Page 67 - Automotive Engineering

CH AP TER 3 .1 Emissions control

into which the extra air is pumped, to join the gas stream is required. The warm air is taken from a jacket around the
before it enters the Pt, or Pt-P1, stage. exhaust manifold.
With three-way conversion, a closed-loop control Ford have developed a similar system. In this case,
system is essential, for regulating the supply of fuel ac- however, the thermostat senses under-bonnet tempera-
curately in relation to the mass air-flow into the engine. ture,andprovisionismade, bymeans ofavacuum-actuated
This entails installing an oxygen sensor in the exhaust, override, to enable maximum power output to be obtained
and an on-board microprocessor to exercise control, both during warm-up.
to correct continuously for divergencies from the stoi- The Austin-Rover design is outstanding for its sim-
chiometric ratio and to ensure good driveability. plicity. It is a banjo-shaped pressed steel box assembly,
the handle of which is represented by the air intake duct
to the air cleaner. In both the upper and lower faces of
3.1.10 The electronic control the box is a large diameter port, and a flap valve is poised
system between them. This flap valve is mounted on a bimetal
strip which, when hot, deflects to close one port and,
In practice, the electronic control system has to be more when cold, to close the other. The latter port simply
complex than might be assumed from the preceding lets air at the ambient temperature into the intake, while
paragraph. When the engine is being cranked for starting the former is connected by a duct to a metal shroud
it has to switch automatically from a closed- to an open- over the exhaust manifold and therefore passes hot air
loop system, to provide a rich mixture. In this condition into the intake. It follows that the temperature of the
the air supply for the second converter bed is diverted to incoming mixture from both ports is regulated by its
the exhaust manifold to oxidise the inevitable HC and effect on the bimetal strip, which deflects the flap valve
CO content, thus avoiding a rapid rise in temperature in, towards the hot or cold port, as necessary.
and overloading of, the second stage of the converter.
Owing to the low temperatures in the combustion
chambers of the engine, NO x production is minimal or 3.1.12 Evaporative emissions
even zero, so no conversion is required in the first stage.
During warm-up the mixture strength has to be mod- The evaporative emissions are mostly hydrocarbons
ified for the transition from rich to stoichiometric mix- though, with some special fuels and those that have been
ture. However, to cater to heavy loading, such as modified to increase octane number, alcohols may also be
acceleration uphill, it may again have to be enriched, present. In general, the vapour comes from four sources:
perhaps with EGR in this condition to inhibit the forma-
tion of NO x . The on-board microprocessor capabilities, (1) Fuel tank venting system.
therefore, must include control over idle speed, spark (2) Permeation through the walls of plastics tanks.
timing, EGR, purge of hydrocarbons from carbon canister (3) The carburettor venting system.
vapour-traps, early evaporation of fuel by air intake
heating, torque converter lock-up, and a fault-diagnosis (4) Through the crankcase breather.
system. Fumes from the fuel tank venting system are absorbed in
carbon canisters which are periodically purged (Section
3.1.17). Permeation through the walls of plastics tanks is
3.1.11 Warm-air intake systems
controlled by one of four methods. These are:
(1) Fluorine treatment.
Apart from setting the coolant thermostat to open at
higher temperatures to improve combustion in cold (2) Sulphur trioxide treatment.
conditions, several manufacturers have introduced au- (3) Du Pont one-shot injection moulding (a laminar
tomatic control of the temperature of the air drawn into barrier treatment).
the carburettor. The GM system is built into a conven-
(4) Premier Fuel Systems method of lamination.
tional air cleaner. There are two air valves, operated by
vacuum-actuated diaphragm mechanisms, and controlled Plastics tanks are generally moulded by extrusion of what
by a thermostat. One valve lets warm and the other cold is termed a parison (a large-diameter tube), which is
air into the intake. suspended in a female mould into which it is then blow-
The thermostat, mounted on the air cleaner, senses the moulded radially outwards. The chemical treatments are

temperature inside, maintaining it at 40–45 C. This applied internally, either in the parison or in the blow-
thermostat operates a two-way control valve directing moulding. With either procedure, problems arise owing
either induction system depression or atmospheric pres- to the toxicity of the barrier chemicals and in the disposal
sure to the actuator, according to whether cool or warm air of the chemical waste.

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