Ricardo announced at November 2021’s Professional Motorsport World Expo in Cologne continuation of its relationship with DS Performance in Formula E. It will continue supplying the transmission for the team’s car through season eight (2022) and will then equip its new ‘Gen Three’ car through seasons nine (2022-23) and 10 (2023-24).
DS Performance’s bespoke Formula E transmissions are designed by Ricardo in conjunction with the Satory, France-based team, and are manufactured at Ricardo’s Midlands Technical Centre at Leamington Spa in the UK. This arrangement has already embraced seasons five (2018-19) and six (2019-20), when DS Techeetah won both the drivers’ and teams’ titles (it finished third in the season seven’s teams’ title chase).
Ricardo presented its title-winning Gen Two Formula E transmission at the 2021 PMWE
Formula E used a spec car for its inaugural 2014-15 season, the powertrain of which incorporated a sequential-change five-speed transmission. The Gen One car was raced for four seasons, with scope provided from season two onwards for teams to develop their own e-motor and inverter plus associated transmission while retaining a spec battery.
The same scope for powertrain development was continued with the Gen Two car that followed in 2018. The spec battery was increased in performance such that one car could now be used throughout a 45-minute race and with permitted maximum power output increased from 200 kW to 250 kW in qualifying.
The Gen Three will add a spec 250 kW front axle generator to boost recuperation, with the rear axle output increased to 350 kW. Again there will be a spec battery, but rear powertrain development will otherwise continue to be open to the teams. Steve Blevins, head of engineering for Ricardo’s performance products division, is understandably unable to shed light on the powertrain DS Performance plans to use for its Gen Three car. However, he is able to provide insight into the development work Ricardo has done for the team’s Gen Two car before this year.
The powertrain freedom from season two saw more powerful motors developed and the number of gears used fall to typically just two by the time the Gen One car was in its final season (2017-18). Blevins reported that, exploiting the Gen Two’s more potent battery and higher permitted power, the rotor speed range and output of the motor rose to an extent that it became feasible to go from standstill to maximum car speed (typically around 280 kph) using only a single gear ratio.
That was a function of the shape of the e-motor torque curve, which rather than humped is at its maximum from zero to a high-rpm figure, well before which the power curve has to flatten out to remain within the mandated limit. The use of a two-speed transmission means the motor can be geared to reach the permitted maximum power at a lower rpm, but that theoretical advantage is only apparent at low speed, when in reality the car might not have sufficient traction to fully exploit it.
Using only a single-speed brought considerable benefit in removing the need for a shift mechanism. There is no de-clutching of gears, no call for the complexity of a seamless shift mechanism. Indeed, there is no clutch at all, while this direct drive transmission does not even need a reverse gear since the motor can be spun in either direction.
Compared with a two-speed gearbox the single-speed unit is much lighter and easier to package, it inherently reduces transmission losses and intrinsically enhances reliability. Blevins noted that across the paddock, single-speed Gen Two cars have either a longitudinal or a transverse motor orientation.
Fig. 1 (upper) – screenshot from the SABR modelling process. Pink represents the gears, green the shafts and blue the bearings. The intermediate gear has a smaller one welded to it, unseen here, which meshes with the output gear. Parameters such as shaft deflection and bearing loads can be identified via the modelling process
Fig. 2 (lower) – A model of the Formula E case. The red areas indicate the highest vibration of the case, allowing the design to be tuned, for example via rib placement, to minimise it (blue areas).
Having the motor shaft on the same longitudinal axis as the gear shafts calls for a bevel gear arrangement within the gearbox. The loss inherent in that is the drawback of the layout, which on the other hand, thanks to the location of the motor, can prove advantageous in terms of underbody aero potential.
Another fundamental choice is between a motor running at about 15,000 rpm versus one running twice as fast. Typically, the Gen Two car’s rear axle runs at up to 2500 rpm, meaning that a 30,000 rpm drive would require a 12:1 speed reduction.
Blevins explained that any spur gear having fewer than 12 teeth is inherently subject to tooth distortion, meaning efficiency drops off quickly. It follows that if the input gear had the minimum feasible 12 teeth the output would need 144 teeth as a single-stage 12:1 reduction – and in turn the diameter of that output gear would be impractical to package in a Formula E car.
