Gas turbine engines
Gas turbine engines have a wide variety of applications and the most common are automobile engines. They work on a simple principle of compressing an air-fuel mixture at high pressures than igniting it. The explosive force that is produced acts as thrust and it is used to induce mechanical motions on the pistons of the engine which rotate the wheels. The same principle is used in aircraft but the major difference is the thrust is used to turn the blades of a propeller which pushes the plane forward. Its other uses are in generators and some water pumps. (Treager, p. 36)
How it Works
When you turn on the ignition key in your car, an electric motor that is powered by the car battery immediately starts spinning. This motor has fans and starts drawing air from the atmosphere pushing it into the chambers of the engine. This air passes through the blades of the fans at a high velocity and enters the compression chamber which is of lower volume than the surrounding atmosphere. The decrease in volume according to Boyle’s Law increases the pressure of the air. (Pressure is inversely proportional to the volume in which it is contained). The turning on of the ignition also subsequently causes the petrol in the tank to start moving towards the combustion chamber in the engine.
The fuel passes through a narrow nozzle before it mixes with the air. The sudden decrease in volume as the petrol passes through the nozzle causes it to “change phase into an almost vapor phase by a process called atomization” (Kerrebrock, p. 124). Now the fuel in the vapor phase starts mixing with the compressed air and the mode of ignition of the fuel depends on the type being used. For diesel-powered engines, they work on the principle of compression ignition whereby the pressure itself of the fuel-air mixture in the combustion chamber causes the diesel fumes to self-ignite hence initiating the combustion phase. For petrol engines, the atomization of the petrol at the nozzles creates a “fuel-air mixture (flue gas) and almost immediately the spark plugs release a spark which ignites the petrol and marks the beginning of the combustion process.” (Kerrebrock, p. 125) It is worth noting that the entire process from turning the ignition key to the combustion of the air-fuel mixture takes a matter of seconds.
The ignition of the air-fuel mixture is an explosive process and a lot of power is released. This power is converted to mechanical force by the pistons that are attached to the combustion chamber. The pistons have an up-down motion and they work opposite to each other such that when one pair of pistons for a four-piston engine is up, the other pair is down. The total combination of the forces of the piston produces a cylindrical motion on the crankshaft which for the case of the automobile, goes to spin the fly-wheel which in turn spins the wheels of the car. For an airplane, the spinning crankshaft is used to turn the propeller which produces the necessary thrust for flight. Higher power output is obtained from the engine by compressing more air into the combustion chamber and initiating something of a chain reaction in the already existing combustion process. Switching gears alters the degree of rotation on the wheels and in a way the amount of power that is transferred to the pistons and crankshaft to the wheels. A high gear causes a deficit in the torque that is rotating the wheels and this extra power is drawn from the engine. A lower gear and the opposite happens whereby the opposite happens and the pistons relax since there is enough torque at the wheels. (Kerrebrock, p. 126).
Works Cited
- Treager Irwin, Aircraft Gas Turbine Technology, Career Education, 3rd Edition, pp 36-39
- Kerrebrock Jack L, Aircraft Engines and Gas Turbines, MIT press, 2nd Edition, pp 123-126