1. Introduction
Small turbine engines play a crucial role in modern aerospace applications, powering a variety of aircraft such as unmanned aerial vehicles (UAVs), small reconnaissance planes, and certain types of missiles. These engines are known for their high power – to – weight ratio, compact size, and relatively low fuel consumption. However, one of the key challenges in their operation is related to the main bearings’ performance during starting, especially under starved – oil conditions.
During long – term storage, the lubricating oil in the main bearings of small turbine engines may volatilize, leading to a situation where the bearings have insufficient lubrication during startup. This problem is exacerbated by the requirement for rapid startup in a matter of seconds in many aerospace applications. Additionally, in low – temperature environments, the viscosity of the lubricating oil increases, and the flow rate slows down, further extending the oil – supply delay time and intensifying the starved – oil issue.
The consequences of starved – oil conditions during engine startup can be severe. They may cause intense rotor acceleration, high bearing friction torque, and even lead to problems such as bearing slip, scuffing, sharp temperature rises, and in extreme cases, bearing “lock – up.” Therefore, understanding and verifying the fast – starting ability of main bearings under starved – oil conditions is essential for ensuring the reliable operation of small turbine engines and extending their storage life.
2. Research Background
2.1 （Foreign Research Status）
- （American J402 Turbojet Engine）：The J402 turbojet engine, which is stored on shipboard launchers or under the wings of aircraft like the Lockheed P – 3 or Boeing B – 52 bombers, may remain untested or unchecked for several years. It is required to have a starting reliability of at least 99% and be able to accelerate to maximum thrust within 4 – 5 s and operate well during the power – cruise phase of combat missions. The lubrication system of the J402 engine consists of a self – enclosed, non – recirculating system. The front support uses an angular contact ball bearing (thrust bearing, bearing a large axial load), which is lubricated by drip – oil from an oil reservoir, while the rear support is a cylindrical roller bearing lubricated by grease. The aircraft has strict requirements for the lubrication system, expecting the engine to be able to accelerate to a maximum speed of 41,200 r/min within 8 s after being thoroughly cooled to – 55 °C.
- （French TRI 60 Series Turbojet Engines）：Considering factors such as the aging of non – metallic materials, the corrosion of metallic materials, and bearing lubrication, the storage life of the TRI 60 series turbojet engines is 7 – 8 years. During the service period, an A – level maintenance is carried out every 2 years, and a B – level maintenance is carried out every 4 years. Both types of maintenance can maintain and detect the rotor system.
- （Russian TRDD – 50 Twin – Shaft Turbofan Engine）：To ensure the starting reliability of the engine in high – altitude and low – temperature environments, the TRDD – 50 engine has made structural improvements to its lubrication system. After the lubricating oil enters the intermediate casing, an additional branch is added to directly inject oil into the front – support bearing (which has a high speed and large load). This direct – injection method simplifies the oil flow path, reduces pressure loss, and shortens the oil – supply delay time.
It can be seen that foreign countries have already paid attention to and conducted research on the fast – starting performance of small turbine engines under starved – oil conditions. However, relevant research in China is still in its infancy.
3. Bearing Starved – Oil Fast – Starting Working Condition Research
3.1 （Analysis of Influencing Factors）
There are many factors that affect the starting of bearings under starved – oil lubrication, including lubrication conditions (oil – supply delay time), load conditions, speed change rate, and assembly interference. Load conditions and speed change rates are related to the starting characteristics of the engine and can be considered without significant influence from the use environment. The assembly interference of the bearing is an objective attribute of the engine state, which can affect the bearing clearance and thus the bearing’s working ability under starved – oil lubrication. It is necessary to study the boundary state of the assembly interference. The oil – supply delay time is significantly affected by the ambient temperature, so the starting performance of bearings under starved – oil lubrication in low – temperature conditions should be studied emphatically.
3.2 （Analysis of Low – Temperature Oil – Supply Delay Time）
The low – temperature oil – supply total delay time consists of three parts: the filling time of the pipeline before the oil pump \(T_{1}\), the filling time of the injection hole \(T_{2}\), and the time from the jet injection port to the bearing gap \(T_{3}\). \(T_{1}\) and \(T_{2}\) are significantly affected by the low – temperature flow resistance of the pipeline, and \(T_{3}\) is the time required for the injection hole outlet flow to reach the critical flow (only when the lubricating oil outlet flow reaches the critical value can it be ensured that it can be injected into the bearing gap to form an effective jet; otherwise, it is still in a starved – oil lubrication state). After the engine starts, the temperature of the lubricating oil will gradually increase under the action of heat generated by the meshing of the oil – pump gears, heat generated by the generator, and the absorption of hot air, and then enter the normal lubrication state.
