Excellent Question:
1. The loads in jet engines are mostly thrust loads, and not radial loads as found in gas and diesel engines. There are no "impulse" loads in the radial direction in which to resist. There is mostly rotational motion.
2. The flame of combustion never touches the oil. So the oil never oxidizes from combustion or becomes contaminated with blowby gasses. Acids can only form by metal activation, and metal deactivators are used to kill any corrosion. The oil may oxidize from heat due to thermal energy being conducted from the combustor to the turbine bearing. Antioxidant additives, and very expensive ones at that, are used to protect the oil from oxidizing due to thermal cooking.
3. The fluids used for lubrication are low viscosity fluids because jet engines need swift lubrication to the bearing "cells," and because the bearings are cooled by the oil, using heat exchangers. There are two heat exchangers. One is the fuel-to-oil heat exchanger, and the other is the air-to-oil heat exchanger. The fuel-to-oil AND the air-to-oil heat exchangers are used when air flow through the nacelle is insufficient, or when the fuel flow is low, as when when taxiing or during take-off thrust when heating is high. While in the air and at high altitudes, the air-to-oil heat exchanger functions by removing oil heat and transferring that heat to the fuel, so that the fuel flows better and preheats the fuel as well.
4. Extreme pressure additives (and very expensive as well) are added when the jet engine is to be used in TurboProp configuration. The same oil not only lubricates the bearing cells in the jet engine, but lubricates the gears and bearings in the speed reduction transmission for the prop, and also lubricates the gears and bearings for Accessory Gearbox. I should add that many of these EP/AW additives are advanced esters, themselves.
The lubricant is usually a Pentaerythritol, a Di-Pentaerythritol, TMP, TME or a combination of polyol esters with just a few percentages of additives. Very expensive fluids.
Most jet engines usually run 10,000 hours on one oil change and consume about a 1/2 to one pint of oil per hour.
There are generally small screen-type (mesh) filters at each bearing cell and one larger filter for filtering the bulk oil. Oil pressure is generally about the same or lower than a high performance automobile engines pressure.
EDIT: Advanced and exotic jet engine (turbine) fluids are being developed as we speak to increase lifetime of the fluids and to withstand the higher temperatures those fluids will have to endure in the future. Higher turbine inlet temperatures relates to higher thrust and fuel efficiency.
[ July 01, 2003, 04:35 PM: Message edited by: MolaKule ]
