Basic emission control systems and what they do
Automotive-generated pollutants come from three sources: tailpipe,
crankcase blowby vapors and fuel vapors that evaporate from the fuel
tank and carburetor.
Crankcase blowby vapors are eliminated as a source of pollution by
recirculating the vapors into the engine for reburning through the
Positive Crankcase Ventilation (PCV) system.
 
The PCV valve acts like a small calibrated vacuum leak, allowing
manifold vacuum to siphon air through the crankcase, taking with it
moisture and blowby gases that would otherwise pollute the
atmosphere. As a side benefit, it extends motor oil life.
Evaporative emissions have been eliminated by sealing the fuel
system and storing vapors in a charcoal canister. When the engine
starts, a purge valve on the canister opens, allowing manifold
vacuum to siphon vapors into the intake manifold to be burned in the
engine.
There are 3 primary tail pipe pollutants:
1) Carbon monoxide (CO) is formed whenever there is not enough
oxygen to completely burn the fuel. The richer the mixture, the
greater the quantity of CO produced.
2) Carbon monoxide is the worst pollutant of the three because it is
deadly. CO emissions are reduced by keeping the air/fuel ratio lean,
by preheating incoming air and manifold to aid fuel vaporization,
and by converting the remaining CO into harmless carbon dioxide in
the catalytic converter.
3) Hydrocarbon (HC) emissions are unburned gasoline. HC is not
directly harmful, but it contributes to smog formation. A fouled
spark plug, a leaky exhaust valve, or a fuel mixture so lean it
won't ignite (lean misfire) can all allow unburned fuel to enter the
exhaust.
HC is reduced by maintaining a balanced air/fuel mixture, by making
sure compression and ignition are OK, and by reburning any HC
remaining in the catalytic converter.
Oxides of Nitrogen (NOX) are formed in the combustion chamber when
temperatures rise above 2,500>1|F and nitrogen begins to react with
oxygen. Lean air/fuel mixtures burn hotter and increase NOX.
Though not as poisonous as carbon monoxide, NOX irritates the eyes,
nose and lungs, and contributes to ozone depletion and acid rain
formation.
NOX is reduced by the Exhaust Gas Recirculation (EGR) system, and by
three-way catalytic converters.
The EGR system allows a small amount of exhaust gas to be siphoned
back into the intake manifold to slightly dilute the incoming
air/fuel mixture. This lowers combustion temperatures to reduce NOX.
It also helps prevent detonation.
The catalytic converter contains a ceramic honeycomb or ceramic
pellets coated with a thin layer of platinum and palladium metal. In
three-way converters, a third catalyst (rhodium) is included to
reduce NOX.
The converter acts to reburn pollutants. To do so, it needs extra
oxygen received from an air pump or an aspirator valve.
Arrows show air flow through the system. Air travels from the air
filter to each cylinder where it mixes with hydrocarbons and carbon
monoxide and then escapes into the atmosphere through the exhaust
system.
The air pump is belt-driven and feeds air to the exhaust manifold
through a diverter valve and check valve. The diverter valve dumps
excess air back into the atmosphere when it's not needed (during
deceleration, for example).
On some engines, a gulp valve is another part of the plumbing. The
gulp valve diverts air from the pump into the intake manifold. This
momentarily leans out the mixture during deceleration, preventing
backfiring in the exhaust from too much fuel.
On some engines, an aspirator is used in place of an air pump. An
aspirator is a one-way valve that allows air to be siphoned into the
exhaust system between exhaust pulses.
When computerized engine controls and three-way catalytic converters
were added, the air pump gained yet another control valve. When the
engine is cold, air is routed to the exhaust manifold to help reduce
the initial HC and CO emissions.
NOX is not a problem when the engine is cold. As the engine warms up
and NOX starts to rise, the flow of air is diverted from the exhaust
manifold directly to the converter where it enters a chamber between
the two catalysts.
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