CHAPTER 5
CHAPTER 5 – FUEL SYSTEM
CONTENTS
PAGE
System Description
02
Low Pressure (LP) Fuel Pump
04
Fuel Oil Heat Exchanger and LP Filter
06
Multi Plunger Pump
08
Multi Plunger Pump Fuel Control
10
Electronic Controlled Fuel Systems
12
Gear Type Pump
14
Gear Type Pump Fuel Control
16
FUEL SYSTEM – Hydro-Mechanical Systems
System Description
Fuel Control Unit (FCU)
Tanks and Booster Pumps
In this system, the FCU controls the output of the HP fuel
pump. The control unit has a mechanical drive from the
engine (off the gearbox), so that the control unit knows what
speed the engine is running at, and can therefore control the
HP pump output as required. HP fuel pump flow control is
achieved by a metered fuel pressure signal from the FCU to
the HP fuel pump.
Fuel is pumped from the wing tanks by electrically powered
booster pumps. The booster pumps are simply to push the
fuel through the extensive aircraft system to the engines.
Low Pressure (LP) Fuel Pump
The purpose of the LP fuel pump is to provide a positive
pressure to the HP fuel pump. It is a centrifugal type pump
driven mechanically by the engine.
Distribution System
This is the system of pipes around the engine which carries
the metered fuel pressure to the fuel spray nozzles (FSN’s) in
the combustion chamber. The FSN’s have a very small orifice
to force the fuel through, this is what achieves the atomization
required for efficient combustion.
Fuel Oil Heat Exchanger
This works like a car radiator, in that 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 the duel advantage of cooling the engine oil
and, preventing fuel blockage due to ice build up in the fuel.
Some systems have a dual distribution system, to provide
good atomization at low rpm’s via a ‘primary’ orifice and the
higher fuel flow and atomization at higher rpm’s via a
‘secondary’ orifice.
LP Fuel Filter
A simple unit which ensure the fuel is free of damaging
contaminants.
The FCU splits the fuel flow into primary (higher Pressure)
and secondary (lower, but still high, pressure) and each FSN
has both the primary and secondary orifices built into it.
High Pressure (HP) Fuel Pump
The purpose of the HP fuel pump is to deliver the fuel at high
pressure to the fuel spray nozzles. It 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.
2
MECHANICAL
DRIVE FROM
ENGINE
OIL
IN
FUEL/OIL
BOOSTER
PUMP
LP FUEL
PUMP
HEAT
OIL
OUT
WING
FUEL
HP FUEL
PUMP
LP FUEL FILTER
TANK
PUMP OUTPUT
CONTROL SIGNAL
FUEL
CONTROL
UNIT
MULTI PLUNGER (SWASH-PLATE) PUMP
TYPICAL FUEL SYSTEM - GAS TURBINE ENGINE
3
FUEL DISTRIBUTION AND SPRAY NOZZLES
EXCHANGER
FUEL SYSTEM – LP fuel Pump
Low Pressure (LP) Fuel Pump
Purpose
The purpose of the LP fuel pump is to provide a positive
pressure to the HP fuel pump.
Type
It is a centrifugal type pump driven mechanically by the
engine. It is a non-positive displacement type pump, which
means that fuel flow through the pump can be varied at a
constant rpm; and, if the pump stops rotating, fuel can
continue to flow.
Operation
The LP pump is simple in operation, the rotational force is
supplied by the engine via the accessory gearbox.
Like the turbo/super charger, the fuel enters the eye of the
rotor, into a ‘scroll’ on modern designs (a coarse slightly
conical screw thread not featured in older designs), then the
fuel passes into vanes in the rotor. Rotation of the pump
forces the fuel to the rim of the rotor by centrifugal force, then
on to the pump outlet port.
The flow through the pump is not proportional to the pump
rpm, i.e. at high rpm and low fuel flow (when the throttle is
closed to slow the engine down) the flow through the LP pump
will reduce before rpm reduces. Flow through the pump will
always be the same as the combustion flow in the system
described here.
4
PUMP
INPUT
DRIVE
LP
FUEL
IN
PUMP
IMPELLOR
LOW PRESSURE PUMP
TYPICAL FUEL SYSTEM - GAS TURBINE ENGINE
5
FUEL SYSTEM - Fuel/Oil Heat Exchanger and Fuel Filter
Purpose
consumption) rather that the potentially dangerous situation of
volatile fuel leaking into the oil system.
To ensure that the oil is cooled and the fuel is heated, and
ensure damaging and/or blockage forming contaminants pass
through the fuel control and distribution systems.
