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Rear silencer of a car
Part of an exhaust system with two rear silencers
Rear silencer of a motorcycle
Exhaust on a Laurin & Klement motorcycle (1898), exhibited in the Škoda Muzeum

The exhaust system of an internal combustion engine in a motor vehicle is referred to as an exhaust (Pl. Exhausts) .


Components of a car exhaust system (petrol engine)

The components of a common passenger car with a gasoline engine are in the direction of the exhaust gas flow:

With smaller engines like mopeds, the manifold leads directly into the rear silencer.

Exhaust manifold

The exhaust manifold is connected directly to the cylinder head . The seal is made using embossed metal seals or coated fabric seals. The manifold is made of gray cast iron or stainless steel. The shape varies between a simple manifold and complicated shapes that combine the outlet channels in different ways. When and which exhaust ports are merged depends on the number of cylinders, the firing order and the design of the engine. On turbocharged engines, for example, the exhaust lines are combined in front of the turbocharger . When designing manifolds , the exhaust pipes are designed to be as long as possible until they join. This form prevents a negative influence on the gas exchange . If the merging of the exhaust gases after the manifold has not yet been completed, a Y-pipe follows the exhaust manifold. This component is not required for the simple variants. In order to meet the strict exhaust gas standards and further reduce fuel consumption, newer engines have exhaust gas recirculation . The valve for this is located on the exhaust manifold and connects it to the intake system.


The Y-pipe is rigidly screwed to the exhaust manifold. The sealing takes place as on the manifold. The name Y-pipe is derived from the mostly double-flow construction, similar to trousers . Like a pair of pants, the entrances end in a single exit. The lambda probe is often found at the end of the downpipe. The temperatures required for proper function are reached here early on. The transition to the next component, the catalytic converter, takes place via a flexible connection, especially in newer vehicles. This can be a flexible metal hose or a cone or ball seal. This connection decouples the vibrations and movements of the engine caused by load changes from the exhaust system attached to the underbody. In older vehicles, there is usually a rigid, inflexible screw connection between the Y-pipe and the catalytic converter. However, in order to still be able to absorb vibrations, the Y-pipe is also resiliently mounted.


The vehicle catalytic converter is the component that cleans the exhaust gases in conjunction with the lambda control . The component known colloquially as Kat is barrel-shaped and has a funnel-shaped entrance and exit. Since it can reach a very high temperature, it is generally surrounded by a heat shield . Inside there is a porous block made of ceramic or metal foil with small channels running parallel to the longitudinal axis. The inner surface is coated with the precious metals platinum , rhodium and palladium and causes chemical reactions in the exhaust gas , in which nitrogen oxides and hydrocarbons are broken down into nitrogen, carbon dioxide and water.

Soot filter

Particulate filters are being used more and more frequently in diesel engines . The particle filter is installed in front of the exhaust system. The structure, with an inner monolith, resembles a normal catalytic converter. The soot with the adhering polycyclic aromatics is collected in the filter and burned off at regular intervals by adding additives to the fuel or heating the filter above the ignition temperature.

Front and middle silencer

Before catalytic converter technology found its way into motor vehicles, the central or front silencer was installed instead of the catalytic converter. Middle silencers absorb the sound through the use of insulating material . The structure is similar to a large tin can. The exhaust pipe goes straight through the lid of the bin. The inside of the tube is perforated and changes its cross-section. The space between the pipe and the outer wall is filled with steel wool, basalt fibers or, in older vehicles, with asbestos-containing fibers. The exhaust gas flow is widened and slowed down through the perforated pipe and the vibrations are weakened. The main function of front silencers is the absorption (neutralization) of high-frequency sound waves.

Two-stroke engines in particular, like the Trabant , also have a front silencer in addition to the center silencer. This has no sound insulation , but expands the exhaust gas flow and reflects the sound waves so that they mutually reinforce each other through interference . A suction is created that favors the outflow of combustion gases from the combustion chamber.

Rear silencer

The rear silencer combines the construction of the front silencer ( reflection silencer) and the middle silencer ( absorption silencer ), although this is not the norm. Rear silencers are also often produced in a pure reflection or absorption design. Absorption silencers are very often used in the form of sports silencers, which, due to their lower dynamic pressure and more favorable flow properties, can help to improve sound and increase performance. Its external shape varies between the different vehicles in order to adapt to their underbody. Inside, the reflection damper is divided into several chambers that are insulated from the outside. The exhaust gas is thus forced to split into several partial streams of different lengths, whereby the sound waves partially cancel each other out ( interference effect due to phase-shifted superposition). The result is high damping with low back pressure. Exhaust system developers can influence the background noise and engine performance through precisely defined cross-sections and shapes.


An important (emotional) factor for some buyers when buying a car is the sound of a vehicle, which is significantly influenced by the exhaust. For this reason, the automobile manufacturers have so far tailored the exhaust sound to the vehicle with great development effort. It is now also possible to change the sound with special loudspeakers in the exhaust: The exhaust can be considerably quieter, but sound components can also be mixed in. The first production vehicle to have such a special loudspeaker in the exhaust is the Audi A6 Avant .


