A novel lubricating method of oil pump with self-adaptive adjustment function for transmission | Scientific Reports

Scientific Reports volume 15, Article number: 10223 (2025) Cite this article

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At present, the lubrication of transmission in vehicles cannot be adjusted according to driving conditions, especially vehicles’ speed, resulting in low lubrication efficiency and high energy loss. This paper aims to develop a mechatronic system for gearbox lubrication to replace traditional forced and splash lubrication. The mechatronic system can adaptively adjust the flow and pressure of lubricating oil based on the driving conditions to achieve an optimal lubrication effect with less loss. The system mainly includes a Brushless Direct Current (BLDC) motor, inner rotor pump, and drive control circuit. BLDC motor drive control circuit can obtain the current flow of oil pump by monitoring its own speed in real-time, through the pressure sensor and the real-time acquisition of automobile speed so as to self-adaptively adjust the best output flow and pressure based on PID trapezoidal wave control algorithm, which can highly increase the life span of transmission and reduce the energy loss. The results show that the best lubrication effect can be achieved under different working conditions. For example, a vehicle in idle condition consumes only 3.1% power with a conventional oil pump, In addition, an electric lubricating oil pump is monitored in real-time and can alert the driver as long as the pump is in failure. Futher more, the new lubrication method can reduces the gearbox temperature to different degree which is more beneficial to its life extension.

At present, with the continuous development of the automotive industry, each actuator tends to be intelligent and energy-saving, and lubrication systems are executed by independent components. The development and widespread adoption of electric vehicles will still take some time due to battery life problems and various incidents that are gradually being reported. In this context major auto giants have launched plug-in hybrid electric vehicles (PHEVs), unlike pure electric vehicles, they can work in either single engine mode or both. An PHEV that uses gasoline as its main power source and an electric motor with a high-voltage battery as its secondary power source is being actively developed as a leading eco-friendly, low pollution vehicle. Transmission is the core of the powertrain and combines and distributes the power of the engine and the motor continuously. Variable transmission and dual-clutch transmission have been selected as the transmission for HEVs1,2,3. Automatic transmission (AT) is used in Honda HEVs due to its lower cost with the use of existing production equipment. Some large trucks usually use automated manual transmissions, which usually have more gears to switch. This is why gearboxes play

This work was supported by the Joint Fund for Aerospace Advanced Manufacturing Technology Research Key Program (Grant No.U1937203) (Corresponding author: Shengdun Zhao).

The authors are with the School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an710049, China (email: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]).

a very important role in the automotive sector. So reliable lubrication is needed to make the gearbox obtain the longest service life span regardless of the transmission type. The pressure angle of gear is increased to improve its lubrication, lower processing costs, and enhance the lubrication effect of low speed4. An oil pump plays an extremely important role in the automobile gearbox, the lubricating oil pumps used in all automobiles are mainly internal meshing gear oil pumps, inner rotor oil pumps and vane oil pumps, but the inner rotor pump is the most widely used. Inner rotors are powder metallurgy parts widely used in automobile engine oil pumps, whether in ATs, continuously variable transmissions, or hybrid vehicle transmissions5,6. They mainly perform the corresponding lubricating calculation to reduce gear friction and wear. The working pressure, friction torque, speed, oil temperature, and other conditions of the inner rotor oil pump affect the output torque and the service life of the lubricating oil pump7. We must start from the lubrication mode of the gearbox to thoroughly improve the lubrication conditions. The lubrication mode of the gearbox has two types of lubrication as following.

(1) Forced lubrication is used in some parts with large load and high movement speed. The friction surface of these parts forms a thick oil film to play the purpose of lubrication.

(2) Splash lubrication is used in some small load, small movement speed parts by rotating parts, such as gear or crankshaft, which splashes lubricating oil into oil star to lubricate bearings, and moving parts when rotating with oil droplets and oil mist lubrication.

However, splash lubrication has many disadvantages, such as uneven lubrication of the gear when the car is going up and down the hill and even idling without intrusion of the gear into the oil, thereby reducing the lubrication condition of the gear and significantly reducing the life of the gearbox. Splash lubrication increases the lubricating oil temperature and results in a poor lubrication effect due to insufficient lubrication8.

Currently, on the market, there is a transmission electric pump, which is a dc motor-driven hydraulic pump, it is controlled by the vehicle ECU to connect its power to run. But there is no way to adjust the output flow and pressure, the electric motor has been working in the rated speed and torque conditions9.

However, with different working conditions of the transmission, the lubrication condition will be different10, In high-speed light load conditions, for example, relying on the gearbox’s own splash lubrication can satisfy the using performance and will not affect its life, Therefore, only the electric pump needs to provide a little flow to meet the lubrication requirements11,12, while the conventional electric oil pump is still running at high speed and large flow, resulting in redundancy and waste of energy. And in the low-speed heavy load condition, it needs an extra electric pump to participate in the work, to the automobile transmission gear mesh surface with good lubricating oil film lubrication and allowing enough to ensure safe and reliable operation of the gearbox. With the conventional pump it will not reach the upper limit, that is, it does not achieve optimal lubrication. in the idle time of the car, because there is not much load, only a certain flow of lubricating oil is needed to the meshing surface of the gearbox, it only needs a lower limit of the power for lubricating, so the current conventional brushed DC or BLDC-driven hydraulic pump will cause a waste of electric energy. With a conventional pump, no matter what condition the car is in, it always runs at its rated working condition, usually, this working condition is lower than the working condition when the car is in heavy load. But when the self-adaptive adjustment lubricating pump is used, it will adjust the output flow and pressure according to the different working conditions, that is, different car speeds, so that the gearbox is always in the best lubrication state. Its specific working mode is shown in Fig. 1. It represents the different working conditions of the vehicle at different times, the upper limit represents the output power of the lubricating oil pump required by the automobile under the condition of heavy load, the output power of the lubricating pump is related to the output flow and pressure of the oil pump. The lower limit represents the output power of the lubricating oil pump required by the automobile under the condition of idle condition.