LP Fuel Filter
The filter is a disposable paper element type, and has a
filtration rate such that damaging or blockage forming
debris/particles are prevented from flowing into the fuel control
and distribution system.
Fuel/Oil Heat Exchanger (FOHE)
This unit is also known as the Fuel Cooled Oil Cooler –
FCOC.
In earlier systems the oil would be cooled by air (ACOC), but
this could leave the problem of fuel icing. So the FOHE has a
duel purpose: to cool the oil and prevent fuel icing by the
transfer of heat from the hot oil to the cold fuel.
The filter has a bypass valve which automatically opens to
allow some fuel flow in filter blocked situations, allowing the
engine to continue in operation.
The fuel in the wing tanks is cooled by the very cold
temperatures that are found at high altitude. In prolonged
altitude cruise flights, the water content of the fuel can
become frozen and ice particles can occur which could collect
in the filter and cause a blockage in the fuel system preventing
sufficient fuel reaching the engine.
On modern systems, both the FOHE and Filter each have a
Differential Pressure switch (differential = the difference
between the upstream and downstream pressure of the FOHE
and Filter) to pre-warn the flight crew that a problem is
occurring before the either bypass valve opens, allowing
contingency plans to be put into operation i.e. reducing power
and therefore flows, and/or diverting to an airfield for
maintenance checks and corrective action.
Indications
In cold ambient temperatures during engine start, the oil can
have high viscosity until heated up. To prevent high oil
pressures (remember the oil pump starts to pump oil as soon
as the engine starts to rotate), a pressure relief valve will
automatically open to allow the oil to bypass the cooler
element, preventing damage by excessive pressure.
Designers ensure that at the FOHE, the oil is always a higher
pressure than the fuel so that, should a leak occur, oil passes
into the fuel system and is burnt off (resulting in higher oil
6
HOT OIL
IN
COLD LP
FUEL IN
FCOC PRESSURE
RELIEF VALVE
FUEL COOLED OIL
COOLER (FCOC)
FUEL
TUBES
COOLED
OIL OUT
OIL BAFFLE
PLATES
FILTER
PRESSURE
RELIEF
VALVE
LP FUEL
FILTER
HOT LP
FUEL OUT
FILTER PAPER
ELEMENT
FUEL/OIL HEAT EXCHANGER AND FUEL FILTER
TYPICAL FUEL SYSTEM - GAS TURBINE ENGINE
7
FUEL SYSTEM – Multi Plunger (Swash-plate) Pump
Description
controller). Changing the HP output pressure and the metered
pressure, changes the camplate angle and therefore sets the
pump output to match exactly the combustion flow.
A Positive variable displacement pump, which means that flow
is proportional to pump rpm and camplate angle, and if the
pump stops rotating, the fuel flow stops as well.
The pump consists of a rotating body which has a number of
pistons or plungers, typically in jet engines 7 or 9 depending
on the pump size.
Operation
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.
Each piston has a ‘slipper pad’ on a ball joint which is in
contact with the camplate, also called a ‘swash-plate’. The
camplate does not rotate, but its angle can be changed by
servo pressure moving the servo piston.
Therefore, pump flow can be changed at a fixed rpm by
changing the camplate angle (a low angle means the pistons
have less travel therefore less fuel is drawn in etc), or by
changing the pump rpm at a constant camplate angle.
In reality, the pump flow is matched to the combustion flow
requirements by changing the camplate angle to provide the
correct flow at the speed the pump is driven at by the engine.
The camplate angle is set by HP delivery pressure (HP pump
output flow to the fuel flow controller) acting on one side of the
servo piston, with a spring and servo pressure (the metered
pressure delivered to the engine FSN’s from the fuel flow
8
‘SWASH’ PLATE
SERVO PISTON
SERVO (CONTROL) PRESSURE
FUEL PUMPED OUT
MECHANICAL
INPUT DRIVE
KIDNEY
PLATE
FUEL DRAWN IN
OPERATING
PISTON
KIDNEY
PORT
MULTI PLUNGER (SWASH-PLATE) PUMP
TYPICAL FUEL SYSTEM - GAS TURBINE ENGINE
9
FUEL SYSTEM – Multi Plunger Pump Flow Control
Flow Requirements
Further Discussion/Observation
It is a characteristic of jet engines that controlling the fuel flow
is the be all and end all of controlling the engine rpm and
therefore power.
Study the graph below, it can be seen that the camplate angle
would be approximately the same at idle as it is in cruise, the
increase in fuel flow is achieved by running the engine (and
therefore the pump) at a higher rpm.
