Reduction of Testing Time of PTCE/HTOE Tests Based on Real Road Load Profiles 2022-01-0176

HTOE (High Temperature Operation Endurance) and PTCE (Power Thermal Cycle Endurance) tests are typically performed according automotive group standards, such as LV 124 [1], VW80000 [2], FCA CS.00056 [3] or PSA B21 7130 [4]. The LV 124-2 group standard, composed by representatives of automobile manufacturers like Audi AG, BMW AG, Volkswagen AG and Porsche AG describes a wide range of environmental tests and their requirements. In addition, calculation parameters and a method are given in the standard. These group standard tests are often attributed to IEC 60068-2-2 [5] for HTOE and IEC 60068-2-14 [6] for PTCE. As both of these tests are typically of long duration, fundamentally linked to reliability (therefore requiring a statistically significant number of samples) and of considerable importance to power electronic, they are worthy of additional scrutiny for automotive developers as most automotive development moves towards electrification.

HTOE and PTCE tests are accelerated tests simulating the thermal and thermomechanical exposure of components resulting from the temperature changes that occur during the vehicle service life. The tests are intended to verify the quality and reliability of the component with respect to faults that occur due to thermal and thermomechanical exposure. From this it is evident that the test must either be linked directly to the lifetime expected thermomechanical exposure or it must be assumed to have some proportionality to it. Without this link, it would not be possible to evaluate sample reliability and confidence level validated through testing and determine if the product would be likely to fail in real world application [7].

A multi-disciplinary vehicle system simulation tool (AVL CRUISE^TM M) is used to build a thermal model of an e-axle including all thermal-energy relevant components like the e-motor, inverter and transmission gear box as well as the cooling systems (including ambient air flow) and heat exchangers. These components are modelled as a multi body “lump mass” system and interact via thermal couplings. In addition, efficiency maps for e-motor, inverter and transmission are considered to provide energy losses during operation which are assumed to equate to the thermal energy generation in the system.

Based on real road load profiles (speed, torque and vehicle velocity), coming from an extensive vehicle usage space and supplemented by simulated track profiles, losses are simulated and result in an extensive collection of e-axle compartment and cooling circuit temperature profiles. The simulated track profiles are generated using AVL Smart Mobile Solutions™.

These temperature distributions, of the individual components, are used as thermal boundary conditions for the HTOE and PTCE test generation which provides a direct link between the test difficulty and the expected usage. This process has, additionally, demonstrated a significant reduction of the required HTOE and PTCE testing times in comparison to the recommendations in automotive group standards.

The testing time reduction goes hand in hand with an enormous cost reduction over a full design validation process, in particular as many test samples are needed to reach a full reliability and confidence level requirement.