Comparation between conventional pump and adaptive adjustment pump.

From above picture, we can clearly see that under the same working conditions, the adaptive lubricating oil pump has better lubrication effect and optimal energy utilization than the conventional lubricating oil pump.

Compared with traditional lubricating oil pump, adaptive lubricating oil pump has the following advantages:

(1) Adaptive lubricating oil pump can adjust the output flow of lubricating oil pump according to the speed of the car. It can be seen that the optimal lubrication of automobile gearbox is closely related to the flow rate and pressure of lubricating oil13,14. The adaptive lubricating pump can do all this so that the gearbox can always work in the best lubrication conditions and temperature which further improve the service life of the gearbox.

(2) The traditional lubricating oil pump is a constant flow pump, which is driven by a brushed DC motor or BLDC motor with constant flow. The automobile power supply is connected to the lubricating oil pump, which can not adjust the flow of output lubricating oil according to the speed of the gearbox gear, so that the gearbox can not always work in the best lubrication conditions.

(3) The brushed DC motor used by traditional lubricating oil pumps will seriously reduce the service life of the motor because of the use of carbon brush, and the carbon brush will be frequently replaced15. However, the current labor maintenance cost is further increased, all of which increase the maintenance cost of owners. Now some manufacturers have begun to replace the brushed DC motor with brushless DC motor, but either way is a constant flow lubrication pump, it can not fully meet the lubrication performance under different working conditions. The variable flow BLDC-driven oil pump not only improves energy efficiency, but also keeps the gearbox operating in the optimum temperature range, thus significantly increasing the life span of the transmission.

In this paper, a novel lubricating oil pump for automobile gearbox is designed, including motor structure, pump body structure and control system, overall, the major contributions of this paper are summarized as follows:

1) It can adjust the output flow and pressure according to the speed of the car in real time to meet the best lubrication conditions of the car.

2) Provide insights for all-electric actuator components of electric vehicles, and provide a new idea to replace other motor actuator components with brushless DC motor.

3) To better understand the BLDC control algorithm and the details of the design of the motor driver board.

4) The lubrication theory of transmission and the theory of oil film formation are established. According to the different speed and load of the gearbox, the lubrication test and temperature measurement were done, and it was concluded that the BLDC motor driven lubricating oil pump could better meet the lubrication performance under different working conditions by controlling different flow and pressure output.

The rest of this paper is organized as follows. The structure of the lubricating oil pump is described in section II. Section III introduces the control algorithm of the BLDC motor. In section IV the experiment of lubricating oil pump and transmission were done and the results are analyzed. Finally key conclusions are summarized in Section V.

The correct lubrication method is essential for the lubrication of the gearbox. Many factors affect the final lubrication effect, and the most important is the viscosity of the fluid.

High fluid viscosity results in the insufficient flow of lubricating oil to every part of the gear. Thus, it does not provide adequate lubrication and cooling. However, extremely low fluid viscosity reduces the adhesion of lubricating oil on the gear surface, thereby reducing the oil film thickness and resulting in dry friction. The lubrication effect is reduced and leads to gear failure. Therefore, the minimum oil film thickness must be guaranteed to ensure the lubrication of gears.

On the basis of a large number of systematic numerical calculations, Dowson et al. proposed the formula of minimum oil film thickness for linear contact elastohydrodynamic lubrication twice. Experiments show that the oil film thickness calculated by using Dowson–Higginson formula is extremely close to the measured value. The proposed formula in 1967 is as follows13:

where \(\:{H}^{*}\) represents the oil film thickness, \(\:{G}^{*}\) represents the material, \(\:{U}^{*}\) represents the speed, and \(\:{W}^{*}\) represents the load.

To further quantify these parameters, Dowson proposed dimensionalized parameters, which can be expressed as follows13:

where R represents the equivalent radius of curvature, \(\:{E}^{{\prime\:}}\) represents the equivalent modulus of elasticity, L represents the length of the object in contact, W represents the mutual loads between bodies, \(\:{\eta\:}_{0}\) represents the inlet viscosity of lubricating oil, U represents the flow of lubricating oil caused by the relative sliding speed between the contact surfaces, velocity flow for short, and \(\:\alpha\:\) represents the adhesion coefficient. The dimensional form of the Dowson–Higginson formula can be obtained by substituting the above equations into Eq. (3).

In accordance with the above equation, the minimum film thickness \(\:{h}_{min}\) of linear contact elastohydrodynamic lubrication is the most significant with the increase in contact surface inlet viscosity \(\:{\eta\:}_{0}\) and velocity flow. Therefore, pressure lubrication can increase the velocity flow, thereby improving the minimum film thickness and lubrication performance. Different relative speeds of gear box and different flow rates of lubricating oil pump can make the parameter U reach an optimal value to ensure the best lubrication effect. Therefore, the output flow and pressure of adaptive lubricating oil must be optimally related to the running speed of the vehicle. In order to further investigate the lubrication theory of gears, the meshing model of gears is now established as shown in Fig. 2 below.

Gear mesh model for transmission.

