Acoustic effects of the coolant mass flow of an electric machine of a hybrid drive train
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In this application paper, the influence of the coolant mantle on the acoustic radiation behaviour of a hybrid drive train is investigated. This was done on an electric machine on an acoustic component test bench. The coolant mass flow around the electric machine stator was varied and then completely drained. The electrical machine remained mechanically unchanged; any variations were made to the feed pumps on the test bench side. Triaxial acceleration sensors are glued to the machine housing and reviewed as evaluation criteria. For the evaluation, the square mean value of all three spatial directions of the glued acceleration sensors was calculated. The evaluation shows that there is no significant acoustic difference between an active stator cooling jacket and a stationary stator cooling jacket. If the stator cooling jacket is pumped out empty so that air remains in it, there is a strong reduction in surface acceleration. The observations are confirmed by analytical literature values. The results presented serve as a basis for further work and developments.
KeywordsElectric engine NVH Fluid-structure Automobiles Test-bench Noise Vibration Acoustic
Hybrid electric vehicles are part of the next development stage of the automotive industry. In these vehicles, the drive system will be extended by a further electric drive system. One of the challenges is the acoustic emissions of such a drive system. Although the electric machine is quieter than a conventional combustion engine, it has a wider frequency spectrum that can put a stress on the occupants. To reduce occupant’s stress and increase vehicle and driving comfort in the passenger compartment, it is important to ensure a low noise level during development.
In the context of this application paper, the influence of the coolant mantle on the acoustic radiation behaviour of a hybrid powertrain is therefore investigated. The architecture of a hybrid vehicle considered in this paper is a P2-hybrid. In this architecture, the electric machine is positioned between the conventional combustion engine and the manual transmission. This makes it possible both to increase the power of the combustion engine and to drive the vehicle completely electrically without using the combustion engine .
The electrical machine is operated on a component acoustic test bench for the purposes of this study. On this test bench, the coolant mantle of the electric machine is varied. The acceleration of the machine’s surface during operation is measured and the change is evaluated by the coolant mantle variation.
2 Previous work
2.1 Electric machine
The tangential tension component and the reaction force in the rotor of the electric machine are used as power output into the gearbox. The radial component, on the other hand, is considered parasitic as it leads to structural excitation in the radial direction. The periodic force excitations also cause periodic oscillations in the machine and the housing. These oscillations in its amplitudes and frequencies result in the acoustic emissions of the drive unit. For an initial estimation of the structure-borne sound, it is therefore sufficient to consider the sound velocity on the machine surface [7, 8].
Overview of the longitudinal impedance ZF for chosen materials
Longitudinal impedance ZF (Ns/m3)
Air (20 °C)
1483 × 106
111 × 106
45 × 106
Overview of reflectance \(\rho\) and transmittance \(\tau\) for chosen material transfers
Reflectance \(\rho\) (%)
Reflectance \(\rho\) (dB)
Transmittance \(\tau\) (%)
Transmittance \(\tau\) (dB)
Steel → air
Steel → water
Steel → oil
The characteristic sound radiation of the electrical machine can be influenced and changed in different ways. On the one hand, the housing can be shielded and encapsulated. On the other hand, measures can be found to reduce the parasitic electromagnetic excitation forces . Finally, the transmission path between the excitation forces and the radiating surface can be manipulated.
2.3 Proposed approach
Overview of the considered variants and operating conditions
Volume flow (L/min)
The entire assembly (electric machine inside the transmission housing) is operated on a component acoustic test bench. This is a reverberation chamber. It is equipped with sound-absorbing walls and ceiling, only the floor consists of a rigid and sound-reflecting floor. On this test bench, the coolant liquid can be varied on the test bench side so that the electrical machine can be operated under different conditions without mechanical changes or modifications. The machine is flanged to a brake. This is located in the adjacent technical room, outside the measuring room. A connecting shaft is sound-absorbing encapsulated. The electric machine is supplied with oil for cooling and lubrication via connected hydraulic lines. Feed pumps and the tank for this oil are also located in the adjacent technical room. The electrical high-voltage supply and control of the electrical machine are provided by the power electronics. This is commanded from the control room.
