The oil system for an aero engine is a full recirculatory system and its main function is to supply oil to the engine’s internal drives, gears, and bearings, where the main rotating assemblies are (Figure 6.3). The oil is used to lubricate these locations and remove all unwanted heat throughout all operating conditions and power settings. The system is designed to ensure that the oil supplied to these drives, gears, and bearings is in the correct condition with regards to cleanliness, pressure, temperature, and quantity. The complete oil system for an aircraft gas turbine can be divided into three main subsystems:
• Oil feed, lubrication, and cooling (pressure side)
• Return oil (scavenge side)
• Breather system (vent)
Within these areas are the necessary components that ensure that each of the subsystems functions as designed (Figure 6.4).
The architecture of the oil system is designed such that bearing chambers and gearboxes will be at different pressures relative to each other and, therefore, the flow from the pressure system will have to be controlled to ensure each area of the engine receives the correct oil flow. The pressure pump delivers the total oil flow for the whole engine at one pressure and this is distributed around the engine by a network of pipes. Restrictors are placed in oil supply lines upstream of the oil jets. The size of the restrictor is controlled so that the pressure drop across the oil jets is correctly maintained and within a specific range;
this ensures adequate targeting and oil velocity.
6-4HandbookofLubricationandTribology
L.P turbine bearing chamber
HP/IP turbine bearing chamber
Internal gearbox drive
LP/IP compressor
bearings
Pressure filter Pressure
pump
Oil temp sensors
Scavenge filter
Master MCD
Centrifugal breather Scavenge
pumps AOHE
FOHE
Quantity transmitter
Oil tank
Gearbox input drive
assembly External gearbox Intermediate
gearbox Strainer
Pressure transmitter Pressure switch Relief valve
Pump Magnetic chip detector Scavenge return HP feed Vent air/oil mist
FIGURE 6.3 Oil system schematic.
© 2006 by Taylor & Francis Group, LLC
Pressure relief valve
Centrifugal breather
Pressure filter Strainer
From oil tank
Air cooled oil cooler
Torquemeter pump Oil pump pack
De-aerator tray
FIGURE 6.4 Turboprop oil system.
Finally, before the oil enters into the oil jet, it can be passed through a “last chance” filter to ensure that no debris generated downstream of any other filtration system can block the oil jet. This type of filter (or strainer) is usually a screw thread filter and will be built into the engine upstream of the oil jet.
Generally the circulation of oil is far in excess of the capacity within the system. For example, in a large turbofan engine of approximately 70,000 lbs of thrust, the nominal oil quantity contained in the oil tank is approximately 23 L (24.3 US quarts), of which approximately 18 L (19 US quarts) is usable. The complete oil system, including pipes, bearing chambers, and gearboxes, will have a capacity of approximately 36 L (38 US quarts) in total. To supply the various parts of the engine that need lubricating, the approximate flows required are as follows:
• The oil supply to all the bearing chambers (including any internal gearboxes) is approximately 2800 L/h (or 2960 US quarts/h).
• The oil supply to an external gearbox is approximately 210 L/h (or 220 US quarts/h).
Therefore, the entire contents of the oil tank circulate around the oil system approximately every 29 sec.
Because the design of the engine has different parts of the oil system operating at different oil pressures, all the areas that are supplied with oil will also have their own dedicated oil scavenge pump to recover the oil back to the tank. Generally, the oil scavenge system will be constructed so that there is only one pipe between the sump and each scavenge pump; this prevents recirculation and preferential scavenging.
In an aero engine, there are bearing chambers that are required to be sealed using bleed air from the compressors. The air pressure outside the chamber is always greater than the oil pressure inside the chamber; thus the air is “allowed” to leak across seals into the bearing chamber and, as a con- sequence, these chambers will have to be ventilated. As this air is no longer required by the engine’s systems, it will be discarded overboard. However, because of the environment within these bearing cham- bers, the vent air will contain small amounts of oil as an air/oil mist. As the vent air will be exhausted to atmosphere, the oil that is suspended in the vent air needs to be recovered and returned to the tank;
otherwise the oil consumption of the engine will increase dramatically as the oil will be wasted. To achieve
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Air to atmosphere
Gear shaft air outlet slots
De-aerator segments
Air/oil mist entry holes
Driven gears
Return oil to gearbox FIGURE 6.5 Centrifugal breather.
this, the compartments in the oil system that require venting are connected to a centrifugal breather (Figure 6.5).
There are two other separate oil systems on a civil aero engine, one for the engine’s starter and one for the integrated drive generator (IDG).
The starter for many aero engines is an air turbine starter and, as the name suggests, this is the main component for starting the engine. The oil system on the start is self-contained and the operation of the starter is controlled by its duty cycle, which is defined in one of the operating manuals for the aircraft. The IDG is the main unit that supplies AC power to the aircraft. Whenever the engine is running, the IDG is usually running under load and generating a large amount of heat. Like the starter, the IDG has its own self-contained oil system; however, because of the time for which the IDG will be used, the oil system for the IDG will have its own dedicated oil cooler and filtration system.