For a certain point A of the tooth meshing position, the friction coefficient \(\:{\mu\:}_{A}\) can be defined as:

where \(\:{F}_{TA}\) is the tangential load at the bonding point A, \(\:{X}_{R}\) represents the roughness of the tooth surface. \(\:{V}_{1 A}\) and \(\:{V}_{2 A}\) represent the absolute velocity of the master and slave wheel meshing teeth at the contact position. \(\:{R}_{EA}\) represents the integrated radius of curvature of the meshing point A at any position.

In addition the viscosity coefficient \(\:{\eta\:}_{0}\) of the lubricant is:

where \(\:\rho\:\) represents the density of lubricant, \(\:{kg/m}^{3}\); \(\:{\theta\:}_{B}\) represents the average gear temperature, ℃.

Pump output flow, pressure and power calculation formula are as follows16:

where N represents the power of the pump, unit kW. \(\:\varDelta\:P\) represents pressure rise of hydraulic pump, unit MPa. Q represents the actual flow rate of the pump, unit L/min, \(\:{\upeta\:}\) represents total oil pump efficiency, where the flow Q is related to the following formula16.

where q represents the displacement, unit mL/r. n represents the motor speed, unit r/min, \({\eta\:}_{1}\) represents the volumetric efficiency corresponding to different output flow rates.

So changing the motor speed will directly change the flow of the lubricating pump, and the output pressure can be adjusted by an electronically controlled relief valve.

This paper directly considers that the flow rate U of lubricating oil on the final meshing surface is the speed of gear rotation, which will superimpose the speed of lubricating oil itself, and the speed of automobile is directly related to the engine speed and the transmission ratio of gearbox. Therefore, when the automobile runs with heavy load, the gear box rotates slowly. In order to meet the minimum oil film thickness requirements, pressure lubrication must be added to make the lubrication more complete.

First, according to the lubricating oil flow and output pressure required by the gearbox, the power of the hydraulic pump is calculated through Eq. (9). Considering the volumetric efficiency and mechanical efficiency of the hydraulic pump, the motor power is usually larger than the power of the hydraulic pump. By selecting the rotational speed of the motor can determine the rated speed of pump through Eq. (10), so as to determine the displacement of the pump, thus it can be designed the corresponding rotor cavity, considering the hydraulic pump need to pass a large torque, so we can’t only consider hydraulic pump cavity which is corresponding to the displacement of hydraulic pump, we also need to consider the size of the hydraulic pump rotor, because it determines torque size of the hydraulic pump. Through flow field simulation calculation, the cavity of the inner rotor pump can be determined. In this way the motor and hydraulic pump design is completed. The motor adopts BLDC motor, the pump adopts the inner rotor gear pump, which can output higher pressure. The inner and outer rotors are made of titanium powder metallurgy. The seal of the pump adopts the skeleton oil seal 10*16*4. In addition, the inner rotor has 6 teeth and the outer rotor has 7 teeth. Fig. 3 shows the main scheme of the motor pump.

Main scheme of the motor pump.

Motor and hydraulic pump through flange connection and fixed with bolts. The motor output shaft and the gear pump adopt the flat shaft connection, the intermediate coupling is omitted, and the integration degree is higher. It contains Hall sensors that monitor the rotor position in real-time and feedback to the microcontroller unit (MCU) to determine which motor phase should be switched on at the moment.

It is well known that automotive electrification has become a trend, but different motors are suitable for different occasions. Ac asynchronous motors are usually used in industrial equipment or in high-power application17. Ac permanent magnet synchronous motors are usually used in occasions requiring higher precision and higher power18. Switched reluctance motors are often used in occasions requiring high temperature resistance and higher power because of their salient poles of stator and rotor, it has no permanent magnet19. However, in some low-power used occasions, there are usually brushed DC motors, stepper motors and BLDC motors. since stepper motors are used for motion control, repeatability of the steps is desirable. This means a large number of magnetic poles, A stepper motor typically has hundreds of steps per revolution, that is, if you start at one step, then to another, then back to the first, it should ideally return to exactly where it was previously. Stepper motors are usually designed for maximum holding torque first, and speed second20. BLDC motor has the advantages of low temperature rise, low noise, large torque, high speed, high efficiency, low energy consumption, no spark, long life and so on21. Some of the current independent motor pumps are mostly driven by a brushed DC motor or BLDC motor. However, the carbon brushes in the brushed DC motor are easy to wear and fail22,23,24, repeated worse carbon brushes cause unnecessary trouble and increase the cost. So BLDC motors have a tendency to replace brushed DC motors.

At the same time, a brushed DC motor lubrication pump with no H-bridge control circuit cannot adjust the pump flow rate and output pressure in accordance with the transmission gear speed in real time25,26,27,28,29,30. However, if the brushed DC motor uses H-bridge speed regulation, it is not as good as using the DC brushless motor because of its own structural defects.

The BLDC motor realizes the rotation of the stator magnetic field through brush commutation so that the rotor can rotate continuously31. BLDC motors are usually classified into two types, namely, permanent magnet synchronous motor and permanent magnet BLDC motor, and their output characteristic is different. The permanent magnet DC motor is usually used in vector control mode, and the output of the back electromagnetic field (EMF) waveform is a sine wave. The permanent magnet BLDC motor usually uses the trapezoidal wave control, and the back EMF waveform is a trapezoidal wave32,33,34,35,36,37. The analysis model of the BLDC motor is established on the RMxprt module of the electromagnetic finite element software Ansoft Maxwell. The windings are set as double-layer windings with a span of 2. Machine windings can be divided into single-layer windings and double-layer windings. Single-layer windings put only one coil edge in one slot, and double-layer windings put two coil edges in one slot. Single-layer windings mainly include overlapping windings, concentric windings, chain windings and cross-chain windings. Single-layer windings are mainly used in small and medium-sized motors. However, the double-layer short-pitch winding type is used in this study, because it can choose suitable short-pitch to reduce harmonic magnetic potential, and the winding ends are arranged neatly. The rated parameters of the motor are inputted, and the upper limit of the slot full rate is set to 75%. The main structure of the BLDC motor is shown below in Fig. 4.

