Research on Transient Characteristics of an Aviation Starter Motor Under Low Temperature and Pressure

Abstract

To enhance the performance of aviation piston engine starters in high-altitude environments, this study investigated their transient starting characteristics under low-temperature and low-pressure conditions using a high-altitude simulation chamber. Experiments were conducted across ambient temperatures from −60 to 15 °C and pressures from 35 to 95 kPa. The results show that the ambient temperature significantly impacts the transient characteristics of the motor, particularly at the initiation stage, whereas pressure has a weaker effect. For every 5 °C drop in temperature, the peak starting current increases by 1.95 A. From 15 to −60 °C, the maximum difference in the starting current pulse width is 11.64 ms. Furthermore, as the ambient temperature decreases, the average current after the motor stabilizes increases, and the average speed decreases.

1. Introduction

Application scenarios for unmanned aerial vehicles (UAVs) are becoming increasingly diverse [1]. For example, UAVs have been employed for reconnaissance, deception, interference, electronic countermeasures, and direct combat engagements for military purposes [2,3]. Piston engines have emerged as a significant power source for UAVs in modern military operations, owing to their numerous benefits [4,5]. The permanent magnet brushless direct current (DC) motor has become the preferred starter motor for aviation piston engines because of its compact size, high power output, stable torque, and low noise level [6]. In some special application scenarios, such as high-altitude air-dropped UAVs and restarts, piston engines must be started at high altitudes [7,8]. Low-temperature and low-pressure environments significantly affect DC motor performance [9,10,11,12,13]. In addition, piston engines face challenges when starting at high altitudes [14]. Milojevi, S. et al. [15] indicated that engines require the use of variable systems, which can facilitate cold starts depending on the ambient temperature, the type and temperature of the fuel, etc. Hence, substantial adjustments had to be made to the compatibility requirements between the piston engine and starter motor under the aforementioned conditions.

Mo et al. [16] noted that low temperatures reduce the fit clearance between the engine crankshaft and bearing as well as that between the piston and cylinder, which leads to an increase in the frictional resistance torque of the engine. Kang et al. [17] confirmed that as the ambient temperature decreased, the viscosity of the lubricating oil increased, leading to significant friction losses. Skulić A et al. [18] pointed out that the increase in oil viscosity in low-temperature environments leads to the increase in engine transmission loss. Baranski, M et al. [19] studied the influence of temperature on the magnetic properties of the permanent magnets as well as on the electric and thermal properties of the materials. Agudelo et al. [20] studied the combustion characteristics of piston engines at different altitudes and found that the intake airflow of piston engines decreases with a reduction in atmospheric pressure, leading to an insufficient final pressure in the engine cylinder and a decrease in the performance of the starting process. The aforementioned research indicates that both the starting resistance torque and starting speed of a piston engine increase in high-altitude environments. Wang et al. [21] noted that sharp changes in the motor’s working environment temperature can cause rapid deformation of the motor material and result in a “breathing effect”. This will accelerate the aging and deformation of the material. Chen et al. [22] demonstrated that the impact of temperature on the initial permeability of a motor’s stator core is almost linear in low-temperature environments; specifically, the permeability decreases as the temperature decreases. The research conducted by Guobao Xu [23] indicates that the insulation material of motor windings is particularly susceptible to accelerated aging at ambient temperatures ranging from 20 °C to 40 °C. Hu et al. [24] investigated the starting parameters of single-phase capacitor start motors and three-phase induction motors at low temperatures. Their findings indicated that the starting voltage and current of the motors both increased as the ambient temperature decreased. Liu et al. [25] investigated the resistance torque of micro permanent magnet DC motors used in vehicles under low-temperature conditions. They concluded that the increases in the starting voltages and starting currents of these motors in such environments were attributable to increases in the bearing resistance and cogging torque. The literature [26,27,28,29,30] primarily investigates the impact of ambient temperature on various motor characteristics, including the efficiency, electromagnetic torque, overload performance, output power, cogging torque, maximum demagnetization points of permanent magnets, torque fluctuations, and motor service life. Hence, a low-temperature environment significantly affects motor performance, which leads to notable changes in the starting characteristics of the motor.

To improve the high-altitude starting performance of aviation piston UAVs, it is necessary to study the impact of low-temperature and low-pressure environments on the aviation starting motor performance. In contrast to data simulation technology [31], this study replicated and analyzed the starting processes of aviation starting motors under low-temperature and low-pressure conditions using a high-altitude environmental simulation chamber. The aim of this study is to investigate the starting transient characteristics of aviation starting motors in such environments, which can provide rules for matching aviation piston engines and starting motors [32,33,34].

2. Experimental Setup and Methodology

2.1. Experimental Apparatus

Figure 1 shows a schematic of the low-temperature, low-pressure test platform for aviation starting motors. The platform consists of a high-altitude environment simulation system, control system, motor starting system, and data acquisition system.

Figure 2 shows a schematic of the working principle of the high-altitude simulation chamber. The chamber features five main control systems, covering environmental pressure, ambient temperature, intake pressure, intake temperature, and intake flow. It also includes auxiliary systems, such as water circulation systems, electrical systems, safety detection systems, and drag control systems. The complete system of the high-altitude simulation chamber can ensure the stability of the experimental conditions, ensure the safety of the experimental bench, and warn of the fault in time.

