Section 1 EO M232.01 – IDENTIFY TYPES OF AIRCRAFT ENGINES

A powered aircraft needs a means of propulsion to overcome drag and allow the wings to generate sufficient lift to overcome weight.

The propeller and jet engine are very closely related, providing thrust by the same means – the acceleration of a mass of air. The propeller generates thrust by acting on a large mass of air, giving it a relatively small acceleration. The jet engine does exactly the same thing by giving a larger acceleration to a smaller mass of air.

The most common engine types used for aviation propulsion employ internal combustion and they include:

Show the cadets a slide or handout of rocket engine applications in Figures A-1 and A-2.

The earliest vehicle engines were rocket engines used to power ancient Chinese fire arrows. This method of propulsion proved so effective that, with many improvements, it is still commonly used today for many applications including space exploration. Self-contained with their own oxidizer, rockets have the great advantage of being able to function in a vacuum such as outer space.

All propulsion systems are reactive, meaning that they all employ an equal and opposite reaction predicted by Newton’s third law of motion.

Piston-powered internal combustion engines were developed in the late nineteenth century. They were available to Orville and Wilbur Wright, who designed their 1903 flyer with a four-cylinder piston-powered engine.

Show the cadets a slide or handout of a Harvard piston-powered engine application in Figures A-3 and A-4.

Piston-powered engines are the most common vehicle engine of all and the one that Proficiency Level Two cadets will explore in most detail. In many ways, pistons are the most complicated system of converting the chemical energy of fuel into the energy of motion but they are found in many places, including aircraft, automobiles, boats and lawnmowers.

Show the cadets a slide or handout of a gas turbine jet engine application in Figures A-5 and A-6. Point out the airflow path and combustion location in the schematic.

Gas turbine jet engines are improvements upon simple ramjets. The ramjet is a liquid-fuelled rocket-like engine, which uses atmospheric oxygen to burn fuel. One of the most limiting aspects of a ramjet is that it requires high velocity to work. Therefore it cannot start combustion until it is up to speed – it must be launched from a speeding vehicle. Air-launched missiles are one of the few applications of ramjet engines.

Any turbine converts the energy of moving liquid or gases, such as jet exhaust or wind, into rotary motion to turn a shaft. A windmill is a turbine which uses wind energy to turn a shaft. Among other advantages, adding a turbine to the simple ramjet allows a compressor to generate high-pressure air so that the gas turbine jet engine can be started from a resting, or static, position. This is the secret of the modern gas turbine jet engine, which still relies on the ejection of hot gases to produce thrust. Until the turbine and compressor are functioning and delivering high-pressure air to the engine, however, the engine cannot start. Even gas turbine jet engines, therefore, must be started with a starting motor.

Show the cadets a slide or handout of the CT-114 Tutor turbojet engine application Figures A-7 and A-8.

A gas turbine jet engine that provides thrust, with no rotating shaft output, is a TURBOJET engine.

Show the cadets a slide or handouts of the C-130 Hercules turboprop engine application Figures A-9 and A-10.

A gas turbine jet engine that provides thrust and also drives a propeller is a TURBOPROP engine.

Show the cadets a slide or handout of the CH-146 Griffon turboshaft engine application Figures A-11 and A-12.

A gas turbine engine that drives a helicopter rotor is usually a TURBOSHAFT engine. In a turboshaft helicopter engine, the output driveshaft is separate from the compressor turbine shaft so that engine speed is not tied to the helicopter’s main rotor speed.

Show the cadets a slide or handout of the CC-150 Polaris (A310-300 Airbus) turbofan engine application Figures A-5 and A-6. Point out the fan location.

The most common variation of the gas turbine jet engine is the TURBOFAN, which is a hybrid of a turbojet and a turboprop. The turbofan has a fan that provides thrust with bypass air, in place of a propeller, adding to the reactive thrust of the ejected exhaust gases. This application allows the aircraft to go faster than normal propellers could go, while also reducing engine noise and allowing the aircraft to make efficient use of fuel. The noise reduction and fuel efficiency of turbofans make them very effective for commercial aviation.

