1. JET PROPULSION
Part 6
Jet Engine Operation
Fuel Systems

2. Introduction 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

3. Fuel Systems Hydro-Mechanical Systems
MECHANICAL DRIVE FROM ENGINE
BOOSTER PUMP
LP FUEL PUMP
Fuel is pumped from the wing tanks
by electrically powered Booster Pumps.
These pumps simply push the fuel through
the extensive aircraft system to the engines.
The Low Pressure (LP) Fuel Pump
provides a positive pressure to the
high pressure fuel pump.
(further on in the system)
It is a centrifugal type pump
driven mechanically by the engine.
WING
FUEL
TANK

4. Fuel Systems Hydro-Mechanical Systems
MECHANICAL DRIVE FROM ENGINE
OIL IN
BOOSTER PUMP
LP FUEL PUMP
FUEL/OIL
HEAT
EXCHANGER
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.

5. Fuel Systems Hydro-Mechanical Systems
MECHANICAL DRIVE FROM ENGINE
OIL IN
BOOSTER PUMP
LP FUEL PUMP
FUEL/OIL
HEAT
EXCHANGER
OIL OUT
WING
FUEL
TANK
LP FUEL FILTER
The Low Pressure (LP) Fuel Filter is
a simple unit which ensure the fuel is free
of damaging contaminants.

6. Fuel Systems Hydro-Mechanical Systems
MECHANICAL DRIVE FROM ENGINE
OIL IN
BOOSTER PUMP
LP FUEL PUMP
FUEL/OIL
HEAT
EXCHANGER
OIL OUT
WING
FUEL
TANK
HP FUEL PUMP
LP FUEL FILTER
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.

7. Fuel Systems Hydro-Mechanical Systems
MECHANICAL DRIVE FROM ENGINE
OIL IN
BOOSTER PUMP
LP FUEL PUMP
FUEL/OIL
HEAT
EXCHANGER
OIL OUT
WING
FUEL
TANK
HP FUEL PUMP
LP FUEL FILTER
The Fuel Control Unit (FCU) controls
the 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.
FUEL CONTROL UNIT

8. Fuel Systems Hydro-Mechanical Systems
MECHANICAL DRIVE FROM ENGINE
OIL IN
BOOSTER PUMP
LP FUEL PUMP
FUEL/OIL
HEAT
EXCHANGER
OIL OUT
WING
FUEL
TANK
HP FUEL PUMP
LP FUEL FILTER
PUMP OUTPUT CONTROL SIGNAL
FUEL CONTROL UNIT
The HP fuel pump flow control is achieved
by a metered fuel pressure signal
from the FCU to the HP fuel pump.

9. FUEL DISTRIBUTION AND SPRAY NOZZLES
Fuel Systems
Hydro-Mechanical Systems
MECHANICAL DRIVE FROM ENGINE
OIL IN
BOOSTER PUMP
LP FUEL PUMP
FUEL/OIL
HEAT
EXCHANGER
OIL OUT
WING
FUEL
TANK
HP FUEL PUMP
LP FUEL FILTER
FUEL DISTRIBUTION AND SPRAY NOZZLES
PUMP OUTPUT CONTROL SIGNAL
FUEL CONTROL UNIT
Finally, the HP fuel pump delivers the fuel,
at high pressure,
to the Fuel Spray Nozzles.

10. FUEL DISTRIBUTION AND SPRAY NOZZLES
Fuel Systems
Hydro-Mechanical Systems
MECHANICAL DRIVE FROM ENGINE
OIL IN
OIL OUT
BOOSTER PUMP
LP FUEL PUMP
FUEL/OIL
HEAT
EXCHANGER
WING
FUEL
TANK
HP FUEL PUMP
LP FUEL FILTER
FUEL DISTRIBUTION AND SPRAY NOZZLES
PUMP OUTPUT CONTROL SIGNAL
FUEL CONTROL UNIT
We shall now examine these units in detail

11. Hydro-Mechanical System
The Low Pressure (LP) Pump
PUMP INPUT DRIVE
PUMP IMPELLOR
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.

12. Hydro-Mechanical System
The Low Pressure (LP) Pump
PUMP INPUT DRIVE
LP FUEL IN
PUMP IMPELLOR
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.

