There are basically two types of lubrication system at present in use in gas turbine engines:-
a) Recirculatory. In this system, oil is distributed and returned to the oil tank by pumps. There are two types of recirculatory system:-
(i) Pressure relief valve system.
(ii) Full flow system.
b) Expendable. The expendable or total loss system is used on some small turbo-jet engines, eg. RB 162 in which the oil is spilled overboard after lubricating the engine.
10.3.1 PRESSURE RELIEF VALVE RE-CIRCULATORY SYSTEM
In the pressure relief valve type of recirculatory lubrication system the flow of oil to the various bearings is controlled by a relief valve which limits the maximum pressure in the feed line. As the oil pump is directly driven by the engine (by the HP spool in the case of a multi-spool engine), the pressure will rise with spool speed.
Above a pre-determined speed the feed oil pressure opens the system relief valve allowing excess oil to spill back to the tank, thus ensuring a constant oil pressure at the higher engine speeds.
A typical relief valve type of recirculatory lubrication is shown in the figure 10.3.
The oil system for a typical turbo-prop engine is similar but, as it supplies the propeller control system, it is more complicated. The oil supply is usually contained in a combined tank and sump formed as part of the external wheelcase. Oil passes via the suction filter to the pressure pump, which pumps it through the air-cooled oil cooler to the pressure filter. A pressure regulating valve upstream of the filter controls the oil pressure. Both oil pressure and temperature indications are transmitted to the cockpit. The oil flows through pipes and passages to lubricate the main shaft bearings and wheelcases. The main shaft bearings are normally lubricated by oil jets and some of the heavier loaded gears in the wheelcases are also provided with oil jets, while the remaining gears and bearings receive splash lubrication.
An additional relief valve is fitted across the pump in the lubrication system of some engines to return oil to pump inlet if the system becomes blocked.
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A Pressure Relief Valve Lubrication System for a Two Shaft Turbojet.
JAR 66 CATEGORY B1 MODULE 15 GAS TURBINE
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10.3.2 RECIRCULATORY OIL SYSTEM – FULL FLOW TYPE
The full flow lubrication system is an alternative to the pressure relief valve oil system and full flow systems are in use as a means of lubricating many modern high power gas turbine engines.
The full flow system is similar in many ways to the pressure relief system just discussed – i.e. oil is drawn from a tank by a pump and delivered, via a pressure filter, to various parts of the engine; the oil is then returned by scavenge pumps, via the oil cooler to the tank; also, air is separated from the oil by a de-aerator and centrifugal breather.
The major differences from the pressure relief type of recirculatory system are as follows:-
The flow of oil to the bearings is determined by the speed of the pressure pump, the size of the oil jets and the pressure in each of the bearing housings.
A metered spill of feed oils is fed back to the tank. This spill is calibrated to match the pump output to ensure that the oil flow to the bearings, via the oil jets, is the same at all engine speeds.
The relief valve in this system is set to prevent excessive oil pressure in the feed side of the system.
A filter by-pass is not normally fitted. The pressure drop across the filter is sensed by a differential pressure switch, any increase in the pressure difference being indicated to give advance warning of a blocked filter.
10.3.3 ADVANTAGES OF FULL FLOW LUBRICATION
The advantages of full flow lubrication are those of suitable oil flow and oil pressure at all engine speeds. A study of the graph will reveal a difference in oil pressure between the pressure relief system and the full flow system and, it will also show that the pressure difference continues throughout the speed range of the engines, with a crossover point at cruising speed. The relief valve system provides too much oil pressure at idle rev/min, but because of the relief valve, the oil pressure is below optimum at maximum engine speed. In contrast the pressure provided by the oil pump of a full flow system rises progressively with increased engine speed and is nearer to the optimum value throughout the rev/min range of the engine.
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Full Flow Oil System ( RR Gem).
Figure 10.6.
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10.3.4 EXPENDABLE SYSTEM
An expendable system is generally used on small engines running for periods of short duration. The advantage of this system is that it is simple, cheap and offers an appreciable saving in weight as it requires no oil cooler, scavenge pumps or filters. Oil can be fed to the bearing either by a pump or tank pressurisation. After lubrication the oil can either be vented overboard through dump pipes or leaked from the centre bearing to the rear bearing after which it is flung onto the turbine and burnt.
An Expendable Oil System.
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In any aircraft oil system, we have a number of components that may be thought of as the main components and we have some that are incorporated to safeguard the system (ie. to act as safety devices). The main components, on which the operation of the system depends, include the oil tank, the oil pump and the oil cooler; these are considered in the paragraphs immediately following. The safety devices, which include the various valves and filters, are considered later.
10.4.1 OIL TANK
The oil tank is usually mounted on the engine; it may be a separate unit or part of an external gearbox called the sump. It has provision to allow the system to be
A de-aerator tray is mounted in the top half of the tank and receives the return oil from the scavenge pumps. The oil in its passage through the system will become aerated