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: 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.
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. 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 degrees 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. Exhaust Gas Recirculation (EGR) Valve NOX is reduced by the Exhaust Gas Recirculation (EGR) system, and catalytic converter(s). 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 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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