Measurement technology is also used to record the acoustic response variables on the transmission housing. The surface of the transmission housing and other parts of the test stand are covered with 16 triaxial acceleration sensors. These are used to record the structure-borne sound. The positions of the sensors are chosen to be on critical structures like ribs and oscillating surfaces, far from any ribs and stiffeners. For this paper only the sensor data from the oscillating surfaces are considered. For the airborne sound nine microphones are systematically distributed in the room. The data from these microphones are not relevant for this application. The electrical machine is operated successively under several load levels, while the speed increases linearly over time. The data of the measurement are recorded with the software PAK of the company Müller-BBM and evaluated afterwards. These data for the structure-borne noise are broken down into the characteristic engine orders. During all measurements, care was taken to ensure that the temperatures in the stator are identical at the beginning of each measurement.
In the first version of the measurement, a volume flow of 10 L/min flows through the coolant thin film. The feed pumps remain in operation. In the next variant the volume flow is reduced to 0 L/min. The feed pumps are switched off and the remaining coolant mass remains in the cooling jacket between the stator of the electric machine and the transmission housing. For the last variant of the measurement, the oil is sucked out of the transmission housing with the aid of the feed pumps, so that the gap with ambient air remains. The feed pumps are also deactivated.
This section presents the results of the experiments carried out. Results from all three variants are plotted for two load cases: 100 Nm and 350 Nm. The vector products of the measured surface velocities above the increasing speed are shown.
Multiple surface velocities are determined by calculating the first integral of the acceleration from the multiple triaxial acceleration sensors. Afterwards one single surface velocity is generated by calculating the root mean square from the multiple surface velocities and plotted on a logarithmic scale. The calculation considers only the eight sensors, which are located on free and oscillating surfaces of the transmission housing (see Sect. 2). By application of the Fourier transformation the data from the surface velocities were broken down into their characteristic engine orders 20, 40 and 60. This whole process was done for several loads.
Identical to the engine orders 20 and 40, variant 3 “pumped out” results in the lowest surface speeds of the considered variants. The 2700 min−1 increases can also be seen in the third variant. For higher speeds from approx. 4000 min−1, the reductions due to variant 3 “pumped out” are lower than in the preceding engine orders 20 and 40.
A comparison of the results with the numerical values for the reflectance and transmittance gives a uniform picture. While only 0.003% of the energy is transferred during the transition from steel to air, it is already 9.3% of the energy during the transition from steel to oil. This means that an intermediate layer filled with a liquid allows more energy to penetrate from the inside of the system to the outside and thus emit into sound. A fluid with a lower impedance leads to better insulation of the inner structures.
This application paper presents the influence of a coolant mantle on the acoustic radiation of an electric drive machine.
In the experiments carried out, the coolant mantle was varied and pumped out so that the thin film was filled with ambient air. It was found that without the coolant the machine had the lowest surface velocities and thus the lowest acoustic radiation. As a result, a fluid-filled thin film couples the two components and has negative effects on the acoustic radiation of a machine. The results were justified and confirmed with the analogy of the mechanical impedance of the different media. A larger impedance jump in the phase transition results in less energy being transmitted from the exciter component into the excited medium. The effect of structure resonances and mode shapes on the surface velocity could not be considered in this paper and will be part of further research.
The results presented serve as a basis for further work and developments. The acoustic suitability of other fluids has to be tested, while focus is on fluids with lower longitudinal impedances. In addition, the influences of the fluid must be integrated into the structural-dynamic FEA simulations of the electrical machine.
The author is thankful to Paul Künstler, M.Sc. in providing support and help to realize the measurements necessary for this application paper.
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