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.