Jet Propulsion

Fuel

Recap

• What are the two basic classifications of turbine engine lubrication systems?

• What are the two types of dry-sump lubrication systems?

• Briefly Explain the difference between them.• What are the three basic sub-systems?• Briefly explain them?

What are the two basic classifications of turbine engine lubrication systems?• Wet-sump and Dry-sump

What are the two types of dry-sump lubrication systems?• Hot-tank system• Cold-tank system

Briefly Explain the difference between them.• In a Hot-tank system, the oil is cooled before

lubricating.• In a Cold-tank system, the oil is cooled before

re-entering the tank

What are the three basic sub-systems?• Pressure system• Scavenge system• Breather/Vent system

Briefly explain them?• Pressure system – This is the pressurised oil being

pumped to the bearings.• Scavenged system – This is the return oil after it has

done its lubricating.• Breather/Vent system – This the steady airstream

which has come from the bearings. This has been contaminated with lubrication oil during operation and must be de-oiled with a component called de-oiler.

Fuel System

• The Fuel Distribution system supplies ice-free filtered fuel at the pressure and flow rates necessary to meet all engine operating requirements.

• The fuel distribution system permits fuel to flow from the aircraft fuel tank through the engine fuel pump to the fuel metering unit.

• The fuel control unit then supplies metered fuel to the fuel injectors for combustion.

• Fuel not sent to the fuel control unit goes to the servo fuel heater to supply servo fuel pressure for engine component actuation.

The LP Stage

• The LP fuel pump has a one stage centrifugal impeller with an axial inducer.

• It has sufficient capacity to supply the HP fuel pump through all rates of HP pump output.

• The impeller/inducer is installed on a driveshaft which is held in precision bearings. These bearings are lubricated and kept cool with a flow of fuel from downstream of the impeller/inducer.

• This flow of fuel is supplied through a flow cleaned strainer. The aircraft supply goes into the pump at the centre of the impeller.

• When the impeller/inducer is turned, the fuel is caused to flow radially (because of centrifugal force) and axially through the pump. This also increases the speed of the flow.

• At the pump outlet the speed of the flow is decreased which increases the fuel pressure.

• This increase in pressure is necessary to prevent cavitation of/or a fumes blockage in the supply to the HP fuel pump.

• The fuel then flows at this low pressure to the fuel oil heat exchanger.

LOW PRESSURE PUMP

PUMP IMPELLOR

PUMP INPUT DRIVE

LP FUEL IN

The HP Stage

• The HP fuel pump is a positive-displacement spur-gear type pump. It has sufficient capacity to supply the fuel control for all rates of metered flow.

• Two spur gears are turned with a driveshaft which is held in precision bearings. The same as the LP stage.

• The driveshaft is installed to the LP pump driveshaft and its related output shaft in the external gearbox module. Therefore, the mechanical power to turn the LP pump is transmitted through the HP pump driveshaft.

• A dry area isolates the pumps from the external gearbox module. Seals around the HP pump driveshaft and the external gearbox output shaft keep the area dry. A usually dry drain connects the area to the power plant drains system.

• Leakage from this drain gives a visual indication that one of the shaft seals has become defective.

• The LP supply goes into the open spaces between the teeth of the adjacent spur gears.

• When the gears are turned, the fuel is caused to flow between the teeth and the pump housing. The rate of flow is in proportion to the speed of the gears.

• At the pump outlet the speed of the flow is decreased which increases the fuel pressure. The fuel then flows at this high pressure to the fuel control unit.

FLOW OUT

‘SPUR’ GEARS

PUMP FLOW AND RESTRICTION TO FLOW IN CONTROLLER CAUSES PRESSURE TO INCREASE

Flow Controller

GEAR TYPE PUMP

Multi Plunger (Swash-plate) Pump

• 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.

• 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.

• Pump flow can be changed at a fixed rpm by changing the camplate angle, or by changing the pump rpm at a constant camplate angle.

FUEL DRAWN IN

FUEL PUMPED OUT

MULTI PLUNGER (SWASH-PLATE) PUMP

MECHANICAL INPUT DRIVE

‘SWASH’ PLATE SERVO (CONTROL) PRESSURESERVO PISTON

OPERATINGPISTON

FUEL DRAWN IN

FUEL PUMPED OUT

KIDNEY PLATE

KIDNEY PORT

Multistage Fuel Pump

Fuel-Oil Heat Exchanger

• Fuel flows directly through the tubes in the cooler, and engine oil flows around the tubes. Heat from the oil warms the fuel, and the fuel cools the oil.

Oil Inlet (Oil outlet not visible)

Flange for mounting on the fuel pump Fuel in

Fuel out

FUEL/OIL HEAT EXCHANGER AND FUEL FILTER

FCOC PRESSURE RELIEF VALVE

FUEL COOLED OIL COOLER (FCOC)

LP FUEL FILTER

FILTER PRESSURE

RELIEF VALVE

FUEL TUBES

OIL BAFFLE PLATES

COLD LP FUEL IN

HOT LP FUEL OUT

COOLED OIL OUT

HOT OIL IN

FILTER PAPER ELEMENT

Fuel Control Unit

• The purpose of the fuel control is to maintain a correct combustion zone air-to-fuel mixture ratio of 15:1 by weight. This ratio represents weight of combustor primary air to weight of fuel.

• All fuels require a certain proportion of air for complete burning, but at rich or lean mixtures the fuel will burn but not completely. The ideal proportion for air and jet fuels is 15:1, and it is called the Stoichometric (chemically correct) mixture.

Spill Flow/Icing

• 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.

• When the fuel has passed through the LP and HP pumps and through the Fuel-oil heat exchanger, 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.

• 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.

Fuel Nozzles

• The end point of the turbine engine fuel system is the fuel nozzles.

• In order for the liquid fuel to release its energy, it must be vaporized so it will mix with the air to form a combustible mixture.

Simplex Nozzle

• Fuel under pressure from the fuel control forces the check valve off its seat and enters the nozzle.

• This fuel then passes through a series of tangential flutes, or grooves, and sprays out through the single discharge orifice in very tine droplets in a cone-shaped pattern.

Duplex Nozzle

• Two types of duplex nozzles are used on modern engines. These are: single-line and dual-line nozzles

• Single-line duplex nozzles are supplied with fuel from the fuel control through a single fuel line or manifold.

• These nozzles incorporate a flow divider valve that allows the fuel to spray from a central orifice in a widespray pattern for starting and idling.

• When the fuel control meters sufficient pressure to open the flow divider, fuel flows to the secondary orifice. The large volume of the secondary fuel and the high pressure at which it leaves the nozzle narrow the spray pattern and force the fuel farther down the combustor

Housings for flow divided valve and check valve

Flange for attachment on combustion casing

Nozzle tube 2 spray nozzle tip

Fuel inletSimplex Nozzle

Single-line Duplex Nozzle

Dual-line Duplex Fuel Nozzle