The previous article described the importance of increasing airflow into the engine. In this article I intend to discuss the many factors that affect total intake airflow, and horsepower output.
In the previous article I stated that the horsepower output of an engine is directly proportional to the amount of air and fuel that it can ingest. It stands to reason then, that if the goal is increased horsepower, we must increase the airflow potential of the induction system. The total airflow through any passage (Ports, intake manifold, etc.) is affected by three variables. These variables are: 1. The size of the passage. 2. The pressure differential between the inlet and the outlet. 3. The coefficient of discharge of the passage.
Coefficient of discharge is normally stated as a percentage, and is a measure of how efficiently a passage will allow air to flow through it. A "perfect" venturi, having an inlet angle of 16°, an outlet angle of 7°, and an inlet and outlet area four times larger than the operating cross sectional area (Vena contracta, the smallest point of the venturi.) has a flow efficiency of 100%. A venturi such as this would flow 137.7 cubic feet per minute (CFM), per square inch, at a pressure differential of 25 inches of water. Using this as a baseline, we can determine the efficiency of a port, or intake manifold runner. 100% flow efficiency is not possible in most cases, but knowing the efficiency of a given combination is a large step towards being able to optimize it.
For example, consider two intake manifolds that flow the same amount of air at the same pressure differential, but one of these manifolds has a runner cross sectional area twice that of the other. The manifold with the larger runners will have a coefficient of discharge that is half that of the smaller manifold. In addition to being less efficient in technical terms, the larger manifold will also lower the horsepower output of the engine as compared to the smaller manifold. Why you ask?
Hopefully all of you remember the previous discussion of velocity, and have a good understanding of its effects on unsteady airflow. If not, go back to part one of the exhaust cycle article for a freshen up. As I stated in that article, flow velocity through the exhaust system is not steady, and in many cases, the flow will reverse at some point in the cycle. This is also true of the intake system.
Since most rotary applications utilize a stock, or off the shelf aftermarket manifold, manipulating the pressure waves by changing the length of the induction tract is not as practical as with the exhaust system. For this reason I will not cover this in great detail.
The pressure wave theories that I discussed in the exhaust article apply to the intake system as well, but there are a few differences between the two. 1. The pressure waves will be much weaker, and so their effect will not be as great. 2. Since the intake manifold is typically much shorter than the exhaust system, the pressure waves will be reflected back and forth several times before they arrive at the intake port at the appropriate time in the cycle. Each time they reflect, they will lose some energy which reduces their usefulness. 3. In the case of the induction system, it is the positive, or high pressure waves, rather than the negative, or low pressure waves that are useful for increasing horsepower.
By timing the positive return wave to arrive at the intake port right before it closes, the pressure differential between the port, and the chamber will be increased. This will increase the flow into the chamber at the end of the cycle when it is typically at it lowest.
There are a few basic rules that apply to pressure wave tuning the induction system. A longer manifold will delay the waves for a greater period of time, and so tune the manifold for a lower rpm range, just as with the exhaust system. A longer manifold will also increase the peak torque output of the engine, in addition to the above mentioned effects. This is the result of the manifold containing a greater mass of air. (Remember, energy = mass times velocity squared.) At the end of the intake cycle, when the chamber pressure is increasing, this greater mass (Which is travelling at a high velocity) will better overcome the rising chamber pressure, resulting in greater airflow during that critical period. Additionally, a greater pressure drop will be created at the beginning of the cycle when the chamber begins to expand, because the engine will have to "pull" harder to get this greater mass of air moving. It is this initial low pressure condition which starts the pressure wave cycle, and the result is a pressure wave of greater intensity which if timed correctly, will increase volumetric efficiency.
If you are thinking to yourself that high rpm horsepower is all that matters for your application, consider this. Even with a close ratio racing gear box, you will need to make power over a range of at least 2,000 rpm. If the engine makes a staggering amount of horsepower at redline, but drops off quickly below that, acceleration will suffer. This is relatively common on race engines, and is the result of low velocity, or poor flow efficiency.
Peak horsepower is a measure of the absolute maximum horsepower that the engine can produce. It is a relative indication of an engines performance, but it only tells you what the engine is doing at one particular rpm. It tells very little about the actual performance, unless you will only run the engine at one rpm!
What is important is the average horsepower throughout a specified operating range. This operating range should be specified based on the gear ratios of the transmission.
I have presented quite a bit of information here, and to make all of this easier to visualize, I will once again refer you to the illustration of the rotary engine during its different phases.
1. 45° after TDC. The chamber is slowly expanding, and the air/fuel mixture is just starting to enter the chamber. This is the beginning of the intake cycle for a conventional side port engine, and the intake port has been open for approximately 20°.
A bridge port, or peripheral port engine will have had the intake port open for 150° to 200° at this point. If the exhaust system is working properly, the low pressure wave will have arrived, and initiated intake flow by TDC, or even sooner, replacing the exhaust gasses in the clearance volume with fresh intake charge.
2. 90° after TDC. The rate of expansion is now fairly rapid, and the low pressure in the chamber initiates a negative pressure wave in the induction system.
3. 180° after TDC. The intake port is completely open, and the point of maximum rate of expansion and flow has occured 45° earlier.
4. BDC At this point, the chamber is at its maximum volume, and past this point the chamber volume will decrease as the compression cycle begins.
5. and 6. 45°, and 90° after BDC. It is during this period that the intake port will close, and the effects of inertial supercharging become critical. The period between BDC, and intake port closing has the greatest effect on the volumetric efficiency of the engine. Velocity, velocity, velocity!
Well, I think that just about covers it. For a very thorough understanding of the gas exchange process, review the previous exhaust cycle articles, and consider the intake and exhaust as a complete cycle. Pay careful attention to the entire process, starting with the exhaust cycle, and the pressure condition that is left at TDC, which is where it all starts.
Paul Yaw
Yaw Power Products