3.2.1 （Analysis of Lubricating Oil Viscosity – Temperature Characteristics）
A certain type of turbine engine uses fluorosilicone – based lubricating oil to improve the fast – oil – supply ability during engine low – temperature startup. The low – temperature performance of fluorosilicone – based lubricating oil (as shown in Table 1) is much better than that of general aviation lubricating oil. Therefore, calculations, analyses, and experimental verifications are carried out based on the low – temperature viscosity parameters of fluorosilicone – based lubricating oil.
| Lubricating Oil Brand | Measured Kinematic Viscosity / (\(mm^{2}\cdot s^{-1}\)) | |||
|---|---|---|---|---|
| At 100 °C | At 40 °C | At – 40 °C | At – 54 °C | |
| Fluorosilicone – based Lubricating Oil | 9.343 | 20.0 | 728 | 2364 |
| 4050 General Aviation Lubricating Oil | 5.000 | 17.4 | 9305 | – |
3.2.2 （Analysis of Jet Space State）
The injection path of the injection hole is shown in Figure 1. Under the action of gravity, the path of the lubricating oil after being sprayed from the nozzle is a curve. The lower the outlet speed, the more it deflects downward. When the downward deflection is lower than the outer – edge diameter of the inner ring of the bearing, it cannot enter the raceway for lubrication. The critical flow rate w of the injection hole for spraying to the effective part of the bearing is calculated by the formula \(w=\frac{1}{4}\overline{v}\pi d^{2}=\frac{1}{4}\pi d^{2}\frac{l}{\sqrt{2m/g}}\) where \(\overline{v}\) is the velocity distribution on the cross – section of the injection pipe, d is the diameter of the injection hole, l is the distance in the jet direction, m is the downward deflection distance of the lubricating oil, and g is the gravitational acceleration. When the injection – hole flow rate is less than w, the bearing cannot be effectively lubricated and cooled; when it is greater than w, the bearing can be normally lubricated and cooled.
[Insert Figure 1: Spatial structure diagram of injection hole of QJS206 bearing]
3.2.3 （Analysis of Oil – Supply Pipeline Flow Resistance Characteristics）
- （Simulation Calculation of Flow Resistance Characteristics）：To understand the flow – resistance characteristics of the oil – supply pipeline of the engine lubrication system, a lubrication – system model of a small turbofan engine is built using simulation software. The atmospheric – environment pressure is 0.1 MPa, and the viscosities of the lubricating oil at different temperatures (0, – 10, – 20, – 25, – 30, – 35, – 40, – 45 °C) are shown in Table 2. Taking injection hole 8 as an example, based on the kinematic viscosity of the fluorosilicone – based lubricating oil, the filling time of the pipeline before the oil pump \(T_{1}\), the filling time of the injection hole \(T_{2}\), and the total oil – supply delay time T of the lubrication system at – 45 – 0 °C are calculated, and the results are shown in Table 2. According to the structural size of the injection hole, the critical flow rate of this hole is 33.5 mL/min. The time for the jet to reach the bearing gap is related to the lubricating – oil temperature – rise rate, and the calculation process is relatively complex, about 0.01 s in magnitude, so it is not considered for the time being. If 4050 lubricating oil is used, its kinematic viscosity at – 45 °C is 10 times that of the fluorosilicone – based lubricating oil, and the oil – supply delay time will be more than tens of seconds.
| Lubricating Oil Temperature / °C | Lubricating Oil Kinematic Viscosity / (\(mm^{2}\cdot s^{-1}\)) | \(T_{1}\) / s | \(T_{2}\) / s | T / s |
|---|---|---|---|---|
| 0 | 85 | 0.888 | 2.735 | 3.623 |
| – 10 | 120 | 0.884 | 3.049 | 3.933 |
| – 20 | 200 | 0.880 | 3.696 | 4.576 |
| – 25 | 260 | 0.878 | 4.110 | 4.988 |
| – 30 | 395 | 0.877 | 4.967 | 5.844 |
| – 35 | 550 | 0.876 | 5.817 | 6.693 |
| – 40 | 800 | 0.876 | 7.010 | 7.886 |
| – 45 | 1450 | 0.878 | 9.970 | 10.848 |
- （Measurement of Low – Temperature Oil – Supply Delay Time）：To verify the accuracy of the lubrication – system modeling calculation, the low – temperature oil – supply delay time is measured using a fixture. An oil – collecting box, a stop valve, and a pressure gauge are installed on the inlet pipeline and connected to a gas source to simulate the pressurization of the oil pump. These parts are placed in a low – temperature box and frozen for more than 2 h at – 35 °C and – 45 °C respectively. The measured results of the lubricating – oil temperature inside the oil – collecting box show that it can reach the set value.
