Expansion chamber

An unexpected bonus was that the two stroke engines using tuned exhausts produced far more power than if running with a normal silencer.

[1][2] The high pressure gas exiting the cylinder initially flows in the form of a "wavefront" as all disturbances in fluids do.

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.

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.

When the descending piston first exposes the exhaust port on the cylinder wall, the exhaust flows out powerfully due to its pressure (without assistance from the expansion chamber) so the diameter/area over the length of the first portion of the pipe is constant or near constant with a divergence of 0 to 2 degrees which preserves wave energy.

By keeping the header pipe diameter near constant, the energy in the wave is preserved because there is no expansion needed until later in the cycle.

To help prevent the piston pushing fresh mixture out the open exhaust port the strong acoustic wave (produced by the combustion) from the expansion chamber is timed to arrive during the beginning of the compression stroke.

They arrive in time to block the exhaust port, still open during the beginning of the compression stroke and push back into the cylinder any fresh mixture drawn out into the header of the expansion chamber.

In a well designed tuned exhaust system, the total increase in pressure is in any case much less than the one produced by a muffler.

An erroneous sizing of the stinger will lead either to poor performance (too big or too short) or to excessive heat (too small or too long) which will damage the engine.

The hot gases leaving the port form a "slug" which fills the header pipe and remains there for the duration of that cycle.

This causes a high temperature zone in the head pipe which is always filled with the most recent and hottest gas.

Scooter exhaust with expansion chamber and silencer
Direct comparison between different types of exhausts for the two-stroke engine, on the left you can see the engine and its exhaust, in the center the progression curves of the pressures (effective pressure in atmospheres) to the exhaust port (detection area highlighted in red), on the right the power curves of the various drains.
A) Traditional discharge with constant section
B) Discharge with divergent section
C) Resonant expansion chamber with expansion chamber, in the power graph the influence of the exhaust back pressure valve is also highlighted