Page 57 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 3 .1 Emissions control

deflect some air from over the exhaust manifold to mix
with the remainder to maintain the overall temperature

at about 40–45 C, thus assisting evaporation in cold
weather.
In the 1970s much effort was devoted to the de-
velopment of various stratified charge engines. By the
1980s, however, high-compression lean-burn systems
had been the main practical outcome. With increasing
pressures for fuel economy as a means of reducing CO 2
output, interest in stratified charge began to surface again
in the early 1990s.
Positive crankcase ventilation (PCV), totally elimi-
nated pollution originating from crankcase fumes, and
at a modest cost (Section 3.1.3). By 1968, weakening
the air:fuel ratio, retarding the spark timing, preheating
Fig. 3.1-1 Effect of air:fuel ratio on exhaust emissions. the air passing into the engine intake, and, on some
models, the installation of a pump to inject air for
oxidising the HC and CO in the exhaust system re-
duced the total emissions about 39–41% by compari-
3.1.1 Early measures for controlling son with the 1960 cars. In absolute terms, emissions
from General Motors (GM) cars, for example, had
emissions been reduced to 6.3 and 51 gm/mile, respectively, for
HC and CO. However, there were still no controls on
A basic essential for spark ignition engine emission con- NO x . New developments then being investigated in-
trol is a carburettor or injection system capable of cluded carbon canister systems for the temporary
extreme accuracy in metering the fuel supply relative to storage and subsequent combustion of evaporative
the air entering the engine. Diesel engine emissions will emissions of fuel catalytic converters for controlling
be covered from Section 3.1.18 to the end of the chapter. exhaust emissions.
All modem fuel injection systems have been developed
from the outset specifically for accuracy of metering, for
minimal emissions and best fuel economy. Irregular 3.1.2 Evolution of the US Federal
combustion must be avoided during idling and, on the
overrun, the mixture must either be totally combustible test procedures
or the fuel supply totally cut off. In the latter event
a smooth return to normal combustion when the throttle In 1970 the US Congress had adopted regulations re-
is opened again is essential. Idling speeds are typically quiring by 1975 a reduction of 90% on the then current
750 rev/min with automatic and 550 rev/min with emissions requirements. The Federal Environmental
manual transmission. Protection Agency was formed and introduced a better
A capsule sensitive to manifold depression could be method of sampling. Previously all the exhaust gas had
used to retard the ignition in the slow-running condition, been collected in one huge bag and then analysed. This
the manifold depression tapping being taken from a po- had the disadvantage that it gave absolutely no indication
sition immediately downstream from the edge of the of how the engine behaved under the different condi-
throttle when it is closed. A centrifugal mechanism tions of operation during the test; moreover, in some

may retard the ignition from about 5 to 15 , while the circumstances, some of the gases interacted in the bag,
depression capsule can further retard it by perhaps giving misleading results.

another 15 . The new requirement was for collection into three
Sudden closing of the throttle, and the consequent bags, one for each main stage of the test (Fig. 3.1-2).
rapid increase in the depression over the slow-running The first, termed the cold-transient stage, comprised
orifices, may draw off extra fuel that cannot be burned cycles 1 to 5 of the test, which represented the be-
completely. To overcome this, a gulp valve has been de- ginning of a journey starting from cold. Next came
veloped for admitting extra air into the induction mani- 13 cycles, representing the remainder of the journey
fold in these circumstances. with the engine warm and including some operation at
Coolant thermostats have been set to open at higher high temperature. After this, the engine was shut down
temperatures for improved combustion in cold condi- for 10 min, to represent a hot soak, before starting up
tions. Also, thermostatically controlled air-intake valves again and repeating the first five cycles, for what is

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