The exhaust uses the pressure waves created by the gasses exiting from the combustion chamber into the exhaust port. These waves have the caratteristics of reflecting as a negative wave (suction) if they meet an open end of a tube. On the other way if they encounter an closed end of a tube they reflect as a positive wave (stuffing). The waves move indipendently from the movement of the gas in which they travel. In comparsion to the gas flowing in the exhaust, they have a much higher speed. Similar to waves on the water; the water is still but the waves travel with an certain speed.
The main parts of an exhaust are; the header, the diffuser, the belly, the baffle cone, the tailpipe and the silencer. The diffuser is the “open end” of the pipe; it creates the suction wave. The baffle cone is the “closed end” of the pipe; it reflects the pressure wave, which does supercharge the cylinder. The steepness of the cones defines the strenght and duration of the reflected waves. The header and the belly section act as distancers, this way the needed timing of the waves can be tuned.
The next important thing is the tailpipe. The purpose of the tail pipe is to create and maintain the proper pressure and temperature inside the exhaust. It is very important to select the right diameter of the tailpipe; to big will result in poor performance, to small will result in a melted piston.
The silencer, as his name says, is needed to silence the sound. The most common type of silencer used has a perforated tube in the middle with glasswool around. The good side of this kind of silencer is that it efficiently reduces the sound level while creating little obstruction for the escaping gasses.
(Note; arrows show the gasses and their direction, the arcs show the pressure waves)
After the ignition the gasses in the combustion chamber begin to expand and move the piston towards the BDC.
The piston uncovers the exhaust port. The burnt gasses begin to escape from the combustion chamber at a high speed, this generates a pressure wave, which begin to travel into the header.
At this point the phase of scavenging in the cylinder with fresh mixture begins. The pressure wave reaches the diffuser where it reflects as a suction wave.
The suction wave has reached the cylinder where it helps to pull out spent gasses and fill the chamber with fresh mixture. A part of the fresh mixture can escape into the exhaust. At this time the rest of the pressure wave that was not reflected as a suction wave reaches the baffle cone, where it reflects as a pressure wave
At this point the piston has closed the transfers, but the exhaust port is still open. The pressure wave reflected by the baffle cone reaches the cylinder and stuffs back the fresh mixture escaped into the exhaust. This is the way that a slight supercharge is created in twostroke engines.
Unluckyly the twostroke expansion chamber works only in a limited rpm range. The engine produces the maximum torque when the exhaust pulses enter in resonance with the opening of the cylinder ports. When the rpm are under the resonance of the exhaust, the suction wave reaches the cylinder too early, the transfers aren’t open so the suction wave can’t help suck out spent gasses and fill fresh mixture into the cylinder. On the other end when the rpm are too high the transfers are already closed by the piston moving upwards, so the suction wave is too late.
The rpm at which the pipe will be in resonance depend on the length of it and the temperature of the gasses. With a temperature change also changes the speed at which the waves travel. The higher the temperature, the higher is the speed at which the waves diffund. The higher is the wave speed, the higher are the rpm at which the engine will make power. We can see sometimes at races when the climate is cold, pipes are stripped with insulation that provide the optimal temperature.
The geometry of the pipe dictates the “character”. The length dictates the rpm at which the pipe enters in resonance. In a short pipe, in comparsion to a long one, the waves have to travel a shorter distance, this means that a short pipe will enter in resonance with the piston movement at a higher crank rotation speed. In short words; a long pipe will make power at low rpm while an short one will make power at high rpm.
The conicity of the diffuser and baffle cone determinate the length and amplitude of single waves reflected back to the cylinder. The higher the cone angle is, the shorter but with an greater amplitude the reflected wave will be. So an pipe with higher angle of the cones will make higher power, but in a narrower rpm range. Compared to a pipe with lower cone angles, an bike with such pipe will be more “nervous” to ride, the transmittion will be harder to tune up. It is in base to the type of engine and riding that the pipe has to be designed for. On a scooter with an engine with automatic variable transmission, the rpm range needed is narrower than the one for a bike with manualshift transmission, so pipes with higher cone angles can be used.
This graph shows the difference of shape of the power band on pipes with different angle of the cones.
Internal silencers are often used on pipes with an homologation for road use. On bikes with speed limited by law this means that they won't bring almost any increase in performance. Important is also the reduction of sound. Internal silencers disrupt in some extent the function of pressure waves.
Expansion chambers on direct injected two-stroke engines
There is a myth going around, that expansion chambers can't increase the performance of 2-stroke DI engines. But it's a fact that changing the stock pipe with an expansion chamber on today DI scooters will only slightly or not at all increase the power. The problem isn't the pipe; the pipe does it's job as usual, the problem is in the injection system. Today's DI scooter engines haven't the diagnosis system. Changing the stock pipe with an expansion chamber will change the ammount of air trapped into the combustion chamber. The injection system won't know that, so it will inject the quantity of fuel appropriate for the stock exhaust. This is a problem, because with an increased ammount of air into the combustion chamber, we can have a lean condition, and this is dangerous to the engine.