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Last months article described the basic internal workings of the rotary engine. The next several articles will break this down into separate cycles, and descirbe them in detail. I will begin with the exhaust cycle because it has the greatest effect on the power output of the engine. If the engine cannot exhaust itself completely, further modifications will result in very little improvement. This is true of naturally aspirated, and turbocharged engines. This first article will explain a few basic terms and concepts. Next months article will present some more new information, and then describe how all of this comes together to affect the complete exhaust cycle.
When attempting to increase the power output of the rotary engine, there are three basic aspects that can be improved upon. Volumetric efficiency, combustion efficiency, and reduction of pumping losses. As most of you know, the rotary engine has four separate cycles. Intake, compression, expansion, and exhaust. Of the four, only the expansion cycle contributes to the power output of the engine by exerting force on the output shaft. The other three cycles actually reduce horsepower by resisting the rotating force. This reduction in power is referred to as pumping loss. Pumping losses occur in both the intake and exhaust cycles. This article, and the next will deal with the importance of reducing pumping losses during the exhaust cycle.
Early internal combustion engines opened the exhaust valve at BDC of the expansion cycle. This required the piston to pump, or physically force the exhaust gasses from the cylinder during the period from BDC to TDC. The force required to pump the gasses from the cylinder considerably reduced the power output of the engine. As performance, and rpm requirements increased, it was discovered that by opening the exhaust valve before BDC the residual combustion pressure could be used to help evacuate the cylinder at the beginning of the exhaust cycle. This is referred to as the blowdown period, and is responsible for approximately half of the exhaust flow. In theory, this will reduce thermal efficiency by releasing pressure that is still applying force to the crankshaft. In practice however it was determined that the reduction in pumping losses far outweighed the loss of pressure at the end of the expansion cycle. Since most of the useful work is done in the first third of the expansion cycle, the pressure loss caused by early exhaust valve opening is minimal. This also applies to the rotary engine. Referring to last months article you can see that the exhaust port of a stock engine opens approximately 75 degrees before BDC.
Pressure wave phenomena is probably the least understood aspect of exhaust tuning. Right now I am thinking that it is also the hardest to explain! Entire books have been written on this subject, but I will try to boil it down to a few paragraphs.
Any time there is a pressure change in an elastic meduim (like air for instance) a series of resonances or vibrations will occur. Any time you hear a sound, it is the result of a pressure disturbance in the air. For instance, if someone across the room claps their hands together, the air pressure between their hands will increase. This rise in pressure will be transferred from one group of molecules to the next (at the speed of sound of course) until it finally reaches your ear. While this energy transfer is invisible, you can easily picture it by dropping a stone into an undisturbed pool of water. Pressure waves radiate outward from the center of the disturbance. This same thing happens in the exhaust system, but because of the higher pressures involved it is more like an elephant doing a belly flop in your swimming pool.
The main difference between the swimming pool analogy, and the exhaust system is that the pressure waves cannot travel outward in all directions from the source of the pressure disturbance, beacause they are enclosed by the tubing itself. In the case of the exhaust system, the initial pressure wave, or pulse caused by the exhaust port opening will travel towards the open end of the tube.
So far I have only referred to pressure waves as being positive, or caused by an increase in pressure. In fact, pressure waves can be negative, or caused by a decrease in pressure. Picture a wave in the ocean with the highest point of the wave being positive, or above sea level, and the trough between two waves being negative, or below sea level. This is analogous to the pressure waves in the exhaust system. These waves can also be referred to as high pressure, and low pressure.
These pressure waves can be used to our advantage because they have the effect of moving gas particles along with them. A positive, or high pressure wave will propel gasses in the same direction that it is travelling. A negative, or low pressure wave will propel gasses in the opposite direction that it is travelling. Take a moment to let this sink in, because this simple fact is at the heart of exhaust system tuning. Although the pressure wave is moving at the speed of sound, it will propel the gasses at a much slower speed. An example of this is a boat that catches a wave from another boat that is motoring by. As the wave passes it will propel the boat in the same direction the wave is travelling, but at a much slower speed, and the wave will eventually pass the boat completely. This is the same thing that happens to the gas molecules in the exhaust system as a pressure wave passes through them.
These pressure waves respond in an interesting manner when they reach a sudden area change in the pipe. An example of a sudden area change is the collector, where the two pipes empty into a larger diameter pipe, a megaphone, or the end of the exhaust where the pipe empties into the atmosphere. When a pressure wave reaches a larger cross sectional area, it will reverse its sign (positive becomes negative, and negative becomes positive) and its direction. For instance, when the exhaust port first opens, a strong positive wave will travel to the end of the pipe, change to a negative wave, and travel back to the exhaust port. This is called a reflection. Both the positive wave travelling towards the end of the pipe, and the negative wave travelling towards the exhaust port will propel exhaust gasses towards the end of the exhaust system which is exactly where we want them to go. The amount of time that this cycle takes is dependant on the total distance that the wave has to travel.
By changing the length of the header pipes, you can time the cycle so that the negative return wave arrives at the exhaust port at the end of the exhaust cycle where it is most beneficial. Assuming that the negative return wave is timed correctly for a given engine at 6000 rpm, lengthening the headers will further delay the return wave so that it is timed appropriately for a lower rpm, and shortening the headers will time the return wave so that it is timed appropriately for a higher rpm. The key to header length tuning is simply timing the low pressure return wave to give the greatest benefit for a given rpm.
This is a VERY basic description of pressure waves, and how they affect the exhaust system of an internal combustion engine. For a more detailed analysis, I would suggest researching two stroke exhaust system design. There is a great deal of information in print, and much of it can be found at public, or university libraries.
Velocity refers to the speed at which the exhaust gasses are travelling. The exact speed is not important to this discussion, but an uderstanding of how velocity affects exhaust flow is. There are two ways that velocity can be increased. One, by decreasing the cross sectional area of the orifice that the gasses are flowing through. (Making the headers or exhaust ports smaller) Two, by increasing the volume of air that is flowing through the orifice. (Increasing engine rpm) Velocity will increase proportionally with an increase in rpm. In other words, if you double the rpm, the velocity will also double. Velocity is inversely proportional to an increase in cross sectional area. Doubling the cross sectional are will halve the velocity, and halving the cross sectional area will double the velocity.
Velocity is important for one simple reason. Inertia. Websters dictionary describes inertia as "The property of matter by which it retains its state of rest or velocity so long as it is not acted upon by an external force." In other words, once it is moving, it will continue to move until some external force stops it. If you apply this theory to the gasses in the exhaust system you can see that once they have been accelerated by the pressure in the combustion chamber, It will take a given amount of energy to stop them, and even more to cause them to reverse direction. Since energy equals mass times velocity squared, you can see that doubling the velocity of the gasses will quadruple the amount of energy required to stop them. This is important because the flow of exhaust gasses is not steady. During each exhaust cycle, the gasses are accelerated, and decellerated rapidly. Often in the forward and reverse direction.
Next months article will take all of these concepts, and describe how they affect the exhaust cycle. Like everything else that seems complex, it is just a combination of many very basic theories. If you take the time to fully understand this months article, next months article will leave you with a thorough understanding of the exhaust cycle.
My goal in writing these articles is to inform you, the reader so that you can go faster, and make appropriate decisions when modifying the rotary engine, or buying performance parts. Until next month, have fun, and thank you for reading.