JET PROPULSION

Part 6

Jet Engine OperationFuel Systems

Broadly speaking there are four main categories of aviation fuel

used in the RAF and RN:

AVGAS, AVTUR, AVTAG and AVCAT

and broadly speaking there are two main fuel systems

used in jet engines:

HYDRO-MECHANICAL and

ELECTRONICALLY CONTROLLED

Introduction

Fuel SystemsHydro-Mechanical Systems

BOOSTER PUMP

WING

FUEL

TANK

Fuel is pumped from the wing tanksby electrically powered Booster Pumps.

These pumps simply push the fuel through the extensive aircraft system to the engines.

The Low Pressure (LP) Fuel Pumpprovides a positive pressure to the

high pressure fuel pump. (further on in the system)

It is a centrifugal type pumpdriven mechanically by the engine.

LP FUEL PUMP

MECHANICAL DRIVE FROM

ENGINE

Fuel SystemsHydro-Mechanical Systems

BOOSTER PUMP

WING

FUEL

TANK

The Fuel/Oil Heat Exchanger is like a car radiator, heat is transferred from the hot oil to the cold fuel.

In older systems, air was used to cool the oil, in modern systems,

cold fuel from the wing tanks is used.

This has a duel advantage of cooling the engine oil and preventing fuel blockage due to ice build up in the fuel.

LP FUEL PUMP

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

Fuel SystemsHydro-Mechanical Systems

BOOSTER PUMP

WING

FUEL

TANK

The Low Pressure (LP) Fuel Filter is a simple unit which ensure the fuel is free

of damaging contaminants.

LP FUEL PUMP

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

LP FUEL FILTER

OIL OUT

Fuel SystemsHydro-Mechanical Systems

BOOSTER PUMP

WING

FUEL

TANK

The High Pressure (HP) Fuel Pump is a multi-plunger ‘swash plate’ type pump

driven mechanically by the engine.

It is a positive displacement pump, which means that flow is proportional to rpm,

and if the pump stops turning, fuel will stop flowing.

LP FUEL PUMP

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

OIL OUT

HP FUEL PUMP

LP FUEL FILTER

Fuel SystemsHydro-Mechanical Systems

BOOSTER PUMP

WING

FUEL

TANK

The Fuel Control Unit (FCU) controlsthe output of the HP fuel pump.

It has a mechanical drive from the engine (off the gearbox), so knows the engine speed and

can control the HP pump output.

LP FUEL PUMP

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

OIL OUT

LP FUEL FILTERHP FUEL PUMP

FUEL CONTROL

UNIT

Fuel SystemsHydro-Mechanical Systems

BOOSTER PUMP

WING

FUEL

TANK

LP FUEL PUMP

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

OIL OUT

LP FUEL FILTERHP FUEL PUMP

FUEL CONTROL

UNIT

The HP fuel pump flow control is achievedby a metered fuel pressure signal from the FCU to the HP fuel pump.

PUMP OUTPUT CONTROL SIGNAL

Fuel SystemsHydro-Mechanical Systems

BOOSTER PUMP

WING

FUEL

TANK

LP FUEL PUMP

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

OIL OUT

LP FUEL FILTERHP FUEL PUMP

Finally, the HP fuel pump delivers the fuel,at high pressure,

to the Fuel Spray Nozzles.

PUMP OUTPUT CONTROL SIGNAL

FU

EL

DIS

TR

IBU

TIO

N A

ND

SP

RA

Y N

OZ

ZL

ES

FUEL CONTROL

UNIT

Fuel SystemsHydro-Mechanical Systems

BOOSTER PUMP

WING

FUEL

TANK

OIL IN

OIL OUT

LP FUEL FILTER

PUMP OUTPUT CONTROL SIGNAL

LP FUEL PUMP

FUEL CONTROL

UNIT

HP FUEL PUMP

FUEL/OILHEAT

EXCHANGER

FU

EL

DIS

TR

IBU

TIO

N A

ND

SP

RA

Y N

OZ

ZL

ES

MECHANICAL DRIVE FROM

ENGINE

We shall now examine these units in detail

PUMP IMPELLOR

PUMP INPUT DRIVE

Hydro-Mechanical SystemThe Low Pressure (LP) Pump

The LP Fuel Pump provides positive pressure to the HP fuel pump. If the pump stops rotating, fuel can continue to flow.

The LP pump is simple in operation, rotational force is supplied by the engine via the accessory gearbox.

PUMP IMPELLOR

PUMP INPUT DRIVE

Hydro-Mechanical SystemThe Low Pressure (LP) Pump

Fuel enters the eye of the rotor, then passes into rotor vanes.

Rotation of the pump forces the fuel to the rim of the rotor by centrifugal force,

then on to the pump outlet port.

