RE: Exhaust Cycle
Exhaust Cycle Part One
Paul Yaw, yawpower.com
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.
- Blowdown Period
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 Tuning
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.
Exhaust Cycle Part Two
Let's start by looking at the effects of a less than optimum exhaust cycle.
A motor has fully exhausted itself (When it is really tired?) when the pressure in the chamber is equal to, or below atmospheric at the end of the exhaust cycle. Several things happen when the motor cannot fully exhaust itself. If the pressure is above atmospheric at the end of the cycle, the result is lowered volumetric efficiency, increased pumping losses, and reduced combustion efficiency as compared to an optimized exhaust cycle.
- Swept Volume, Clearance Volume, and Compression Ratio.
In December's port timing article, I stated that top dead center, or TDC refers to the point at which the chamber is at its smallest possible volume. The space in the chamber at TDC is referred to as the clearance volume, and this in part determines the compression ratio. The compression ratio is specified as (Volume at BDC/Volume at TDC) Using an '87 13B as an example, the chamber volume at BDC is 9.4 times greater than the volume at TDC, for a compression ratio of 9.4 to 1. The difference between the volume at TDC, and BDC is referred to as the swept volume, or displacement. This is the volume of gasses that will be displaced in one complete cycle assuming 100% volumetric efficiency. A little bit of high school algebra shows that the volume at BDC is 44.66 cubic inches, and the volume at TDC is 4.75 cubic inches, or 10.6% of the total volume.
- Volumetric Efficiency
The exhaust gasses that occupy the clearance volume will be carried around into the following intake stroke. As you can see, even at 100% volumetric efficiency the mixture will still only be 89.6% fresh intake charge. If the chamber pressure does not reach atmospheric by the end of the cycle, this 10.6 %, or 4.66 cubic inches of exhaust gasses will be pressurized, and will take up even more space once they are allowed to expand as the chamber volume increases during the intake stroke. This will reduce volumetric efficiency considerably, as the exhaust gasses will occupy space that could be used for fresh mixture. These exhaust gasses effectively "take away" from the swept volume, or displacement of the motor. The goal of the exhaust system then, should be to evacuate as much of the spent gasses as possible.
- Inertial Scavenging
Inertial scavenging is easiest to understand if you think of the gasses in the exhaust system as a big piece of elastic. While they are not directly connected, a change at one end of the system will have an effect on the gasses at the other end of the system. For instance, towards the end of the cycle, the flow through the exhaust port slows down, but the high velocity gasses from earlier in the cycle are still travelling through the system. (Note: A system made up of 100" long, 1/34" inside diameter header tubes, as you might see on a race car, will contain about six complete cycles worth of exhaust gasses per pipe.) These high velocity gasses will "pull" on the slower moving gasses near the exhaust port, helping to evacuate the chamber. This is inertial scavenging. Just imagine two cars rolling down the road, connected to each other by a bungee cord. If the car at the back slows, it will not immediately be jerked back to speed, but rather gently pulled back up to speed by the car in front. As some of you may have guessed, a series of resonances will then occur, with each car alternately pulling at the other. This is very much like what happens to the gasses in the exhaust system.
- Pumping Losses
Well, here we are at pumping losses again! Luckily this is quite easy to explain and understand. It all comes down to exhaust flow. Not just the airflow capability of the exhaust port, but of the entire system from the port to the end of the exhaust pipe. Quite simply, if the exhaust flow is insufficient, the blowdown period will only account for a small amount of the total exhaust gasses, and the remainder will have to be squeezed out by the rotor itself. Physically forcing the gasses from the chamber through a restrictive exhaust system requires a substantial amount of horsepower. So much in fact that many diesel truck engines have a mechanism which blocks the flow of exhaust gasses to slow the vehicle down, thus saving wear on the brakes. Just think about slowing an 18 wheeler with nothing but exhaust pressure, and you get an idea how much this can affect your engine.
- Combustion Efficiency
We have already discussed how insufficient exhaust flow reduces volumetric efficiency, but the presence of exhaust gasses in the intake charge (Exhaust gas dilution) causes other problems as well. The rotary engine is known for its poor combustion characteristics. Due to the shape of the chamber, and the location of the spark lugs, a large percentage of the intake charge does not burn in the chamber. The end result is a fair amount of unburned gasses, or hydrocarbons being passed into the exhaust system. This reduces power output, because a portion of the mixture that we tried so hard to put into the engine did not burn. This also reduces fuel economy, and increases emissions. Another effect that is not often realized is excessive exhaust gas temperatures. These hydrocarbons will then burn in the exhaust system raising the exhaust gas temperatures.
The addition of exhaust gasses to the intake charge will reduce the already poor combustion quality. The end result is that the mixture is harder to ignite, and when it finally does light up it will burn at a slower rate further reducing power output. In a turbocharged engine excessive exhaust gas dilution will cause its own unique set of problems.
We tend to think of combustion inside of the engine as a series of explosions, but in fact the combustion occurs at a very slow rate, at least compared to an explosion. In the absence of detonation, the mixture in the vicinity of the spark plugs is ignited first, and the "flame front" travels from that point, through the rest of the mixture in a fairly controlled manner. Detonation occurs after the combustion has initiated, and the pressure, and temperature in the chamber rises to the point that the remaining mixture literally explodes. Anyone who has ever experienced detonation understands that it certainly is an explosion! Detonation is caused by a combination of heat, and pressure, and so it stands to reason that excessive exhaust gas dilution, (remember these are hot gasses) will increase the likelyhood of detonation. As most of you know, detonation will destroy a turbocharged engine in a big hurry.
- A "Perfect" Exhaust Cycle
Now that we have all of the pieces, it is time to put the puzzle together. I personally have a hard time understanding anything unless I can see it in front of me. For that reason I will refer once again to the illustration of the engine during its different phases.
