Gas Turbine Engines

Anyone who has watched a windmill in operation has witnessed the principle of the turbine engine at work. The wind blows against the blades of the windmill and causes them to spin. The blades are connected to a shaft that in turn does some work like pumping water or grinding corn. The amount of work that the windmill can do depends on the strength of the wind. The problem then is to find a way to maintain a constant flow of air through the windmill blades.

Let’s imagine that we have a tube. At one end is a fan that we’ll call the compressor. It has a shaft running through it to the other end of the tube (the exhaust) where there’s a smaller fan also connected to the shaft. Now the middle of the tube contains a nozzle through which we can inject a fuel like kerosene. This area is the combustion chamber. If we start our fans spinning, the compressor fan will force air back into the combustion chamber where it will mix with the fuel that we’re injecting.

If we ignite this mixture, it will burn and produce hot, high-pressure gasses that flow out of the exhaust at high speed. Some of this gas causes the exhaust fan to spin which in turn causes the compressor fan to spin faster and force more air into the combustion chamber. Now we put our windmill blades at the very end of the tube and the hot, high-pressure exhaust gas now is a constant strong wind to make our windmill do work. What we have built is a working gas turbine engine.

The characteristics of the turbine engine (which we’ll discuss in more detail later) made it an obvious choice for aircraft engines. The first flight of a turbine powered aircraft took place in the 1930’s. Shortly after the end of World War II, General Electric, using a design developed by a British inventor, was hard at work on a turbine engine for commercial aircraft. Today almost all commercial aircraft are powered by a special type of turbine engine called a turbofan. Turbine engines also drive the emergency power plants used by hospitals when earthquakes and other natural disasters interrupt the supply of electricity.

Turbines have some characteristics that make them natural choices for these types of uses. Compared to the engine in your car, a turbine of the same size and weight can provide several times as much power and produce lower exhaust emissions too. Turbines can run on kerosene, jet fuel, vegetable oil or just about any flammable liquid. They have few moving parts and are very reliable. Why then aren’t they used everywhere? Well, like everything else, turbines also have their disadvantages.

One of the obvious places that turbines could have proved useful was in automobiles. A lighter, more powerful engine that could burn a variety of fuels cleanly seems like an automobile designer’s dream. Indeed, Chrysler Corp. for many years championed the use of the turbine in cars. In the early 60’s they even manufactured a fleet of turbine-powered cars and gave them to customers to evaluate.

Unfortunately, the results weren’t good enough to justify a turbine car. For all of its great qualities, the turbine engine also presents some unique challenges. Although it has fewer moving parts than other engines, almost all of these parts are exposed to the extreme heat of combustion and exhaust.

This means they all must be made of fairly exotic materials in order to survive. This translates to a high cost. The exhaust must be cooled down before it’s allowed to exit the car to prevent melting the bumper on the car behind and it must be made quiet. The Chrysler engineers were very successful in solving these problems but others remained.

Although turbines operated at very low emissions levels when used in planes, the automotive environment was much different. The engine in a plane runs at a constant speed most of the time but a car’s engine is constantly speeding up and slowing down. Under these conditions, the turbine didn’t show much of an advantage in emissions or fuel economy over a normal internal combustion engine.

But the thing that hurt the turbine most seems almost trivial. When a turbine engine is accelerated, there is a noticeable lag between the time the accelerator is depressed and the engine speeds up. This is because the fan can’t accelerate instantly.

Although this lag is very small, to people who were used to the instant acceleration of the internal combustion engine, it was annoying. This was the most frequently reported flaw in the car. In the end, Chrysler decided that the turbine engine didn’t have enough of an advantage to justify using it as an automotive power plant.

But the turbine remains the mainstay of those applications where a high power to weight ratio is important especially in the airline industry.

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