Exhaust System Basic Components
The exhaust system is used to remove the burned air/fuel mixture, or exhaust, from the engine, away from the passenger compartment and out the back of the vehicle. The major components of the exhaust system are the exhaust manifold(s), exhaust pipes, catalytic converter(s), muffler and oxygen sensors. The AIR (Air Injection Reaction) system and EGR (Exhaust Gas Recirculation) system are part of the exhaust and emission control systems. The exhaust manifold(s) provide a pathway for exhaust gases from the engine’s cylinder head exhaust ports. The exhaust pipes convey the exhaust to the rear of the vehicle. There are single exhaust systems that have one pipe and dual exhaust systems that have two. V6 and V8 engines have dual exhaust or can have single exhaust by utilizing a crossover pipe that connects the exhaust from both banks and channels it into one pipe. Part way down the pipes is at least one muffler that reduces engine exhaust noise. Some systems also have a smaller muffler called a resonator. The last section of exhaust pipe is called the tailpipe.
Exhaust System Emissions Components
All OBD II vehicles (1996- up) utilize emission control devices called catalytic converters, which thermo-chemically react with exhaust gases to reduce emissions, particularly Carbon Monoxide (CO), Hydrocarbons (HC) and Oxides of Nitrogen (NOx). As part of the engine control system, one or more oxygen sensors are placed into the head pipe or exhaust manifold, before the catalytic converter, to detect the amount of oxygen in the exhaust. Another oxygen sensor is placed after the converter to measure the effectiveness of the converter. Some vehicles require additional oxygen to improve emission system efficiency, particularly when the engine is cold. In these situations an AIR injection system may be employed to provide more ambient air into the exhaust stream when it is needed. NOx, a pollutant that helps cause smog, is the result of high combustion temperatures. NOx emissions are controlled by the use of an EGR valve, PCM/ECM software and the addition of a NOx bed within the catalytic converter. EGR system restriction or failure is probably the most common cause of NOx related emissions. The EGR valve is used to introduce a controlled amount of exhaust gas back into the combustion process. The exhaust gas, which contains very little, if any oxygen (because it should have all been consumed during the combustion process), takes up space that would otherwise be occupied by the regular air/fuel charge, which contains oxygen. Replacing the oxygen with the exhaust gas slows and cools the combustion burn. EGR is almost exclusively utilized during light loads such as light throttle cruise.