Emissions System

The purpose of the exhaust system is to control the emissions and exhaust produced by the engine. The idea is to turn the harmful pollutants your car produces into harmless ones that don't ruin the environment. These pollutants include hydrocarbons (unburned), carbon monoxide, carbon dioxide, nitrogen oxides, sulfur dioxide, phosphorus, lead and other metals. Although emissions control systems vary between manufacturers and vehicles, they all have the same goal and use many of the same methods.

Emissions Systems Terms

Exhaust Manifold and Header

The exhaust manifold, usually constructed of cast iron, is a pipe that conducts the exhaust gases from the combustion chambers to the exhaust pipe. It has smooth curves in it for improving the flow of exhaust.
A header is a different type of manifold; it is made of separate equal-length tubes.

Exhaust Pipe

The exhaust pipe is the bent-up or convoluted pipes that connect the entire exhaust system together. Some are shaped to go over the rear axle, allowing the rear axle to move up and down without bumping into the exhaust pipe; some are shaped to bend around under the floor of the car, connecting the catalytic converter with the muffler. Exhaust pipes are usually made out of stainless steel, since the high heat conditions involved with the muffler system will cause rust.

Exhaust Pipe Hangers

Hangers hold the exhaust system in place. They give the system flexibility and reduce the noise level. The hanger system consists of rubber rings, tubes and clamps.

The EGR Valve

The Exhaust Gas Recirculation (EGR) valve is used to send some of the exhaust gas back into the cylinders to reduce combustion temperature.

PCV Valve

The process of combustion forms several gases and vapors; many of them quite corrosive. Some of these gases get past the piston rings and into the crankcase. If left in the crankcase, these substances would cause all kinds of bad things (rust, corrosion, and formation of sludge), so they have to be removed. Back in the old days, they used to be dumped out into the atmosphere through a tube. Once we realized what a problem pollution was in the sixties, the PCV (Positive Crankcase Ventilation) system was developed to take the place of the old "dump tube."

The PCV system uses a hose connected between the engine and the intake manifold to draw these gases out of the engine's crankcase and back into the cylinders to burn with the regular fuel.

The Air Pump

The air pump sends (or pumps) compressed air into the exhaust manifold and in some cases to the catalytic converter. The oxygen in the pressurized air helps to burn quite a bit of any unburned hydrocarbons (fuel) and therby converts the poisonous carbon monoxide into good old carbon dioxide.

The Catalytic Converter

To further help reduce harmful emissions, modern cars (those built after 1977) have a catalytic converter in the exhaust system. The catalytic converter is installed in the exhaust line, between the exhaust manifold and the muffler. Basically, the harmful gases enter the catalytic converter, a kind of stainless steel container, which is lined with chemicals (catalysts - which are chemicals that cause a reaction between other chemicals without being affected itself) such as aluminum oxide, platinum and palladium. These chemicals cause the carbon monoxide and hydrocarbons to change into water vapor and carbon dioxide. Some converters have a third lining of chemicals, platinum and rhodium, that reduce nitrogen oxides (three-way, dual-bed converter). Therefore, the pollutants are changed from harmful gases to harmless ones before they are let into the environment through the muffler and tail pipe.

Mufflers

Exhaust gases leave the engine under extremely high pressure. If these gases escaped directly from the engine the noise would be tremendous. For this reason, the exhaust manifold sends the gases to a muffler. The muffler quiets the noise of the exhaust by "muffling" the sound waves created by the opening and closing of the exhaust valves. Since a muffler cannot reduce the noise of the engine by itself, some exhaust systems also have a resonator. Resonators are like little mufflers. They are added at the end of the exhaust system to take care of any noise that has made it through the muffler.

The Tailpipe

The tailpipe is a long metal tube attached to the muffler. It sticks out from under the body of your car, generally at the rear, in order to discharge the exhaust gases from the muffler of your engine into the air outside the car.