Emission Control System
The emission control system improves the engine performance and reduces the air pollution.
It consists of several subsystems: positive crankcase ventilation (PCV),
exhaust gas recirculation (EGR), catalytic converter and evaporative emission.
The PCV system circulates the escaped gases from the crankcase back to the combustion
chamber. The engine cylinder can not be sealed completely. A portion of the gases will leak through the small gap between the rings
and the cylinder wall and enter into the crankcase. The PCV system
recycles these gases back into the cylinder and burns them again. The PCV valve is the main component that controls the amount of the recycled
gases. This improves the mileage per gallon of gasoline, and prevents
the formation of the harmful gases and sludge in the engine.
The EGR system
prevents the formation of the harmful nitrogen oxides (NOx). NOx
can be produced severely in the combustion chamber at high temperature
(over 2500 F). Lowering the combustion temperature reduces the chance
of the NOx formation. The EGR system circulates a certain amount
of the exhaust gases back into the combustion chamber and dilutes the fuel
mixture. Since there is not much fuel to burn at this time, the combustion
temperature is lowered and the NOx gas can not form. The EGR valve
controls the flow of the recirculated exhaust gases based on the combustion
temperature and the engine operating conditions. A failed EGR valve
may affect the engine performance.
converter changes the harmful carbon monoxide (CO) into non-harmful carbon
dioxide (CO2), and hydrocarbons (CH) into water and carbon dioxide.
It is located in the exhaust system. It contains very expensive catalyst
materials which can be poisoned by the leaded gasoline. Therefore,
unleaded gasoline is required for all converter-equipped cars.
emission system controls the gasoline vapor. A canister is filled
with activated carbon that absorbs the gasoline vapor from the fuel system.
The vapor can be purged from the canister and then burned in the engine.
The canister is usually maintenance-free for lifetime.