I think a brief explanation of the main types of engines would be a good read, especially for people like me who don't have the expertise needed to understand the nuances of these complex machines ;
http://www.boldmethod.com/learn-to-fly/systems/the-4-types-of-turbine-engines/
How The 4 Types Of Turbine Engines Work
Live from the Flight Deck
Gas powered turbine engines have come a long way since 1903. That was the first year a gas turbine produced enough power to keep itself running. The design was accomplished by Norwegian inventor Aegidus Elling, and it produced 11 horsepower, which was a massive feat at the time.
These days, gas turbine engines come in all shapes and sizes, and most of them produce
a lot more than 11 horsepower. Here are the 4 main types of turbine engines, as well as the pros and cons of each.
1) Turbojet Engine
Wikipedia
Heinkel He 178, the world's first turbojet aircraft
Turbojet engines were the first type of gas turbine engine invented. And even though they look completely different than the reciprocating engine in your car or plane, they operate using the same theory:
intake, compression, power, exhaust.
How Does A Turbojet Work?
Turbojets work by passing air through 5 primary sections of the engine:
Step 1: Air Intake
The air intake is essentially a tube in front of the engine. The air intake may look simple, but it's incredibly important. The intake's job is to smoothly direct air into the compressor blades. At low speeds, it needs to minimize the loss of airflow into the engine, and at supersonic speeds, it needs to slow the airflow below Mach 1 (the air flowing into a turbojet needs to be subsonic, regardless of how
fast the airplane is flying).
Step 2: Compressor
The compressor is driven by the turbine in the rear of the engine, and its job is to compress the incoming air, significantly increasing the air's pressure. The compressor is a series of 'fans', each with smaller and smaller blades. As air passes through each compressor stage, it gets more compressed.
Step 3: Combustion Chamber
Next up is the combustion chamber, where the magic really starts happening. The high pressure air is combined with fuel, and the mixture is ignited. As the fuel/air mixture burns, it continues through the engine toward the turbine. Turbojets run very lean, with approximately 50 parts air to every 1 part of fuel (most reciprocating engines run anywhere from 6-to-1 to 18-to-1). One of the main reasons turbines run this lean is that extra airflow is needed to keep a turbojet cool.
Step 4: Turbine
The turbine is another series of 'fans', which work like a windmill, absorbing energy from the high speed air passing through it. The turbine blades are connected to and turn a shaft, which is also connected to the compressor blades at the front of the engine. The turbjet's 'circle of life' is almost complete.
Step 5: Exhaust (aka "I'm outta here!")
The high speed burned fuel/air mixture exits the engine through an exhaust nozzle. As the high speed air exits the rear of the engine, it produces thrust, and pushes the airplane (or whatever it's attached to) forward.
Turbojet takeaway:
- Pros:
- Relatively simple design
- Capable of very high speeds
- Takes up little space
- Cons:
- High fuel consumption
- Loud
- Poor performance at slow speeds
2) Turboprop Engine
Live from the Flight Deck
King Air with turboprop engines
The next three types of turbine engines are all forms of the turbojet engine, and we'll start with the turboprop. The turboprop is a turbojet engine, connected to a propeller through a gearing system.
How Does A Turboprop Work?
Step 1: The turbojet spins a shaft, which is connected to a gearbox
Step 2: A gears box slows down the spinning, and the slowest moving gear connects to the propeller
Step 3: The propeller rotates through the air, producing thrust just like your Cessna 172
Turboprop takeaway:
- Pros:
- Very fuel efficient
- Most efficient at mid-range speed between 250-400 knots
- Most efficient at mid-range altitudes of 18,000-30,000 feet
- Cons:
- Limited forward airspeed
- Gearing systems are heavy and can break down
3) Turbofan Engine
Live from the Flight Deck
Some wide-body turbofan engines can produce more than 100,000 pounds of thrust
Turbofans combine the best of both worlds between turbojets and turboprops. And, you'll probably see these engines when you head out to the airport for your next airline flight.
How Does A Turbofan Work?
Turbofans work by attaching a ducted fan to the front of a turbojet engine. The fan creates additional thrust, helps cool the engine, and lowers the noise output of the engine.
Step 1: Inlet air is divided into two separate streams. One stream flows around the engine (bypass air), while the other passes through the engine core.
Step 2: Bypass air passes around the engine and is accelerated by a duct fan, producing additional thrust.
Step 3: Air flows through the turbojet engine, continuing the production of thrust.
Turbofan takeaway:
- Pros:
- Fuel efficient
- Quieter than turbojets
- They look awesome
- Cons:
- Heavier than turbojets
- Larger frontal area than turbojets
- Inefficient at very high altitudes
USAF
Pratt & Whitney F100 turbofan with afterburner on an F-16
4) Turboshaft Engine
NASA
Bell 206 helicopter with turboshaft engine
Turboshaft engines are primarily used on helicopters. The biggest difference between turboshafts and turbojets is that turboshaft engines use the majority of their power to turn a turbine, rather than produce thrust out the back of the engine.
How Does A Turboshaft Work?
Turboshafts are essentially a turbojet engine with a large shaft connect to the back of it. And since most of these engines are used on helicopters, that shaft is connected to the rotor blade transmission.
Step 1: The engine operates like a turbojet, for the most part.
Step 2: The power shaft attached to the turbine powers the transmission.
Step 3: The transmission transfers rotation from the shaft to the rotor blade.
Step 4: The helicopter, through mostly unknown and magical means, is able to fly through the sky.
Turboshaft takeaway:
- Pros:
- Much higher power-to-weight ratio than piston engines
- Typically smaller than piston engines
- Cons:
- Loud
- Gear systems connected to the shaft can be complex and break down
4 Types Of Engines, Based On The Same Basic Concept
Gas turbine engines have come a long way in the past 100 years. And while turbojets, turboprops, turbofans and turboshafts all have their differences, they way they produce power is essentially the same: intake, compression, power, and exhaust.
