11

Understanding Aircraft Engines

•History •Types Of Engines used on Aircraft •Basic understanding of their operation•Sections / Modules of a Gas Turbine Engine.•Significant Components installed on the Engine.

For Flight Operation Officers

Engr, Zafar I. Jami

22

Short HistoryThe engine that Wright Brothers used to

power the first aircraft was a standard automobile piston engine based on Otto Cycle.

Later, engines were made lighter and more powerful. These requirements lead to development of several versions. More common types in use were:HO, V type liquid cooled and Radial air cooled.

World War II brought rapid developments and advent of the Jet age. These also blossomed into several types:Turbojet, Turboprop and Turbofans.

33

Piston Engines.Piston Engines are based on Otto Cycle. Which is same as that used for most automobile engines. It normally works on a Four Stroke Cycle consisting of:

1. Intake2. Compression3. Power and4. Exhaust strokes.

There are three most common types which are named according to the arrangement of cylinders around the crankshaft. These are

• Horizontally Opposed (4 to 8 Cylinders)-80 to 400 Hp range, aircooled,

• Vee type ( 8-12 cylinders) usually water cooled. Used on high performance aircraft in WW II due lower Drag.

• Radial (7 to 11 in Single Row or) 14 to 18 in Double Row. Aircooled. -900 to 4000 Hp range.

44

Gas Turbine Engines.•They work on Brayton Cycle where:

• The air is compressed in the Compressor.•Then it is burned in the Combustor.•The hot gasses pass through the turbines that extract energy to run the Compressor, Fan and accessories installed on the Engine.•Then the gasses exhaust at very high velocity producing the required Thrust.

Typical Brayton Cycle for a Gas Turbine.

ExhaustNozzle

55

Types of Gas Turbine Engines• The first generation Gas Turbine Engines were Turbojets that sucked air from the front and exhausted them at high speed from rear producing reaction that pushed the aircraft forward.•Next came the Turbofans where a part of air was exhausted without going through the core producing additional thrust. These are also called Bypass Jets and are more efficient than the turbojets.•Concurrently, for lower speed application, the energy from exhaust was used to drive the propeller after reducing its speed through a Gearbox. These are called Turboprops.• The same engine, when used to power the Helicopter’s Rotor instead of a Propeller, is called a Turbo shaft Engine.

Major Modules of a High Bypass ( PW JT9D )Turbofan

Early Bypass jet

Fan/LPC Core LPT

66

How an Engine powers the aircraft to fly?Propeller Aircraft:

Propellers have airfoil shaped blades that generate Lift in forward direction when rotated by the Engine.

Jet Aircraft:It works on the Newton’s

Third Law of Motion:Every Action Has a

Reaction.It discharges a jet of fast

moving gas backward whose reaction pushes the aircraft forward.

7

Engines and PowerplantThe Engine in the form that is Ready For

Installation (RFI) on an Aircraft, is commonly called a Powerplant.

It consists of Engine + QEC (Quick Engine Change):QEC – consists of Plumbing, Harness and

Components peculiar to a position on an aircraft. Commonly installed Components on the engine include: Starter Hydraulic Pump/s. IDG (or CSD + Generators). Anti-icing valves and ducts. Customer Bleeds.

7

Note: - All OEMs have their own divisions and names for Modules.

8

Basic Sections (or Modules ) of a Modern Turbofan Engine – (1).A. Fan and LPC: It consists of a Single Stage fan which

has a much larger diameter than other stages and pushes the bypass air out without passing through the “Core” of the engine. In addition it has a few (usually 3-5 stages) of Low pressure compressor to reduce load on HPC.

B. Core Module: This consists of:a. High Pressure Compressor (HPC): This usually has 7-

14 stages of alternate rows of blades and vanes that progressively increase the pressure to desired value.

b. Diffuser and Combustor: In this section, the passage is widened to slow down the airflow and increase its static pressure. The air is then led to Combustor unit where it is mixed with atomized fuel which burns to provide high energy.

9

Basic Sections (or Modules) of a Turbofan Engine (2).

c. High Pressure Turbine (HPT): It consists of One or Two Stage of Vanes and Blades. First stage vanes face the highest temperature. It extracts power to run the HPC and the Accessory Gear Box.

