While motorsport is still predominantly powered by IC engines, there is growing interest in electrified motorsport, mirroring the transition to the use of electric vehicles (EVs) in mainstream automotive passenger vehicles. With that in mind, I talked to Stuart Cox at Hewland and Pat Troy at Ettractive about motorsport EV transmissions and the challenges of designing and producing a gearbox where the input machine is an electric motor rather than an engine.

 

Typical non-planetary EV transmission internal layout with two high-ratio spur-gear reductions (Courtesy of Hewland)

Although we have seen some pretty impressive engine speeds used in Formula One and MotoGP, EV motor shaft speeds used in passenger cars and motorsport really put them in the shade. In the regulations for Formula E, the rules are refreshingly sparse in terms of technical restrictions, among which is this: “The rotational speed of the rear MGU rotor must not exceed 100,000 rpm”.

I don’t know of anyone using an MGU at even close to that speed, but the rules are framed to recognise a general strong trend toward ever-higher motor speeds and to present no obstacle to increased motor speeds if a manufacturer wants to pursue that line of development.

In order to keep an IC race engine in its often-narrow operating range, it needs a multi-speed transmission – six, seven and eight-speed transmissions are not unusual. The total ratio between the machine providing the drive (engine) to the road wheel speed can be around 10:1. If we take the example of a typical single-seat racecar, for example those used in Formula One, part of the reduction is achieved in driving the differential, and the rest is a simple spur reduction between the input and output shaft in the transmission.

An electric motor has a very wide operating range and effectively, at maximum performance, has maximum torque available at a certain motor speed, and thereafter maximum power is available, with reducing torque. A complex multi-speed transmission is not required, although there have been examples of three and four speeds used.

There is still a proportion of the overall reduction achieved in driving the differential, but there is generally only ever a requirement for a single-speed transmission. However, with a typically greater overall level of reduction for an EV compared to an IC-engined racecar, ahead of the differential drive reduction, the greater reduction may require more than a single-stage reduction.

Each stage of speed reduction comes with various technical and financial penalties. Having more reduction stages involves more complexity, components, friction and inevitably more cost. In considering the technical aspects alone, there can be a choice between using a very high motor speed combined with a transmission with two or more stages, or a larger and heavier, lower speed motor with a simpler single-stage transmission.

Epicyclic (planetary) stages are a natural choice for single-stage reductions, with high input torques requiring such reductions that are too high for two shaft-mounted spur or helical gears. There is a degree of load sharing on the central sun gear, such that the torque is nominally shared between each of the planet gears. The whole planetary assembly is also ‘balanced’ in terms of having no external radial forces to deal with.

Ettractive’s transmission is mated to two electric motors and powers the Scalar SCR-1, an electric touring car aimed at the amateur racer (Courtesy of Ettractive)

A single-stage planetary is not only compact, it is able to handle relatively large reductions per stage compared to a simple two-shaft spur or helical ratio. Cox says 6:1 is Hewland’s general rule of thumb for a maximum stage reduction, although it has produced single-stage planetary reductions much higher than that.

The highest reduction ratio in a planetary with one locked element is achieved when the ring gear (annulus) is stationary, the sun gear provides the input and the planet carrier is the output. Higher ratios are achieved with increased ratios of ring gear teeth to sun (input) gear teeth. Larger ring gears lead to larger transmissions, and smaller numbers of teeth on the input gear soon reach a lower limit where tooth geometries become problematic.

Gear types and efficiency

Spur gears are typically used in the planetary stages and two-shaft reductions instead of helical gears, although planetaries are not always the default choice for an EV transmission. Both Cox and Troy agree on their preference for spur gears for motorsport applications.

Helical gears have a number of advantages over spur gears in terms of reduced noise and transmission of increased torque for a given gear size, but they can have disadvantages in the form of increased friction and a requirement to react axial loads at the bearings. To achieve high efficiency and a lightweight product, motorsport customers are usually quite happy to endure the increased noise levels that result from the use of spur gears. In contrast, for passenger cars, EVs have led to an increased focus on noise, vibration and harshness. In the absence of engine noise, the noise from the motor(s), inverters and transmissions can be very intrusive.

This EV transmission has a splash lubrication system optimised through the use of meshless particle-tracking CFD methods (Courtesy of Hewland)

In motorsport, the lack of noise is a concern for spectators as it rather diminishes the overall spectacle of racing, but Troy comments on the desirability for some noise in the cabin for the driver of a racecar. “The desire with our gearbox was to get as much efficiency as possible, but also to provide noise,” he says. For race drivers and spectators, the extra noise of a spur gear transmission compared to helicals can be welcome.

There is a sharp focus on transmission efficiency. EV racing is very often limited by the amount of energy available (as evidenced by short race durations and the unedifying spectacle of multiple vehicles running out of energy before the race has finished), so it is particularly important that the absolute minimum of energy is converted to heat due to friction in the transmission. A single-stage reduction can have an efficiency of 99% or greater, and the efficiency of the whole transmission (primary reduction and differential drive) can be 97% or greater.