The components of the exhaust system are connected to the exhaust pipe. The tubes are plugged into one another at certain points between the components. They are fastened and sealed with a pipe clamp. For cost reasons, modern exhaust systems are installed as individual parts in new cars, i.e. the pipes between the components (catalytic converter, silencer) are firmly welded together. The pipes must be sawed off to replace individual components. The spare parts are fastened again with pipe clamps. In road vehicles, exhaust pipes are laid under the vehicle to behind the passenger cabin in order to prevent the exhaust gases from reaching the occupants. In trucks , the exhaust can therefore end between the axles .

In the case of larger engines, such as six, eight or twelve cylinders, the exhaust systems are usually designed with two pipes from front to rear (with two pipes). Either catalytic converters and rear silencers are then also available with two branches or twice.

The pipes and mufflers of the exhaust system are elastically attached to the underbody with rubber to counteract cracks in the system and the transmission of vibrations to the body . The underbody is covered with zinc or aluminum sheets to protect against heat in the area of ​​the exhaust.

In some trucks, buses , tractors and other special vehicles, the exhaust gases are directed upwards in order to avoid blowing on people or objects. In such cases, the exhaust ends often have a cap to keep rain out and protect components like catalytic converters and engines. In cars that are manufactured in countries with left-hand traffic (such as Japan ), it is often observed that the tailpipe faces the sidewalk.

If the tailpipes or silencers hang on the left and right on the sides, one speaks of sidepipes . According to the German Road Traffic Licensing Regulations (StVZO), these are only permitted under certain conditions:

  1. The sidepipes are only installed as dummy, i.e. not connected to the exhaust gas flow.
  2. When connecting to the exhaust system, the outlet opening on the right (passenger) side must be placed downwards so that people on the roadside are not bothered with exhaust gases.
  3. The original exhaust system must not be shortened or changed and the connection must be made after the last silencer.

In the event of changes to the exhaust system, approval by the technical inspection association (TÜV / Dekra) is required. In addition, a corresponding entry must be made in the vehicle documents.


Animation of the task of the silencer as a resonance exhaust in two-stroke engines

Two main tasks of the exhaust are to divert the combustion gases from the vehicle and to dampen the pressure surges that occur in the combustion chambers during explosive combustion in order to reduce noise emissions .

If you take a closer look at the processes involved in the operation of an internal combustion engine, the exhaust is very important. The exhaust has a significant influence on the available torque in the usable speed range of the drive unit. In motor sports , where performance and power delivery is important, which is particularly exploited.

The exhaust is passively involved in the gas exchange because its shape determines the vibrations of the exhaust gases that take place in it. These vibrations can support the outflow of combustion gases from the combustion chamber when the exhaust valve is open, but they can also counteract them. The suction can be so strong that even the inflow of fresh gases is supported when the outlet valve is open at the same time. By appropriately designing the exhaust system and coordinating other components such as control times, the filling of the combustion chambers can also be increased and consumption reduced by keeping the scavenging losses low. The gas flow is first supported by the negative part of the gas oscillation in order to push back the gases in the subsequent positive part before the valve is closed. This principle also forms the basis for being able to operate a two-stroke engine at all, since the gas exchange is mainly determined by fluidic processes. There are no valves and the piston cannot, or only very little, support the outflow of the burnt gases due to its stroke movement.

There are a large number of exhaust manufacturers, the best known include Hamann , Eberspächer , Faurecia , Remus-Sebring , Tenneco , Bosal and High Performance Motorsport .

Exhaust colors

As a rule, the exhaust gases emitted by vehicles are colorless or slightly gray-white. However, depending on the engine condition, the following color abnormalities may occur:

White smoke: If conspicuously white exhaust gases emerge from the exhaust, this is due to the very high humidity in the vehicle's combustion chamber. During the winter months, this color of smoke is often seen on a cold start. Even a small amount of condensation is enough to generate this white steam. As soon as the vehicle has reached its operating temperature, the exhaust gases come out again in the usual color.

Black smoke: this color of smoke indicates that the fuel cannot be completely burned. Some of the hydrocarbons contained therein are therefore emitted as soot.

Blue smoke: if blue smoke is emitted from the exhaust, oil has entered the combustion chamber. In this case the vehicle should be parked and a workshop should be visited to avoid further damage to the engine.

See also


  • Wilfried Staudt: Handbook Vehicle Technology Volume 1. Bildungsverlag EINS, Troisdorf 2005, ISBN 3-427-04520-X .
  • Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3. Edition. Westermann Schulbuchverlag, Braunschweig 2000, ISBN 3-14-221500-X .
  • Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition. Verlag Europa-Lehrmittel, Haan-Gruiten 2001, ISBN 3-8085-2067-1 .
  • Peter A. Wellers, Hermann Strobel, Erich Auch-Schwelk: Vehicle technology expertise. 5th edition. Holland + Josenhans Verlag, Stuttgart 1997, ISBN 3-7782-3520-6 .

Web links

Commons : Exhaust  - Collection of pictures, videos and audio files
Wiktionary: Exhaust  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Audi A6 Avant: loudspeakers in the muffler. on: spiegel.de , July 21, 2011
  2. Car guide: Correctly interpreting smoke signals from the exhaust pipe . In: Spiegel Online . January 12, 2011 ( spiegel.de [accessed November 27, 2018]).