Engine Lubrication Systems

Engine Lubrication Systems Engine Lubrication The primary function of the lubrication system is to supply oil to the various parts within the engine which are subjected to friction loads from engine rotation and heat loads from the gas path. Engine heat that radiates outwards is dealt with by the nacelle ventilation system. Engine heat that radiates inwards is dealt with by the lubrication system. The oil is supplied under pressure along the main rotor shaft and to the gearboxes to reduce friction, to cool and to clean. It is then returned by a scavenging system to the oil storage tank to be used again and again. Oil consumption is low in gas turbine engines as compared to piston engines, and this accounts for the relatively small bulk oil storage tanks used. They can be as small as 3–5 L in capacity on business jet-sized engines and 20–30 L on large commercial type engines. The oil is not exposed to great quantities of combustion products and stays fairly clean by filtration. Heat, however, is a problem which can cause rapid oil decomposition, and for this reason temperature is carefully controlled by automatic cooling devices and carefully monitored by the engine operators. Two basic types of self-contained lubrication systems are used in today’s gas turbine engines: Pressure relief valve system Full flow system. The pressure relief valve system controls oil flow by limiting the pressure from the oil pump to a given value. In the full flow system, pressure changes with rpm, and this is the system of choice when the bearing vent cavities contain high air pressures. Both systems will be discussed in detail later. Engine lubrication systems can be classified as either of the following: Wet-sump system Dry-sump system. Oil Reservoir The purpose of the oil reservoir is to hold sufficient oil to allow for engine consumption at maximum flight operation and duration. Wet-Sump Systems The oil supply for a wet-sump system is typically located in the main or accessory gearbox at the lowest point within the engine. From here, oil is pressurised and routed through multiple filters before reaching the main rotor bearings and couplings. It also allows splash lubrication to be used on accessory gears and bearings. Once the oil has lubricated the main bearings, it drains to low-lying areas, where scavenge pumps route it back to the sump or gearbox. Wet-sump oil system Today, the majority of turbine engines use a dry-sump lubrication system consisting of pressure, scavenge and breather subsystems. Dry-Sump Systems Dry-sump systems differ from wet-sump systems in that the oil is stored in a separate oil reservoir mounted either internally within the engine or externally on the engine or in the aircraft. In this type of system, an oil pump pulls oil from the oil reservoir and provides pressure and spray lubrication throughout the engine. The oil reservoir in a dry-sump system is usually constructed of sheet aluminium or stainless steel and is designed to furnish a constant supply of oil to the engine during all approved flight manoeuvres. They come in a variety of shapes and sizes, depending mainly on system requirements and the amount of space allowed around the engine when installed in the nacelle or airframe. System requirements also determine whether the tank is pressurised or unpressurised. Dry-sump oil system Once circulated, the oil accumulates in low-lying areas, where scavenge pumps pick it up and pump it back to the reservoir. To ensure a positive flow of oil to the oil pump inlet, most oil reservoirs are pressurised. Pressurising the reservoir also helps to suppress oil foaming, which in turn prevents pump cavitation. In most cases, pressurisation is accomplished by installing an adjustable relief valve in the oil reservoir vent line. This way, reservoir pressure builds until the relief valve opens to relieve excess pressure. Dry sump reservoir