The structure of the BLDC motor.

In accordance with the flow and pressure output requirements of the transmission lubrication, the rated parameters of the lubricating oil pump and motor are designed, as shown in Table 1.

For better performance and to achieve the special function of the lubricating oil pump, the BLDC motor driver is designed, Firstly, the power requirements of the motor are determined according to the input voltage and current of the motor, and then the appropriate metal–oxide–semiconductor field-effect transistors (MOSFETs) is selected. Usually, two times drain-source voltage and four times drain-source current are selected. The appropriate driver chip is determined according to the source and sink current of the MOSFET and the threshold voltage of the gate driver. The selection of appropriate gate drive resistance is directly related to the speed of transistor on and off, but also affects the heating of the system and the power consumption of the drive chip, so its selection is very important. After selecting the appropriate driver chip and gate resistor, the CPU covering all its functions is selected according to the overall requirements of the control system. Check the communication interface related to CPU, determine the communication mode and design the communication circuit, and then determine the appropriate power supply chip according to the power supply requirements of all chips. At last design the DC/DC conversion circuit, and then design the whole circuit of the system. Altium Designer was used to design the circuit schematic diagram, and drew the PCB drawing, which was sent to the factory for processing, and finally debugging and run. The final driver board as shown in Fig. 5, which contains the alarm output, it uses a relay to switch on the dashboard warning light to achieve the status indicator effect, which includes all the operation statuses of lubricating oil pump, overcurrent, overvoltage, overtemperature of the driver board, etc. The board can use the universal serial bus (USB) to communicate with a personal computer and debug with the man-machine interface, finally, it can download the firmware to the central processing unit.

Motor pump driver board.

The basic principle of the BLDC motor driver is shown in Fig. 6. The MCU receives the control signals from various sensors and generates the required pulse width modulation (PWM). The PWM signal generated by the MCU is isolated and amplified by the gate driver, and then the PWM signal is applied to MOSFET to control its on and off switching. MCU knows the output flow rate and pressure of the lubricating pump according to the speed feedback of the BLDC pump and the pressure feedback of the pressure sensor and calculates the appropriate flow rate and pressure through the detected automobile speed information to meet the optimal lubrication requirements.

A single 24 V power supply is used to power the MOSFETs and the BLDC motor. The DC/DC conversion of the whole lubricating pump system must be reduced to obtain the minimum energy consumption. Thus, we need a buck–boost converter to feed the whole system38. The power supply for the drive chip needs to be converted to 12 V through the input terminal and then through the power module. The 12 V power for the MCU is then converted to 3.3 and 5 V through the buck circuit. We choose the MSP430 system control and communication family of ultralow-power microcontrollers to obtain minimum power consumption. It is an ultralow-power MCU of the MSP430 system control and communication family, and has peripheral interfaces for various applications. Its architecture is combined with a wide range of low power modes and optimized to minimize the energy consumption in system applications. The controller features a powerful 16-bit reduced instruction set computer CPU, a 16-bit register for maximum code efficiency, and an integrated digital clock that activates the CPU at low power consumption in 3.5 µs.

Main scheme of the lubrication oil pump.

The MCU determines the power phase connected through the rotor position signal received by the BLDC motor. With the fast processing MCU, the motor can run continuously at a fast speed. A sampling resistor is designed at the port of MOSFETs’ output current to collect the DC bus current of the motor and prevent the MOSFETs from being damaged by overcurrent. The three-phase output voltage is collected, which can be used for more advanced algorithm controls, such as sensorless BLDC motor control. For lower power consumption, we selected the MOSFETs with low \(\:{R}_{Gint}\).

where \(\:\triangle{V}_{out}\) is the voltage drop when the power tube is switched on and off. For unipolar power supply, this value is the power supply that drives the chip, whereas for bipolar power supply, this value is the voltage difference between the two power supplies. \(\:{P}_{D}\) is the power loss of the bipolar transistor, \(\:{f}_{s}\) is the switching frequency, \(\:{Q}_{G}\) is the gate charge, \(\:{R}_{Gint}\) is the internal resistance of the power device MOSFETs, and \(\:{R}_{G}\) is the gate resistance between the gate driver and MOSFETs.

Under the condition of keeping \(\:{P}_{D}\) as small as possible, we need to meet the junction temperature conditions of bipolar transistors, as shown in the following Eq.

where \(\:{T}_{A}\) is the ambient temperature, \(\:{R}_{THJA}\) is the direct thermal resistance between the node and the environment, \(\:{T}_{Jmax}\)is the maximum node temperature allowed by the bipolar transistor, and the calculated node temperature \(\:{T}_{J}\) must be less than the maximum allowed, otherwise it will burn.

In the control of BLDC, there are also a lot of literature studies on the senseless control mode, which will significantly reduce the cost of the entire electronic oil pump. However, there is a problem that the reversal problem of the sensorless control of BLDC will bring difficulties to the high-precision control of BLDC, for which there are also studies on this20. These studies of commutation error provides a reference for high precision sensorless control of BLDC.

The control strategy of the BLDC motor in this paper is mainly PID trapezoidal wave control. The MCU switches on the corresponding motor phase in accordance with the received Hall sensor signal, so as to realize the continuous operation of the motor. The basic control principle is as follows: the AB phase is switched on when the Hall sensors B and C are high level. When the rotor turns an angle, the Hall sensor becomes the high level of C phase. When the AB phase is low level, the AC phase is switched on. As shown in Fig. 7, it represents the commutation logic of brushless DC motor.

