
conditions of providing enough oxygen to burn all the fuel. chamber, most of the oxygen has been consumed before
Asfuel deliveryisincreased,a criticallimitisreachedabove injection ceases and, therefore, fuel injected into these
which first the CO and then the HC output rise steeply. areas cannot burn completely to CO 2 .
Injection systems are normally set so that fuelling does not
rise up to this limit, though the CO can be removed sub-
sequently by a catalytic converter in the exhaust system. 3.1.23 Particulates
Another potential cause of HCs is the fuel contained
in the volume between the pintle needle seat and the Regulations define particulates as anything that is

spray hole or holes (the sac volume). After the injector retained, at an exhaust gas temperature of 52 C, by a
needle has seated and combustion has ceased, some of filter having certain specified properties. They therefore
the trapped fuel may evaporate into the cylinder. Finally, include liquids as well as solids. Particle sizes range from
the crevice areas, for example between the piston and 0.01 to 10 mm, the majority being well under 1.0 mm.
cylinder walls above the top ring, also contain unburnt or While black smoke comprises mainly carbon, the heavier
quenched fractions of semi-burnt mixture, Expanding particulates comprise ash and other substances, some
under the influence of the high temperatures due to combined with carbon. The proportions, however,
combustion and falling pressures during the expansion depend on types of engine, fuel and lubricant.
stroke, and forced out by the motions of the piston and Measures appropriate for reducing the fuel and oil
rings, these vapours and gases find their way into the content of the particulates are the same as those already
exhaust. mentioned in connection with HC emissions (Section
In general, therefore, the engine designer can reduce 3.1.21). The overall quantity of particulates can be re-
HC emissions in three ways. One is by increasing the duced by increasing the injection pressure and reducing
compression ratio; secondly, the specific loading can be the size of the injector holes, to atomise the fuel better.
increased by installing a smaller, more highly rated, This however, tends to increase the NO x content. In-
engine for a given type of operation; and, thirdly, by creasing the combustion temperature helps to burn the
increasing the rate of swirl both to evaporate the fuel loose soot deposited on the combustion chamber walls.
more rapidly and to bring more oxygen into intimate Various measures have been taken to increase the tem-
contact with it. perature of these particulates, though mostly only ex-
Reduction of lubricating oil consumption is another perimentally. They include insulation by introducing an
important aim as regards not only control of HCs but also, air gap, or some other form of thermal barrier, between
and more importantly, particulate emissions. Whereas oil the chamber and the remainder of the piston, and the
consumption at a rate of 1% of that of fuel was, until the incorporation of ceramic combustion chambers in the
mid-1980s, been regarded as the norm, the aim now is piston crowns.
generally nearer to 0.2%. Using a lubricant containing a low Reduction of the sulphur content of the fuel also re-
proportion of volatile constituents helps too. duces particulates. Although the proportion of sulphate
Avoidance of cylinder-bore distortion can play a sig- þ water is shown in Table 3.1-3 as being only 2% of the
nificant part in the reduction of oil consumption. The total, if the insoluble sulphur compounds are added, this
piston rings tend to ride clear over and therefore fail to total becomes more like 25%. Because most measures

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