In piston engines it is possible to change the mixture without
changing rpm, primarily since rpm, particularly in ground
vehicles, is set by forward speed, i.e. a direct relationship
between engine rpm and road speed through the gearbox.
Also, accelerating from idle to cruise means the camplate
angle would have to first reduce then increase up to the cruise
condition.
In jet engines, because there is no direct mechanical link to
forward speed, i.e. the engines are free to rotate at whatever
speed the fuel flow drives them at (up to governed maximums
for safety), there is no possibility of changing the fuel flow
without changing the engine rpm/power.**
Additional Information/Clarification
*Exponential
Where a quantity changes linearly at a constant time interval, i.e. every minute (or
any constant time interval, or any rpm increase interval) the quantity doubles (or
trebles etc).
The graph below shows the ‘steady state demand line’. This is
the combustion flow, i.e. the amount of fuel required to drive
the engine at the constant rpm when the fuel flow was
recorded; the curve is an exponential* curve.
For instance (imagine cell growth) if 1 is doubled in one minute to 2, another
minute later there is 4, another minute 8, and so on. Therefore, starting at 1, in 10
minutes there would be 1024, 11 minutes 2048 etc..
Plot these quantities at a minute interval (or at any constant time interval) and the
curve is an exponential curve.
The camplate angle has a maximum and minimum angle,
limited by fixed mechanical stops built into the pump.
**Jet Engine Mixture Adjustment
It can be seen from the graph below that at a constant rpm,
such as cruise power as shown, the pump could deliver either
too much fuel or too little fuel to maintain that rpm (too little
fuel/lower camplate angle means the engine speed reduces
and vice-versa), simply by changing the camplate angle.
The exception to this is the Dart engine, where fuel flow can be changed by a
maximum of 4% from the flight deck whilst the engine is running, for corrections for
moisture content in the air i.e. the ‘dew point’. As the engine speed is controlled by
the propeller, increasing the fuel flow increases the prop pitch and therefore thrust.
This is a power restore function due to thrust loss caused by moisture, rather than
a power boost over what would be normal for the conditions.
10
MAX
GAL/HR FUEL
LTR/SEC FLOW
W
CRUISE
FUEL FLOW
M
PU
P
M
IM
X
A
UM
=
M
CA
P
PUM
M
IDLE
M
UM
‘STEADY-STATE’
FUEL DEMAND
‘STEADY-STATE’
CRUISE CAMPLATE
ANGLE
O
FL
C
W=
IDLE
P
TE
A
L
IM
X
A
LE
G
AN
AM
TE
PL A
M
ANG
M
U
INIM
LE
LO
MF
U
INIM
ENGINE RPM
CRUISE
RPM
MAX
MULTI PLUNGER (SWASH-PLATE) PUMP
TYPICAL FUEL SYSTEM - GAS TURBINE ENGINE
11
FUEL SYSTEM – Electronically Controlled Systems
Flow Requirements
The amount of spill is determined by the fuel controller via a
spill valve. This is controlled via parameters such as engine
throttle setting, engine power, air temperature; and limited by
maximum rpm limiters, maximum gas temperature limiters by
the electronic system.
The flow requirements of a jet engine controlled electronically
are exactly the same as a hydro-mechanical system; imagine
a jet engine initially fitted with hydro-mechanical fuel system,
then modernized with an electronically controlled fuel system.
The fuel flow requirements to drive the engine are exactly the
same, only the control method has changed.
Pump Assemblies
On modern systems the LP and HP pumps are housed in one
unit, therefore taking up only one mounting pad on the
accessory gearbox. The mechanical drive passes from the
gearbox to the gear type pump and then through to the LP
pump.
System
The system below is almost the same as the previously
described system for description/training purposes only; in
reality, very few engines have fuel control system changes
due to the cost of the change.
Fuel Control Unit Mounting
The main difference is in the control connection from the fuel
controller to the HP pump. Whereas in the hydro-mechanical
system previously described, this connection is a control
pressure, not a flow. In the electronically controlled system,
the connection is now simply a return line for excess fuel.
The FCU, or Fuel Metering Unit (FMU) is usually bolted to the
pump assembly for convenience. These units no longer have
a direct mechanical rpm input signal, this comes into the
Electronic Control Unit (ECU), and is used in calculating the
position of the FMU throttle valve and spill valve.
This is due mainly to the type of pump fitted rather than the
control method, i.e. some hydro-mechanical systems may
have the gear type pump fitted.
