Emission control systems

When viewed as a whole, emission control systems can be extremely confusing. However, it is possible to ease some of the confusion by dividing the overall emissions system into several easily understood smaller systems.

There are five popular systems used to reduce emissions: the crankcase ventilation system, the evaporative emission control system, the Exhaust Gas Recirculation (EGR) system, the air injection system and the catalytic converter system. In addition to these emission systems, some vehicles incorporate an electronically controlled fuel system (feedback system) which further reduces emissions.

Note: Not all vehicles are equipped with these emission systems.

Crankcase ventilation systems
See Figures 38 and 39

Since the early 1960s, all cars have been equipped with crankcase ventilation systems.

When the engine is running, a small portion of the gases which are formed in the combustion chamber leak past the piston rings and enter the crankcase. Since these gases are under pressure, they tend to escape from the crankcase and enter the atmosphere. If these gases are allowed to remain in the crankcase for any length of time, they contaminate the engine oil and cause sludge to build up in the crankcase. If the gases are allowed to escape to the atmosphere, they pollute the air with unburned hydrocarbons. The job of the crankcase ventilation system is to recycle these gases back into the engine combustion chamber where they are re-burned.

The crankcase (blow-by) gases are recycled as the engine is running by drawing clean filtered air through the air filter and into the crankcase. As the air passes through the crankcase, it picks up the combustion gases and carries them out of the crankcase, through the oil separator, through the PCV valve or orifice, and into the induction system. As they enter the intake manifold, they are drawn into the combustion chamber where they are re-burned.

The most critical component in the system is the PCV valve that controls the amount of gases that are recycled. At low engine speeds, the valve is partially closed, limiting the flow of gases. As engine speed increases, the valve opens to admit greater quantities of air to the intake manifold. Some systems do not use a PCV valve. They simply use a restrictor or orifice in the ventilation hose to meter the crankcase gases.

If the PCV valve/orifice becomes blocked or plugged, the gases cannot be vented from the crankcase. Since they are under pressure, they will find their own way out of the crankcase. This alternate route is usually a weak oil seal or gasket in the engine. As the gas escapes by the gasket, it usually creates an oil leak. Besides causing oil leaks, a clogged PCV valve also allows these gases to remain in the crankcase for an extended period, promoting the formation of sludge in the engine.

Evaporative emission control system
See Figures 40 and 41

The evaporative emission control system is designed to prevent fuel tank and carburetor bowl (if equipped) vapors from being emitted into the atmosphere. Fuel vapors are absorbed and stored by a fuel vapor charcoal canister. The canister stores them until certain engine conditions are met and the vapors can be purged and burned by the engine.

The charcoal canister purge cycle is controlled different ways: either by a thermostatic vacuum switch, a solenoid or by a timed vacuum source. The thermostatic switch is installed in the coolant passage and prevents canister purge when the engine is below a certain temperature. The solenoid is usually controlled by a computer and is used on feedback controlled fuel systems. The computer determines when canister purge is appropriate. Depending on the system, this can be engine operating temperature, engine speed, evaporative system pressure or any combination of these. The timed vacuum source uses a manifold vacuum controlled diaphragm to control canister purge. When the engine is running, full manifold vacuum is applied to the top tube of the purge valve which lifts the valve diaphragm and opens the valve.

A vent located in the fuel tank, allows fuel vapors to flow to the charcoal canister. A tank pressure control valve, used on some high altitude applications, prevents canister purge when the engine is not running. The fuel tank cap does not normally vent to the atmosphere, but is designed to provide both vacuum and pressure relief.

Air injection system
See Figure 42

Introducing a controlled amount of air into the exhaust stream promotes further oxidation of the gases. This in turn reduces the amount of carbon monoxide and hydrocarbons. The carbon monoxide and hydrocarbons are converted to carbon dioxide and water, the harmless by-products of combustion. Some systems use an air pump, while other use negative exhaust pulses to draw air (pulse air).

