Exhaust and Emission Control Systems Emission Control System

     Several different emission control systems are used to reduce the amount of airpollution produced by the automobile. The following major ones are included：

 

    Positive crankcase ventilation (PCV)

 

    A positive crankcase ventilation system uses engine vacuum to draw blowby gases intothe intake manifold for reburning in the combustion chambers, shown as Fig. 7. 2.

 

    Engine blowby is caused by pressure leakage past the piston rings on the powerstrokes. A small percentage of combustion gases can flow through the ring end gaps or thepiston ring grooves and into the crankcase. If not reburned in the engine，these fumeswould contribute to air pollution if vented to the atmosphere. If not vented from thecrankcase, the gases would build to a point where engine damage would occur.

 

    A PCV system keeps the inside of the engine clean and reduces air pollution. Olderengines use an open PCV system. This system was not sealed and gases could leak outwhen the engine was shut off. These systems have been totally replaced by the closed PCVsystem．

 

    A closed PGV system uses a sealed oil filler cap，a sealed oil dipstick, ventilationhoses，and either a PCB calve or a flow restrictor. The gases are drawn into the engine andare burned. The system stores the gases when the engine is not running.

 

 

 

    Evaporative emissions control systems

 

    The evaporative emissions control (EVAP) system prevents toxic fuel system vaporsfrom entering the atmosphere. As mentioned earlier, gasoline and many of its additivesevaporate easily, especially if exposed to the atmosphere.

 

    When the engine is operating above idle speed，intake manifold vacuum causes thevacuum-operated purge valve to open. This allows gases to flow through the purge lineand causes fresh air to be drawn through the filter at the bottom of the canister. Theincoming fresh air picks up the stored fuel vapors and carries it through the purge line.The vapors enter the intake manifold and are pulled into the combustion chambers forburning.

 

    When the engine is shut off, gasoline slowly evaporates，producing unwanted vapors.These vapors flow through the fuel tank vent line and into the charcoal canister.Theactivated charcoal in the canister absorbs the fuel vapors and holds them until the engine isstarted again.

 

    Fig. 7. 4 illustrates evaporative emission control system operation.