Unlike the multi-plunger type pump, the gear type pump is a
positive displacement with a flow proportional to the rpm it is
driven at.
This means that the pump delivers more fuel that is required
by the engine; this ‘excess’ is called the ‘spill flow’ and is
returned to the inlet side of the HP pump.
12
MECHANICAL
DRIVE FROM
ENGINE
OIL
IN
FUEL/OIL
BOOSTER
PUMP
LP FUEL
PUMP
HEAT
OIL
OUT
WING
FUEL
HP FUEL
PUMP
LP FUEL FILTER
TANK
FUEL ‘SPILL’ FLOW
FADEC – ELECTRONIC SIGNAL
FUEL
CONTROL
UNIT
GEAR TYPE PUMP
TYPICAL FUEL SYSTEM - GAS TURBINE ENGINE
13
FUEL DISTRIBUTION AND SPRAY NOZZLES
EXCHANGER
FUEL SYSTEM – Gear Type Pump
Description
restrictions which causes the pressure to rise. Without the flow
controller, the pump would continue to provide a flow but at no
pressure.
intermeshing straight cut (in this example) spur gears rotating
in a close fitting body.
Spill Flow/Icing
A Positive displacement pump and flow is relative to pump
rpm.
In some systems, the spill fuel flow can be used to help
prevent icing in the fuel system and tanks by passing it
through another control valve back to the fuel tanks.
Angled gears can reduce pressure pulsations which can occur
with straight cut gears.
When the fuel has passed through the LP and HP pumps and
through the FOHE, it is heated to very high temperatures. In
certain conditions, this hot fuel can be returned to the tanks to
counter the cold temperatures flown in at high altitude,
particularly in passenger type aircraft.
Operation
One of the gears is the ‘driver’ gear driven by the engine,
which in turn drives the ‘driven’ gear via the intermeshing gear
teeth.
As the gear rotates, the fluid is carried around the outside of
both of the gears between the gear teeth.
At certain flight phases, such as take off, the return to tank
flows are prevented. This is to ensure that faults do not starve
the engine at a critical stage of the flight.
A pressure relief valve (not shown below) is usually included
to limit absolute pressure (around 450 psi) in the system, to
protect against system physical damage, not just to the pump.
When the pressure relief valve opens, the excess fluid is
simply passed around to the inlet side, or LP - Low Pressure
from the HP - High Pressure side of the pump.
Pump Design/Choice
Gear type pumps are usually the choice of pump in modern
electronic controlled systems, and because of the few,
relatively easy and therefore cheaper to produce, moving
parts.
Further Discussion
Pressure Values
The pump provides the flow, but if there were no system to
pump it through there would be no pressure. Pressure is only
achieved because the flow control system works on flow
14
PUMP FLOW AND RESTRICTION TO FLOW IN
CONTROLLER CAUSES PRESSURE TO INCREASE
‘SPUR’
GEARS
FLOW
Flow
OUT
Controller
GEAR TYPE PUMP
TYPICAL FUEL SYSTEM - GAS TURBINE ENGINE
15
FUEL SYSTEM – Gear Type Pump Flow Control
Flow Requirements
The gear type pump is the most favoured pump design as it
consists of just a few easy to produce working parts.
At any given rpm the pump has one output, which is higher
that that required for combustion. The pump capacity is set at
higher than maximum flow at maximum rpm.
The excess flow, the spill, is returned to the LP side of the fuel
system under control of the ECU via the fuel metering and
spill valves.
The combustion flow is determined via the throttle position
(i.e. power demand), engine and ambient conditions by the
ECU.
Acceleration/Deceleration
To accelerate the engine, i.e. increase its speed, the first thing
that changes it the fuel flow to the engine. This is achieved by
reducing the spill flow which increases the combustion flow.
Now the engine is receiving a fuel flow quantity equal to a
higher rpm value, so it accelerates.
Deceleration follows the same principal except of course
reducing fuel flow to bring rpm down.
To achieve maximum acceleration, the fuel flow is increased
by approximately 25%. To decelerate at maximum rate, the
fuel flow is reduced by approximately 50%.
16
A
MAX
‘STEADY-STATE’
FUEL DEMAND
PUMP OUTPUT
GAL/HR FUEL
LTR/SEC FLOW
W
M
PU
P
O
FL
SPILL
FLOW
CRUISE
FUEL FLOW
COMBUSTION
FLOW
IDLE
IDLE
ENGINE RPM
CRUISE
RPM
MAX
GEAR TYPE PUMP
TYPICAL FUEL SYSTEM - GAS TURBINE ENGINE
17