Erbman's Engine Emporium, Part I

Russ Erb

Originally published December 1993

Objective
Terminology Review
Engine Speed
The Basic Answer
References
Erbman's Engine Emporium, Part II
Erbman's Engine Emporium, Part III
Erbman's Engine Emporium, Part IV

Aero majors and homebuilders alike tend to spend a lot of time talking about airframe issues. For instance, we have sheet metal workshops, composite workshops, fabric covering workshops, welding workshops, and workshops on many other subjects. Rarely do we talk about engines, and when we do, it's primarily about how to attach them to our airframes.

However, engines are a very important part of the aircraft. If the engine ain't working, the airplane don't go anywhere (gliders excluded, but then there always is the tow plane to consider...). A look at the history of aircraft shows that development has mostly been limited by engine technology. This has a lot to do with why an F-16 can carry the same bomb load as a World War II B-17.

The inspiration for this set of four articles was discussions amongst chapter members and in Sport Aviation about modifying automotive engines for aircraft use. As an Aero major, I had some propulsion classes in college, but these were heavily slanted toward, if not exclusively, turbine engines (turbojet, turbofan, turboprop). Virtually no one talked about the engineering principles and design of reciprocating engines. This isn't a problem when dealing with Air Force aircraft, but look through Sport Aviation and what do you find? With the exception of the BD-10 and BD-5J, virtually all homebuilts use reciprocating engines. Many people have shown that this is the appropriate technology for aircraft under about 300 horsepower.

There is an excellent article in the April 1993 Sport Aviation on auto engine conversions (Ref 1). I highly recommend that you go back and re-read it. The major point that I got out of this article, other than someone was doing this, was that you CAN'T just take a car engine out of the local salvage yard, slap a reduction drive on the front of it, stick it in your airplane, and go fly. (This point is also reinforced in the April 1993 Sport Aviation about installing two Javelin Ford V-6 conversions into a Defiant.) He said that the first step was to totally tear down the engine and rebuild it, replacing many parts with competition or racing parts. Of course, after doing that, I purport that you no longer have an auto engine, but a custom built aircraft engine.

A major point that was almost glossed over was that he changed the camshaft to adjust the torque and horsepower curves. This tweaked a question that had been flopping around in my brain but had never come out: "How do you set the engine speeds for maximum torque and maximum horsepower?" Or, if you prefer: "What factors of engine design affect the rpm that the maximum torque and horsepower occur at?" Having renewed access to a university library (namely, the U.S. Air Force Academy library), I went in search of some books that would answer this question. I did find several good textbooks on reciprocating engines, but unfortunately none of them simply answered my question directly. As a result, I had to read many sections and synthesize the information together to get the answer. Along the way, I learned more than I had bargained for, which answered a lot of other questions I hadn't thought to ask.

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Objective:

Oh, good. We have an objective. Where would Air Force writing be without objectives? But enough about that. My objective is to help you be able to understand the reciprocating engine beyond the basic cycle of intake, compression, power, exhaust. When you consider the speed at which these strokes take place, the reciprocating engine is a very dynamic environment, and cannot be fully understood from a static analysis. These articles aim to help you understand why there are differences between otherwise similar engines. For instance, can you answer these questions:

  1. Why does an IO-360 (fuel injected) have a higher peak power than a O-360 (carbureted)?
  2. Why is the compression ratio of a TSIO-520 (turbocharged) less than an IO-520?
  3. Why can a Rotax 582UL produce as much power as a Continental A-65 with 1/5 the displacement?
As I mentioned earlier, this article will be presented in four parts, so as to even out the flow of paper through Norm's laser printer. This first article will deal with the basic concepts of reciprocating engine design. The second and third article will seek to answer in reasonable detail the original question of what affects the speeds for maximum torque and horsepower. The fourth article (the "bonus" article) will analyze design parameters of actual engines and their effects on engine performance.

Let me first start out mentioning two assumptions that I had starting this study:

ASSUMPTION 1: Auto engines would not be good for aircraft use because the higher speeds (rpm) required would cause the engines to wear out faster.

ASSUMPTION 2: The speeds for maximum torque and horsepower are affected by the bore of the cylinder. This would be why aircraft engines typically ran slower than auto engines.

The first assumption is true only in a limited sense, and the second assumption would prove to be FALSE.

