Car Parts and Accessories

A CAR exhaust system has two main functions. Firstly, it takes hot, waste gases from the engine to a point where they can be released into the atmosphere without danger to the occupants of the car. Secondly, it reduces the noise made when used gases are expelled by the engine. This is done by a muffler, or silencer.

The gases produced in an engine expand with great force and are released into the exhaust system under pressure. Each time the gases pass into the exhaust manifold, a shock wave is set up. With shock waves occurring at the rate of several thousand a minute, the noise from cars would be socially unacceptable if it were not moderated.

After travelling a short distance down the exhaust pipe, the shock waves, which are initially supersonic, slow down to below the speed of sound.

By the time exhaust gases leave the silencer, they have expanded sufficiently for their pressure to drop to about that of the outside air, and most of the noise has been curbed.

If exhaust gases are not cleared effectively, the incoming flow of petrol/air mixture into the combustion chamber will be impeded, and the mixture will be contaminated by the residual burnt gases. This will reduce the efficiency of the engine.

Exhaust manifolds are designed to avoid interference between the pulses of exhaust gas that leave each cylinder in turn. The object is to allow the gases to flow to the exhaust pipe as freely as possible.

Some back pressure is unavoidable in the exhaust system, due to the restrictive effect of the manifold, piping and silencer. The designer allows for this; his objective is to quieten the exhaust with minimum restriction to the flow of gases.

Silencers/Mufflers

A silencer breaks up or absorbs sound waves, reducing them to a noise level that is acceptable to the law.

The gas flow is usually slowed down by baffles, or metal plates, inside the silencer; these break up and impede the action of the sound waves.

A variation of this design is the perforated, ’straight-through’ silencer in which gases pass through holes into a layer of sound-absorbing material. This type of silencer is often fitted to sports cars to give higher engine power. The perforations do not impede gas flow as much as a system of baffles does.

Silencers and exhaust systems are usually manufactured of mild-steel tubing and sheet steel. Constant exposure to road grit, salt, slush and mud reduces the life of the average car’s exhaust system to little more than one or two years. But the use of aluminised steel or, better still, stainless steel, can give an exhaust system a much longer life.

Silencers and exhaust systems rust from the inside as well as from the outside. Every litre of petrol burnt produces a litre of water along with lead salts and acids, which pass as gas or vapour into the exhaust system.

If the silencer or pipe is cold, as in the first start of the day, these corrosive elements will condense on the interior surfaces of the exhaust system.

They act as weak acids, eventually eating through the metal. Every time a car starts from cold, a minute amount of internal corrosion takes place. This is why a car used only for short trips needs more frequent exhaust replacement than one used regularly for long journeys.

Turbocharging

Compressing the incoming charge of fuel and air increases the power available from an engine. Turbocharging uses the waste exhaust gases to power a turbine which rotates a compressor at very high speed. The compressor forces an increased amount of fuel/air into the engine’s combustion chambers, thus placing a greater strain on pistons, bearings and the drive train. The resulting increase in power shows that it is possible to obtain considerably more drive from an engine of small physical size without causing it to consume the same amount of fuel as a larger, naturally aspirated engine of the equivalent power output. Excessive engine loading is controlled on most modern turbocharged engines by sensors. These monitor engine temperature, the onset of knocking and other factors. Information is relayed to the system’s computer which adjusts timing or controls performance to prevent component damage. Excess turbine pressure operates a wastegate valve which releases exhaust gases directly into the exhaust pipe, bypassing the turbine.

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