Hall sensor output logic.

The proposed control scheme has the following advantages.

Highest maximum speed.

Great for delivering maximum torque.

Lowest switching losses.

Easiest implementation.

However, for the lubricating oil pump, the output torque is stable and its peak value is low is not an advantage, often the inertia of the load side requires the motor to instantly burst greater torque, so this paper chooses the way of trapezoidal wave control.

The integrated control panel has the following functions.

(1). It has wide voltage input range, can accommodate 9–95 V DC voltage, rated 15 A, up to 20 A output current.

(2). The drive board integrates the sampling resistor to the DC bus current sampling and the high-precision divider resistor to the DC bus voltage sampling, real-time monitoring of the voltage and current of the bus, to prevent over-voltage and over-current.

(3). It has the receiving port of Hall sensor and uses the programmable MSP430F5529 master chip.

(4). It integrates the temperature sensor on the board to collect the temperature around the control board for preventing the short-circuit caused by the burning of the MOSFET.

(5). It can perform trapezoidal wave inductive control or noninductive control and sine wave control.

(6). The alarm output port can provide a signal to the automatic transmission control unit (TCU) to inform the current state of the lubricating oil pump. The sensor signal terminal can accept the pulse signal sent by the TCU and convert it into the required speed signal inside the MCU to control the output flow rate.

The drive control board can make the motor run in two control modes of sensored and sensorless algorithms by writing different codes in accordance with the needs. It also integrates the alarm output, and can self-adaptive adjust the speed of the motor to change the output flow of the motor pump in accordance with the requirements. The flow chart of design and control procedure of the proposed system is as the Fig. 8 below.

Flow chart of design and control procedure of the proposed system.

First, the system initialization of MSP430 microcontroller is carried out, including configuring clock, ADC, IO port and interrupt, and then enabling interrupt and timer. second, the CLOCK interface of CPU can receive high-speed pulse signals and continuously monitor the speed instruction given by the vehicle ECU. When the ECU gives the speed instruction, at the same time, the corresponding pulse signal is sent to the proportional valve which mounted on the output end of the lubricating oil pump, and the opening and closing degree of the proportional valve is adjusted through different width duty cycle signals to control the output pressure of the lubricating oil pump. After receiving the signal, the CPU calculates the real-time speed of the car. If the speed of the car is 0 rpm, the vehicle is considered to be in the stop or idle stage, the oil pump will not start, and the motor is not enabled. If the detected speed of the vehicle is not 0 rpm, according to the current vehicle speed to do internal operation of oil flow of the lubricating oil pump, then to determine the target duty circle of the motor needs, if the target duty circle of the motor is different, the motor output speed will be different, then the CPU start to increase the duty cycle from 0 to the target duty circle through the PID adjustment, if it is detected that the system has reached the target duty cycle, the actual speed of the motor is calculated to see whether the motor speed at this time is the same as the target speed. If not, the duty cycle is further increased and the motor speed is calculated in real time until the motor reaches the required speed which represent the required flow of oil pump, so as to meet the requirements of lubrication.

In the future, the ADC interface can be further expanded to control the output of each way with servo valves, so that the flow and pressure output can be controlled in a closed-loop, paving the way for precise control of centralized lubrication in machine tools and automotive fields.

The experiment and analysis are mainly divided into two parts, and the specific experimental plan and scheme are shown in the Table 2 .

The following test platform is set up to verify the feasibility of the whole scheme of electronic oil pump, as shown in Fig. 9.

Overall scheme of electronic oil pump.

When the motor turns, it absorbs oil from the tank through the oil inflow pipeline and then flows back to the oil drum through the oil outlet pipeline to the hydraulic flow sensor and pressure sensor. The flow sensor and pressure sensor output 4–20 mA current signal, which is then converted into 1–5 V voltage signal through the signal acquisition system and transmitted to the data acquisition card in the computer. The output signal of the flow sensor and pressure sensor is collected by the upper computer software of the acquisition system. The data acquisition system can easily obtain real-time data and display it on the man-machine interface through the upper computer software, and also can easily export the data for subsequent processing. The BLDC motor turns through the DC power supply and control board. Because the appropriate program has been burned in MCU, when the whole system is correctly connected, the circuit board only needs to be connected to power, and the motor can operate in a specified way. The output voltage of the control board is collected by an oscilloscope to determine whether the signal is correct. Also, the oscilloscope can be used to collect all kinds of output signals. Different speed instruction signals are given to the motor pump through the external signal so that it can work at different speeds to meet the different lubrication needs of the automobile gearbox for achieving the highest motor pump final lubrication effect and efficiency. Different speed instruction signals are pulse signals of different frequencies sent by the ECU of the car. MSP430F5529 development board receives pulse signals to calculate the current speed of the car in real time, and also calculates the speed that the lubricating oil pump should run at this moment, so as to control the output speed of the lubricating oil pump and adjust the output flow.

In order to adjust the output pressure, the relief valve is used to adjust to meet the lubrication requirements of different vehicle conditions.