As the exhaust port opens, (#13 in the illustration) the high pressure in the combustion chamber will force the gasses through the port and down the exhaust system at a high rate of speed. This, as you remember, is the blowdown period, and a large portion of the gasses will exit the chamber at this time. At the same time that the flow is initiated, a high pressure wave will travel towards the end of the exhaust system at the speed of sound. (Note that this high pressure wave will help to propel the slower moving exhaust gasses with it.)
Further into the cycle (#15) as the pressure differential between the chamber and the exhaust system has decreased, (ie., the chamber has "blown down") the velocity through the exhaust port will also decrease, and the remaining flow will be the result of the decreasing chamber volume. At this point, approximately half of the exhaust gasses will have exited the chamber.
At 135 degrees after bottom dead center, (between #15, and #16) the chamber will be at its maximum rate of decrease of volume. In other words, it is at this point in the cycle that the rotor will be travelling at maximum velocity. If the exhaust flow is insufficient, it will require a great deal of force to expel the gasses from the chamber. This is where the pumping losses during the exhaust stroke will be the greatest. Keep in mind that these losses cannot be eliminated, but they can certainly be lessened by providing sufficient exhaust flow.
Moving on to #17, and #18, the chamber volume is decreasing at a very slow rate, and the motor is doing very little to mechanically expel the gasses from the chamber. It is at this point in the cycle that pressure wave tuning comes into play. The high pressure wave that originated when the exhaust port first opened will have travelled to the collector, and been reflected back as a low pressure wave. (Remember last months section on pressure wave tuning?) If timed correctly, the wave will arrive at this point, just before the intake port opens. This low pressure wave, in conjunction with the "pull" created by the high speed gasses still in the exhaust system will lower the pressure in the chamber to sub atmospheric. When the intake port opens, this vacuum will help to initiate the flow of fresh mixture into the chamber, which will increase volumetric efficiency.
Looking back to December's port timing article, you can see that the intake port does not open until approximately 30 degrees after top dead center. That means that for the first 30 degrees after TDC, (The distance between #18, and #1 in the illustration) the chamber volume is increasing, but because only the exhaust port is open, the chamber will be filling with exhaust gasses by pulling them back out of the exhaust system. This is called exhaust gas reversion. If the exhaust gas velocity is low, (Such as at low rpm) the vacuum created by the increasing chamber volume can easily reverse the flow and pull the gasses back into the chamber. If, on the other hand, the exhaust gas velocity is high, it will take a great deal more energy to reverse their flow, and the result will be less exhaust gas dilution. This is why large exhaust ports, and large diameter exhaust tubing reduce low speed power.
- Low RPM Operation
The above paragraphs describe a "perfect" exhaust stroke, and unfortunately this can only happen over a very narrow rpm range. Let's look at what happens when we halve the rpm. We will assume that the above example is at 8000 rpm. Now let's look at the same cycle at 4000 rpm. Since the exhaust cycle lasts twice as long at 4000 rpm, the chamber will have reached sub atmospheric pressure approximately half way through the cycle, assuming of course that we have sufficient exhaust flow. This sub atmospheric condition will send a low pressure wave travelling towards the end of the exhaust system at the speed of sound. (Remember that a pressure wave is intiated anytime pressure deviates from atmospheric.) This wave will reach the collector, and reflect back as a high pressure wave. Since we have halved the rpm, it is likely that this wave will arrive near the end of the exhaust stroke, (#18) and so the chamber pressure will be above atmospheric when the intake port opens. This will result in excessive exhaust gas dilution as compared to the 8000 rpm example. In addition to this, the exhaust gas velocity will be low, and during the period from TDC, to intake valve opening, the exhaust gas flow will reverse momentarily. This will also add to the amount of exhaust gas dilution.
If we wanted the exhaust stroke to be optimized for this lower rpm, several changes would be necessary.
1. Later exhaust port opening. Since we have more time to exhaust the chamber, the total exhaust duration can be lessend. The result of this will be that we can hold pressure in the chamber for a greater period of time. This will increase the amount of time that torque will be applied to the eccentric shaft.
2. Smaller cross sectional areas. Decreasing the cross sectional area of the port, and the exhaust tubing will increase the velocity of the exhaust gasses. This will result in less reverse flow, or exhaust gas reversion after top dead center, and will make the inertial scavenging towards the end of the cycle more effective.
3. Longer tuned lengths. Since the exhaust cycle occurs over a greater period of time at low rpm, the pressure wave must be further delayed if it is going to arrive at the appropriate time. In the case of optimizing the system for 4000, rather than 8000 rpm, the header lengths would need to be approximately twice as long. This is easiest to understand if you think of the headers as a delay source. What we are trying to do is delay the wave from the time it initiates to the end of the exhaust cycle. The further that the wave travels, later it will arrive at the exhaust port.
As you can see, we can only optimize the exhaust cycle over a fairly narrow rpm range. If at first this seems discouraging, it is important to consider that an optimized cycle over a narrow range is much better than a less than optimum cycle throughout the operating range. A "perfect" intake stroke can also only occur over a fairly narrow rpm range, and so it is important to consider the trade-offs when contemplating performace upgrades. If for instance you wish to "street port" your engine, you must understand that the increase in top end power will be accompanied by a decrease in low speed power.
The intent of these articles is not to make specific reccomendations, but to give you the knowledge to make informed decisions, and sort through the hype. For all of you racers, using the lessons learned from the exhaust system articles will allow you to make sense of exhaust tuning. If you apply these theories, and do some trial and error testing, you will likely unleash some hidden power. Now that you have the facts, you will understand why one system affects the engine differently than another, and this will make it much easier to arrive at the "correct" setup.