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Terminology Review

Three words are typically used to characterize the output of an engine. These are torque, thrust, and power (or horsepower). Torque is simply a moment, or a force times a distance. This is similar to what you measure with a torque wrench when tightening the propeller hub bolts (You DO use a torque wrench, don't you?). If you prefer, it is a measure of how hard is the engine twisting the prop shaft. Well, our buddy Sir Isaac Newton said that for each action there was an equal and opposite reaction. The propeller is typically modeled as a rotating wing, and, like any other wing, it creates drag. In this case, the drag on the propeller blades is trying to slow down the propeller. At a constant rpm, the moment caused by this drag is balanced by the torque generated by the engine. Of course, to spin the propeller faster would require more torque. A well-designed propeller is designed to produce the maximum possible thrust while absorbing the torque of the engine at a specified rpm. It is the drag of the propeller blades that keeps the engine from over-revving.

Thrust, of course, is what moves the airplane forward. Since the propeller blades have a particular lift to drag ratio, or a thrust to torque ratio (this is not a particularly rigorous concept, but only meant to hopefully increase comprehension), an increase in thrust requires an increase in torque. The main point is that while we are used to thinking of the thrust of a propeller, the engine is actually working against the torque required of a propeller.

The thrust an engine produces depends heavily on what propeller is mounted on it. It is much more convenient to characterize the engine by its power. Power, in engineering terms, is defined as thrust times speed. For spinning things, it is equivalently expressed as torque times rpm. The thrust available from the engine/propeller combination will change with speed, as will the thrust required (drag). If the torque of an engine could be held constant, the power output would increase linearly with the rpm. It is possible, and in fact is usually the case, that the torque can decrease with increasing rpm, but if the torque decreases at a slower rate than the rpm increases, the overall power will still increase.

Consider an example: Which takes more power, to move a car at low speed or to move the same car at a higher speed? You probably said more power is required as the speed goes up. Now go out and push your car around the street. First push it at a slow speed. You can produce a lot of force (torque) at a slow speed. Now try to speed it up. You will probably find that even though you are producing more power (you said so up above), each push does not have as much force, but they are coming along much faster. So the power output can continue to go up with higher speeds even though the torque is dropping off.

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Engine Speed

Ask someone how you measure engine speed, and they will probably tell you by the rpm of the output shaft. And in many applications, you would be right. There is another important measure of engine speed, particularly when looking at engine performance. This measure is average piston speed, or how fast is the piston moving up and down. The two are related, as average piston speed is equal to twice the stroke times the rpm. The factor of two comes in because the piston moves twice the stroke (once up, once down) in one revolution. This number will be important when considering how fast the engine is demanding the fuel/air mixture.

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The Basic Answer

Most people think of an engine as a device coupled with a propeller used to produce thrust. This idea works well for determining aircraft performance. However, to answer questions like what determines the speed for maximum horsepower, the engine must be considered from a totally different view. The output of the shaft is ignored, and the engine is looked at as an air pump. The purpose of the engine is to pump air from the intake to the exhaust pipe. The shape of the torque and power curves are indications of how well the engine can breathe. When its ability to breathe drops off, the power output will drop off. According to Obert, "Even for a real engine, the ihp [indicated horsepower] can be considered as being closely proportional to air consumption." (Ref 2)

If you thought the engine power depended on how much fuel it was using, remember that an engine at full rich runs slower than an engine at the same throttle setting with a properly leaned mixture. The ability to move air through the engine is the important factor; the amount of fuel just needs to be in the right proportion to the air.

Again, we can relate this to another example. Go out and run a long distance. After a short period, you will notice that your speed is not limited by your leg strength (fuel). After all, you can sprint to first base at a speed much faster than you can run five miles. You will find that your maximum speed is limited by how fast you can breathe and supply oxygen to your body. Likewise, the engine is limited by how fast it can move the air from the intake to the exhaust pipe.

Next month, we'll look at the dynamics of the air flowing through a reciprocating engine, and see how it changes as the engine speed changes. We will then see how this affects the power and torque outputs.

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References

1. C. Hall "Skip" Jones, "Converting Auto Engines For Aircraft Applications," Sport Aviation, April 1993.

2. Edward F. Obert, Internal Combustion Engines, 3rd ed. (Scranton, PA: International Textbook Company, 1968).

Erbman's Engine Emporium, Part II

Erbman's Engine Emporium, Part III

Erbman's Engine Emporium, Part IV

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URL: http://www.eaa1000.av.org/technicl/engemp/engemp1.htm
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 22 February 1997