13. Hydro-Mechanical System
Fuel/Oil Heat Exchanger & Fuel Filter
HOT LP FUEL OUT
HOT OIL IN
COOLED OIL OUT
FUEL COOLED
OIL COOLER
(FCOC)
FILTER PAPER ELEMENT
FUEL TUBES
OIL BAFFLE PLATES
LP FUEL FILTER
COLD LP FUEL IN
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.

14. Hydro-Mechanical System
Fuel/Oil Heat Exchanger & Fuel Filter
HOT OIL IN
FCOC PRESSURE RELIEF VALVE
HOT LP FUEL OUT
COOLED OIL OUT
FUEL COOLED
OIL COOLER
(FCOC)
FILTER PAPER ELEMENT
FUEL TUBES
OIL BAFFLE PLATES
LP FUEL FILTER
COLD LP FUEL IN
To prevent high oil pressures
a pressure relief valve automatically opens
to allow oil to bypass the cooler element,
preventing damage by excessive pressure.

15. Hydro-Mechanical System
Fuel/Oil Heat Exchanger & Fuel Filter
FILTER PRESSURE RELIEF VALVE
FCOC PRESSURE RELIEF VALVE
HOT LP FUEL OUT
FUEL COOLED
OIL COOLER
(FCOC)
FILTER PAPER ELEMENT
FUEL TUBES
OIL BAFFLE PLATES
LP FUEL FILTER
COLD LP FUEL IN
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.

16. Hydro-Mechanical System
High Pressure Fuel Pump
‘SWASH’ or CAM PLATE
SERVO PISTON
SERVO (CONTROL) PRESSURE
MECHANICAL INPUT DRIVE
KIDNEY PLATE
KIDNEY PORTS
OPERATINGPISTON
This is a Multi Plunger (Swash-plate) Pump
which 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.

17. Hydro-Mechanical System
High Pressure Fuel Pump
‘SWASH’ or CAM PLATE
SERVO PISTON
SERVO (CONTROL) PRESSURE
MECHANICAL INPUT DRIVE
FUEL PUMPED OUT
KIDNEY PLATE
FUEL DRAWN IN
KIDNEY PORTS
OPERATINGPISTON
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.

18. Hydro-Mechanical System
High Pressure Fuel Pump
‘SWASH’ or CAM PLATE
SERVO PISTON
SERVO (CONTROL) PRESSURE
MECHANICAL INPUT DRIVE
FUEL PUMPED OUT
KIDNEY PLATE
FUEL DRAWN IN
KIDNEY PORTS
OPERATINGPISTON
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.

19. Hydro-Mechanical System
Multi Plunger Pump Flow Control
FUEL FLOW
GAL/HR LTR/SEC
IDLE
MAX
PUMP MAXIMUM FLOW = CAMPLATE MAXIMUM ANGLE
‘STEADY-STATE’ FUEL DEMAND
‘STEADY-STATE’ CRUISE REQUIRED CAMPLATE ANGLE
CRUISE
FUEL FLOW
PUMP MINIMUM FLOW = CAMPLATE MINIMUM ANGLE
ENGINE RPM
IDLE
MAX
CRUISE RPM
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).

20. Electronically Controlled Systems
BOOSTER PUMP
WING
FUEL
TANK
MECHANICAL DRIVE FROM ENGINE
FUEL/OIL
HEAT
EXCHANGER
OIL IN
OIL OUT
LP FUEL FILTER
FUEL DISTRIBUTION AND SPRAY NOZZLES
LP FUEL PUMP
HP FUEL PUMP
PUMP OUTPUT CONTROL SIGNAL
FUEL CONTROL UNIT
Flow requirements of an electronically controlled system
are the same as a hydro-mechanical system;
only the control method has changed.

21. Electronically Controlled Systems
BOOSTER PUMP
WING
FUEL
TANK
MECHANICAL DRIVE FROM ENGINE
FUEL/OIL
HEAT
EXCHANGER
OIL IN
OIL OUT
LP FUEL FILTER
FUEL DISTRIBUTION AND SPRAY NOZZLES
LP FUEL PUMP
HP FUEL PUMP
PUMP OUTPUT CONTROL SIGNAL
FUEL CONTROL UNIT
Electronic systems have LP & HP pumps housed in one unit,
only taking up one mounting pad
on the accessory gearbox.

