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Emission control systems are very important in reducing the emission of hydrocarbons from automobile engines. The emission of such gases can be controlled by three methods. The first method is to promote proper combustion so that there will be fewer byproducts. The second method is recirculation of the emitted hydrocarbons back to the engine for further combustion. The third method is to provide additional space in the engine for better oxidation and combustion.
Now, we take a look on history of emission control devices and their working principle.
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1958- Crankcase Ventilation System
The crankcase ventilation system is the oldest emission control system used in automobiles. This system takes the vapor developed in the crankcase during combustion of the air/fuel mixture, and transfers it back into the intake system so that it can be further burned. Prior to this, crankcase fumes and pressure was directly vented via "road tubes."
The main problem with this system is that the vapor may dilute the air/fuel mixture decreasing performance. Therefore, it needs to be controlled and metered properly to avoid any reduction in performance. This controlling of vapors is done by the positive crankcase valve or PCV valve. Damage in the PCV valve may cause air leaks and the engine may become rough from inside.
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1966- Air Injection System
The air injection system was invented because of the presence of unburned fuel in the exhaust manifold after combustion. It is impossible to get 100 percent efficiency from any engine, which means some amount of unburned fuel must be present in exhaust. This is the cause of extra hydrocarbon emission. To prevent this emission, this system injects some amount of air into the exhaust manifold. This air starts the combustion of unburned fuel. This combustion doesn’t increase the power of the engine because the combustion occurs in the exhaust manifold, and unlike advanced turbocharger systems there is no any device like turbine to utilize the energy developed from this combustion. Besides, proper burning of the unburned fuel reduces the hydrocarbon emission.
Large amounts of the unburned fuel may cause backfire in the exhaust. It is because of excessive pressure and extra air present inside the exhaust. This problem is prevented by a diverter valve that controls the air flow and releases the excessive pressure through silencer.
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1970- Evaporation Control Systems
The evaporation control system traps and stores the emissions from the gas tank and carburetor. The trapping of vapor is performed by a charcoal canister because fuel vapor adheres to the charcoal. The process occurs until the engine is started. The engine vacuum is then used to draw the trapped vapor into the engine to burn with the air fuel mixture.
Vapor flow is controlled by a purge valve present in the engine. When the valve stops working, the vapor moves directly into the intake manifold and dilutes the air/fuel mixture and thereby reduces the performance.
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1973- Catalytic Converter System
The catalytic converter is an additional area provided in the exhaust system for oxidation or combustion. This area looks like a muffler and is located in or close to the exhaust manifold. Platinum or palladium pellets or a honeycomb are present inside the catalytic converter. These elements work as catalysts to accelerate the oxidation of hydrocarbons, which produces inert carbon dioxide and water vapor.
The oxidation of hydrocarbons produces an excessive amount of heat. The more fuel present in the exhaust the more heat will be produced. This excessive amount of heat may destroy the exhaust system and it's the main problem with this system. Furthermore, the fuel puts a coating on the platinum and palladium pellets which may cause the failure of the catalytic converter system.
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1973- Exhaust Gas Recirculation System
The exhaust gas recirculation system reduces the emission of nitrogen oxides by diluting the air/fuel ratio using very small amount of air. An EGR valve is used in it to control the quantity of air. Actually, the dilute mixture lowers the temperature of the combustion chamber below 750 0C. The reduced temperature doesn’t allow producing nitrogen oxides because this reaction requires more than750C temperature to occur.
Exhaust gas recirculation system is the most effective system to control the emission of nitrogen oxides. Even so, the dilution in the air/fuel mixture causes decrease in the performance of the engine. The cost of the system is very high due to the presence of a series of electrical and vacuum switches, and vehicle computer.
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1977- Advanced Turbocharging System
The advanced turbocharging system is used in modern high speed cars and aircrafts. A turbine needs to be installed in the exhaust manifold. The turbine develops power from the heat energy of exhaust gases, and the power thus developed is transferred to the crankshaft of the engine. It reduces the exhaust gas emission and also provides extra power to the engine for more speed.
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1980- Electronic Injection System
Electronic injection systems were made possible by the introduction of engine control modules (ECM), a computerized device with many functions. Among these were monitoring the amount of oxygen in the exhaust in order to regulate, indirectly, temperatures in the catalytic converter. The ECM also controls fuel flow, ignition timing (or delay if used with a knock sensor), idle speed, and monitors items such as engine temperature, airflow into the engine via a mass air or hot wire sensor, throttle position changes, etc.
The electronic injection system directly injects the fuel and air inside the cylinder in atomized form so as they can mix properly. Air and fuel do not mix completely in the carburetor. The emission of hydrocarbons depends on the quality of the air/fuel mixture. If the air and fuel are mixed correctly then the mixture will burn properly and proper combustion will take place. The proper combustion of the mixture reduces the emission of hydrocarbons and increases the efficiency of the engine.
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This is the emission control history for automobiles. Modifications to these systems is ongoing, and the evolution of automotive emission control devices is a stepping stone to protect the environment and human beings from pollution.
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- "Emission Control System", Encyclopedia Britannica, http://www.britannica.com/EBchecked/topic/185892/emission-control-system
- Bordoff, Fred, "Emission Control System", Family Car Parts, http://www.familycar.com/classroom/emission.htm
- Kiran, "Emission Control Devices and Measures", Engi Blogger, Sept 17, 2009, http://www.engiblogger.com/non-conventional/emission-control-devices-and-measures-ii/
- Image- Turbochargers, Fotopedia, by- TRD JZX100