Erbman's Engine Emporium, Part III

Russ Erb

Originally published February 1994

Erbman's Engine Emporium, Part I
Erbman's Engine Emporium, Part II
Fuel Injection
Spark Timing
Stroke/Bore Ratio
Compression Ratio
Supercharging
Horsepower
Next Time
References
Erbman's Engine Emporium, Part IV

When last we met, we determined that the key to producing high torque from a reciprocating engine was to get the maximum possible amount of fuel/air charge into the cylinder. How well the engine does this is measured by its volumetric efficiency. In general, the more air we can pump through the engine, the greater the torque we will get out. Last time we looked at the two biggest factors affecting volumetric efficiency, valve overlap angle and intake valve closing angle. These are directly related to the choice of camshaft for your engine, and changing the camshaft will change the torque curve, and thus change the power curve. This time we will look at other lesser factors affecting the torque output of the engine, and then look at how the torque output affects the power output.

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Fuel Injection

So why does an IO-360 (fuel injected) have a higher peak power than a O-360 (carbureted)? The answer is that fuel injection reduces losses in the intake system. The first reason is that the venturi in the carburetor is another constriction in the flow, which manifests itself as a pressure drop in the intake manifold. This pressure drop is eliminated with a fuel injection system, thus allowing a higher pressure to reach the cylinders, and thus a larger amount of fuel/air charge to enter the cylinder.

The second reason is that the fuel/air charge is colder, and thus denser when it reaches the cylinder, again allowing a larger amount of fuel/air charge to enter the cylinder. Just like when you add carb heat, the density of the fuel/air charge is reduced when it is heated. So you're asking "Why would it be heated?" In some carbureted engines, the intake manifold is heated to assist distribution. Even without intake manifold heating, the intake manifold will be hotter than the ambient air simply because it is attached to the engine. Heat transfer studies have shown that the liquid fuel on the walls on the intake manifold increases the rate of heat transfer. (Ref 1) Thus, in a carbureted engine, the small drops of fuel in the fuel/air charge cause the charge to heat up more passing through the intake manifold than dry air would passing through the same intake manifold. Therefore, the density of the fuel/air charge is decreased, reducing the amount of charge entering the cylinder. Experiments have shown that volumetric efficiency may be increased by 10% by direct injection of the fuel into the cylinders. This also prevents loss of fuel because of valve overlap. Fuel injection into the intake port (just outside the intake valve) shows a smaller, but appreciable improvement. (Ref 1)

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Spark Timing

If you've ever set the timing either on your aircraft engine or your car engine, you've probably noticed that the spark fires before the piston reaches top dead center. At first glance, it would seem that the force of the burning fuel/air charge, which is supposed to push the piston down, would be fighting the piston which is still coming up. Of course, this will happen if the spark is advanced too much. Once again, we must remember that nothing happens instantaneously. It takes a finite, though very small, amount of time for the fuel/air charge to burn and reach a maximum pressure. To get the maximum useful work out of the expanding gasses, the maximum pressure should occur just as the piston reaches top dead center. Thus, the burning must start prior to the piston reaching top dead center. (Ref 2)

So why is a spark sometimes retarded from the position for best power? Within limits, a retarded spark is a powerful way of controlling detonation. This allows use of a higher compression ratio than would be possible if the spark were always set for best power. When high power is required of the engine, such as during a climb, the spark can be retarded to control the detonation. At lower power settings when detonation is not imminent, such as cruise, the spark can be advanced to the best power position. The result of this setup is a better over-all fuel economy than with a lower compression ratio.

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Stroke/Bore Ratio

As it turns out, the stroke to bore ratio has no relation to the speediness of the engine. "Low-speed engines with a short stroke-bore ratio are about as common as long-stroke high speed engines." (Ref 2) The size of the bore does not affect the volumetric efficiency, assuming the valves are properly sized. The big factor with regards to the stroke is not the length of the stroke, but the piston speed (because the inlet Mach index is proportional to piston speed). An engine with a short stroke running at high rpm can have the same piston speed, and thus similar performance, to a long stroke running at a low rpm. We'll see examples of this in next month's article.

