Track testing of the Spark-Renault racing car is expected to begin this month. Former Brawn and Mercedes AMG F1 engineer Chris Vagg explains what to look out for.
“As far as we know so far, the Spark-Renault racing car will include motors made by McLaren, batteries by Williams and a transmission by Hewland. But no one will know if everything works together properly until the car hits the track.
Many electric powertrains use a single speed transmission, for efficiency and durability, but if you want to extend your speed envelope while maintaining high wheel torque at low speed then having a multi-speed transmission is a good option. The motor will have a top rotational speed which is a property of the whole power system. Although direct drive may be more efficient and may reduce the chances of a gearbox failure, it could also reduce acceleration. Using a gearbox will boost wheel torque, in exactly the same way as you see in a conventional internal combustion engine car, helping the acceleration needed for racing.
We’re expecting the car to be roughly similar to GP3 in performance terms, although perceived performance will be much more impressive as the cars will be running on short street circuits rather than enormous tracks. Top speed is determined by the motor properties, gearing, and the system voltage – which is in turn determined by the number of battery cells. The more cells there are, the higher the voltage in the system. The higher the voltage, the higher the top speed. The Spark-Renault will run at around 800V, with probably some 250 to 500 cells in the battery depending on the capacity of each.
It will be particularly interesting to see how the ‘Frankenstein’ system works together in the Dallara chassis as electric powertrains are temperature limited. You can put lots of power through an electric motor but drawing current to produce torque to turn the motor generates heat. Too much heat will damage the system. Running the powertrain near its limit, which is what you do with a racing car, will require significant cooling to keep everything working.
Think of it this way: in an internal combustion engine, you can keep pumping more and more fuel into the engine, but you’re limited by the combustion pressure and the strength of the cylinder head. Keep increasing that pressure, and eventually something will go bang.
Similarly, in an electric powertrain you can keep turning up the current to achieve more torque, but that will increasingly run the risk of exceeding your cooling capacity and burning out the motor windings. Either of these could lead to a failure of the powertrain in the space of a single lap. You’d be reduced to walking speed and that would be the end of your race.
The car will be built with control systems designed to cut power if temperature exceeds set parameters and teams will monitor temperatures constantly. These principles are already common practice in motorsports, but the new technology in Formula E may trip teams up in the first year. We may well see some failures on the test track and on the race track.
The consortium partners will be running all sorts of aero simulations to predict cooling requirements in order to size components. But until you try the systems in the real world, with real cooling systems, it will be very difficult to predict performance.
The perfect racing car fails on the finish line. It’s been precisely designed for that exact competition distance and time: any more would simply be waste. The shakedown of the Spark-Renault will show just how likely the new car is to make it to the finish line.”