Turbine Engine Types

Question 1

What are the main types of turbine engines? 4

Answer 1

Turboshaft, turboprop, turbojet, turbofan.

Question 2

What is the definition of the bypass ratio of a turbofan engine?

Answer 2

The ratio of the mass of air bypassing the engine core, to the mass of the air passing through the engine core.

Question 3

What is the main similarity between a turbine engine and reciprocating engine?

Answer 3

Both the turbine and reciprocating engines are internal combustion.

Question 4

What is the main advantage of a turbine engine over reciprocating engine of the same power output?

Answer 4

The turbine engine has a much higher power to weight ratio than a reciprocating engine of a similar power output.

Question 5

How does the combustion cycle of a turbine engine compare with that of a reciprocating engine?

Answer 5

A turbine engine combustion takes place at a near constant pressure, while in a reciprocating engine combustion takes place at near constant volume.

Question 6

What is the functional differences between turboprop and a Turboshaft?

Answer 6

A turboprop engine has an output shaft which drives a gearbox for a propeller while a Turboshaft engine has an output shaft which drives a gearbox for something other than a propeller, e.g. a helicopter’s main rotor gearbox.

Question 7

How is a free power turbine characterised?

Answer 7

It is mechanically completely independent of the other spools in the engine.

Question 8

What are the advantages of a free turbine propulsion system? 4

Answer 8

1. Easier to start.
2. The spools can run at their optimum speeds, independent from each other.
3. Design flexibility of layout.
4. The ability to stop the propeller or helicopter rotors while the engine is still running.

Question 9

Why does a free turbine turboprop engine require a smaller starter motor than direct-drive and compound turboprops?

Answer 9

Because only the high pressure turbine and compressor need to be rotated on start.

Question 10

Which part of a turboprop engine usually powers the accessory gearbox?

Answer 10

The high pressure compressor (N2) section.

Question 11

What drives the main drive gearbox in a free turboprop or Turboshaft?

Answer 11

The low pressure turbine(s).

Question 12

What is the meaning of the term “spool”?

Answer 12

A spool is an independent compressor, shaft, turbine combination.

Question 13

What is compressor/ turbine combination called?

Answer 13

A spool

Question 14

Why do turbine engines have multiplayer spools?

Answer 14

So that each can operate closer to their design rpm.

Question 15

In a twin spool turbine engine, which are the high pressure and low pressure stages of the compressor and turbine?

Answer 15

The first stage(s) of the compressor and the last stage(s) of the turbine are low pressure, while the last stage(s) of the compressor and the first stage(s) of the turbine are high pressure.

Question 16

Within an engine’s spools, what drives the intermediate pressure compressor?

Answer 16

The intermediate pressure turbine.

Question 17

Within an engine’s spools, what drives the low pressure compressor?

Answer 17

The low pressure turbine.

Question 18

What is the description of a turbofan engine?

Answer 18

A fan/ low pressure compressor supplying supercharged (pressurised) air to the engine core and to the bypass duct. A ducted propeller, suitable for a powering high speed aircraft.

Question 19

What is the ‘core’ of a turbofan engine comprised of? 4

Answer 19

The compressor, combustion chamber, turbine and exhaust section.

Question 20

What is the bypass ratio if 20% of the intake air passes through the engine core?

Answer 20

4:1

Question 21

In terms of bypass ratio, how does a pure turbojet compare to a turbofan?

Answer 21

A pure turbojet has no bypass air.

Question 22

What percentage of bypass air passes through the compressor of a turbofan engine?

Answer 22

0%

Question 23

Why is a turbofan engine more efficient than a turbojet engine?

Answer 23

Because turbofan accelerate a relatively larger amount of air a relatively lower amount.

Question 24

Why are turbofan engines more resistant to FOD ingestion than pure turbojet engines?

Answer 24

Because the fan blades are bigger and stronger than the first stage compressor stages of the turbojet, and because most of the air bypasses the engine core.

Question 25

What are the advantages of a twin spool over a triple spool bypass engine? 2

Answer 25

Lower cost.

Easier maintenance.

Question 26

What are the advantages of a triple spool over a twin spool bypass engine? 2

Answer 26

Shorter and lighter.

Better range/ fuel consumption.

Question 27

What is the name given to turbine engine which has propeller blades mounted radially from the low pressure turbine?

Answer 27

An aft fan turbojet.

Question 28

What output is provided by an APU?

Answer 28

AC electrical power and bleed air.

Question 29

What is the primary purpose of an APU?

Answer 29

To provide AC electrical power and pneumatic bleed on the ground, and in flight in the event of a main engine failure.

Question 30

What section of an APU provides pneumatic bleed air for engine start?

Answer 30

The compressor