Lesson 2: Example of requirement: maximum wheel torque

Introduction:

  • We will explore a use case surrounding passenger car powertrain development.
  • Our focus: “Maximum Wheel Torque” requirement.

Setting the Scene:

  • A vehicle manufacturer (OEM) is developing a Battery Electric Vehicle (BEV) for the European market.
  • Drawing from prior experience with combustion vehicles, they have preliminary specifications and understand the needs of drivers, passengers, and legislations.

Initial Requirements:

  1. Car tire size: 205/55 R16.
  2. The vehicle will have front-wheel drive only.
  3. The car must achieve maximum torque and power of 100 kW momentarily at 90 km/h.

Deriving the Wheel Torque Requirement:

  • Engineers determined that the powertrain must deliver a total torque of 1260 Nm.
  • This translates to 630 Nm for each front wheel.

Calculating Motor Torque:

  • To determine the torque needed from the e-motor, consider the system’s components.
  • The OEM decides to repurpose the differential from its combustion vehicles.
  • Using a stock reduction gear set, the complete reduction ratio from e-motor to wheel is established at 11.63.
  • Therefore, the e-motor’s torque requirement can be deduced.

Figure Layout of a Battery Electric Vehicle (BEV)

Validation and Verification:

  • E-Motor Testing:
    • The e-motor is installed on a test bed, powered by an ideal electrical source, and linked to a dynamometer to regulate speed.
    • The dynamometer sets the speed, and the motor’s torque is measured. For our case, assume it met the needed torque.
  • Inverter Testing:
    • The inverter’s role is to supply current to the e-motor.
    • It’s tested under various conditions, using passive or active electrical loads.
    • Active loads, like e-motor emulators, can mimic an e-motor’s realistic behavior.
    • Successful tests confirm component-level requirement fulfillment.

Integration Process:

  • The task now is to integrate the inverter and e-motor.
  • This involves “teaching” the inverter to drive the e-motor under various conditions, such as:
    • Varying driver inputs.
    • Different driving scenarios.
    • Changing environmental conditions.
    • Interactions with other powertrain components.
  • Integration ensures consistent performance within specified tolerances, leading to a uniform user experience and compliance with standards.

Figure Example of a simple derating strategy allowing the maximum possible power e.g. during an acceleration period

Integration Testing:

  • Utilizing both measurements and simulation tools, the e-motor’s performance is characterized.
  • Simulations, if mature and validated, can sometimes replace certain testing phases.
  • The purpose: Cover all operation conditions, such as:
    • Battery voltage ranges.
    • Cooling scenarios.
    • Thermal conditions.
  • Successful tests result in an integrated system, with the inverter able to accurately drive the e-motor.

Advanced Integration:

  • Progressing up the V model, the next step involves integrating the e-drive with the gearbox and differential.
  • Modern powertrains are becoming increasingly integrated, making individual component validation challenging.
    • Example: A shared cooling circuit for the gearbox and motor, necessitating special samples for component testing.

Conclusion:

  • “Maximum Wheel Torque” is an essential requirement in BEV design, impacting various components.
  • Ensuring each component meets its requirement and then successfully integrating them is pivotal to achieving the desired vehicle performance.
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