The flow rate and output pressure of the motor are measured at different speeds. The BLDC motor is verified during vehicle operation in accordance with the different transmission speed instructions sent by the car corresponding to different flow requirements, so as to achieve the best lubrication state of the gearbox. Figure 10 shows the voltage waveform outputted by the motor control board at the speed of 150 rpm. The voltage is strictly trapezoidal in the output phase, but relatively poor in the reflux phase. EMF is generated in the idle phase due to the permanent rotor magnet. At a certain time, there is a permanent magnet on the rotor of the brushless DC motor, so there is still a voltage on the C phase measured at a certain moment. However, the trapezoidal wave control principle of the brushless DC motor tells us that only two phases are connected at a certain moment, because the motor pump speed is low when testing the output voltage, it is only 150 rpm, so the measured output voltage itself is very low, and due to the large inductance between the motor phase windings and the winding resistance is not exactly the same, so there is a sharp peak of the B phase voltage, and there is a certain difference between the maximum peak voltage. However, the overall effect is sufficient to meet the requirements of use.

Electromotive force of motor pump.

Figure 11. shows the output flow and pressure of the motor pump when the motor speed is 600 rpm. This corresponds to when the car is driving at high speed and does not need large oil pressure. When the speed of the motor is changed in approximately 5 s, the output pressure drops in an instant and then recovers to the specified pressure quickly. The output flow rises with the increase in speed and stabilizes at 600 rpm. The design system shows slow dynamic performance, it takes around 5 s to track the step response. Because the flow and pressure sensor itself has a great delay, in addition, the terminal board of the acquisition card uses the analog signal isolator and digital to analog converter which can convert the pulse frequency to analog output, so that the pulse signal output by the pressure sensor is converted into an analog signal by the converter and then input to the data acquisition card, which itself has a great delay. In addition, the delay of data acquisition system is also very large, and the length of pipeline connecting pressure sensor also has a great influence on the dynamic characteristics of the system.

Figure 12. shows the output flow and pressure curve of the motor pump when the motor speed is 800 rpm. This corresponds to when the gearbox needs a higher lubricating oil flow but does not need a larger output pressure. An external speed signal is given at approximately 5s. Thus, the motor speed quickly increases to 800 rpm, enabling the lubrication of the gearbox to reach the optimal effect at the moment. Similarly, the output pressure drops slightly and then returns to the original given value when the speed increases. The output flow of the lubricating oil pump can rise to a given speed value in a short time.

Motor pump with high flow of 2.69 L/min.

Motor pump with flow of 3.21 L/min.

Figure 13. shows the corresponding output flow and pressure curve when the output of the lubricating oil pump does not need pressure but only needs flow. This corresponds to a car operating at a low speed rather than a heavy load, the lubricating oil pump operates at the speed of approximately 900 rpm and output pressure of 0.068 MPa.

Motor pump with high flow but no output pressure.

Figure 14. shows the test result when the lubricating oil pump needs to meet the requirements of flow rate and output pressure. The lubricating oil pump operates at the pressure of 0.61 MPa and speed of 750 rpm. This corresponds to the car driving at low speed and heavy load conditions, it not only needs a large lubrication flow, but also needs a large lubrication pressure to ensure that the oil can take away the heat in time to ensure the safety and reliability of lubrication. With the given speed instruction, the output speed of the lubricating oil pump and the output pressure increase. The rising process of the pressure sensor is slightly slow due to its certain lag, but stabilizes at the specified value.

Motor pump with high flow output also with high pressure output.

Figure 15. shows that the motor only needs to meet the flow requirements, and the output pressure of the lubricating oil pump is not required under the condition that the motor operates at 850 rpm. When a short speed down command is given suddenly, the speed of the lubricating oil pump rapidly drops to 400 rpm within approximately 1 s operates for approximately 3s, and then returns to the initial speed. The lubricating oil pump can quickly feedback the speed down and the speed up instructions. The lubricating oil pump operates at the specified speed, so that the most ideal state of lubrication effect is achieved.

Motor pump output with changing flow but stable pressure output.

It can be seen from the above experimental results that the self-adaptive lubricating oil pump can adjust the output flow and pressure to meet the requirements of transmission lubrication and improve its reliability. Comparation between output pressure and output flow of conventional and adaptive adjustment pumps just like Table 3 . The traditional lubricating oil pump rotates at a constant speed, so it can only output a fixed flow of 3.21 L/min, and there is no relief valve or proportional valve on the output oil path of the pump, so it can not control the output hydraulic oil pressure, and the 0.05 MPa output is only imposed because of the resistance of the pipeline. However, the output flow of novel adaptive lubricating oil pump can be adjusted linearly from 0.1 L/min to 3.71 L/min according to the speed of the transmission, also the output pressure can be adjusted linearly from 0.05 MPa to 0.61 MPa according to the requirements of the transmission.

From Figs. 11, 12, 13, 14 and 15, as can be seen from the above experimental results, the output flow and pressure vibrate during the rising process. This is because the connection of the hydraulic pump, the hydraulic flow leads to a large moment of inertia at the load end of the motor. Therefore, the modeling of the entire control system and accurate parameter identification are the core of building a high-precision controller, and accurate parameters will construct an accurate transfer function, so that the control can be more accurate after the Laplace transform and discrete, which will make the system respond more quickly39,40,41. Then the motor pump would run more smoothly, and the power consumption will further reduced.

According to the above experiments and combined with the demand of lubricating oil flow and pressure of the automobile under different working conditions the comparison between conventional oil pump and novel adaptive lubricating oil pump is shown as Fig. 16. It mainly divided into four parts, section ①represents the high speed cruising condition, in this section, the car cruises at high speed and does not require large amounts of oil and output pressure. section ② represents the low speed highest torque condition, at this stage, the transmission of a large torque car gearbox requires a large oil flow and pressure output from the lubricating oil pump. section ③ represents the higher speed higher load condition, at this stage, the car gearbox transmits a large torque and runs at a high speed, so the lubrication pump needs to output a large output flow and pressure. section ④ represents the idle speed condition, at this stage, the gearbox of the car just operates in a low speed and transmits little torque, so only the low flow rate of the lubricating pump is needed.