C. Low Pressure Turbine(LPT): This usually has 3-6 stages of alternate rows of blades and vanes that progressively extract power to rotate the Fan/LPC.

D. Accessory Drive Gear Box (AGB): It provides a drive for all components and accessories installed on the engine. It gets power from the HP Shaft through a series of gears and shafts.

10

Some significant Components installed on a Jet Engine…. (1)Fuel Pump : It gets fuel from the

aircraft when the fuel shut off valve is in ON position. It usually have two stages. It delivers more fuel than required in the engine. The next unit bypasses the excess fuel back to output of the first (centrifugal) stage.

Engine Fuel Control: In general, it receives fuel from the fuel pump, then computes the fuel required by the engine and bypasses the remaining fuel back to Fuel Pump. It can be Fully Mechanical, Hybrid or Fully Electronic (Digital)- which are also called EEC or FADEC .

11

Some significant Components installed on a Jet Engine…. (2)Starter : They are either Electric or Pneumatic

driven. Pneumatic starters are more common on larger engines. But the latest trend is again towards Electric starters.

They are mounted on Accessory Gear Box and drive Engine’s (HP) shaft through a gear train till the engine reaches a self sustaining RPM.

Pneumatic supply can be obtained from following sources.Ground Start CartAuxiliary Power Unit

Which itself has an electric starter that can be run on Aircraft batteries.

Through Cross Bleed from another running engine.Through air bottles in emergency.

12

Significant Engine Components..(3)Electric Generator: They are also mounted on the Accessory

Drive Gearbox.As they have to run at constant RPM, power

to run the Generator is transmitted from engine’s HP shaft through a Constant Speed Drive Unit.On newer aircraft, this unit is integral with the

Generator and housed in the same unit. Together, they are called IDG.

Usual supply from the generator is at 115V /400Hz.

13

Significant Engine Components… (4)Hydraulic Pumps and Plumbing.Hydraulic power is needed in aircraft to

operate Landing Gears and several other components.

Larger twin engined aircraft usually have two pumps on each engine, while the smaller ones have a single pump.

On most aircraft they provide hydraulic fluid at 3000 psi.

There are three tubes for pressure, scavenge and drain fluids which are joined with tubing from aircraft near top of the pylon.

14

Significant Engine Components… (5)Engine Anti-icing and Customer Bleeds. As engine compressor is a ready source of

high temperature and high pressure air, some of it is bled out for different uses.

Anti-icing air is bled from later sages, which is very hot. As and when selected, it runs through the lip of nose cowl or wing Leading edge and exhausts to atmosphere after heating it.

Customer Bleed air is often taken from alternately from one of two stages in compressor (depending upon requirement), and fed to the aircraft’s air-conditioning and pressurization system for controlling Cabin’s pressure and temperature.

1515

What a Flight Dispatching Officer must know about the Engines. As the weight of the

aircraft increases, the engine has to work harder, not only during takeoff, but also during Cruise and Landing.

This can result in two things:

1. Engine under certain conditions may fail.

2. Repeated applications of higher power will lower its life on wing.

So remember, the Engines that work harder die young.

Also, when such engines go to the Shop, they are in more depilated condition and cost much more.

As such, always try to minimize All Up Weight and ensure that it never exceeds the weight permitted by the OEM’s manuals.

1616

How to be a good ‘Weight watcher’ of an AircraftBe very vigilant to look for

the following: Unaccounted weights

specially in passengers’ baggage and hand-carries.

Higher temperatures than expected at the time of takeoff. Specially, a delayed

morning flight.o Excess fuel load even

within spec limits is bad for engines’ health.

.Avoid fuel tankering for economic reasons unless it will pay for added engine maintenance costs.

Consider safety first. If the recommended weight is being exceeded, never hesitate to recommend offloading of passengers, baggage or cargo even if it means loss of revenue and delays.

17

Thank You!Warning

The presentation has been specifically prepared by the author for a Pakistan CAA approved course

conducted at Academy for Aviation Excellence and material or pictures may not be copied or used without

specific authority from the author the Academy. Some material may also have a copyright.