A two-stage reduction is therefore necessary using a circa 30,000 rpm motor, whereas it is feasible to use a 15,000 rpm motor in conjunction with a single-stage transmission. Blevins noted that a pair of meshing gears can be as high as 99% efficient but that is a practical limit, so adding a second stage will cost at least a further 1% of efficiency. On the other hand, the 30,000 rpm motor is inherently more efficient, and as an overall package DS Performance found that the more efficient solution.
Blevins reported that the DS Performance Gen Two car has been developed around a transversally mounted, circa 30,000 rpm motor and a direct drive, two-stage gearbox with transverse shaft axes. Consequently the layout is fundamentally simple [Fig. 1] and it fits within a notably slim transmission case [Fig. 2].
The gearbox feeds advanced composite driveshafts via a limited-slip differential with adjustable preload that is fitted inside the output gear, hence it is housed within the case. The case does not take chassis loads; it sits within a rear structural element designed to take chassis loads. For season five (2018-19), as displayed by Ricardo in Cologne, the case was machined from billet aluminium.
The case carries what Blevins described as “a range of bearings, suited to the speed and load that they see within the gearbox. Some of them are of the ‘conventional’ steel rolling element type, but we are using some ceramic rolling element types where shaft speeds are higher.”
He noted that Ricardo uses “a range of aerospace specification steels” for its gears, “which are carburised and hardened”. He adds, “We use a few surface processes on all our geared products. Formula E uses some of the most extreme types of these to reduce gearbox losses to a minimum.”
We put it to Blevins: in theory, through coating, surface treatment and/or metallurgy, might it be possible to run the gears without any lubricant? “We use a number of techniques to reduce any lubrication-related losses to a minimum. But at the moment we still need to rely on the oil system to cool the gearbox – although as losses are so low, oil heating is greatly reduced compared with more conventional gearbox designs,” he said.
Blevins reported that the gearbox has an external, electrically driven oil pump. “This evacuates oil from the gearbox sump, pushing it through an external cooler before reintroducing it back into the gear case. We feed oil around the case through some internal drillings and oil jets.”
In respect of bearing oiling, Blevins said, “We feed a small amount of oil to the bearings for lubrication and cooling. This is controlled throughout the gearbox via oil restrictors to ensure no bearing is ‘overfed’ with oil, which would decrease overall gearbox efficiency.”
In seeking to maximise efficiency, Ricardo’s design and development process was assisted by the use of its own well-proven and sophisticated computer-based modelling. Blevins reported that SABR – Ricardo’s shaft and bearing software – and its accompanying GEAR software allows all or part of a transmission system to be modelled at a level of detail appropriate to the current design phase.
Fig. 3 – Depending on the speed of operation, the efficiency can be predicted by Ricardo’s modelling – here blue is 90% efficient and deep red is 98%. Efficiency is lost at low torque, as the losses attributable to bearings and so on are proportionally greater
It also allows sensitivity studies to be carried out to determine the effect of different geometric features, bearing types and gear positions. The Ricardo software further allows the optimisation of gear geometry, right down to tooth contact analysis that allows the micro-geometry to be refined in order to minimise contact stress and keep the contact centred in the middle of the tooth.
In the case of the DS Performance transmission, the lightweight spur gears produce high excitation across the revs range and have multiple harmonics when compared with helical gears, Blevins said. He explained that the software allows a WOT speed sweep to be run to see where any casing resonances are excited by the meshing frequencies. This dynamic model can be combined with an FE model of the casing to ensure it is as stiff as possible, minimising the loss implicit in case flexing.
Thus shaft misalignment can be avoided, as well as shaft deflection, factors that have a negative impact on gearbox efficiency. Bearing loads can also be investigated, together with how overall efficiency is influenced by oil seals, the oil itself and how it is deployed, and how the oil distribution affects churning losses and component running temperatures.
Blevins said that a “loss map” and an “efficiency map” across the torque and speed range of the motor generated by the software [Fig. 3] have correlated well with testing. That testing uses Ricardo’s sophisticated in-house transmission rigs, and the upshot has been the very impressive performance on track.