The temperature of the introduced airflow within the high-altitude simulation chamber is regulated by an intake temperature system, and the temperature ranges from −60 °C to 50 °C. The cabin air pressure is determined by the airflow, which can be controlled by adjusting the vacuum pump system. The minimum air pressure in the high-altitude chamber is 20 kPa. During the experiment, the ambient temperature error is allowed to be within ±2 °C, and the ambient pressure error is within ±1 kPa.

Table 1. Basic parameters of the high-altitude simulation test cabin.

Item	Parameter
Model	GY10000
Cabin size (mm)	3500 × 3500 × 6000
Temperature range (°C)	−60–50
Temperature fluctuation (°C)	±0.5
Pressure limit (kPa)	20–95
Total flow (m3/min)	70
Mains voltage (V)	380
Gross power (kW)	150

lists the basic parameters of the aero-engine altitude simulation test chamber.

Figure 3 illustrates the low-temperature and low-pressure starting test platform for aviation starting motors. The starting system includes a permanent magnet brushless DC motor, crankcase, connecting shaft, and gear disk. The permanent magnet brushless DC motor used in this experiment has an outer rotor structure.

Table 2. Main parameters of the aviation starting motor.

Item	Parameter	Item	Parameter
Starting voltage (V)	26–32	Power (Kw)	1.25 (29 V)
Stator outer diameter (mm)	138	Length of air gap (mm)	0.75
Rotor cooling diameter (mm)	152	Permanent magnet width (mm)	7.9
Rotor inner diameter (mm)	139.5	Permanent magnet thickness (mm)	4
Number of pole pairs	20	Core length (mm)	26.5

lists the DC motor parameters. The experimental setup employs an engine crankcase as a generator carrier. The crankshaft, piston, and other components of the aviation piston engine are removed from the crankcase. This modification prevents these components and their associated lubricating oils from influencing the transient characteristics of the aviation starting motor, thereby increasing the reliability of the experimental results. The test platform connects the motor and gear disk through a shaft. The gear disk simulates rotational inertia during the engine starting process in order to reproduce the transient process of the motor driving the engine during startup. In addition, a gear disk is used to capture the instantaneous rotational speed of the starting motor. Finally, an 18 kW spark-ignition two-stroke aviation piston engine is matched with the permanent magnet brushless motor used in the system. The main parameters are shown in

Table 3. Main structural parameters of 18 kw class-ignited two-stroke aviation piston engine.

Item	Parameter
Cylinder diameter/mm	66
Stroke/mm	54
Displacement/ml	340
Effective compression ratio/-	6.715
Crank radius/mm	27
Connecting rod length/mm	31/6000
Volume before compression/mm3	64,005.62
Volume after compression/mm3	9532.29

.

2.2. Experimental Conditions and Data Processing

Figure 4 shows the experimental setup, data acquisition, and processing diagram. The working conditions for the aviation starting motor experiment are configured as shown in the figure. These conditions were established based on the altitude starting requirements of an aviation piston engine. The data acquisition system of the test platform included the following components: a signal acquisition device, a current sensor, an angular velocity sensor, and temperature and pressure sensors. The main parameters of all sensors are shown in

Table 4. The main parameters of the main test sensor.

Equipment	Range	Precision
Current sensor	0–300 A	0.01 A
Temperature sensor	−200–+200 °C	1.00 °C
Pressure sensor	0–100 kPa	1.00 kPa
Angular velocity sensor	50–5000 HZ	1.00 HZ

. The angular velocity sensor operates based on electromagnetic induction. The gear disk shown in Figure 4 is a physical representation of the angular velocity sensor and the gear disk itself. The gear disk had 72 teeth, and the tooth thickness was equal to the width of the space. The calculation formula for the speed of the aviation starting motor is also shown in Figure 4.

3. Results and Discussion

3.1. Impact of Low Temperature on Transient Startup Characteristics

When the ambient pressure is 95 kPa and the ambient temperature during the starting process of the aviation starting motor varies from −60 °C to 15 °C in 15 °C increments, the speed and current curves of the motor during startup are shown in Figure 5. Across different ambient temperatures, the trends in current and speed during the starting process of the aviation starting motor exhibited similar patterns. At startup, the motor experiences a peak in the starting current. As the speed increased, the current began to decrease and eventually stabilized within a certain range. Once the speed reached its peak, it stabilized within a specific range.

Figure 6 illustrates the relationship between the peak current of the aviation starting motor during the startup process and the ambient temperature, which ranges from −60 °C to 15 °C. As the ambient temperature decreases from 15 °C to −60 °C, the peak starting current rises from 246.50 A to 275.16 A. The fitting curve presented in the Figure 6 shows the correlation between the peak current and changes in the ambient temperature. These results indicate that on average, the peak starting current increases by 1.95 A for every 5 °C decrease in the ambient temperature.