The lubricating oil in the entire pipeline is blown clean to simulate the real state of the engine lubrication system. After freezing for 2 h, the lubricating oil in the oil – collecting box is pressurized to 0.8 MPa (simulating the typical pressure of engine low – temperature startup), the stop valve is quickly opened, and the time is recorded until oil comes out of the injection hole.
Two tests are carried out. In test 1, the oil – supply delay tests at – 35 °C and – 45 °C are carried out. The measured oil – supply delay times are [specific values] less than the simulation – calculated values (\(T_{2}\)) by 1.067 s and 2.350 s respectively, and the errors are both less than 25%, indicating that the model – calculation results are reliable. It is determined that the oil – supply delay time of the bearing at – 35 °C is 8 s, and the oil – supply delay time at – 45 °C should exceed 7.62 s.
In test 2, to verify whether an effective jet can be formed at – 45 °C, the test fixture is locally improved. A fixture that simulates the gap between the bearing cage and the inner ring is designed, and the outflow of lubricating oil from the gap is used as a sign of an effective jet. The oil – supply delay test at – 45 °C is carried out again. It is found that although there is lubricating oil spraying out of the injection hole at [a certain time], there is no lubricating oil flowing out of the bearing gap, indicating that an effective jet has not been formed. Considering a 25% safety margin, the oil – supply delay time of the QJS206 angular – contact ball bearing at – 45 °C is set to 10 s.
4. Bearing Starved – Oil Fast – Starting Bench Test
4.1 （Test Scheme）
The test is carried out on a ZYS – 103 type bearing testing machine to simulate and test the bearings under different speeds, loads, and lubrication conditions. As shown in Figure 2, 1# and 4# bearings are test bearings of the QJS206 angular – contact ball bearing type, which bear axial and radial loads. 2# and 3# bearings are companion – test bearings of the cylindrical – roller bearing type, which are used to support the main shaft. An HT – 150 – 55000/30 type electric spindle with a rated power of 30 kW is selected, which can increase the speed from 0 to 40,000 r/min within 8 s. At the initial stage of the testing – machine startup, the supply of lubricating oil is delayed for several seconds to simulate the starved – oil lubrication condition during the fast startup of the engine.
Before the test, the test shaft system is removed, blown with high – pressure air for 30 min, and cleaned with gasoline to ensure that the oil film on the steel balls, raceways, and other parts is in a starved – oil state. Then the shaft system is installed for the starved – oil startup test.
[Insert Figure 2: Structure diagram of bearing test tooling]
4.2 （Test Conditions）
The test conditions and load spectrum of the QJS206 angular – contact ball bearing are formulated according to the common working profile of small turbine engines, as shown in Table 3. During startup, the starved – oil startup is carried out according to the specified starved – oil time in the load spectrum. After a successful startup, a 4 – h working assessment test is completed according to the profile.
| Serial Number | Radial Load | Axial Load | Bearing Speed / (\(r\cdot min^{-1}\)) | Working Time / min | Lubricating Oil Pressure / MPa | Lubricating Oil Flow Rate / (\(L\cdot min^{-1}\)) | Lubricating Oil Inlet Temperature / °C |
|---|---|---|---|---|---|---|---|
| 1 | 0.55 | 1 | 0 – 40000 | Set starved – oil times of 8, 10, 12, 15 s respectively | 0 | 0 | 0 |
| 2 | 1.50 | 3 | 40000 | 48 | 0.25 | 0.9 | 85 |
| 3 | 1.50 | 3 | 50000 | 48 | 0.25 | 0.9 | 85 |
| 4 | 1.50 | 3 | 40000 | 48 | 0.25 | 0.9 | 85 |
| 5 | 1.50 | 3 | 45000 | 48 | 0.25 | 0.9 | 85 |
| 6 | 1.50 | 3 | 40000 | 48 | 0.25 | 0.9 | 85 |
4.3 （Test Data Analysis）
A total of 4 groups of tests are carried out. The first 3 groups of tests pass the assessment. The test – data curves of the first 120 s are shown in Figure 3. By comparative analysis, it can be seen that the change trends of the test data of the 3 groups of bearings are basically the same:
- In the 0 – 10 s stage, the bearing temperature rise is basically zero. The speed increases from 0 to 40,000 r/min. Although the bearing is in a starved – oil lubrication state, due to the ambient – temperature environment, it takes a certain time for the heat conduction between the internal metal parts of the bearing, so the temperature rise of the outer – ring temperature measuring point of the bearing increases slowly.
- In the 10 – 20 s stage, the bearing is starved of oil, and the heat generated by friction accumulates and is then conducted to the outer ring of the bearing.
