ADD-ON SYSTEMS FOR CONTROL OF ENGINE-OUT EMISSIONS

Crankcase Emission Control (PCV System)

A small amount of charge in the cylinder leaks past piston rings into crankcase of the reciprocating engines.   Near top dead centre (TDC) when the rings change their position in the grooves at the end of compression stroke, combustion has already begun and the cylinder pressures are high.  A significant part of charge stored in the piston- ring-cylinder crevice leaks into the crankcase. These gases are known as ‘crankcase blow by’ and their flow rate increases as the engine is worn out and the piston - cylinder clearances and ring gaps increase. In the homogeneous charge engines, the crankcase blow by gas is high in HC concentration. Only a small fraction of the gas stored in the ring crevices and hence blow by gases may consist of partially burnt mixture. This source contributes about 20 percent of total hydrocarbons emitted by an uncontrolled car.

The crankcase blowby gases in the uncontrolled engines were ventilated to atmosphere under the effect of pressure difference occurring naturally between the crankcase and atmosphere. For control of crankcase emissions, the blowby gases are recycled back to the engine assisted by a positive pressure drop between the crankcase and intake manifold. When engine is running and intake charge is throttled the intake manifold is at a lower pressure than the crankcase. The blow-by gases mix with the intake charge to be burned inside the engine cylinder to CO₂ and H₂O. A typical PCV system is shown in Fig. 5.6.  A tube connects crankcase or cylinder head cover to the intake manifold below throttle valve, which leads the blowby gases back to the engine. Due to suction effect of intake manifold as the pressure in the crankcase falls, ventilation air
from the air cleaner is drawn into the crankcase that continuously purges it. A one-way valve (PCV valve) is used to control the flow of blowby gases PCV valve restricts flow of blowby gases during idling and very light loads which otherwise would cause excessive leaning of the charge by ventilation air.Under normal engine operation, PCV valve is fully open providing free flow of the gases while under high intake manifold vacuum the flow is restricted.