When the exhaust port first cracks open toward the end of the combustion stroke, the burnt (burning) products of combustion escape at the speed of sound. Ever wonder why engines are so noisy? You have a miniature sonic-boom every time the exhaust port opens.
Instead of immediately trying to muffle the sonic boom, we harness this incredible energy, by allowing the shock wave to race down a carefuly shaped pipe, preserving the wave-front as much as possible. As the gases travel down the pipe, it expands, trying to get back to atmospheric presure. This part of the pipe expands, or tapers outward (divergent), to keep conformity of this shock wave.
As gases expand, they cool down adiabatically. The speed of sound is directly related to the density of the gasess, i.e. pressure and temperature, which are both dropping rapidly. The shape of this "expansion pipe" must cater for this constantly changing speed of sound, or lose some inherent energy.
This exhaust gas has considerable momemtum, due to its mass and substantial velocity. It creates an enormous vacuum behind it, which "scavenges" all the burnt gases from the combustion chamber, emptying it fully of stale gas and heat.
This scavenging effect literally boosts power by clearing out all the waste products of combustion, making more room for fresh fuel and air. Without this scavenging, as much as 20% of burnt gas may still be present with the fresh charge, preventing a 100% fresh charge being introduced into the cylinder.
Furthermore, removing the last dregs of burnt gas, also carries away the heat contained in this burnt gas, helping to keep the engine relatively cool.
This high-vacuum inspiration causes more fuel/air to enter the combustion-chamber in a shorter space of time, delivering more energy in the form of a greater "charge" to be ignited.
This completes the first part, and a silencer/muffler may follow shortly after this.
However, a fully resonant system goes a bit further.
Even after the chamber has filled to capacity, there is still a huge vacuum in the expansion pipe, the tranfer port and the exhaust port is still open, so the expansion pipe itself starts filling with fresh fuel/air as well, while the burnt gases are further along, in the parallel part known as the belly. The two masses of gasses are kept seperate by the sonic shock-front, and their vastly different densities which do not mix easily.
Eventually the shock wave strikes the Taper, or Reflector, which causes some of it to bounce back, and race backwards toward the cylinder, pushing the fresh fuel/air mixture that was in the exhaust system ahead of it, literally "stuffing" this extra gas, back into the combustion-chamber, just as the transfer port closes.
You now have a much greater "charge" of fresh fuel/air in the combustion chamber, hence the term super-charged or more corectly, pulse-charged.
After the reflector, there is usually a short length of narrow parallel pipe, the stinger, followed by the silencer, to bleed pressure out of the system. The length and diameter of the stinger is tuned to control the internal pressures and, indirectly, the temperatures in the combustion chamber.
Some points to ponder:
If the shock wave bounces back too early, some burnt gas will also end up back in the chamber along with all the extra fresh gas. It also introduces heat back into the chamber, which is not desirable.
On the other hand, if it bounces back too late, not all the fresh fuel/air in the expansion pipe gets back into the chamber before the exhaust port closes, resulting in wasted, unburned fuel vented out the exhaust.
There are a number of possible problems to be overcome in the design; for example, the exhaust itself must never get hot enough to ignite fuel, as it contains an explosive mixture of fuel/air, unlike ordinary exhausts.
To function properly, it must be "resonant" with the engine through a certain range of RPM.
If the engine is running at say 5000 RPM, it acts as a brasswind musical instrument making a sound with a pitch of 5000 cycles per minute or 83 Hertz (cycles per second).
The exhaust can be tuned to suit any specific needs. It can be used to boost RPM, top end, or to boost mid-range, or for smoother bottom end with improved low RPM torque, or a combination of these.
Just like turbo-chargers on conventional 4-stroke engines, it can be set up to increase brute power (fuel thirsty), or to produce the same power but more economically (lower fuel consumption), or anywhere inbetween.
On our Xplorer Paramotor, we boosted the mid-range and broadened the power-curve, without boosting the top RPM. The Torque-Peak and the Horsepower-Peak are moved further apart, to offer a wide range of usable engine RPM. Both the torque peaks and the hp peaks are increased in size considerably, but with the emphasis on boosting the torque across the range. This extra torque generated is used to drive a higher pitched propeller, at a slower RPM, delivering smoother, more controlled power, but with almost 50% extra thrust.
This results in a noticably more powerful motor, with a much improved power-curve, but with less vibration, less noise, less heat retention, less wear and tear, and a great sound to boot!
What more could you want?
The alternative would be to build a larger engine with higher cubic capacity, which would weigh much more, be noiser and much more expensive. So essentially, a properly designed tuned exhaust system allows a smaller engine to do the job of a larger engine, with many other added benefits.
The art of designing fully resonant tuned exhausts has been developed almost into a science in some circles, most notibly racing motorcycles. There are even computer programs that allow you to punch in all the technical data of the engine, specify the type of performance you require, and the program calculates the correct lengths and angles of all the sections. Some even draw life-sized templates to cut your metal to.
However, a big Caveat here... all these programs, and most of the literature on designing tuned exhausts, are aimed at road vehicles, usually with multiple gears. Aircraft with propellers require considerable modification to these calculations, even more so seeing as we usually have only a single reduction ratio without the luxury of keeping the RPM in its peak band and working through the gears.
I will not go into the various forms of the actual manufacturing process here, as it is irrelevant to the pilot.
Address comment to Keith Pickersgill at email@example.com