High input speeds

At the beginning of this article, I mentioned that the present Formula E regulations allow a motor speed of 100,000 rpm.  however, those questioned for this article have not produced a motorsport or mainstream automotive transmission with an input speed approaching that. There are mainstream (albeit low-volume) high-power applications for transmissions with very high input speeds, such as aerospace auxiliary power unit (APU) gas turbines, which can have transmissions with input speeds of 75,000 rpm or more.

There are difficulties associated with the support and sealing of shafts operating at high speeds in terms of bearing and seal selection. The advent of high-speed electric motors has led to the development of new bearings in order to support loads at higher speeds.

Shaft surface speeds are a challenge for seals where the shafts are transmitting high torque at ever-increasing speeds. In order to remain within the limits of existing seal materials while accommodating higher rpm, shaft diameters must decrease. Seal materials will need to improve to allow an increase in surface speed.

Hewland produces EV transmissions designed for input speeds of up to 40,000 rpm, and Cox confirms that these are for motorsport. The design, cost and efficiency of the transmission have provided a counterbalance to high input speeds for decades, and the trend for higher shaft speeds for aerospace APUs has resulted in heavier and more complex transmissions with high demands for gear accuracy*. The same applies to electrified motorsport, but with the additional constraint of efficiency being a high priority.

Ettractive caters for lower input speeds, but high input torque means that shaft sealing remains a problem according to Troy. “The trend is to increase motor rotor speeds – 15,000 to 20,000 rpm are in regular use now,” he says. “That can pose a challenge for seal design, so we leave the realm of most off-the-shelf shaft seals at these speeds.”

While there is a general trend towards higher transmission input speeds in passenger cars and motorsport, for any application there will inevitably be an optimal combination of motor and transmission design that strikes the best balance between efficiency, mass and the volume of the motor/ transmission system.

Is a single ratio always the answer?

In the infancy of electrified motorsport, there were transmissions with multiple speeds, but the trend is very much towards single-speed transmissions being the norm, as is the general case for passenger cars. However, there is a wide range of on and off-highway automotive applications where multi-speed transmissions are required: examples include utility vehicles, goods vehicles and buses required to climb steep gradients, and mining trucks.

Compared to some applications, most motorsport transmissions are for applications where the fastest lap or race is the aim. That means a primary aim is for high vehicle speeds and therefore high motor speeds.

Transmission manufacturers are presented with a relatively narrow operating range, as defined by the ratio of maximum input speed to average input speed, which means that a single transmission ratio is often sufficient. However, the Porsche Taycan for example is equipped with a two-speed transmission, meaning there is no fundamental choice to be made between maximum acceleration from standstill and a high top speed.

The transmission and motors are shown here mounted in a cradle ready for installation into the Scalar SCR-1 (Courtesy of Ettractive)

With battery energy density and range improving all the time, there will be less requirement to limit top speeds to make batteries last. While the challenge of deciding how best to deploy limited energy in a race is interesting for a strategist, it is vexing for drivers and spectators alike.

Electric motors, like engines, do not have a single figure for efficiency, and the efficiency of an electric machine varies across its load/speed range. It is important that the transmission design recognises that, and where possible allows the motor to operate close to its maximum efficiency for the maximum amount of time or, conversely, to limit its operation in the least efficient areas of the load/speed range to the absolute minimum.

Lubrication

The matter of transmission lubrication is complex, and the optimum system would take account of pitch line speeds, gear face widths and so on, and meter the oil in exact quantities into those areas of the transmission where it is required and with measures in place to otherwise limit the interaction of gears with the lubricant. While oil is required in small quantities for lubrication and larger quantities for cooling, any excessive churning or shearing of the oil reduces transmission efficiency.

However, such precision lubrication systems are necessarily complex and require extra, electrically powered pumps that then require their own control. The advantages of such an optimal system can be outweighed by the extra mass, complexity and cost they bring. By introducing extra components and systems, there is also the risk of creating unreliability.

It might surprise readers that those at the top levels of electrified motorsport do not necessarily avoid splash-lubricated transmissions. There are many design features that can be incorporated in order to reduce the friction penalties commonly associated with splash lubrication. Cox says Hewland has made extensive use of CFD in order to help design the most effective splash-lubrication system.

Troy notes, “Our gearbox is splash-lubricated, with provisions for an external pump and cooler if needed. “We expect the differential to be the main driver of temperature rise in the box, so the cooler will help reduce coolant temperatures. In passenger car applications, there are combined motor/ gearbox forced-lubrication systems, but motorsport applications typically separate the motor cooling/lubrication from that of the gearbox.”

Summary

With an increasing number of EV race series and EVs entering open competitions, gearbox suppliers will be busy providing efficient transmissions to take the often high input speeds and convert them to road wheel speed with the minimum of friction and weight. Compared to the very complex six, seven and eight-speed transmissions used in top-level IC-engined motorsport, EV transmissions are very simple, often requiring only one ratio and two stages of reduction.

In addition to efficiency, noise is an additional factor, but there is not necessarily a requirement to attenuate noise for racing.  Seals and bearings represent continuing challenges, given the high torque and high input speeds.

* Rogers, C., “Small Auxiliary Power Unit Design Constraints”, AGARD Conference Proceedings, No 352, 1983, ISBN 9-2835-0339-2