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Compression Ratio

Why are engines built with the highest possible compression ratio? Higher compression ratios give a higher thermal efficiency, allowing the engine to get more useful work out of the heat energy in the fuel. Compression ratio has only a small effect on the volumetric efficiency of four-stroke engines. (Ref 1) The upper limit on compression ratio is set by detonation, where the fuel/air charge is heated sufficiently in the compression stroke to self ignite. Higher compression ratios can be used by using higher octane fuel.

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Supercharging

Supercharging increases the volumetric efficiency of an engine simply by providing the fuel/air charge to the cylinders at a higher pressure, thus allowing more of the fuel/air charge to be drawn into the cylinders. However, supercharging cannot simply be added to an existing engine without considering the other factors that have been discussed.

As just mentioned, the upper limit on compression ratio is detonation. When supercharging is added, the starting pressure of the fuel/air charge is higher, and thus the compression pressure will be higher if nothing else is changed. At this point, one of two things must be done. The first possibility is to increase the anti-knock characteristics of the fuel (i.e. higher octane). This may be fine if you're racing at Reno, but to the average pilot higher octane fuel means more buck$ at the pump, and the question of fuel availability for anything higher than 100LL. The other possibility is to reduce the compression ratio so that the compression pressure will be the same as before supercharging. This can be done simply by increasing the volume in the cylinder head. This will result in a slightly lower thermal efficiency, but the power output will be increased, since more fuel will be burned. "An engine operating with natural aspiration with a compression ratio 7:1, when supercharged should have a compression ratio about 6:1." (Ref 2)

Another consideration arises when supercharging a carbureted engine. If the valves are overlapped, as they generally will be, some fuel may be lost out the exhaust manifold during the overlap period. Even so, many supercharged aircraft engines use considerable overlap to achieve high values of volumetric efficiency under take-off conditions. The fuel loss is not important because the take-off covers only a short time. In cruising flight, the pressures in the intake manifold and exhaust manifold are close to equal (because of the back pressure from the turbine on the supercharger), and little fuel is lost due to the overlap. (Ref 1)

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Horsepower

So far we've talked about volumetric efficiency and torque produced by the engine. Even so, you probably don't remember ever asking someone about the torque rating on his engine. The reason I haven't talked about horsepower specifically is that volumetric efficiency is directly related to the torque, and the torque is related to horsepower simply by rpm. Specifically, torque * rpm = horsepower (with the appropriate units conversions).

The torque will always peak at a lower rpm than the horsepower. As rpm is increased above that for maximum torque, the indicated horsepower will continue to increase as long as the rpm increases faster than the torque decreases. Note that I said indicated horsepower, not brake horsepower. The difference between these is the friction horsepower. Friction horsepower is the power required to overcome the friction from sliding pistons up and down in cylinders and turning shafts in bearings. The friction horsepower increases rapidly at high speeds. As a result, the brake horsepower (the power available at the output shaft of the engine) will eventually peak when the friction horsepower increases faster than the indicated horsepower.

Since high power requires high rpm, this is why your engine produces its maximum horsepower generally at the redline rpm. The maximum torque typically occurs at about half the rpm of maximum horsepower. (Ref 2)

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Next Time...

In the next (and last) installment, we'll look at actual engines past and present, and see how the topics we've discussed affect their performance. We'll also finish answering the questions raised in Part I.

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References

1. V.L. Maleev, Internal-Combustion Engines: Theory and Design, 2nd ed. (New York: McGraw-Hill Book Company, Inc., 1945).

2. Charles Taylor, The Internal Combustion Engine in Theory and Practice, 2nd ed. (Cambridge: The M.I.T. Press, 1966), I.

Erbman's Engine Emporium, Part I

Erbman's Engine Emporium, Part II

Erbman's Engine Emporium, Part IV

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Revised -- 22 February 1997