The engine lubrication system is designed to deliver clean oil at the correct temperature and pressure to every part of the engine. The oil is sucked out the sump into the pump, being the heart of the system, than forced through an oil filter and pressure feeded to the main bearings and to the oil pressure gauge. From the main bearings, the oil passes through feed-holes into drilled passages in the crankshaft and on to the big-end bearings of the connecting rod. The cylinder walls and piston-pin bearings are lubricated by oil fling dispersed by the rotating crankshaft. The excess being scraped off by the lower ring in the piston. A bleed or tributary from the main supply passage feeds each camshaft bearing. Another bleed supplies the timing chain or gears on the camshaft drive. The excess oil then drains back to the sump, where the heat is dispersed to the surrounding air.
If the crankshaft journals become worn the engine will have low oil pressure and throw oil all over the inside of the engine. The excessive splash will probably overwhelm the rings and cause the engine to use oil. Worn bearings surfaces can be restored by simply replacing the bearings inserts. In good maintained engines bearing wear occurs immediately after a cold start, because there’s little or no oil film between the bearing and shaft. At the moment that sufficient oil is circulated through the system hydrodynamic lubrication manifests and stop the progress of bearing wear.
Piston rings – cylinder
Piston rings provide a sliding seal preventing leakage of the fuel/air mixture and exhaust from the combustion chamber into the oil sump during compression and combustion. Secondly they keep oil in the sump from leaking into the combustion area, where it would be burned and lost. Most cars that “burn oil” and have to have a quart added every 1,000 miles are burning it because the rings no longer seal properly.
Between the piston rings and the cylinder wall of a well maintained engine hydrodynamic lubrication prevails, essential for the lowest friction and wear. In the top and bottom dead centre where the piston stops to redirect, the film thickness becomes minimal and mixed lubrication may exist.
To realize a good head transfer from the piston to the cylinder, an optimal sealing and a minimum of oil burning, a minimal film thickness is desirable. The film thickness is kept minimal by a so called oil control ring. This ring is situated beyond the piston rings so that the surplus of oil is directly scraped downwards to the sump. The oil film left on the cylinder wall by the passage of this ring is available to lubricate the following ring. This process is repeated for successive rings. On the up stroke the first compression ring is lubricated by the oil left behind on the cylinder wall during the down stroke.
Leakage of the fuel/air mixture and exhaust from the combustion chamber into the oil sump result in oil degradation. This is the reason why, despite of frequent replenish of oil, oil change remain essential or even become more essential.
TURBINE ENGINE DRY-SUMP LUBRICATION
In a turbine dry-sump lubrication system, the oil supply is carried in a tank mounted externally on or near the engine. With this type of system, a larger oil supply can be carried and the oil temperature can be controlled An oil cooler usually is included in a dry-sump oil system (Figure 5-l). This cooler may be air-cooled or fuel-cooled. The dry-sump oil system allows the axial-flow engines to retain their comparatively small diameter. This is done by designing the oil tank and the oil cooler to conform to the design of the engine.
The following component descriptions include most of those found in the various turbine lubrication systems. However, not all of these components will be found in any one system.
The dry-sump systems use an oil tank which contains most of the oil supply. However, a small sump usually is included on the engine to hold a supply of oil for an emergency system. The dry-sump system usually contains–
- Oil pump.
- Scavenge and pressure inlet strainers.
- Scavenge return connection.
- Pressure outlet ports.
- Oil filter.
- Mounting bosses for the oil pressure transmitter.
- Temperature bulb connections.
A typical oil tank is shown in Figure 5-2. It is designed to furnish a constant supply of oil to the engine. This is done by a swivel outlet assembly mounted inside -the tank a horizontal baffle mounted in the center of the tank, two flapper check valves mounted on the baffle, and a positive-vent system.
The swivel outlet fitting is controlled by a weighted end, which is free to swing below the baffle. The flapper valves in the baffle are normally open. They close only when the oil in the bottom of the tank rushes to the top of the tank during deceleration. This traps the oil in the bottom of the tank where it is picked up by the swivel fitting A sump drain is located in the bottom of the tank. The airspace is vented at all times.
All oil tanks have expansion space. This allows for oil expansion after heat is absorbed from the bearings and gears and after the oil foams after circulating through the system. Some tanks also incorporate a deaerator tray. The tray separates air from the oil returned to the top of the tank by the scavenger system. Usually these deaerators are the “can” type in which oil enters a tangent. The air released is carried out through the vent system in the top of the tank. Inmost oil tanks a pressure buildup is desired within the tank. This assures a positive flow of oil to the oil pump inlet. This pressure buildup is made possible by running the vent line through an adjustable check-relief valve. The check-relief valve normally is set to relieve at about 4 psi pressure on the oil pump inlet.
