April 24, 2008
Some history and basics of aircraft turbocharging systems

Since the beginning of time, man has always looked for ways to add power to the machines that he’s built. The combustion engine is a prime example of a power source that was just waiting to be improved upon. Since the very beginning of the 20th century, inventors and industrialists alike have looked for ways to boost the performance of engines.

As surprising as it may seem, the typical piston engine converts only about one-third of its potential energy from fuel into useful work. The remaining energy is lost to other forces such as friction and cooling losses. A major part of the potential energy is simply wasted out the exhaust.

Fortunately for us some early inventors found the power boost they were looking for. They found it in the form of turbochargers and superchargers.

Historical perspective
Looking at turbocharging from a historical perspective, you could actually go back as far as the late 1800s, to the German inventor Gottlieb Daimler, or even Rudolf Diesel, who was credited with designing the mechanical supercharger way back in 1896. But for the sake of this discussion, we’re going to start with Swiss engineer Dr. Alfred Buchi. In 1905 Buchi was granted the first patent for a practical turbocharger — a supercharger driven by exhaust gas pulses.

General Electric began to manufacture turbochargers in 1910. In 1915, working as chief engineer for Sulzer brothers research department, Dr. Alfred Buchi proposed and developed the first prototype of a turbocharged diesel engine. Unfortunately, it wasn’t very efficient. It wasn’t efficient enough to maintain adequate boost pressure.

1918 was another important year, probably the benchmark year for aviation-related turbocharging. It was in this year that Dr. Sanford Moss, an engineer for General Electric, carted a 350-horsepower engine to the top of Pike’s Peak in Colorado. And there, in the thin air of the summit of the second-highest mountain in Colorado, at 14,109 feet, Moss was able to boost the power output of that engine to 356 horsepower.

1920 was also an important year in the ongoing history of turbocharging. A turbocharged 12-cylinder Liberty engine was installed on a LaPere bi-plane for altitude tests. They selected this plane, surprisingly enough, because they believed it would be less likely to break up in the event of a long fall from altitude or a violent pull out. A young man named Lt. John Macready was selected to fly the plane. He took it to 33,113 feet! Macready didn’t stop with that altitude. In fact, in a very short time, he became one of the most experienced high-altitude flyers in the world, testing turbochargers from 1917-1923. The highest he flew in his open-cockpit plane was an indicated 40,800 feet on Sept. 28, 1921.

Turbo technology evolved rapidly during the war years. The full strength of blowers was certainly tested during World War II. As you can imagine, the B-17 and the B-29 bombers, along with the P-38 and P-51 fighters, were fitted with turbochargers and controls. The B-36 bomber had six piston engines, each with 28 cylinders. The flight engineer’s handbook for the B-36 states that without turbochargers, the B-36 would require 90 cylinders per engine to achieve the same performance as the turbo supercharged design.

In the late 1980s and early 1990s, the Grob Strato 2C set an unofficial record for piston-powered manned flight when it reached an altitude of 54,574 feet. This unique plane was fitted with the world’s largest all-composite wing at a wingspan of 185 feet. The aircraft was designed to perform missions for communications monitoring, geo-physical research, and pollution and weather observation.

In terms of unmanned flight, in 1986 the Boeing Condor set an altitude record for recip engines at 66,980 feet.

Engine horsepower
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 turbocharge?
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.

About the Author

Joe Escobar