Emission Controls

Emission controls are an essential and integral part of today's cars. Prior to the late 1960's, environmental concerns were essentially non-existent. There were no regulations on vehicle emissions. The byproducts of burning gasoline were permitted, unhindered, to flow freely into the atmosphere and onto the ground. Cars today burn fuel much more cleanly, producing a mere fraction of the pollutants that were present just 30 years ago. Here are some of the devices that have helped contribute to this.

Electronic Controls: (see electronics) There is little doubt that the electronics revolution has brought about superior engine control systems. Computerized control of many functions of an engine's operation including ignition timing and fuel metering based on the demands on the engine have resulted in a much cleaner, more fuel efficient vehicle. Older mechanical and electromechanical devices that were replaced by the computer did a mediocre job of providing the proper incremental changes necessary to accomodate operating conditions. Electronics have also helped to more effectively operate existing emission control devices.

Catalytic Converter: The catalytic converter turned out to be one of the most useful devices for emissions due to its relatively minimal impact on engine operation. Several other devices introduced on vehicles in the 1970's had negative effects not only on performance, but also on fuel efficiency. The converter had no such effect. 

The catalytic converter functions by chemically changing exhaust hydrocarbons (essentially unburned fuel) and carbon monoxide into more environmentally benign compounds. The chemical reaction is based on a catalyst that, when heated, changes the molecular bonds of certain materials. The catalyst chosen for a car's catalytic converter reacts with the unburned fuel and carbon monoxide to change their chemical compounds. Newer catalytic converters, called three-way catalysts lower the levels of oxides of nitrogen as well as hydrocarbons and carbon monoxide.

Exhaust Gas recirculation (EGR): This system recirculates a small amount of exhaust gasses, under certain conditions, to allow for more a reduction of oxides of nitrogen. When these gasses are recirculated into the intake system, the effect is to cool the combustion process. Overheated combustion results in an increase in oxides of nitrogen, so by cooling the process, these gasses can be effectively reduced. The function of the EGR system is now controlled by the car's computer, though in its original state, it operated mechanically based on intake manifold vacuum conditions (see intake system).

Evaporative Emissions Control: This system is designed to contain fuel vapors that occur in the fuel tank that would otherwise enter the atmosphere. These fuel vapors are vented into a charcoal canister, and are introduced into the intake system under certain driving conditions to be burned as fuel. This function is now controlled by the computer as well.

Positive Crankcase Ventilation (PCV): This system vents fuel vapors from the crankcase into the intake system to be burned as fuel, in a manner similar to that of the evaporative emissions system. Fuel vapors enter the crankcase in small amounts during the compression stroke (see engine), as the fuel-air mixture is compressed. This is due to the imperfect seal between the piston and the cylinder walls. These unburned vapors, which had previously been vented to the atmosphere, are drawn into the intake system to be burned.

Thermactor System: This system, though not used to any extent today, serves to reduce carbon monoxide and hydrocarbon emissions by injecting fresh air into the hot exhaust gasses as they leave the engine. The fresh air causes oxidation of the hydrocarbons and carbon monoxide causing them to change chemically into the more environmentally benign carbon dioxide and water. This function has been less useful today due to the use of computer controls and three-way catalytic converters.

Inlet Air Temperature Control: This device introduces warm air into the intake system during warm-up, resulting in the engine warming up to operating temperature faster. Colder engines do not operate as efficiently as warmer ones, and require a richer fuel-air ratio to operate properly. The use of preheated air allows not only for faster warm-up, but also permits a leaner fuel-air mixture to be used during this time period. The air is preheated by passing across the exhaust manifold, which heats up very quickly after starting the engine.

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