Torque Shop: Driving an EV when the roads are slippery

Without the option to “downshift”, how can I maintain control of my electric vehicle (EV) when driving in slippery conditions?

EVs need not be driven any differently from other types of cars.

It is true that you do not have the option of downshifting gears for engine braking, but EVs have different inherent characteristics from cars with internal combustion engines (ICEs).

An ICE’s torque output rises with engine speed and begins to tail off beyond its peak. The exact variation of torque output is unique to each engine. In any case, the torque going to the driven wheels will be in the exact pattern as the engine fitted in the vehicle.

In contrast, an EV’s motor is at its peak torque output almost from the moment it starts to spin and stays at that level for the entire range of motor speed, as long as there is electrical energy supply.

Electronic traction control and stability management systems on all cars function effectively by controlling individual wheel brakes and the energy going to the wheels, whether it is fuel for the engine or electricity to the EV’s motor.

Since the braking system is essentially the same on all cars, the big difference between the stability management in EVs compared with conventional vehicles is in the modulation of torque at the wheels.

For this, the stability management relies on a number of sensors, including those for individual wheel speed, steering angle and lateral acceleration.

The electronic control unit evaluates if the vehicle’s actual motion matches the driver’s desired input and determines if a condition of impending instability exists. If so, the system will intervene by selectively braking wheels individually while simultaneously modulating engine power output.

This last corrective intervention – engine power output – is what is drastically different between EVs and ICE cars.

From the throttle pedal to torque at the wheels, an ICE has a number of variables including fuel injection, throttle opening, engine rpm and transmission status. The system needs to evaluate and control these parameters electro-mechanically.

In contrast, the EV’s stability system basically needs to control power flow which is managed fully electronically. This makes for a relatively high level of precision with a far more instantaneous response.

Just as accelerating in an EV on a wet road is precisely modulated to prevent loss of traction due to wheelspin, sudden throttle lift-off is also carefully controlled to prevent sudden retardation that could induce instability.