Comparison between conventional pump and adaptive adjustment pump.

In section ①, the automobile was lubricated at a flow rate of 3.71 L/min and a pressure of 0.05 MPa in the first 5 min. 0.05Mpa represents the pressure from the tubing, at this time, the traditional lubrication mode was difficult to carry out full load due to the operation under rated working conditions, so a low flow rate was used as the section ①in the first stage from Fig. 16, and depend on the pressure of the pipeline to provide their own to provide lubrication pressure, so at this time and can’t meet the demand of the best lubrication. During 10 to 25 min in the second stage as section ②, at this point, the car runs at a lower speed in heavy load, requiring a larger lubrication flow and pressure to make the heat generated by the gear meshing of gearbox can be taken away in time to ensure the best lubrication effect, while the traditional lubrication only provides a large flow, and the output pressure can not be adjusted, also, the lubrication effect is poor. Because of the motor power limit, causes in the case of the rated power output, when the output flow rate increased, the output pressure of oil pump will reduce, if choose more powerful motor, if not full output, so in the case of guarantee the output flow, still can make output lubricating oil pressure increase. At this stage, as the output flow of the lubricating oil pump decreases, its output pressure increases to achieve pressure lubrication. Pressure lubrication belongs to forced lubrication, which can significantly improve the lubrication effect between friction pairs and improve the performance of products. In the third stage as the section ③, the vehicle runs at a higher speed and lower loads requires a certain oil pressure to ensure safe and reliable lubrication. However, the traditional oil pump still runs at a constant flow and output pressure, which is difficult to meet the optimal lubrication requirements. At this stage, the speed of the gearbox is higher, but the load is small, so the gearbox relies on its own splash to provide lubrication on the one hand, and on the other hand depends on the external lubrication pump to provide a certain lubricating oil flow to meet the lubrication requirements of this stage. In the fourth stage as the section ④, the vehicle runs in the idle condition, because no load and speed of gear box is not high, lower lubrication flow and pressure are required, but the traditional oil pump still operate in conventional mode which cause waste of energy, according to the power of the Eq. (9) can be calculated that a novel lubrication method of power consumption accounts for only 3.1% of the traditional, calculated as follows, 0.1 * 0.05 / (3.21 * 0.05) = 3.1%. Considering the urban vehicle operation, especially the morning and evening rush hour traffic jam in some developing countries, the actual time under this working condition will be long, so under this working condition, the new lubrication method will undoubtedly bring more benefits.

Considering the time period and working conditions experienced by urban vehicles from starting at home to stopping at work, the power at different speeds can be calculated according to the data tested in Fig. 17. The power consumption values of conventional and proposed motor pumps at different speeds are shown in the following Fig. 18. It should be noted that the traditional lubricating oil pump always works at a constant speed of 2000 rpm, so its power consumption is constant. Considering its output pressure is 0.05mpa, the power calculated by Eq. (9) is 6.7 W. The adaptive oil pump adjusts the output flow at any time according to the speed of the gearbox to ensure safe and reliable lubrication.

Although the power consumption of the adaptive pump is higher at some times, such as in the speed of 800 rpm, 750 rpm, 600 rpm, etc., the power consumption of the adaptive pump exceeds the traditional lubricating pump because of the pressure lubrication required at lower speeds to ensure that the transmission gears are always lubricated in optimal conditions. Furthermore, In the future, the control of the electronic oil pump will be further optimized, and the transfer function of the electronic oil pump control system will be obtained more accurately through parameter identification and other methods, and the controller will be upgraded by discretization, which will make the electronic oil pump control more accurate and the power consumption will be further reduced39,40,41.

Considering the fact that the vehicle will rarely be used in heavy loads, So Compared with the traditional lubricating oil pump, the new adaptive lubricating oil pump needs to provide such functions to ensure the safety and reliability of lubrication. In addition, the vehicle is lubricated with large flow and low pressure in most working conditions. Therefore, when the speed of the lubricating oil pump is higher than 900 rpm, the maximum speed of the lubricating oil pump will not be higher than 2000 rpm, and its output pressure is 0.05Mpa as a constant, so its energy consumption will always be lower than 3 W. That means it will always be below 6.7 W which represents the conventional oil pump power consumption as Fig. 17 shows. So the proposed topology is advantageous in terms of efficiency compared to conventional pump.

The power consumption values of conventional and proposed motor pumps at different speeds.

Further more, a lubrication test rig for the gearbox was built to verify the high value of the lubrication solution as shown in the Fig. 18 below.

Gear box temperature rise test bench.

The gear box is a pair of teeth meshing, the modulus is 2, the number of teeth are 40 and 20 respectively, the top of the cover is an acrylic plate, which can be easily removed to measure the temperature through the handheld infrared thermometer. The speed measurement range of the torque sensor is 0–6000 rpm, and the measurement range of the magnetic powder brake is 0-2000Nm, the power of driver motor is 30 kW. The drive motor drives the input gear of the gearbox and then connects to the speed and torque sensor through the output of the gearbox reducer, whose output shaft is then connected to the rotating inertia disk and finally to the magnetic powder brake. The magnetic powder brake is loaded with different currents through a constant current power supply to provide different load torques.

In order to reduce the influence of experimental randomness on experimental results, the following experiments are the results of repeated tests. Testing the output shaft of the gearbox at different speeds and torques, as shown in Fig. 19 below, it can be seen that as the speed of the gearbox output continues to increase, the temperature of its output shaft gears gradually increases and finally level off, which is attributed to splash lubrication, and the temperature of the gears is also measured after about 200s of operation as the base speed increases as shown in Fig. 20.

