Expansion chamber

On a two-stroke engine, an expansion chamber or tuned pipe is a tuned exhaust system used to enhance its power output by improving its volumetric efficiency.

Expansion chambers were invented and successfully manufactured by Limbach, a German engineer, in 1938, to economize fuel in two stroke engines. Germany was running short of petrol, which was at that stage produced using coal and sewerage transformation. An unexpected bonus was that the two stroke engines using tuned exhausts produced far more power than if running with a normal silencer. After the end of the second world war, some time passed before the concept was re-developed by East German Walter Kaaden during the Cold War. They first appeared in the west on Japanese motorcycles after East German motorcycle racer Ernst Degner defected to the west while racing for MZ in the 1961 Swedish Grand Prix. He later passed his knowledge to Japan's Suzuki.

The high pressure gas exiting the cylinder initially flows in the form of a "wavefront" as all disturbances in fluids do. The exhaust gas pushes its way into the pipe which is already occupied by gas from previous cycles, pushing that gas ahead and causing a wave front. Once the gas flow stops, the wave continues, passing the energy to the next gas down stream and so on to the end of the pipe. If this wave encounters any change in cross section or temperature it will reflect a portion of its strength in the opposite direction to its travel. For example, a strong acoustic wave encountering an increase in area will reflect a weaker acoustic wave in the opposite direction. A strong acoustic wave encountering a decrease in area will reflect a strong acoustic wave in the opposite direction. The basic principle is described in wave dynamics. An expansion chamber makes use of this phenomenon by varying its diameter (cross section) and length to cause these reflections to arrive back at the cylinder at the desired time in the cycle.

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