Vehicle Emissions Control Systems

Below are general descriptions of emissions control systems that are inspected during an emissions inspection. Not all vehicles will have all of these systems installed on them. The inspection process will only check for systems that were installed on the vehicle by the vehicle manufacturer. Different vehicle manufacturers may use slightly different versions/configurations of these systems. For more detailed specifications for your vehicle, visit a local dealership or repair shop.

Air Injection System

The air injection system is designed to introduce outside air into the exhaust stream to assist in burning the gases produced by the engine. Air is injected into the exhaust system at one of several locations including the cylinder head, exhaust manifold, or directly into the catalytic converter. Outside air is drawn by an air pump (sometimes referred to as a smog pump), a pulse air valve, or a reed valve. The air pump is run by a fan/accessory belt on the front of the engine or, in some cases, a small electric motor. The pulse air and reed valves work by sensing the drop in manifold vacuum produced by an exhaust valve closing. Each time the drop in vacuum is noticed, a 'puff' of fresh air is pulled into the exhaust stream.

Some related parts of an air injection system can include, but are not limited to: air pump, check valve(s), gulp valve, diverter valve, air supply tube, and/or an air manifold.

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Exhaust Gas Recirculation System

An exhaust gas recirculation system is designed to reduce nitric oxide emissions from an engine. Nitric oxide is produced when the temperature of the combustion chamber rises above 2,500 degrees Fahrenheit. At these high temperatures, the nitrogen (78% of outside air) and the oxygen (21% of outside air) chemically combine to create nitric oxide, a major contributor to ground level ozone. The name of the system describes its operation. During times of slight acceleration or constant load (the dynamometer simulates this load), a small amount of exhaust gas is recirculated back into the combustion chamber of the engine. This helps cool the temperature of the combustion chamber which in turn lowers the amount of nitric oxide produced. High temperatures in the combustion chamber can also produce a knocking or pinging sound in the engine, indication of a problem which can, over time, damage the engine.

Some related parts of an exhaust gas recirculation system can include, but are not limited to: the EGR valve, EGR solenoid, vacuum line(s), and intake manifold (EGR passages within).

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Positive Crankcase Ventilation System

The positive crankcase ventilation system keeps blow-by gases from escaping from the engine. During combustion, a portion of the burned air/fuel mixture is forced past the piston rings and ends up in the crankcase and oil pan. These gases are a raw and untreated pollutant. The PCV system is designed to draw these untreated gases from the crankcase and reintroduce them into the intake stream. Once they enter the intake stream, they can be burned off by the combustion process and other emissions control components (such as an air injection system and/or a catalytic converter). Blow-by gases that are not evacuated from the crankcase and oil pan can contribute to engine corrosion, oil dilution, and engine deposits or sludge. Since the system must be sealed to operate properly, oil fill caps and dipsticks need to be in place and properly seated or tightened.

Some related parts of a positive crankcase ventilation system can include, but are not limited to: PCV valve, vacuum hose(s), sealed oil cap and dipstick, and fresh air intake tube.

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Catalytic Converter

The catalytic converter (or catalyst) is probably the simplest and most effective emissions control system on a vehicle. As exhaust gases pass through the honeycomb or pellets contained inside the shell, a chemical reaction takes place. The platinum and/or palladium that coats the honeycomb or pellets speeds the change of hydrocarbons (unburned fuel) and carbon monoxide (partially burned fuel) into water and carbon dioxide. Some catalytic converters also contain the metal rhodium. These catalysts are referred to as 'three-way catalysts.' The rhodium's purpose is to reduce amounts of nitric oxide. In many cases, an air injection system may be routed directly to the catalytic converter. The extra oxygen introduced by the air injection system helps the catalyst perform better under certain conditions.

An air/fuel mixture that is overly rich (too much fuel, not enough air), can destroy a catalytic converter over time, as can any lead in the fuel. This can create extremely high temperatures in the catalyst and can ultimately damage and destroy the honeycomb or pellets inside the catalyst. In extreme cases, the outside shell of the catalyst actually glows orange or red from the extreme heat inside, and the contents of the catalytic converter can actually melt and clog the exhaust system creating an undriveable vehicle.

Many people think that they can simply replace the catalytic converter and pass the emissions inspection. If the problem is the catalytic converter, of course this would be the right solution. But, in many cases, a new catalytic converter will simply 'mask' an underlying problem with the air/fuel ratio, ignition system, or some other problem with the engine. A catalytic converter should only be replaced if it is operating inefficiently or not at all. There are tests that can be performed on a catalytic converter to determine whether or not it needs to be replaced.

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Fuel Evaporative System

About 20% of hydrocarbon emissions emitted by a vehicle can be caused by the evaporation of fuel. The fuel evaporative system is designed to hold the hydrocarbon vapors and introduce them into the engine to be burned. A vapor line running from the fuel tank and carburetor (if equipped) transports fuel vapors to an evaporative canister (or charcoal canister). The hydrocarbon molecules attach themselves to the charcoal contained in the canister. When the engine is started (or shortly thereafter), the vapors are drawn from the canister and pulled into the engine to be burned along with the air/fuel mixture. Gas caps are also part of the fuel evaporative system. They prevent pressurized vapors from exiting the fuel tank through the filler neck.

Some related parts of a fuel evaporative system can include, but are not limited to: a charcoal canister, vapor hose, purge hose, purge solenoid, gas cap, and vapor-liquid separator.

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Thermostatic Air Cleaner System

This system regulates air temperature flow into the engine. The thermostatic air cleaner system is designed to draw heated air from around the exhaust manifold of the engine during a cold engine 'start-up' and as the engine warms up to normal operating temperature, a valve changes position to let cool air enter the engine.

Some related parts of a thermostatic air cleaner system can include, but not limited to: exhaust manifold heat shroud, hot air pipe (tube), air door (valve) assembly, vacuum diaphragm and hoses, and a temperature sensor.

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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000


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