Bearings and cooling
Rotational speed achieved by the engine will probably be above 40000rpm. At these speeds, regular ball bearings may overheat or suffer from a too fast wear. In real turbine engines, bearings are constantly lubricated by oil jets, which poses problems with regards to oil pressurization and leaks in other parts of the engine. Seals are consequently placed close to bearings to prevent leaks, generally carbon seals. To enforce the seal, oil is contained in a casing with an internal pressure lower than the external pressure built from compressed air. That way, air can enter the oil casing, but oil cannot leak outside, in other parts of the engine.
Accessories like oil pumps, pipes, fixations, filters, tanks, heat exchangers and so on, are also required.
Ball or roller bearings are the obvious way to guide rotating parts. They can handle high mechanical constraints radially or even axially, they are inexpensive and their integration is reasonably simple.
Silicon nitride bearings have lots of improvements over regular metal ball bearings. Balls are more than 60% less heavy, thus having a lower inertia at high speeds, implying a more softer contact with the tracks, allowing longer lifetime or higher reachable speeds. They also require less lubrication. Fortunately, silicon nitride bearings have reached market with a large production, and are not over-expensive.
Fluid or magnetic bearings should be considered. They allow much higher rotation speeds and lower friction, but have two main drawbacks. At standby state, they release the radial constrain on moving parts. In reduced-size turbomachinery, where rotor and stator have to be adjusted to tens of microns, it seems quite complicated to use those bearings. The second drawback is that they require more external hardware, to pressurize the fluid or to provide magnetic energy.
However, magnetic bearings have been demonstrated in this paper , in which axial position accuracy is measured below 150µm for a 4kg rotor at around 2000rpm. The rotor position sensor has a resolution of 2µm per mV. Unfortunately, no indication is given about the resting position of the rotor and how it impacts the clearance between rotor and stator.
Foil bearings are a particular type of fluid bearing, that "Unlike aero or hydrostatic bearings, foil bearings require no external pressurisation system for the working fluid, so the hydrodynamic bearing is self-starting". In this other paper , a small centrifugal turbojet is built to evaluate the ability of MiTi's product, a foil bearing, to sustain very high rotation speeds (120'000rpm) and high temperature (800°C). The bearing has a low spacing between the rotor's journal and the stator fixation, but it is secured, in this paper, using a ball bearing on the compressor side, where the temperature is low. They planned to make a dual-foil bearing, we'll need to check on that. MiTi also demonstrated a hybrid foil magnetic bearing, that has the advantages of magnetic bearings at low speeds and those of foil bearings at high speeds.
Air bearing are used by Bladon Jets for example, in their small turbines.
Use of lubricating oil for cooling
In real-world jet engines, cooling is the primary function of oil when conventional bearings are used, even more important than lubrication. That's well explained in this AgentJayz video. A high flow rate of oil is then required, with a heat exchanger somewhere along the oil path. The other fluid for the heat exchanger can be air from the bypass duct or fuel, but in our highly size-constrained engine environment, we'll probably have to move some of the engine's equipment to the wings. But that will have to be studied after the bearing type has been chosen obviously.
Oil displacement without external pumping
Screw pumping is being assessed as the way to displace the oil throughout the engine. There will probably be two bearings in the engine, both requiring an oil bath and the shaft itself can probably also use a little refreshment. The idea is to use the rotation of the shaft to actually displace the oil without requiring external accessories. That would be a very lightweight solution and perhaps not that hard to implement, since our shaft is not hollow. The principle first has to be verified, then be tested with such high rotation rates in order to verify that the drag generated on the shaft is acceptable.
That does not solve the fact that cooling the oil requires external hardware (a heat exchanger), and that sealing is mandatory in the oil inlet and outlet areas, generally where the bearings are.
Other hardware required for lubrication and bearing cooling
Sensors will be required too, at least for oil temperature and displacement confirmation. Oil temperature informs about the status of the engine's bearings. Oil displacement sensor is required to ensure that there is no problem with the oil/cooling flow in the engine and that the measured temperature is not bogus.
Simple oil filters should also be put somewhere on the oil lines to prevent the more obvious failures.
- S. Jana, V. Arun Kumar and M. Ananda. 5-axes levitation of a rotor towards indigenization of the magnetic bearing technology. In Air breathing engines and aerospace propulsion: proceedings of NCABE 2004, november 2004.
- Hooshang Heshmat, Michael J. Tomaszewski, James F. Walton II. Small gas turbine engine operating with high temperature foil bearing. In proceedings of GT2006 ASME Turbo Expo 2006: Power for land, sea and air, may 2006.