Figure 7 shows the curve depicting the relationship between the average current of the aviation starting motor after it reaches stable operation and the ambient temperature, ranging from −60 °C to 15 °C. As the ambient temperature decreases from 15 °C to −60 °C, the average current increases from 5.31 A to 6.85 A. Figure 7 includes a fitting curve that illustrates how the average current varies with the ambient temperature. The average current of the aviation starting motor increases as the ambient temperature decreases. This occurs because the resistance of the copper windings in the motor stator decrease as the temperature decreases.

Figure 8 depicts the curve representing the starting current pulse width during the starting process of the aviation starting motor as a function of the ambient temperature, ranging from −60 °C to 15 °C. At an ambient temperature of 0 °C, the starting current pulse width is 19.28 ms, whereas it extends to 30.92 ms at −60 °C, resulting in a difference of 11.64 ms between the maximum and minimum pulse width. Moreover, Figure 8 includes a fitting curve that illustrates how the starting current pulse width changes with the ambient temperature. As the ambient temperature decreases, the starting current pulse width for the aviation starting motor increases progressively, and the rate of increase accelerates as the temperature decreases.

Figure 9 shows the curve illustrating the change in the average speed of the aviation starting motor as a function of the ambient temperature, ranging from −60 °C to 15 °C. The average speed of the motor decreases from 1350 r/min at an ambient temperature of 15 °C to 1243.80 r/min at −60 °C. When the aviation starter motor drives the engine under a larger load, the decrease in the motor speed becomes more pronounced as the ambient temperature decreases.

3.2. Impact of Low Pressure on Transient Startup Characteristics

Figure 10 depicts the corresponding speed and current curves of the aviation starting motor at an ambient temperature of 0 °C with an ambient air pressure that gradually increases from 35 kPa to 95 kPa in 20 kPa increments during the starting process of the motor.

Figure 11 illustrates the corresponding speed and current curves of the aviation starting motor at an ambient temperature of −15 °C as the ambient air pressure gradually increases from 35 kPa to 95 kPa in 20 kPa increments during the starting process of the motor.

Figure 12 illustrates the curve showing the variation in the peak current of the aviation starting motor with changes in the ambient pressure during the starting process under ambient temperatures of 0 °C and −15 °C and ambient pressures ranging from 35 kPa to 95 kPa. At an ambient temperature of 0 °C and an ambient pressure of 35 kPa, the peak starting current of the aviation starting motor is 252.28 A. When the ambient pressure increases to 95 kPa, the peak starting current decreases slightly to 249.11 A, resulting in a maximum difference of 3.17 A. At an ambient temperature of −15 °C and an ambient pressure of 35 kPa, the peak starting current is 252.53 A. When the ambient pressure increases to 95 kPa, the peak starting current rises to 256.78 A, with a maximum difference of 4.25 A.

Figure 13 presents the variation curve of the average current of the aviation starter motor as a function of the ambient pressure after achieving stable operation at ambient temperatures of 0 °C and −15 °C, with pressures ranging from 35 kPa to 95 kPa. The average current of the aviation starter motor shows minimal variation at both 0 °C and −15 °C across the different ambient pressures of 95, 75, 55, and 35 kPa. The rates of change between the maximum and minimum values are 1.21% and 1.98%, respectively.

Figure 14 illustrates the variation curve of the starting current pulse width of the aviation starter motor during the starting process, in relation to the ambient pressure, at ambient temperatures of 0 °C and −15 °C, with pressures ranging from 35 kPa to 95 kPa. At both 0 °C and −15 °C, the differences between the maximum and minimum values of the starting current pulse width during the starting process under varying ambient pressures are 1.08 ms and 0.88 ms, respectively.

Figure 15 shows the curve depicting the average speed of the aviation starting motor after reaching stable operation as a function of the ambient pressure at ambient temperatures of 0 °C and −15 °C. At both temperatures, the average speed fluctuation of the aviation starter motor remained minimal across ambient pressures of 95, 75, 55, and 35 kPa. The rates of change between the maximum and minimum values were 0.48% and 0.17%, respectively, compared with the average value.

4. Conclusions

When the ambient pressure is 95 kPa and the ambient temperature ranges from −60 °C to 15 °C, the starting current of the aviation starter motor increases as the temperature decreases, whereas the average speed decreases as the temperature decreases. When the ambient temperature is 0 °C or −15 °C, changes in ambient pressure between 35 kPa and 95 kPa have little effect on the starting current and starting speed.

• For every 5 °C decrease in ambient temperature, the peak starting current increases by 1.95 A. In contrast, the peak starting current was less affected by changes in ambient pressure, with a maximum difference of no more than 4.50 A.
• Within the temperature range of −60 °C to +15 °C, the maximum difference in the starting current pulse width is 11.64 ms, whereas the fluctuations in the starting current pulse width caused by changes in the ambient pressure do not exceed 2 ms.
• As the temperature decreased, the average current after the motor stabilized during startup increased. The changes in the ambient pressure had a relatively small impact on the average starting current, with a variation rate of no more than 2%.
• Within the temperature range of −60 °C to +15 °C, the average speed decreases by 106.20 r/min as the temperature decreases. The impacts of ambient pressure changes on the average starting speed were minimal, with a variation rate of no more than 0.5%.