LP FUEL IN

Hydro-Mechanical SystemFuel/Oil Heat Exchanger & Fuel Filter

FUEL COOLEDOIL COOLER

(FCOC)

OIL BAFFLE PLATES

FUEL TUBES

The Fuel/Oil Heat Exchanger or Fuel Cooled Oil Cooler (FCOC)not only ensures the oil is cooled,

but also that the fuel is heated.It also ensures damaging and/or blockage forming contaminants

pass through the fuel filter systems.

COOLED OIL OUT

HOT OIL IN

HOT LP FUEL OUT

COLD LP FUEL IN

FILTER PAPER ELEMENT

LP FUEL FILTER

Hydro-Mechanical SystemFuel/Oil Heat Exchanger & Fuel Filter

FUEL COOLEDOIL COOLER

(FCOC)

OIL BAFFLE PLATES

FUEL TUBES

To prevent high oil pressures a pressure relief valve automatically opens to allow oil to bypass the cooler element,

preventing damage by excessive pressure.

COOLED OIL OUT

HOT OIL IN

HOT LP FUEL OUT

COLD LP FUEL IN

FILTER PAPER ELEMENT

LP FUEL FILTER

FCOC PRESSURE

RELIEF VALVE

Hydro-Mechanical SystemFuel/Oil Heat Exchanger & Fuel Filter

FUEL COOLEDOIL COOLER

(FCOC)

OIL BAFFLE PLATES

FUEL TUBES

The filter also has a bypass valve which automatically opens in filter blocked situations,

to allow some fuel to flow and the engine to continue in operation.

HOT LP FUEL OUT

COLD LP FUEL IN

FILTER PAPER ELEMENT

LP FUEL FILTER

FCOC PRESSURE

RELIEF VALVE

FILTER PRESSURE

RELIEF VALVE

This is a Multi Plunger (Swash-plate) Pumpwhich means that flow is proportional to pump rpm,

and if the pump stops rotating, the fuel flow stops as well.The pump consists of a rotating body

with 7 or 9 pistons or plungers,depending on the pump size.

MECHANICAL INPUT DRIVE

‘SWASH’ or CAM PLATE SERVO (CONTROL)

PRESSURESERVO PISTON

OPERATINGPISTON

Hydro-Mechanical SystemHigh Pressure Fuel Pump

KIDNEY PLATE

KIDNEY PORTS

Each plunger works the same as a bicycle pump, when it extends it sucks fuel in;

when it is compressed, fuel is pushed out under pressure.

MECHANICAL INPUT DRIVE

SERVO PISTON

OPERATINGPISTON

Hydro-Mechanical SystemHigh Pressure Fuel Pump

FUEL PUMPED OUT

FUEL DRAWN IN

KIDNEY PLATE

KIDNEY PORTS

‘SWASH’ or CAM PLATE SERVO (CONTROL)

PRESSURE

MECHANICAL INPUT DRIVE

SERVO PISTON

OPERATINGPISTON

Hydro-Mechanical SystemHigh Pressure Fuel Pump

FUEL PUMPED OUT

FUEL DRAWN IN

KIDNEY PLATE

KIDNEY PORTS

‘SWASH’ or CAM PLATE

Each piston is in contact with the ‘swash-plate’ or camplate.

The camplate does not rotate, but pump flow can be changed by the camplate angle (a low angle means less fuel is drawn in).

Changing the camplate angle sets the pump output to exactly match the combustion flow.

SERVO (CONTROL) PRESSURE

Multi Plunger Pump Flow Control

‘STEADY-STATE’ FUEL DEMAND

ENGINE RPMIDLE MAX

FUEL FLOW

GAL/HR LTR/SEC

IDLE

MAX

PUMP M

AXIMUM F

LOW =

CAM

PLATE M

AXIMUM A

NGLE

PUMP MINIMUM FLOW = CAMPLATE MINIMUM ANGLE

‘STEADY-STATE’ CRUISE REQUIRED CAMPLATE

ANGLE

CRUISE RPM

CRUISE FUEL FLOW

Hydro-Mechanical System

The graph shows the ‘steady state fuel demand line’. It also shows that at a constant rpm, such as cruise power,the pump could deliver too much/ too little fuel for that rpm,

simply by changing the camplate angle. (too little fuel/lower angle means reduced engine speed

and vice-versa).

Electronically Controlled Systems

Flow requirements of an electronically controlled system are the same as a hydro-mechanical system;

only the control method has changed.

BOOSTER PUMP

WING

FUEL

TANK

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

OIL OUT

LP FUEL FILTER

FU

EL

DIS

TR

IBU

TIO

N A

ND

SP

RA

Y N

OZ

ZL

ES

PUMP OUTPUT CONTROL SIGNAL

FUEL CONTROL

UNIT

LP FUEL PUMP

HP FUEL PUMP

Electronically Controlled Systems

Electronic systems have LP & HP pumps housed in one unit,only taking up one mounting pad

on the accessory gearbox.

