EV POWERTRAIN

EV Powertrain Architecture

High Voltage Systems

DC current out of the battery converted into AC current by the inverter for the motor to use it.

• Battery stores energy (in kWh), output a direct current (DC) using internal chemical reaction

• BMS (battery monitoring system), checks the health state of the battery and report cell voltages, temperatures, state of charge.

• Inverter (or MCU: Motor Control Unit) converts DC current into AC current at the required frequency.

• The motor converts the AC current into mechanical power in kW (it is an electrical actuator).

• Transmission (single speed) reduces the speed of the MGU to wheel speed.

Battery

Battery Pack

EV batteries are made of modules. Modules are made of cells. The number of cells in series defines the battery voltage. Measuring the voltage of the battery enables to evaluate its state of charge.

Each cell has a voltage range. Fully charged it’s usually around 4.2V and the lower limit is typically 2.5V to 2.7V. You have to connect a number of cells in parallel to adjust capability without changing output voltage. In this way, you design a battery pack which meets the usable energy target as well as the output voltage target.

There is a trade off between power and energy density of batteries: F1 ERS cells have a higher power density (approximately 10-17 kW/kg) but lower energy density (approximately 90-120Wh/kg). Formula E battery have high energy density (approximately 2.2kW/kg and 232Wh/kg respectively). These differences arise from the different cell chemistries used in F1 and Formula E. 

424 battery should be a 750kW battery pack with about 600 cells that weighs less than 400kg. Nevertheless, it has yet to be precisely defined and optimized as battery technology evolves very fast. Battery development is at its early stages and further steps are required before cars like 424 can race at Le Mans 24h. Between 2007 and 2019, the F1 ERS system has seen an 81% weight reduction, a 56% efficiency increase whilst achieving 12 times the power density and twice the energy density.

Based on the different challenges our car will be facing, different solutions may be chosen. For example, for Nürburgring lap record, battery cooling system will be reduced to its minimum whereas for the 24 hours of Le Mans, switching to a hydrogen Fuell Cell is still an option to take into account. Indeed, battery charging time is not short enough to be able to compete in endurance racing for now...but we can expect to have superchargers in a few years thanks to continuous innovation in Formula E, for example.

Temperature Management is crucial

The hotter the battery, the faster it will degrade. But reduce the temperature too much and the battery loses performance (below 10°C typically).

There are two ways in which a battery can degrade:

• Cycle ageing– the charge of a battery becomes less and less the more you use it over time.

• Calendar ageing– the lithium-ion cells lose capacity even when they are not in use which is extremely difficult to model and predict.

For the Formula E battery which needs to last for two seasons, it is a factor. In addition, there is also the risk of catastrophic failures such as thermal runaway when your battery goes over approximately 80°C or 90°C. This triggers chemical reactions within the battery which are exothermic and so release a large amount of heat. You end up in this loop where the exothermic reactions generate heat which then fuels the thermal runaway even further. Often, the battery then catches fire because the electrolytes used in batteries are flammable. Cooling down the battery while charging will become more and more challenging too as we start using superchargers (2MW, over 2000A).

Battery Cooling

There are three ways to cool a battery:

• Air cooling

• Two phase cooling

• Single phase coolants (liquid cooling)

In motorsport, single phase coolants such as liquid cooling are mostly used. You can flow that liquid through a cold plate (electrically isolated from the cells) or you can use flood cooling. With flooded cooling you use a dielectric fluid, because these are non-conductive so you are taking advantage of the fact that this fluid can be in direct contact with the cells. A cold plate is normally a metallic heat sink, with a cooling fluid flowing inside it, which is attached to the bottom of the cells via a thermal adhesive. For this, you would ideally use water as it is a much more efficient cooling medium than dielectric fluid and it is less dense and therefore of lighter weight. Once the cooling fluid has flowed into the battery, extracted the heat and then exited the battery, it flows to a conventional radiator where that excess heat is rejected to ambient air.

MGU

EV powertrain does not require multiple gears transmission because an MGU can deliver constant power over a large speed range. For example, most Formula E cars are running single speed gearboxes to reduce weight and increase overall mechanical efficiency.

Efficiency of EV powertrain is mainly limited by copper losses, due to internal resistance of engine components. Increasing the system voltage enables to reduce the current (while keeping the same power) and therefore increase efficiency as copper losses are decrease. Regulations limit race cars maximum voltage to 1000V for safety reasons.

There are several types of electric machines: direct current motors, induction motors and permanent magnet motors. Below is an overview of the characteristics for each one of them.

DC Direct Current Motor

• DC supply to the stator winding as well as the rotor winding

• Commutator ring with brushes

=> Only used for small appliances, not suitable for cars.

Induction Motor

• 3 phase AC supply to the stator winding only produces a rotating magnetic field RMF

• Alternating flux is produced around the stator winding due to AC supply

• Asynchronous motor - there needs to be some slip between rotating field and rotor in order to create induced current and mechanical torque.

• In drive, rotor speed is slightly less than the rotating field speed (2% to 6% slip typically). In regen it is the opposite.

• Most common motor today but not used in cars due to efficiency.

=> No permanent magnet, no brushes, no commutator rings, no position sensor

=> Rugged and reliable design (used in some Tesla cars)

=> Not the best choice for racing cars due to lower efficiency and lower power density

Permanent Magnet Motor

• Magnet field from PM affixed on rotor

• Synchronous motor - a position sensor is measuring speed of rotor and the inverter is making sure the rotating magnetic field is staying at a fixed optimum angle to the rotor for maximum torque, therefore at same speed

=> No current in rotor - no current losses, most efficient motors

=> High power density

=> Simple rotor, low inertia

=> No magnetising current, more torque per amp

=> Relatively simple construction, brushless

=> Magnets are expensive, one country dominates market (China)

=> Magnets lose magnetism at temperatures around 150-250°C

=> Cannot be operated without a power converter

There are Brushless AC motors (sinusoidal current excitation) and Brushless DC motors (square wave current excitation). Windings must be cooled, either by water on the outside surface of the motor (water jacket) or by oil directly in contact with the windings inside the slots.

In-hub Motors: only for road cars

In-hub motors are traction motors in that they change rotary motion to linear motion. They are actually a special case because they are part of the wheel itself instead of being attached to the wheel through gearing. This limits these direct-drive motors to a size that will fit inside the wheel.

In-hub motors are not used in racing cars mainly due to the need to reduce the weight of unsprung masses to improve ride and overall car weight. Regulations do not permit the use of in-hub motors right now. This is something that could evolve in the future if this technology becomes lighter and road car relevant.