In issue 135 (November 2021), we briefly covered Porsche’s prototype electric racecar, the Mission R project. Now Porsche has shown us what might be the first racing application of the technology developed as part of the project, the 718 Cayman GT4 ePerformance.
The car made its motorsport competition debut at the Goodwood Festival of Speed in June. The event is becoming increasingly important – not only is it a great place to see the best racecars of the past 100-plus years in action, it is becoming an important event at which to debut new cars and technology. Speaking at Goodwood, Porsche factory driver Richard Leitz described the car as “scarily fast”.
It is quite likely that we will soon see an electric Porsche racing series. The 718 Cayman GT4 ePerformance features technology developed in the Mission R concept car (Images courtesy of Porsche)
The 718 Cayman GT4 ePerformance is, as the name suggests, an EV racer based on the 718 Cayman, but with a number of striking differences. Much of the powertrain is taken straight from the Mission R concept car; there are what Porsche calls permanently excited synchronous machines (PESMs) powering front and rear axles.
PESMs are what we often refer to as ‘permanent magnet motors’ and are very often used for the most power-dense and lightweight motors. Small volume and low mass are prized in motorsport, so most electric machines designed for motorsport – and many high-performance road vehicles too – are of this type.
The powertrain is capable of delivering a peak output of 800 kW (1072 bhp) but the aim of this vehicle is to be a real racecar, and simulations show that with 450 kW available, race durations can be around 30 minutes with the 82 kWh battery, which provides sensible race distances on real-world racetracks.
The car looks like a production racer ready for delivery to a customer, and that is clearly the aim. Comments made by Porsche staff point towards it being introduced as a customer car, probably to race in one of the many successful one-make race series supported and supplied by Porsche around the world.
I asked Scholz if we would see this car racing in future, and he said, “The GT4 ePerformance is a well-tested prototype. It will not be built in higher numbers, and it will only be used for showcasing Porsche’s vision/approach in electric GT customer racing.
“We call this the GT4 ePerformance tour (2022-24) and yes, maybe we’ll see this car in a one-make series race running together with IC-engined cars during this demo tour. During the tour we want to get feedback from customers, teams and partners to then know exactly what to do as the next step, because in the end we want to design an electric customer racecar and expand our racecar portfolio with an electric, entry-level racecar, like the GT4 Clubsport but in an electric way.
“The specifications of that car are very much connected to the use case. That could be track-days, one-make series or a decathlon-like mixture of different formats, just as our GT4 ePerformance Tour, which began at Goodwood with a hillclimb.”
The Mission R project used motors developed from the Porsche Taycan passenger car, and that remains the case for the 718 Cayman GT4 ePerformance, although the internals of the motor are completely different to the passenger car. The motors were developed in Zuffenhausen, Germany, where Porsche has a dedicated plant for the rapid development of electric machines.
I asked Matthias Scholz, director of GT racecars at Porsche, whether the motor windings are closer to the hairpin type of the Taycan roadcar, or whether they might resemble the ‘thin-wire and many turns’ strategy of the hybrid motors in the 918 Spyder.
He said, “It’s definitely more comparable to the Taycan layout, but the power density [power-to-weight ratio] of the motors is four times higher and it will never run into thermal derating. So, there’s no ‘peak power’ for 10x accelerating; only constant power until the battery is empty. Same for the batteries and inverters – no thermal derating.”
Most electric machines and major powertrain components will have a continuous operating performance and a much higher short-term performance. Porsche clearly has enough performance in its powertrain such that the continuous performance is sufficient to make this car very fast.
The powertrain features direct oil cooling of the electric motors. If the windings get too hot, their insulating layers can become degraded, leading to internal short-circuits that in turn lead to loss of performance and total motor failure. Scholz says the heat is removed not only from the windings, but also from the periphery of the stator back-iron. Porsche also flows coolant through the rotor to remove heat from the permanent magnets.
This type of attention to cooling detail allows Porsche to run the motor at high levels of output continuously. Scholz explains that the coolant “is a specific dielectric fluid for this application. The development is based on a technical partnership with Exxon Mobil.”
The battery again takes advantage of direct oil cooling to remove heat from the cells. The challenge of removing heat from cells comes from their low temperature limit: to reject heat to atmosphere, the coolant temperature must be between cell temperature and ambient. The temperature differentials (between cell/coolant and coolant/atmosphere) are therefore modest, and heat transfer is therefore limited. The focus is always on reducing the thermal resistance between the heat source and the coolant. Cooling battery cells in high-duty motorsport applications always takes careful engineering.
The 718 Cayman GT4 ePerformance achieved the second-fastest time at the 2022 Goodwood Festival of Speed in the hands of factory driver Richard Lietz
The cooling system is a single circuit and carries around 30 litres of cooling oil. Although the failure of a motor is quite a serious event, the failure of a lithium-ion battery through overheating is far more dangerous. The cells have a very strict operating temperature ceiling, beyond which they can go into an unrecoverable thermal runaway, resulting in a very exothermic battery failure.
By cooling the battery cells directly and monitoring cell temperatures, the battery can be run at the peak of its performance. Lack of sufficient cooling or knowledge of cell temperatures means extra safety factors need to be applied to prevent breaching a thermal limit, and that means always having to run the battery below its real performance potential. This is also a feature of the Electroflight battery (RET 141, August/September 2022).
Carried over from the Mission R project is the use of the same high-voltage (900 V) technology. The front and rear batteries are used in parallel, with each being rated at 908 V. Each battery has 216 pouch cells connected in series; from that we can infer that Porsche rates the cells at a little in excess of 4.2 V, which is a common voltage for lithium-ion cells when fully charged. The cells are NMC 622 types from Microvast.
The process of configuring a battery of given capacity to be of a different voltage is ‘simply’ a matter of calculating the amount of cells required to act in parallel and series, but the real technology behind the increase in voltage lies in the inverter, which converts battery DC to the carefully controlled AC needed by the propulsion motors (and vice versa when the motor is used for regenerative braking).
Passenger car applications are moving from 400 V to 800 V in order to take advantage of the well-documented benefits of much faster charging and greater efficiency. In the case of the 718 Cayman GT4 ePerformance, this allows battery state of charge to be increased from 5% to 80% in around 15 minutes using a 350 kW charger.
High voltage means lower current, and that can be taken advantage of in two ways – maintaining conductor areas, which produces lower ohmic losses (and therefore greater efficiency), or maintaining the same level of losses and using smaller (and thus lighter) conductors. In terms of the design of the propulsion motors, this can be significant with more compact windings, smaller stators and therefore smaller overall package sizes.
The car is a significant 140 mm wider than the 718 Cayman GT4 Clubsport, which is powered by an IC engine. Porsche has taken the concept of environmentally friendly GT racing a step further by making this wider body from natural fibre composites.
The electric powertrain has also offered Porsche aerodynamics experts new possibilities. With no rear engine to contend with, underbody aero is less compromised and a much more effective rear diffuser has been designed.
This highly developed concept racer now gives us a clear view of what Porsche’s first EV racecar will look like, along with its technology and its capabilities. It is also priming Porsche’s customers, showing them that battery EV racers and roadcars can be extremely fast and remain true sportscars.