OK, here we go with part two of the intake
cycle. To all of you who regularly follow these monthly articles, I apologize
for making you wait so long for the last half.
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.
Total Intake Airflow
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
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?
Velocity
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.
Pressure Wave Tuning
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 traveling 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.
The Peak Horsepower Myth
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 occurred 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
This site is meant to give information related to the 1993
(o)Mazda RX-7 Twin Turbo. Anything from rotary engines to wiring diagrams and turbo upgrades to tuning info, this site has it all! efini 93 rx7 13b anfini Turbo RX-7 Turbo RX7 turbo rx7 rx7tt rx-7tt As well as the Rotary Engine Pickup Truck aka REPU repu
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