Tuned exhaust

A tuned exhaust system is an exhaust system for an internal combustion engine which improves its efficiency by using precise geometry to reflect the pressure waves from the exhaust valve or port back to the valve or port at a particular time in the cycle.

Tuned exhaust systems are common on motor racing engines, light aircraft engines,model engines and two-stroke motorcycle engines, and are not restricted to these applications. They are of two main types:

In either case, the efficiency of the system is greatest at a particular engine speed, which is determined by the exhaust system geometry. Tuning the system for best effect is typically done both by calculation during the design of the exhaust system, and by trial and error during engine development.

An important recent development is variable geometry exhaust systems, both for two-stroke and four-stroke engines, which allow the tuning to be effective over a wider range of engine speeds.

In many two-stroke engines, the exhaust port is opened and closed directly by the position of the piston rather than by a separate valve, which restricts the timing of its operation; Typically, the port remains open long after is optimum, allowing some of the incoming charge to escape. This can be partly addressed by use of a tuned exhaust system to deliver a pulse of positive pressure prior to the port closing, to retain the charge.

Direct-injection two-stroke diesel engines tend to use exhaust valves actuated either by camshafts or electronic control, rather than exhaust ports. This system is called uniflow scavenging. Opposed piston engines are inherently uniflow-scavenged, but these do use piston-controlled cylinder ports. Two-stroke opposed piston engines such as the Napier Deltic and Junkers Jumo 204 engines use one piston to control the inlet port and the other the exhaust, allowing more flexibility in timing. A variation of this approach is taken by the split-single engine, in which two cylinders share one combustion chamber, with the piston in one cylinder controlling the transfer port and the other the exhaust port.

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