Variation of gear temperature with speed.

Variation of maximum gear temperature with speed.

The motor is given a closed loop speed so that the speed of its gearbox output shaft is stabilized at 250 rpm, and the temperature change at different torques is measured under splash lubrication. It can be seen that as the load torque becomes larger, its temperature also increases one by one, and finally it also tends to be smooth, as shown in Fig. 21 below. In addition, the maximum temperature of the motor when running at different load torques to a steady state is shown in the following Fig. 22.

Testing the temperature of gears at different torque at 250 rpm.

Adjusting the speed of the gearbox lubricating oil pump and thus changing the flow rate of the pump output to make the flow rate of the lubricating oil at the nozzle different, it can be seen from Fig. 23 that with the change of the flow rate, the temperature of the gears at the output end of the gearbox gradually decreases, and the change pattern is consistent through different loads to observe. So it can be concluded that changing the output flow rate of the oil pump can significantly improve the lubrication condition of the gears, thus changing their temperature rise. And the temperature will change the service life of the gearbox.

Testing the maximum temperature of gears at different torque at 250 rpm.

Effect of lubricant output flow on gear temperature at different load torques.

Comparing the three ways of splash lubrication, constant flow lubrication and adaptive lubrication, it is found from Fig. 24 below that splash lubrication can make the maximum gear temperature close to 100 degrees, while the temperature under constant flow lubrication is close to 90 degrees, and the temperature under adaptive lubrication conditions is close to constant at 52 degrees up and down. Doing almost constant temperature control can significantly improve the temperature rise of the gearbox, thus improving the service life of the transmission.

Comparison of gear temperature under three types of lubrication.

Therefore, it can be seen from the Fig. 24 that the temperature of gears changes differently under three different lubrication methods, under splash lubrication and load torque of 140Nm, the maximum temperature of the gear can reach about 95°, under constant flow lubrication and load torque of 140Nm, the temperature can reach 85°, but under adaptive control, the temperature of the gear under different loads is basically stable at about 52°. Moreover it can adjust the output flow in real time according to the temperature you need, so that the gearbox always works in the optimal temperature range. Compared with traditional lubrication and splash lubrication, the new lubrication method reduces the gearbox temperature and power consumption by percentage are shown in Table 4.

It is worth mentioning that temperature is one of the key factors affecting the life of gears according to the Eq. (8), so it can be seen from Table 4 that the new lubrication method can reduces the gearbox temperature and power consumption to different degree. Especially in the case of 140Nm heavy load, the adaptive lubrication method can reduce the temperature rise of the gearbox by 35.5% compared with constant flow lubrication, and can reduce 42.3% compared with splash lubrication. In addition, during stage 4 in Fig. 16, the novel lubrication method can reduce consume of the power by 96.9% compared with the conventional oil pump. Temperature control of gears is more accurate under the effect of self-adaptive variable flow lubricating oil pump, which is more beneficial to its life extension.

Considering the current market, a constant flow of DC motor pump or BLDC motor pump waste the energy to a certain extent regardless of the working condition of automobile fuel supply characteristics. Thus, a variable flow rate of lubricating oil pump with a BLDC motor is developed. The output flow of the lubricating oil pump is adjusted according to the real-time speed of the vehicles, this condition must be met to match the output pressure and flow of the lubricating oil pump for achieving the best lubrication of the gearbox, to effectively improve the friction performance of the gearbox and the utilization rate of energy. From the experimental point of view, the lubricating oil pump with BLDC motor obtains a good use level through the multidimensional analysis of hardware and software algorithms. From stage 1 to 3, the novel lubricating oil pump can provide better lubrication performance, during stage 4, the novel lubrication method can consume 3.1% of the power of the conventional oil pump use. In addition, the effects of splash lubrication, constant flow lubrication and variable flow lubrication on gear temperature under different torque loads were tested, and it was concluded that variable flow lubrication can control gear temperature well. Therefore, the viscosity of the lubricating oil is controlled, and thus the thickness of the lubricating oil film is controlled, so that the gear is always in the best lubrication state, thereby extending the service life of the gearbox. Further more, considering the increasing electrification and intelligence of vehicles, and the requirements of global carbon emissions, the development of electric equipment that meets the requirements of optimal use and optimal efficiency is necessary and will become a trend. Future work will provide a set of such solutions to provide a reference.

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The present work is financially supported by the Joint Fund for Aerospace Advanced Manufacturing Technology Research Key Program (Grant No: U1937203).

School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an, 710049, Shaanxi, China

Yangfeng Cao, Dazhou Liu, Peng Zhang, Hao Zhou & Shengdun Zhao

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Yangfeng Cao.:Conceptualization, Methodology, Software, Investigation, Writing- Original draft, Writing - Review & Editing.Dazhou Liu.: Data curation, preparation, Writing - Review & Editing.Peng Zhang: Visualization, Investigation, Writing - Review & Editing.Hao Zhou.:Supervision, Resources, Writing - Review & Editing.Shengdun Zhao.: Writing- Reviewing and Editing, Project administration, Funding acquisition.

Correspondence to Shengdun Zhao.

The authors declare no competing interests.

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Cao, Y., Liu, D., Zhang, P. et al. A novel lubricating method of oil pump with self-adaptive adjustment function for transmission. Sci Rep 15, 10223 (2025). https://doi.org/10.1038/s41598-025-94966-3

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Received: 01 March 2024

Accepted: 18 March 2025

Published: 25 March 2025

DOI: https://doi.org/10.1038/s41598-025-94966-3

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