All three of these engine types, rocket, gas turbine jet and piston-powered engines, use internal combustion to capture the energy of expanding hot gases in a closed container.

All rocket, jet and piston-powered engines are internal-combustion engines because they all use burning fuel to generate power from expanding gases in a closed container. However, all these engine systems have important differences that distinguish them from one another.

Show the cadets a slide or handouts of combustion in Figure B-1.

When fuel is oxidized it gives off heat. The heat causes expansion of the gases that result from the oxidization. If oxidization is very slow it is usually referred to simply as oxidization, or rusting. If the oxidization is faster, it is often referred to as burning. If it is very rapid, it is referred to as an explosion. All these processes result from fuel chemically combining with oxygen. The distinguishing characteristic between them is the speed of molecular combination.

Show the cadets a slide or handout of combustion locations in Figures B-2 and B-3.

By capturing the expanding hot gases of combustion in a tightly closed container, such as a piston-powered engine’s combustion chamber or a gas-turbine jet engine’s combustor, the energy of the hot gases can be put to useful work. All the engine types discussed here contain the energy of expanding gases in a tight closed container, so all are said to be internal-combustion engines.

There have been many methods developed that direct and transmit this power. The most common is the turning of a shaft, such as a turbine shaft or a crankshaft. That shaft can then be used to turn an aircraft’s propeller, the impellor of an air compressor, or an automobile’s wheels.

A rocket applies the energy of the combustion’s expanding gases in the most direct manner, by simply ejecting them to get the equal and opposite reaction. Gas-turbine and piston-powered engines apply the energy indirectly through moving machinery.

The simplest system of combining fuel with oxygen is the self-contained system of the rocket and the most intricate is the internal combustion engine, with gas turbine jets between those extremes.

The rocket carries its own fuel and oxygen and combines them in a closed container at a rate that will generate the energy needed at any given moment. Of course, the rocket will have to start out with enough oxygen to finish the mission, since it cannot get more from outside its closed container. So, the fuel and the oxygen must be carefully calculated and loaded before launch. The hot expanding gases that result from an explosion in the rocket’s combustor are blasted out the back of the rocket at high speed through a nozzle. The nozzle applies the equal and opposite reaction of the moving gases to the body of the rocket, propelling it upward.

A jet engine is similar to a rocket engine but, because a jet engine uses air for oxidization, it must allow for the fact that air is mostly nitrogen and only about 20 percent oxygen. Therefore, obtaining enough oxygen for efficient combustion in a jet engine requires that air be somehow compressed before combustion takes place.

A gas turbine jet engine typically resembles a hollow cylinder with air sucked in the front and blasted out the back. These can be seen slung under the wing of most airliners. The fuel to be burned is stored in tanks, often in the aircraft’s wing. The air, which is sucked by a compressor fan into the front of the engine, contains the oxygen that is needed for oxidization of the fuel. The fuel is combined or mixed with the pressurized air and the mixture is detonated in a container within the engine called a combustor. As in a rocket, the hot expanding gases are blasted out the back of the engine through a nozzle that applies the equal and opposite reaction of the moving gases to the body of the engine, propelling it forward. Significant in this is the presence of a turbine beside the combustor, which uses a portion of the hot expanding gases to spin a shaft. That shaft drives the compressor fan to suck the air into the engine. This system of generating propulsion power has proven so useful and reliable that many variations of the basic theme have been developed and given names such as “fan-jet”, “turboprop” and “turboshaft”.

Specialized gas-turbine applications are explored in complementary lessons of Proficiency Level Two.

The most intricate method of generating power by oxidizing fuel is the most common. The reciprocating piston-powered engine is encountered in many applications. In a four-stroke piston-powered engine, air is carefully mixed with atomized fuel droplets and then either sucked or injected into cylinders where the mixture is detonated to drive pistons in a cycle of intake, compression, power, and exhaust. These cycles will be examined in EO M232.03 (Participate in a Discussion on the Cycles of a Four-Stroke Piston-Powered Engine).