Last year’s Santiago E-Prix was the hottest ever Formula E race, pushing the Gen2 cars to their absolute limits. We caught up with Jaguar Racing’s Mitch Evans about what happens to a Formula E car in high temperatures – and what a driver can do about it.
At over 60C (140F) in the pit lane before the race, last year’s Chilean round was blisteringly hot – in fact, it was the hottest ever Formula E race. All racecars suffer in extreme heat, already pushing to the limits of their technology in order to be competitive, heat increases stresses and can make it more likely that there’s a failure.
Electric race cars have some unique challenges – although they don’t suffer from effects like altitude as badly as combustion cars, even the most rugged batteries have upper operating temperature limits that, if exceeded, cause or can risk critical problems. Several drivers hit the limits or went beyond them last year.
This is the big one. Or at least, the factor that a lot of the other management issues stem from.
Formula E cars don’t have telemetry – the detailed sensor data about the way a car is working that many race cars transmit back to their garage in real-time, to let engineers advise the driver better. But a few crucial numbers are shared with the teams during the race, critically battery and brake temperature for safety reasons.
All the management of that temperature is on the driver’s decisions, however. Evans told us, “Most of it’s just thermally, with the battery, trying to keep it under control. It’s quite sensitive to temperature and there’s a threshold we can’t go over and then there’s a big consequence if we go over a certain temperature.
“So to manage that, especially with the amount of regen we use now, with the Gen2 car, it’s a lot of demand and energy going through the powertrain. It’s not easy because you can start conservative but you don’t know if that’s going to be enough, you don’t know how the battery’s going to cope and once you have some sort of situation where the temperature is approaching critical, it’s harder to manage than if you stop it reaching that point.
“Obviously, when it’s super hot it’s quite exponential and the whole system ramps it up so you don’t want to lose pace and it turns into an overheated chess match with other competitors.”
Regenerating energy from the rear brakes is incredibly important in Formula E. Under regen braking, the powertrain doesn’t stop, it reverses so that instead of taking energy from the battery and turning it into movement, it takes heat from the brakes and feeds energy back into the battery. It’s the same process as MGU-K in Formula 1 but without a combustion engine providing the main drive, a Formula E powertrain sends much, much more energy each way.
…if it’s a super hot race – anything sort of north of 35C ambient – you have to be really careful…
If you’ve ever tried to use your phone while it’s charging, you’ll know that putting energy back into a rechargeable battery makes it get hot, but as Mitch explained to us, “Negative 250kW is a lot. The way you manage that, if it’s a super hot race – anything sort of north of 35C ambient – you have to be really careful with how you optimise your regen strategy because you won’t be able to use the full-on 250kW at every corner. You have to be more strategic with how you use it.
“It’s very hard because you know you’re leaving energy on the table but the consequence of trying to manage the battery temp, in terms of lap time, can be really big. As we saw last year, there was a big drop off with so many cars basically down to critical battery temp and if we go over 72C then the car can just switch off so it’s quite a big thing to keep an eye on.
“You can, obviously, completely turn off the regen but you then wouldn’t be getting anything back so you’ve got to get on top of it quite early, you need to lift to try and reduce the demand on the battery early. There can be some corners where turning the regen down low is not going to have a huge effect in terms of time or energy penalty and you can let the system just breathe a bit, so there’s a lot of ways of skinning the cat.”
Turning regeneration completely off is possible in a Formula E car but only as a total last resort – without it, temperatures might actually get much worse.
Because a Formula E car’s braking relies on the use of regeneration to absorb the energy, the mechanical brakes alone aren’t very strong. And since regen actually removes heat from the brakes, the situation would rapidly get much worse.
Mitch explained it to us, “The rear brakes are actually very small because we don’t rely on the manual braking, really – we use the brake by wire and the negative torque through the rear axle to slow down the car so if you rely only on the mechanical braking then it immediately becomes a problem with brake temps.
“If you’re at that point, where you turn the regen off, then you’re in big trouble anyway. So you obviously try and predict it from an early stage, don’t clip the negative torque completely and turn it down rather than off to manage things strategically and isolate the lap depending on the track characteristics.”