JET PROPULSION Part 5 Jet Engine Operation Oil Systems

Introduction ‘Dry Sump, Recirculating System’ Early jet engines had a ‘Total Loss’ oil system, that is oil was taken from the tank, used for lubrication and was then lost overboard. Now, jet engines have what is known as ‘Dry Sump, Recirculating System’ In jet engines oil is sucked (scavenged) out of the sump and returned to a separate tank. The scavenge pumps have a greater capacity than is required to make sure that all the oil is actually taken out of the sump, hence the term ‘dry’ sump.

Engine Bearing Chamber Dry Sump Recirculating System Early jet engines were a total loss system Engine Bearing Chamber Pump Filter Tank SUMP Scavenge Pump Scavenge pumps make sure all the oil is actually taken out of the sump, hence the term ‘DRY-SUMP’. i.e. oil was lost in the gas stream

Engine Bearing Chamber Dry Sump Recirculating System Early jet engines were a total loss system Engine Bearing Chamber Pump Filter Tank SUMP Scavenge Pump Oil scavenged from the engine, is replaced in the tank ready to be used again, hence the term ‘RECIRCULATING’.

Oil Systems Typical Oil System SUPPORT BEARINGS As we know, location bearings support the engine rotor, which have to be lubricated to maintain smooth operation. These bearings are contained within bearing chambers; the number of bearings inside a chamber can vary with different designs of jet engines.

Oil Systems Typical Oil System OIL COOLER OIL PUMP ASSEMBLY PRESSURE OIL TANK Oil is sucked out of the tank by the pressure pump, which then forces it through an oil cooler. Coolers are placed before the tank, a ‘Cool Tank system’ or after, a ‘Hot Tank system’.

Oil Systems Typical Oil System OIL COOLER OIL PUMP ASSEMBLY PRESSURE OIL TANK Oil flows via individual pipes, to the bearing chambers. Oil distribution is set to the correct flow because bearings near hotter engine parts require the greater oil flow for cooling as well as lubrication.

Oil Systems Typical Oil System SCAVENGE OIL COOLER OIL PUMP ASSEMBLY PRESSURE OIL TANK Each chamber is scavenged by its own pump ensuring no oil rests in the bearing chamber sump. The outlet from the scavenge pumps is combined to a single return flow back to the oil tank.

Oil Systems Typical Oil System SCAVENGE OIL COOLER MCD MASTER MCD OIL PUMP ASSEMBLY PRESSURE OIL TANK If a bearing starts to disintegrate, it generates particles. As bearings are made from magnetic metals, magnets are fitted into the scavenge system called Magnetic Chip Detectors (MCD’s).

The Bearing Chamber Bearing Oil Supply and Scavenge Let’s take a closer look at what happens here

The Bearing Chamber Bearing Oil Supply and Scavenge Shaft Oil Jet The oil is delivered under pressure to a jet. Once the oil has passed through the jet, its pressure becomes the same as the bearing chamber pressure.

The Bearing Chamber Bearing Oil Supply and Scavenge Shaft Oil Jet The jet is directed to a rotating reservoir, filling by centrifugal force when the engine is running. The oil evens out providing a feed through internal drillings to all areas of the bearings.

The Bearing Chamber Bearing Oil Supply and Scavenge Shaft Oil Jet Scavenge Oil to tank After the oil has lubricated the bearing, it drops to the bottom of the bearing chamber sump, and is taken away by the scavenge pumps and returned to the oil tank.

The Bearing Chamber Bearing Oil Supply and Scavenge Shaft Oil Jet Scavenge Oil to tank In order to prevent the loss of oil, the bearing chambers need to be sealed, allowing the oil to be continuously recirculated through the system.

The Bearing Chamber Bearing Oil Supply and Scavenge Shaft Oil Jet Scavenge Oil to tank As the bearings are situated between the rotating and static parts of the engine, the seals cannot be of contact type, so air sealing vents are added.

The Bearing Chamber Bearing Oil Supply and Scavenge Shaft Pressurised air from compressors Higher pressure air forced into the Bearing Chamber prevents oil loss Shaft Oil Supply Oil Jet Scavenge Oil to tank The seals basically comprise of a small gap between the rotating and static part of the engine, to which an air pressure is applied from the outside.

Oil Systems Oil System with Air Sealing SCAVENGE PRESSURE OIL PUMP ASSEMBLY MCD MASTER MCD OIL COOLER OIL TANK Scavenge Oil passes through Air/Oil Separator Returning oil passes through an air/oil separator :- A Settlement tray allows air to bubble to the oil surface A Vortex separator which squeezes the air to the centre and then to the top of the tank.

Oil Systems Oil System with Air Sealing SCAVENGE PRESSURE OIL PUMP ASSEMBLY MCD MASTER MCD OIL COOLER OIL TANK Clean(?) Air Overboard Gearbox Drive Centrifugal Breather Scavenge Oil passes through Air/Oil Separator The air in the tank passes to a Centrifugal Breather, with a vent line from each bearing chamber. Driven by the engine gearbox, the air is throws out and the oil returned to the tank.

Oil Systems Oil System with Air Sealing SCAVENGE PRESSURE OIL PUMP ASSEMBLY MCD MASTER MCD OIL COOLER OIL TANK Clean(?) Air Overboard Gearbox Drive Centrifugal Breather Scavenge Oil passes through Air/Oil Separator Centrifugal Breathers are never 100% effective, and oil loss can be visible as a blue-ish smoke. Too much smoke shows over filling of oil or other problems like bearing chamber seal failure.

Check of Understanding What type of oil system do modern jet engines have? Sealed Sump Redirecting System Dry Sump Recirculating System Sealed Sump Centrifugal System Dry Sump Scavenger System

Check of Understanding What is fitted into a scavenge system to check if bearings are disintegrating? Magnetised Coil Detectors Magnetised Chip Detectors Magnetic Coil detectors Magnetic Chip Detectors

Check of Understanding In the bearing oil supply, the oil is delivered under pressure to a jet. Once the oil has passed through the jet, which of the following applies? Its pressure becomes the same as the bearing chamber pressure Its pressure becomes less than the bearing chamber pressure Its pressure becomes more than the bearing chamber pressure Its pressure remains the same

Check of Understanding In the bearing oil supply, why do the bearing chambers need to be sealed? To prevent the loss of oil To maintain the oil pressure To maintain oil temperature To withstand outside air pressure

Check of Understanding In the bearing system, air sealing vents are added, contact seals cannot be applied because . . . . The oil and air pressure is too high The bearings are situated between rotating and static parts of the engine The oil and air temperatures are too high The bearings would be inoperative if contact seals were used

Check of Understanding If blue-ish smoke is exiting an engine, which of the following could apply? Damage to the centrifugal breather Too little oil in the system Bearing chamber seal failure Bearing particles in the system

JET PROPULSION End of Presentation