TEXT: Gas Turbines and Other Types of Engines

The idea of an internal combustion engine to drive a turbine, rather than pistons, evolved on paper as long ago as 1791 but it needed modern technology to make such an engine possible. The problem was that the blades of a turbine could not with­stand the great heat resulting from the combustion in such an engine. Now new alloys or mixtures of metals, as well as some ceramics and crystals that do not fail or disintegrate at high degrees of heat have been developed.

In a basic gas turbine engine air is taken in and compressed un­til it becomes extremely hot. The compressed air is then mixed with fuel, usually kerosene, which ignites. The expanding gases caused by the combustion cause the turbine to turn. The turbine then turns a shaft which performs the actual work. In a turboprop engine for aircraft that shaft turns a propeller.

Experiments to adapt gas turbines for other forms of transportation have not been entirely successful. A major drawback, especially in view of today's energy crisis, is the necessity for large amounts of fuel; the combustion is continuous in gas turbines rather than intermittent as in piston engines.

The first successful turbojet engine was designed by Frank Whittle, an English officer in the Royal Air Force. Jets were in use as military aircraft before the end of World War II but they did not come into commercial use until the 1950s. They have enormously increased the speed, range, and size of modern airplanes. Some types of turbojets can propel a plane at speeds faster than sound. The use of engines at supersonic speeds is one of the more controver­sial subjects of our time.

In a turbojet engine air is taken in by a blower that operates on a shaft from a turbine; the waste gases are expelled from the rear of the engine at an extremely high rate of speed. The plane is driven forward by a practical illustration of Newton's Third Law of Mo­tion: for every action there is an equal and opposite reaction; therefore the thrust of the gases backward pushes the plane for­ward.

The type of turbojet in common commercial use is the fennec. The intake blower is in effect a highly sophisticated version of tin-everyday electric fan. Two other types, used for military, are the pulsejet and the ramjet. The pulsejet, used by the V-l bombs to attack London during World War II, has lateral intake valves that produce a pulsing or vibrating movement; it pressure opens the valves, the explosions of fuel close them and push against them to provide forward propulsion; no turbine is necessary in this type of engine. In the ramjet, air is rammed or forced into the intake at such pressure that no blower or turbine is necessary, only a means of injecting the fuel.

A word about the energy crisis: the world's supply of petroleum was created millions of years ago and it cannot be replaced or renewed in our time. Estimates vary on how long the supply will last but according to some experts it may not be much more than thirty years at the present rate of consumption. Automobiles, diesels, and jets use enormous amounts of fuel derived from petroleum as do households and power plants that produce electricity. Petroleum is also the basis for petrochemical products including many of today's plastics, fertilizers, and insecticides.

Therefore there is a mounting interest in engines that do not use petroleum as fuel. Some power plants are already converting from oil to coal, but while coal is in much greater supply than petroleum it is another nonrenewable energy source which will eventually be exhausted. Experiments are under way to harness such energy sources as the wind, the tides, and the sun. Nuclear fusion —the release of energy when atoms join together—is being ex­plored as a safer alternative to nuclear fusion with its hazardous by­products of radioactive wastes that pose a serious threat to the environment and to human life. The difficulty with fusion is that it requires an enormously high degree of heat to start the reaction; to date it has not been possible to generate that much heat even under laboratory conditions.

There is much interest today in hot-air and rotary engines. The hot-air engine has a long history: a Scotsman, Robert Stirling, built one in 1827 so the hot-air engine is often called the Stirling engine. Then John Ericsson, a Swede who became a citizen of the United States (he is best remembered as the designer of the ironclad ship Monitor during the Civil War), built and marketed thousands of hot-air engines. The Stirling engine has two cylinders, one of which compresses air; when the air is heated it expands and pushes down a piston in the other cyl­inder. Engineers today are working to improve the ba­sic Stirling engine and this offers a promise of greater fuel efficiency.

A rotary engine should more properly be called a rotating internal combus­tion engine. Instead of the reciprocating motion of pistons, a rotary motion is produced directly. The best-known rotary engine is the Wankel engine, named for its German inventor, Felix Wankel. The combustion in the Wankel engine turns a rotor that is triangular, though the outer edges are curved outward or convex. It produces almost no vibration because it has fewer parts; for this reason it is cheaper to manufacture but its fuel efficiency is still in question. Automo­biles with Wankel engines use gasoline as fuel since that is what is available; if other fuels become commercially feasible the Wankel engine may be more economical than it is at present. Other inventors have produced other types of rotating combustion engines which offer interesting possibilities in experimental models.