No matter what type of piston engine you are working with, engine horsepower is always dependent on the amount of fuel and air the engine burns. It’s the density of the mixture, not the volume, that determines the power that the engine is capable of generating. Power is not a function of the volume of air, it’s a function of the mass, or weight, of the air — the actual number of molecules entering the combustion chamber. This is an important factor to keep in mind when discussing turbocharging. Let’s look at an example.
Let’s assume a standard day, and we’ve got a TSIO-550-cubic-inch displacement engine at sea level. At sea level field elevation, that engine will inhale 550 cubic inches of air for every two revolutions of the crank. It would also inhale around 550 cubic inches of air in Denver, at 5,000 feet altitude, on a hot day. However, the actual number of air molecules entering the combustion chamber, and of course the resulting power, is going to be very different in these two examples. In Denver, there’s fewer air molecules at that altitude to support combustion than there are at sea level. The result is less power for the same volume of air.
The bottom line is this: We can only burn more fuel if we build a larger engine, or we artificially cause a small engine to breathe as if it were larger than it really is. That’s what we do with turbocharging. We cause a small engine to breathe as if it were a larger engine.
Let’s go over a few principles of turbocharging. Keep in mind it’s not only the cubic-inch displacement of the engine that it’s rated at and its rated manifold pressure that determine the engine’s performance. Power is also affected by the temperature of the air as it’s swept into the cylinders. The temperature of the air greatly affects the density of the air. It’s the weight of the air, not the volume, that produces the power.
Now at sea level, assuming a standard day, sea level air density is 0.0765 pounds per cubic foot whereas at 10,000 feet, on a standard day, air density drops to 0.0565 pounds per cubic foot. So in a naturally aspirated engine, let’s say it’s rated at 100 horsepower at sea level. It generates only 73.9 horsepower at 10,000 feet.
Why do we bother turbocharging? It’s for the simple reason that power diminishes with an increase in altitude. How much manifold pressure is lost for every 1,000 feet of altitude gained? Most of you know the answer to that. It is approximately 1 inch for every 1,000 feet of altitude. That calculates to around 3 to 4 horsepower lost for every 1,000 feet gained. Remember, power is inversely proportionate to altitude gained. Increased altitude comes with a price — loss of power. At about 18,000 feet, the air pressure and the oxygen molecules are about half that of sea level pressure and air density. The bore stroke of the pistons hasn’t changed. We are still drawing in the same volume of air. However, there’s less mass, so there’s less oxygen to mix with the fuel — there’s less to burn. It shouldn’t come as a surprise that a normally aspirated engine is only going to produce about 50 percent of its maximum rated power at 18,000 feet. We need some method to pump more air into the induction, to increase that air going into the induction at increased altitudes. Turbocharging provides that additional mass of air required to boost an engine’s power output at these varying altitudes.
This introduction to turbocharging is based on portions of AMT’s virtual IA seminar “Boosting Your Knowledge of Turbocharging” by Randy Knuteson, Director of Product Support for Kelly Aerospace Power Systems. In his presentation, Knuteson gives an overview of the many facets of aircraft engine turbocharging, including principles and definitions of turbocharging, the four basic system components, acceptable and unacceptable overboost, and maintenance of a turbo system. For more information on this and other virtual IA seminars, go to www.amtonline.com.
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