In parallel with the world of passenger transport, motorsport is turning to electrification in many ways, either by hybridisation or by introducing new race series that are fully electric.
It is fair to say though that there has been a mixed level of acceptance from fans and manufacturers. Hybridisation partially satisfies the manufacturers whose vehicle ranges include powertrains that are propelled partly by an IC engine and partly by an electric motor. It is not especially limited by considerations of power or race duration, and maintains the spectacle of racing.
Low-traction, airborne cars and huge power make for quite a quite a spectacle in NitroCross (Photos courtesy of NitroCross)
Battery technology is advancing rapidly, but it is often a limiting factor when it comes to racing. Lightweight cars with an excess of power are a tantalising prospect, but that often comes at the expense of races which turn out to be energy-limited.
Electric propulsion allows motorsport to go to places where IC-engine powered motorsport often cannot, but if spectators are to be denied the sound of engines, the racing needs to provide speed or at least the illusion of it. Some series have not managed that and so have failed to gain the support that would have sustained them.
One electrified rallycross series that does provide exciting viewing is Nitro RX, now renamed NitroCross. It is not beset by some of the strange racing tactics that come from excessive energy management, nor does it require any gimmickry to keep the spectator interested.
I spoke to NitroCross series technical director Robin Shute (who is also featured in the article on page 40). He is very aware of the relative benefits of each type of powertrain, but says “NitroCross suits an EV powertrain”. He adds that it was also important not to create a car that is too different from what has gone before, because many of the NitroCross drivers will have graduated from other rallycross series. “The aim is to have cars handle like conventional rallycross,” he says.
Four-motor powertrain
Olsbergs MSE produced the FC1-X NitroCross car; all cars have the same specification and share an unusual powertrain, each comprising four motors and giving a total of over 750 kW (1000 bhp) in maximum power ‘push to pass’ mode. It is permanent all-wheel drive, but features a different drive arrangement front to rear.
There are two motors at the front and two at the rear, provided by Magelec/Omni Powertrain, and are typical of the type they supply to other motorsport series. Unusually, they are an axial-flux type but run at a relatively high speed for this type of machine, at a maximum of 12,500 rpm. The rotors are typical of motorsport in using permanent magnets.
Shute notes that the front and rear motors are different, but does not say whether this is limited to mechanical differences or whether there are electromagnetic differences affecting the performance of the motors.
The power electronics (inverters) and DC-DC converters are also supplied by Magelec/Omni Powertrain. There is one inverter unit per motor, and they are based on silicon carbide (SiC) rather than the traditional silicon technology.
It is beyond the scope of this article to go into the technical reasons why SiC is replacing Si in many automotive and motorsport applications, but the ability to operate at higher voltages is a significant driver. This allows higher rates of charge and discharge. Another key reason for SiC is the higher switching frequency required by high-speed, high pole-count motors.
Cooling
Cooling electric drivetrains is always an important consideration. The motors and inverters in the FC1X are liquid-cooled, and each motor-inverter pair is on the same cooling circuit. In some applications, motors and inverters have their own pumps and use very different cooling fluids, but being able to use a common coolant in a single circuit simplifies the design as well as the car’s plumbing and packaging. The front and rear powertrains each have their own separate coolant pumps and coolers.
NitroCross is unusually exciting for a pure EV series. Short races, close racing and speed to match IC-engined cars have made the series a success
Transmission
The 12,500 rpm motor output speed clearly requires a speed reduction to turn the drive shafts at road speed. However, the rear part of the powertrain has a three-speed transmission – unusual enough for an electric motorsport application – while the front transmission is only single-speed.
The basic transmission layout is similar front to rear, and the front transmission is permanently driving at the same ratio as top gear in the rear. If we assume that there is no slip at the wheels (unlikely in such a traction-limited car), the wheel speeds are the same, and there is therefore a speed mismatch between the front and rear motors. The transmissions are supplied by Sadev.
Battery
The lithium-ion battery pack weighs around 500 kg and stores 51 kWh of energy. Its nominal voltage is stated as 800 V, but varies between 590 and 865 V depending on the state of charge. To reduce complexity and weight, the battery is only cooled during charging.
The battery is based on pouch cells and is designed for high power density rather than energy density: the energy density is 0.102 kWh/kg and the power density is 1.5 kW/kg. Comparing these to the Voxan Wattman electric motorcycle land speed record holder (0.052 kWh/kg and 3.75 kW/kg), the NitroCross battery has a higher energy density, but much lower power density.
There is a balance to be struck. Based on the Wattman battery it would be possible to provide a 200 kg battery for 750 kW output, but it would have only 10.4 kWh of capacity, which would reduce race duration by 80%.
A more relevant battery comparison is with the Ducati MotoE race bike, which has a 110 kW powertrain and a 110 kg battery storing 18 kWh of energy, giving 1 kW/kg power density and 0.164 kWh/kg. Compared to the Ducati battery, the NitroCross unit has far better power density but lower energy density.
While the powertrain can operate at 750 kW, the normal output is 450 kW. This lower rate of discharge is good for battery durability and reduces the cooling requirement, hence the lack of cooling on the car.
With 750 kW available on a push-to-pass function, the NitroCross powertrain has four axial flux motors
It is relatively common for racecars to regenerate energy at lower rates than they can deploy it. According to Shute, NitroCross can regenerate at a maximum of 100 kW, which is only 13% of the maximum deployment power.
The relatively low energy use in each race means the batteries can be quickly recharged, although liquid cooling is required during recharging.
Outlook
Although the US market has not displayed the same enthusiasm for rallycross as Europe, NitroCross seems to be breaking that cycle. It has been enthusiastically received and the 2023-24 calendar is entirely in North America, with four of the five rounds already announced in the US and another in Canada. There is a sixth, yet to be finalised, season finale.
NitroCross is an example of choosing the right application for an electric powertrain in motorsport. It is therefore more likely to succeed by providing a genuinely exciting type of motorsport.