How Two-Stroke Expansion Chambers Work, and Why You Should Care.
You know that changing the exhaust pipes on your two-stroke motorcycle can have a marked effect on the engine's power characteristics, but do you know why?
Simply put, it's because the two-stroke exhaust system, commonly referred to as an 'expansion chamber' uses pressure waves emanating from the combustion chamber to effectively supercharge your cylinder.
In reality, expansion chambers are built to harness sound waves (created in the combustion process) to first suck the cylinder clean of spent gasses--and in the process, drawing fresh air/gas mixture (known as 'charge') into the chamber itself--and then stuff all the charge back into the cylinder, filling it to greater pressures than could be achieved by simply venting the exhaust port into the open atmosphere. This phenomenon was first discovered in the 1950s by Walter Kaaden, who was working at the East German company MZ. Kaaden understood that there was power in the sound waves coming from the exhaust system, and opened up a whole new field in two-stroke theory and tuning.
An engine's exhaust port can be thought of as a sound generator.
Each time the piston uncovers the exhaust port (which is cut into the side of the cylinder in two-strokes), the pulse of exhaust gases rushing out the port creates a positive pressure wave which radiates from the exhaust port. The sound will be be the same frequency as the engine is turning, that is, an engine turning at 8000 rpms generates an exhaust sound at 8000 rpms or 133 cycles a second--hence, an expansion chamber's total length is decided by the rpm the engine will reach, not displacement. Indeed, the only advantage to this crude pipe system was that it was easy to tune: You simply started with a long pipe and started cutting it off until the motor ran best at the engine speed you wanted. Of course those waves don't radiate in all directions since there's a pipe attached to the port. Early two strokes had straight pipes, a simple length of tube attached to the exhaust port. This created a single "negative" wave that helped suck spent exhaust gases out of the cylinder. And since sound waves that start at the end of the pipe travel to the other end at the speed of sound, there was only a small rpm range where the negative wave's return would reach the exhaust port at a useful time: At too low of an rpm, the wave would return too soon, bouncing back out the port. And at too high of an rpm, the piston would have traveled up the cylinder far enough to close the exhaust port, again doing no good.
Indeed, the only advantage to this crude pipe system was that it was easy to tune: You simply started with a long pipe and started cutting it off until the motor ran best at the engine speed you wanted.
So after analyzing this cut-off straight-pipe exhaust system, tuners realized two things: First, that pressure waves could be created to help pull spent gasses out of the cylinder, and second, that the speed of these waves is more or less constant, though it's affected slightly by the temperature of the air. Higher temperatures mean that the air molecules have more energy and move faster, so sound waves move faster when the air is warmer.
A complicating factor here is that changes in the shape of the tube cause reflections, or changes, in the sound waves: Where the section of the tube grows in diameter, there will be sound waves reflected back towards the start of the tube. These waves will be the opposite of the original waves that they reflected from, so they will also be negative pressure waves.
Aha! The next important discovery was made--by gradually increasing the diameter of the tube, a gradual, more useful negative wave could be generated to help scavenge, or pull spent gasses out of, the cylinder.
Adding Divergent Tubes, which used to be called "Megaphones," to Two-Stroke Pipes Helped Make Useful Power
Putting a divergent cone on the end of a straight pipe lengthens the returning wave, broadening the power band and creating a rudimentary expansion chamber.
So, to sum up, when the negative wave reaches the exhaust port at the correct time, it will pull some of the exhaust gases out the cylinder, helping the engine to scavenge its spent exhaust gas. And putting a divergent cone at the end of the straight (parallel) "head" pipe broadens the returning wave. The returning negative wave isn't as strong, but it is longer, so it is more likely to find the exhaust port open and be able to pull out the exhaust gases. As with plain, straight pipes, the total length of the pipe with a divergent cone welded on determines the timing of the return pulses and therefore the engine speed at which they are effective. The divergent cone's critical dimensions are where it starts (the distance from the exhaust port to the start of the divergent cone is called the "head" pipe), while the length of the megaphone and the rate at which it diverges from the straight pipe determine the intensity and length of the returning wave--A short pipe which diverges at a sharp angle from the head pipe gives a stronger, more straight-pipe-like pulse. Conversely, a long, gradual divergent cone creates a smaller pulse of longer duration.
In addition, the negative wave is also strong enough to help pull fresh mixture up through the transfer ports.
And while adding a divergent cone to the head pipe produced great tuning advantages, it had its limitations, too: The broader negative wave from a megaphone can still arrive too early and pull fresh mixture out of the cylinder. That's exactly the problem that Walter Kaaden had with the factory MZs. He realized that putting another cone, reversed to be convergent, on the end of the first divergent pipe would reflect positive waves back up the pipe. These positive waves would follow the negative waves back to the exhaust port, and if properly timed would stuff the fresh mixture that was pulled into the pipe back into the exhaust port right as the piston closed the port.
In addition to head pipe length, divergent and convergent cone lengths, an expansion chamber has three more crucial dimensions. The length of the straight 'belly' between the divergent and the convergent cones, the length of the tailpiece 'stinger', or muffler, and the diameter of the belly section. The stinger acts as a pressure bleed, allowing pressure to escape from the pipe. Back pressure in the pipe, caused by a smaller-diameter or longer stinger section, helps the wave action of the pipe, and can increase the engine's performance. This, presumably, happens since the greater pressure creates a more dense, uniform medium for the waves to act on--waves travel better through dense, consistent mediums. For instance, you can hear a train from a long way away by putting you ear to the steel railroad track, which is much denser and more uniform than air. But it also causes the engine to run hotter, usually a very bad characteristic in two-strokes.
Kaaden immediately realized a large power gain, and the expansion chamber was born.
The length of the belly section determines the relative timing between the negative and positive waves. The timing of the waves is determined by the length of the straight pipe. If the belly section is too short, positive waves have a shorter distance to travel, and return to the exhaust port sooner. This is good if the engine is running at a higher speed, bad if you want to ride on the street. The diameter of the belly section is crucial for one simple reason: ground clearance. It's hard to keep big, fat pipes off the ground, though V-Fours have solved that for now since two of the pipes exit directly out the back.
A complete two-stroke pipe has properly tuned header, convergent, belly, divergent and stinger sections--a difficult process.
As the forces in a two-stroke pipe design have become more well-understood, designers have been able to create engines that take more advantage of them and in fact require an expansion chamber to run at all. For instance, a modern pipe has a gently divergent head pipe to keep gas velocity high near the port, a second cone of "medium" divergence, and a third divergent cone with a strong taper. A belly section connects to multi-angled convergent cones, which should exit in a straight line into the stinger for good power. As you can see, modern two-stroke expansion chambers create a complex scenario and are quite difficult to tune. Stay tuned to upcoming issues of Motorcycle Online for reviews of software that promises to simplify pipe design.