BOOSTER PUMP

WING

FUEL

TANK

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

OIL OUT

LP FUEL FILTER

FU

EL

DIS

TR

IBU

TIO

N A

ND

SP

RA

Y N

OZ

ZL

ES

PUMP OUTPUT CONTROL SIGNAL

FUEL CONTROL

UNIT

LP FUEL PUMP

HP FUEL PUMP

Electronically Controlled Systems

The FCU, or Fuel Metering Unit (FMU) now has an electronic signal to calculate the position

of the FMU throttle valve and spill valve.

BOOSTER PUMP

WING

FUEL

TANK

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

OIL OUT

LP FUEL FILTER

FU

EL

DIS

TR

IBU

TIO

N A

ND

SP

RA

Y N

OZ

ZL

ES

PUMP OUTPUT CONTROL SIGNAL

FUEL CONTROL

UNIT

FUEL MERERING

UNIT FADEC ELECTRONIC SIGNAL

FUEL SPILL FLOW

LP FUEL PUMP

HP FUEL PUMP

Electronically Controlled SystemsThe Gear Type Pump

‘SPUR’ GEARS

BODY

The gear type pump is the most favoured pump design,because it consists of just a few easy to produce working parts.

Intermeshing straight cut spur gears rotating in a close fitting body.

BOOSTER PUMP

WING

FUEL

TANK

MECHANICAL DRIVE FROM

ENGINE

FUEL/OILHEAT

EXCHANGER

OIL IN

OIL OUT

LP FUEL FILTER

FU

EL

DIS

TR

IBU

TIO

N A

ND

SP

RA

Y N

OZ

ZL

ES

Electronically Controlled Systems

An HP Gear Type Pump is fitted to electronic systems;a positive displacement pump with a flow proportional

to the rpm it is driven at.

FADEC ELECTRONIC SIGNAL

HP FUEL PUMP

FUEL MERERING

UNIT

FUEL SPILL FLOW

HP FUEL GREAR TYPE

PUMP

LP FUEL PUMP

We shall now examine this unit

FLOW OUT

Electronically Controlled SystemsThe Gear Type Pump

‘SPUR’ GEARS

Flow Controller

As the gears rotate the fluid is carriedbetween the gear teeth.

A control valve is set to limit the absolute pressure in the system,

(around 450 psi) to protect against system damage.

FLOW OUT

Electronically Controlled SystemsThe Gear Type Pump

‘SPUR’ GEARS

Pump flow restrictionin the controllercauses pressureto increase

Flow Controller

Without the flow controller, the pump would provide a flow,

but at no pressure. Pressure is only achieved

because the flow control system works on flow restrictions

which causes the pressure to rise.

COMBUSTION FLOW

FUEL FLOW

GAL/HR LTR/SEC

IDLE

MAX

Gear Type Pump Flow Control

ENGINE RPMIDLE MAX

PUMP M

AXIMUM F

LOW =

CAM

PLATE M

AXIMUM A

NGLE

PUMP MINIMUM FLOW = CAMPLATE MINIMUM ANGLE

‘STEADY-STATE’ CRUISE REQUIRED CAMPLATE

ANGLE

CRUISE RPM

CRUISE FUEL FLOW

Electronically Controlled Systems

At any given rpm the Gear Type Pump has one output, which is higher that that required for combustion.

The excess flow, spill, is returned to the LP side of the systemunder control of the ECU via the fuel metering and spill valves.

Power demand, engine & ambient conditionsdetermine the combustion flow.

Remember the Multi Plunger Pump Flow Control graph

STEADY PUM

P FLO

W

PUMP OUTPUT

SPILL FLOW

COMBUSTION FLOW

SPILL FLOW

‘STEADY-STATE’ FUEL DEMAND

Check of Understanding

What pumps the fuel from the wing tanksinto the engine systems?

Fuel Metering Units

High Pressure Fuel Pumps

Electric Booster Pumps

Low Pressure Fuel Pumps

A direct feedgear-type pump

A multi-plunger swash plate type pump

A multi-rotational cam plate type pump

A revolving gear-type pump

In a Hydro-Mechanical Fuel Systemwhat type of pump is the

High Pressure (HP) Fuel Pump?

Check of Understanding

The Fuel Cooled Oil Cooler (FCOC)

The Fuel Metering Unit (FMU)

The Fuel/Oil Heat Exchanger

The Fuel Control Unit (FCU)

In a Hydro-Mechanical Fuel SystemWhat controls the output of the HP fuel pump?

Check of Understanding

The oil is filtered

The fuel is cooled

The fuel is heated

The pressure is kept high

The Fuel/Oil Heat Exchanger not only ensures the oil is cooled,

but also . . . .

Check of Understanding

Too much/ too little fuel delivered

The pump would provide a flow, but at no pressure

The pump would provide a flow, but at an increased pressure

A steady state fuel demandwould be maintained

In a multi plunger fuel pump what effect could be produced

by changing the camplate angle?

Check of Understanding

JET PROPULSION

End of Presentation