There is little need for an oil-dilution system. If the air temperature is abnorrnally low, the oil may be changed to a lighter grade. Some engines may provide for the installation of an immersion-type oil heater.
In some engines the lubrication system is the wet-sump type. Because only a few models of centrifugal-flow engines are in operation, there are few engines using a wet-sump type of oil system.
The components of a wet-sump system are similar to many of a dry-sump system. The oil reservoir location is the major difference.
The reservoir for the wet-sump oil system may be the accessory gear case, which consists of the accessory gear casing and the front compressor bearing support casing. Or it may be a sump mounted on the bottom of the accessory case. Regardless of configuration reservoirs for wet-sump systems are an integral part of the engine and contain the bulk of the engine oil supply.
The following components are included in the wet-sump reservoir:
- A bayonet-type gage indicates the oil level in the sump.
- Two or more finger strainers (filters) are inserted in the accessory case for straining pressure and scavenged oil before it leaves or enters the sump. These strainers aid the main oil strainer.
- A vent or breather equalizes pressure within the accessory casing.
- A magnetic drain plug may be provided to drain the oil and to trap any ferrous metal particles in the oil. This plug should always be examined closely during inspections. The presence of metal particles may indicate gear or bearing failure.
- A temperature bulb and an oil pressure fitting may be provided.
This system is typical of all engines using a wet-sump lubrication system. The bearing and drive gears in the accessory drive casing are lubricated by a splash system. The oil for the remaining points of lubrication leaves the pump under pressure. It passes through a filter to jet nozzles that direct the oil into the rotor bearings and couplings. Most wet-sump pressure systems are variable-pressure systems in which the pump outlet pressure depends on the engine RPM.
The scavenged oil is returned to the reservoir (sump) by gravity and pump suction. Oil from the front compressor bearing in the accessory-drive coupling shaft drains directly into the reservoir. Oil from the turbine coupling and the remaining rotor shaft bearings drains into a sump. The oil is then pumped by the scavenge element through a finger screen into the reservoir.
The oil system components used on gas turbine engines are–
- Pressure pumps.
- Scavenger pumps.
- Oil coolers.
- Relief valves.
- Breathers and pressurizing components.
- Pressure and temperature gages lights.
- Temperature-regulating valves.
- Oil-jet nozzle.
- Fittings, valves, and plumbing.
- Chip detectors.
Not all of the units will be found in the oil system of any one engine. But a majority of the parts listed will be found in most engines.
Tanks can be either an airframe or engine-manufacturer-supplied unit. Usually constructed of welded sheet aluminum or steel, it provides a storage place for the oil. In most engines the tank is pressurized to ensure a constant supply of oil to the pressure pump. The tank can contain–
- Venting system.
- Deaerator to separate entrained air from the oil.
- Oil level transmitter or dipstick.
- Rigid or flexible oil pickup.
- Coarse mesh screens.
- Various oil and air inlets and outlets.
Both gear- and Gerotor-type pumps are used in the lubricating system of the turbine engine. The gear-type pump consists of a driving and a driven gear. The engine-accessory section drives the rotation of the pump. Rotation causes the oil to pass around the outside of the gears in pockets formed by the gear teeth and the pump casing. The pressure developed is proportional to engine RPM up to the time the relief valve opens. After that any further increase in engine speed will not result in an oil pressure increase. The relief valve may be located in the pump housing or elsewhere in the pressure system for both types of pumps.
The Gerotor pump has two moving parts: an inner-toothed element meshing with an outer-toothed element. The inner element has one less tooth than the outer. The missing tooth provides a chamber to move the fluid from the intake to the discharge port. Both elements are mounted eccentrically to one another on the same shaft.
These pumps are similar to the pressure pumps but have a much larger total capacity. An engine is generally provided with several scavenger pumps to drain oil from various parts of the engine. Often one or two of the scavenger elements are incorporated in the same housing as the pressure pump (Figure 5-3). Different capacities can be provided for each system despite the common driving shaft speed. This is accomplished by varying the diameter or thickness of the gears to vary the volume of the tooth chamber. A vane-type pump may sometimes be used.
Oil Filters and Screens or Strainers
To prevent foreign matter from reaching internal parts of the engine, filters and screens or stainers are provided in the engine lubricating system. The three basic types of oil filters for the jet engine are the cartridge screen-disc and screen (Figures 5-4, 5-5 and 5-6). The cartridge filter is most commonly used and must be replaced periodically. The other two can be cleaned and reused. In the screen-disc filter there are a series of circular screen-type filters. Each filter is comprised of two layers of mesh forming a chamber between mesh layers. The filters are mounted on a common tube and arranged to provide a space between each circular element. Lube oil passes through the circular mesh elements and into the chamber between the two layers of mesh. This chamber is ported to the center of a common tube which directs oil out of the filter. Screens or strainers are placed at pressure oil inlets to bearings in the engine. This aids in preventing foreign matter from reaching the bearings.
