Types of Aircraft Engines

Aircraft engines differ in two respects:

1. How they are powered
2. How that power results in thrust

For the first difference, power is created either through traditional piston-powered engines or turbines.

1. Piston engines are like you would find in an automobile, through with different configurations and durability – pistons move up and down (or back and forth) and power a crankshaft. For example, the Cessna 172 is powered by a four-cylinder engine quite similar to those in cars.

   Multiple configurations were used – inline, V, radial, and rotary. Radial was where the pistons were in circle around a central crankshaft. These engines were beneficial because it could sit right behind the propeller, and each cylinder could be air-cooled by the airflow. Rotary engines didn’t refer to Wankel rotaries, but rather to a piston engine where the crankshaft was stationary, and the cyclinders rotated around it.

   The biggest challenge with these engines, historically, was weight and cooling. They tended to be heavy for the amount of power they produced, and it was a constant battle to keep them from overheating. Air-cooling didn’t require much extra weight, but it introduced drag. Liquid cooling was all internal, but added weight.

2. Turbines create rotational power by compressing air through a series of vanes, then injecting fuel into it. This spins the turbine, and the hot gases are expelled through the rear of the engine, used to turn a shaft, or both. Turbines are considered more reliable than piston engines due to fewer moving parts and fewer points of failure.

3. …Electric: I hesitate to mention this, because it’s just starting to get some traction and remains mostly untested, but some planes are using battery-powered electric motors to generate rotational power.

Once we have rotational and/or thrust power, there are different ways to use that to move a plane forward.

1. Propeller: this can be powered by a piston engine, or by a turbine engine, in which case it’s called a turboprop. Piston engines can drive propellers directly, but turbines spin much faster, so they’re “geared down” to a lower speed.

   Most propellers “pull” the aircraft through the air, either from the nose or the front of the wings; these are known as “tractors.” However, some aircraft have propellers in the back which “push” the aircraft; these are known as “pushers.” Very few aircraft have had propellers mounted in the middle of the plane, rotating within or around the fuselage.

   A propeller is the only option for a piston-powered plane. All the options below require gas turbines.

2. Bypass Fan or Turbofan: this is a lot like a propeller, except that a “fan” rotates around the turbine – air coming into the front of the engine is either sent through the turbine to be ignited to create rotational power, or it “bypasses” the turbine to be accelerated by the fan. Air that is ignited, is also directed out the back of the engine to provide thrust.

3. Jet Exhaust or Turbojet: fuel and air ignited by a turbine can be pushed out the back of the engine to provide rocket thrust (essentially a 0% bypass). More fuel can be pumped into the exhaust jet after it’s ignited to provide extra power, which is known as an afterburner. A type of jet engine that uses the forward motion of the engine at high speed to compress air (rather than rotational compressor) is known as a ramjet.

4. Rockets: these do not compress air at all, but rather simply ignite solid or liquid fuel to produce thrust. They do not have any rotating parts.

Note that there’s sometimes overlap as to what the explosive power of a turbine is used for. In the case of a turbofan, it’s used both to turn the fan (the so-called “bypass air”), and to provide thrust via exhaust. With a jet engine, the rotational power generated is simply used to continue turning the compressor – all the thrust is generated by exhaust gases.

Which engine a plane uses is governed by factors like how slow or fast it needs to fly, what elevation it will fly at, and how expensive it is.

In general, propellers enable a plane to fly slower, but turbofans are much more efficient. Jet engines enable planes to fly the fastest, but require a certain amount of speed, which can be a problem in some situations.

(Example: the A-10 Thuderbolt is designed to attack targets on the ground. Thus, it needs to fly slower than a typical jet. It’s therefore one of the few combat planes with turbofan engines instead of jet engines.)

Today, airliners generally all use turbofans because they’re very efficient and relatively quiet. In the past, some passenger airliners used turbojets – the most common being the original Boeing 707, and the most famous being the Concorde. Over the years, turbofans proved more efficient and displaced turbojet for passenger aircraft, leaving pure jet engines mostly reserved for combat aircraft.

In general, engine sophistication goes like this:

1. Piston-powered propeller
2. Turbine-powered propeller (“turboprop”)
3. Turbofan
4. Turbojet