22. Electronically Controlled Systems
BOOSTER PUMP
WING
FUEL
TANK
MECHANICAL DRIVE FROM ENGINE
FUEL/OIL
HEAT
EXCHANGER
OIL IN
OIL OUT
LP FUEL FILTER
FUEL DISTRIBUTION AND SPRAY NOZZLES
LP FUEL PUMP
HP FUEL PUMP
PUMP OUTPUT CONTROL SIGNAL
FUEL SPILL FLOW
FUEL MERERING UNIT
FUEL CONTROL UNIT
FADEC
ELECTRONIC SIGNAL
The FCU, or Fuel Metering Unit (FMU)
now has an electronic signal to calculate the position
of the FMU throttle valve and spill valve.

23. Electronically Controlled Systems
The 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.

24. Electronically Controlled Systems
BOOSTER PUMP
WING
FUEL
TANK
MECHANICAL DRIVE FROM ENGINE
FUEL/OIL
HEAT
EXCHANGER
OIL IN
OIL OUT
LP FUEL FILTER
FUEL DISTRIBUTION AND SPRAY NOZZLES
LP FUEL PUMP
HP FUEL GREAR TYPE PUMP
HP FUEL PUMP
FUEL SPILL FLOW
FUEL MERERING UNIT
We shall now examine this unit
FADEC
ELECTRONIC SIGNAL
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.

25. Electronically Controlled Systems
The Gear Type Pump
‘SPUR’ GEARS
Flow Controller
FLOW OUT
As the gears rotate the fluid is carried
between the gear teeth.
A control valve is set to limit
the absolute pressure in the system,
(around 450 psi)
to protect against system damage.

26. Electronically Controlled Systems
The Gear Type Pump
Pump flow restriction
in the controller
causes pressure
to increase
‘SPUR’ GEARS
Flow Controller
FLOW OUT
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.

27. Electronically Controlled Systems
Gear Type Pump Flow Control
COMBUSTION FLOW
SPILL FLOW
FUEL FLOW
GAL/HR LTR/SEC
IDLE
MAX
PUMP MAXIMUM FLOW = CAMPLATE MAXIMUM ANGLE
STEADY PUMP FLOW
PUMP OUTPUT
‘STEADY-STATE’ FUEL DEMAND
SPILL FLOW
‘STEADY-STATE’ CRUISE REQUIRED CAMPLATE ANGLE
CRUISE
FUEL FLOW
PUMP MINIMUM FLOW = CAMPLATE MINIMUM ANGLE
COMBUSTION FLOW
Remember the
Multi Plunger Pump
Flow Control graph
ENGINE RPM
IDLE
MAX
CRUISE RPM
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 system
under control of the ECU via the fuel metering and spill valves.
Power demand, engine & ambient conditions
determine the combustion flow.

28. Check of Understanding
What pumps the fuel from the wing tanks
into the engine systems?
High Pressure Fuel Pumps
Low Pressure Fuel Pumps
Electric Booster Pumps
Fuel Metering Units

29. Check of Understanding
In a Hydro-Mechanical Fuel System
what type of pump is the
High Pressure (HP) Fuel Pump?
A multi-plunger swash plate
type pump
A revolving
gear-type pump
A multi-rotational cam plate
type pump
A direct feed
gear-type pump

30. Check of Understanding
In a Hydro-Mechanical Fuel System
What controls the output of the HP fuel pump?
The Fuel Metering Unit (FMU)
The Fuel Control Unit (FCU)
The Fuel/Oil Heat Exchanger
The Fuel Cooled Oil Cooler (FCOC)

31. Check of Understanding
The Fuel/Oil Heat Exchanger
not only ensures the oil is cooled,
but also
The fuel is cooled
The pressure is kept high
The fuel is heated
The oil is filtered

32. Check of Understanding
In a multi plunger fuel pump
what effect could be produced
by changing the camplate angle?
The pump would provide a flow,
but at no pressure
A steady state fuel demand
would be maintained
The pump would provide a flow,
but at an increased pressure
Too much/ too little fuel delivered

33. JET PROPULSION
End of Presentation