The air pump, usually driven by a belt, simply pumps air under a pressure of only a few pounds into each exhaust port. Between the nozzles and the pump is a check valve to keep the hot exhaust gases from flowing back into the pump and hoses thereby destroying them. Most pumps also utilize a gulp valve or a diverter valve. Early systems used a gulp valve, while later systems use diverter valves. They both operate on the same principle. During deceleration, as the throttle is closed, the fuel mixture tends to get too rich. If the air continued to be pumped during deceleration, an explosion in the exhaust system could occur that could blow the muffler apart. During deceleration, the air is either diverted into the atmosphere or into the intake system.

On pulse air systems, clean air (from the air cleaner) is drawn through a silencer, the check valve(s) and then into the exhaust ports. The negative exhaust pulses opens the reed valve in the check valve assembly, allowing air to flow into the exhaust port.

Some feedback controlled vehicles utilize an oxidizing catalytic converter. Under certain operating conditions, the air is diverted into the catalytic converter to help oxidize the exhaust gases.

Exhaust gas recirculation (EGR) systems
See Figures 43, 44 and 45

The EGR system's purpose is to control oxides of nitrogen (NOx) which are formed during the combustion process. NOx emissions at low combustion temperatures are not severe, but when the combustion temperatures go over 2500°F, the production of NOx in the combustion chambers shoots way up. The end products of combustion are relatively inert gases derived from the exhaust gases. These are redirected (under certain conditions) through the EGR valve and back into the combustion chamber. These inert gases displace a certain amount of oxygen in the chamber. Since not as much oxygen is present, the explosion is not as hot. This helps lower peak combustion temperatures.

The EGR valve can either be actuated by a vacuum diaphragm, a solenoid or a stepper motor. On feedback controlled vehicles, the EGR system is controlled by the computer.

Catalytic converter
See Figure 46

The catalytic converter is a muffler-like container built into the exhaust system to aid in the reduction of exhaust emissions. The catalyst element is coated with a noble metal such as platinum, palladium, rhodium or a combination of them. When the exhaust gases come into contact with the catalyst, a chemical reaction occurs which reduces the pollutants into harmless substances such as water and carbon dioxide. Oxidizing catalysts require the addition of oxygen to spur the catalyst into reducing the engine's HC and CO emissions into H2 O and CO2.

While catalytic converters are built in a variety of shapes and sizes, they all fall into two general types, the pellet, or bead type and the monolithic type. Construction may differ slightly, but the object is the same - to present the largest possible surface area to passing exhaust gases. Older vehicles use bead/pellet type converters. The exhaust gas must pass through a bed of these pellets. This type of converter is rather restrictive. The cross-section of a monolithic type converter resembles a honeycomb. The exhaust gases are exposed to a greater amount of surface area in these converters, as a result they are more efficient. They also tend to be less restrictive.

Catalytic converter precautions

1. Use only unleaded fuel.
2. Avoid prolonged idling; the engine should run no longer than 20 min. at curb idle and no longer than 10 min. at fast idle.
3. Don't disconnect any of the spark plug leads while the engine is running. If any engine testing procedure requires disconnecting or bypassing a control component, perform the procedure as quickly as possible. A misfiring engine can overheat the catalyst and damage the oxygen sensor.
4. Make engine compression checks as quickly as possible.
5. Whenever under the vehicle or around the catalytic converter, remember that it has a very high outside or skin temperature. During operation, the catalyst must reach very high temperatures to work efficiently. Be very wary of burns, even after the engine has been shut off for a while. Additionally, because of the heat, never park the vehicle on or over flammable materials, particularly dry grass or leaves. Inspect the heat shields frequently and correct any bends or damage.
6. In the unlikely event that the catalyst must be replaced, DO NOT dispose of the old one where anything containing grease, gas or oil can come in contact with it. The catalytic reaction may occur with these substances, which can start a fire.

Continue to page 6 of Fuel systems

To top of page

Return to iCARumba Car Care Encyclopedia index