Exhaust System Design

Exhaust system design is primarily about managing the gases ejected from an engine as waste products are transported away from the engine using an exhaust system. The main features that must be considered are:

  • Acoustic design – Acoustic Ducts, Acoustic Filter Elements (eg Silencer), Flow Noise, Shell radiated noise
  • Construction and manufacturing process
  • Durability
  • Dynamic tune
  • Emissions control
  • Exhaust system backpressure
  • Orifice position
  • Packaging
  • System mounting and structural coupling
  • Thermal stress
  • Turbo/super charging

The layout shape of the exhaust system will be determined by the position of the exhaust outlet on the cylinder head, the obstacles the exhaust has to negotiate on its way around the vehicle, the position of silencers and the final position of the orifice itself.

At this stage the first acoustic mode of the system has been set by the overall length of the pipe (first mode of a pipe closed at one end valve face and open at the other exhaust tailpipe orifice), although there will be some changes depending on silencer internals, exhaust gas temperature along the length of the system (affecting the basic speed of sound in the gas), exhaust pipe diameters (hence gas flow rate) and a number of other minor effects.

If we look at the exhaust as a system then we have: a collection of pipes, silencers, sensors and catalysts or filters that make a system to channel the waste exhaust gases away from an internal combustion engine.

  • The exhaust valve opens into a port in the cylinder head that transports exhaust gas to the exhaust manifold.
  • The manifold normally consists of a set of primaries, one for each cylinder.
  • Primaries collect together into secondary pipes and then into the main exhaust pipe.
  • At this point a catalyst or filter is used to clean up the exhaust gas.
  • Exhaust silencers are used to attenuate the acoustic noise.
  • Further ducts allow the exhaust gas to be transported to an exit point, the exhaust tailpipe.

There are different forms of exhaust system depending on the type of engine and the application.

The primary function of the exhaust system design is to channel the waste exhaust gases from the exhaust valve to a suitable point. These waste gases are the products of combustion which are discharged from the cylinder after doing work on the piston. The exhaust from internal combustion engines contains CO2, CO, NOx, SO2 and unburnt hydrocarbons along with some soot. All these have bad effects.

There is no way to avoid CO2 emissions but the other emissions can be reduced by various methods:

  • fuel to air ratio control
  • secondary air injection into the exhaust manifold
  • exhaust catalyst
  • water injection
  • particulate filters
  • urea injection into the exhaust

CO2 (Carbon Dioxide) – At stoichiometric Air/Fuel ratio CO2 is at a maximum of 13%. When the Air/Fuel ratio is high then CO2 is lowest at around 4%. CO2 and CO (Carbon Monoxide) in the exhaust gas vary opposite to each other.

Carbon Monoxide – At stoichiometric Air/Fuel ratio CO is low and around 2%. When the Air/Fuel ratio is high then CO can be as high as 16%. In the case of car engines CO is high during deceleration events.

NOx (Nitrogen Oxides) – is high during acceleration and during cruising.



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