To allow for oil flow in the event of filter blockage, all filters incorporate a bypass or relief valve as part of the filter or in the oil passages. When the pressure differential reaches a specified value (about 15 to 20 psi), the valve opens and allows oil to bypass the filter. Some filters incorporate a check valve. This prevents reverse flow or flow through the system when the engine is stopped Filtering characteristics vary, but most filters will stop particles of approximately 50 microns.
Magnetic Chip Detector
One or more magnetic chip detectors are installed on gas turbine engines. They are used to detect and attract ferrous material (metal with iron as its basic element) which may come from inside the engine. This ferrous material builds up until it bridges a gap. Whenever there is a requirement, the chip detectors may be collected and analyzed to determine the condition of the engine. Most engines utilize an electrical chip detector, located in the scavenger pump housing or in the accessory gearbox. Should the engine oil become contaminated with metal particles, the detector will catch some of them. This causes the warning light on the caution panel to come on.
Tubing, Hose, and Fittings
Tubing, hose, and fittings are used throughout the lubricating system. Their purpose is to connect apart into a system or to connect one part to another to complete a system.
Oil Pressure Indicating System
In a typical engine oil pressure indicating system the indicator receives inlet oil pressure indications from the oil pressure transmitter and provides readings in pounds per square inch Electrical power for oil pressure indicator and transmitter operation is supplied by the 28-volt AC system.
Oil-Pressure-Low Caution Light
Most gas turbine engine lubricating systems incorporate an engine oil-pressure-low caution light warning device into the system for safety purposes. The light is connected to a low-pressure switch. When pressure drops below a safe limit, the switch closes an electrical circuit causing the caution light to burn. Power is supplied by the 28-volt DC system.
Oil Temperature Indicating System
In a typical engine oil temperature indicating system, the indicator is connected to and receives temperature indications from an electrical resistance-type thermocouple or thermobulb. These are located in the pressure pump oil inlet side to the engine. Power to operate this circuit is supplied by the 28-volt DC system.
The oil cooler is used to reduce oil temperature by transmitting heat from the oil to another fluid usually fuel. Since the fuel flow through the cooler is much greater than the oil flow, the fuel is able to absorb a considerable amount of heat. This reduces the size and weight of the cooler. Thermostatic or pressure-sensitive valves control the oil temperature by determining whether the oil passes through or bypasses the cooler. Oil coolers are also cooled by air forced through them by a blower/fan.
Breathers and Pressurizing Systems
Internal oil leakage is kept to a minimum by pressurizing the bearing sump areas with air that is bled off the compressor (Figure 5-7). The airflow into the sump minimizes oil leakage across the seals in the reverse direction.
The oil scavenge pumps exceed the capacity of the lubrication pressure pump They are capable of handling considerably more oil than actually exists in the bearing sumps and gearboxes. Because the pumps area constant-displacement type, they make up for the lack of oil by pumping air from the sumps. Large quantities of air are delivered to the oil tank. Sump and tank pressures are maintained close to one another by a line which connects the two. If the sump pressure exceeds the tank pressure, the sump vent check valve opens, allowing the excess sump air to enter the oil tank. The valve allows flow only into the tank; oil or tank vapors cannot back up into the sump areas. Tank pressure is maintained little above ambient.
The scavenge pumps and sump-vent check valve functions result in relatively low air pressure in the sumps and gearboxes. These low internal sump pressures allow air to flow across the oil seals into the sumps. This airflow minimizes lube oil leakage across the seals. For this reason it is necessary to maintain sump pressures low enough to ensure seal-air leakage into the sumps. Under some conditions, the ability of the scavenge pumps to pump air forms a pressure low enough to cavitate the pumps or cause the sump to collapse. Under other conditions, too much air can enter the sump through worn seals.
If the seal leakage is not sufficient to maintain proper internal pressure, check valves in the sump and tank pressurizing valves open and allow ambient air to enter the system. Inadequate internal sump and gearbox pressure may be caused by seal leakage. If that occurs, air flows from the sumps, through the sump-vent check valve, the oil tank, the tank and sump pressurizing valves to the atmosphere. Tank pressure is always maintained a few pounds above ambient pressure by the sump and tank pressurizing valve.
The following addresses two types of lubrication systems used in the Army today: the General Electric T-701 turboshaft engine and the International/Solar T-62-series engine.