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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.

    Exhaust gas recirculation (EGR)

    Exhaust gas recirculation(EGR) is a nitrogen oxide(NO=)emissions reductiontechnique used in petrol/gasoline and diesel engines. EGR works by recirculating a portionof an engine's exhaust gasback to the engine cylinders. In a gasoline engine,this inertexhaust displaces the amount of combustible matter in the cylinder. In a diesel engine, theexhaust gas replaces some of the excess oxygen in the pre-combustion mixture. BecauseNO= forms primarily when a mixture of nitrogen and oxygen is subjected to hightemperature,the lower combustion chamber temperatures caused by EGR reduces theamount of NO= the combustion generates. Most modern engines now require exhaust gasrecirculation to meet emissions standards.

    The exhaust gas recirculation system allows burned exhaust gases to enter the engineintake manifold to help reduce NO= emissions. When exhaust gases are added to the air-fuel mixture, they decrease peak combustion temperature. For this reason, an exhaust gasrecirculation system lowers the amount of N仪in the engine exhaust. EGR systems can becontrolled by engine vacuum or by the engine control module.

    Air injection system

    An air injection system forces fresh air into the exhaust ports or catalytic converter toreduce HC and CO emissions. The exhaust gases leaving an engine can contain unburnedand partially burned fuel. Oxygen from the air injection system causes this fuel to continueto burn in the exhaust manifold or the catalytic converter.

    When the engine is running,the spinning vanes in the air pump force air into thediverter valve. If not decelerating, the air is forced through the diverter valve, checkvalve,air injection manifold,and into the engine exhaust ports. The fresh air blows on theengine exhaust valves to keep any fuel burning as it leaves the engine.

    During periods of deceleration,the diverter valve blocks airflow into the engineexhaust manifold. This prevents a possible backfire,which could damage the vehicle'sexhaust system. When needed,the diverter valve's relief valve releases excess pressure.

    Catalytic converter

    A catalytic converter burns the remaining HC and CO emissions that pass into theexhaust system. Extreme heat(700 C)ignites these emissions and changes them intoharmless carbon dioxide and water.

    A catalyst is any substance that speeds a chemical reaction without itself beingchanged. A catalytic converter contains a catalyst agent,usually the element platinum,palladium,rhodium, or a mixture of these material. Platinum and palladium treat the HCand Co emissions. Rhodium acts on the NO= emissions. Some newer converters also usecerium to attract and release additional oxygen into the exhaust stream.

    The converter's catalyst agent is coated on either a ceramic honeycomb shaped blockor small ceramic beads. The catalyst is encased in a stainless steel housing that is designedto resist heat. The catalyst operating temperature is attained when the catalyst agents arehot enough to start treating emissions.

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