FE: Season4 Formula E Powertrain Analysis
As Hong Kong hosted the first two rounds of the fourth ABB FIA formula E season, this season will prove to be a seminal point for the fledgling championship. This will be the last season with the current Spark SRT01e chassis, the Williams Advanced Engineering (WAE) battery and with it, the mid race car swaps.
Equally this will be perhaps the last season where the original independent teams that formed the grid back in 2014s first season will continue to race. As the major automotive manufacturers step up to race and promote the electric formula. Yet this shaping up to be the most competitive season yet, with all the teams all converging on similar design ideas and performance.
Season4 Technical Regulations
Technical rule changes for this year are sparse, the race power limit has been raised by 10kw to 180kw, that’s a 13hp rise, to some 241hp. While qualifying remains at 200kw (270hp) and plans to increase the regenerative braking level have been held back, keeping the 150kw per lap limit.
To aid this hike in race power, the WAE batteries have had refresh of their internals. But otherwise there’s no other technical rule changes for this season. Allowing teams to focus on development on the all new Season5 car.
NEW POWERTRAINS: TEAM BY TEAM
Very much the leader on the technology front in FE, Renault already have the definitive powertrain layout, being a single radial flux motor, running a single speed to the differential. It can be argued that the price Renault have paid over the past two seasons is a less adaptable set up. Without a choice of a higher gear, the motor is forced to run over wider RPM range, which risks it having to run at a less electrically-efficient speed and compromising top speed. This has left them struggling with qualifying power and on longer straights (such as Mexico). But it’s clear the compromise in the single-motor\single-gear, pays off over a season, on most circuits and especially in races. Now most other teams are following Renault’s layout, as they develop higher torque motors that are electrically efficient over a wide enough range to support the ‘gearless’ set up.
Renault's transverse motor during directly through gears to the differential, everything is mounted close to the ground for a low CofG
If other teams are running the same powertrain set up, this should level the playing field and Renault should not see the same compromises in performance relative to the competition. What’s more as they do not have to re-invent their powertrain, they can spend their development time in other areas, to boost their performance.
Both the motor and differential bolt to the heavily machined aluminium side plate, that in turns bolts into the side of the carbon rear case
Thus, Renault has looked at the finer details, the packaging of the powertrain is impressive, already lightweight and requiring a tiny amount of cooling, the new car still mounts the inverter inside the carbon case, sat down low behind the (what is expected to still be a Zytek) motor. Although all the other teams that have followed Renault with a carbon case and transverse motor, Renault remain unique, in that the motor/differential bolt in through the side of the carbon case on a common aluminium side plate. While this is a neat set up, it could be argued that the large opening in the side of the carbon case reduces rigidity compared a fully enclosed case. It may be this is a lighter set up overall, as well as rapid access being somewhat easier as the entire rear end doesn’t need to be removed.
One packaging gain Renault have always enjoyed is a low centre of gravity of the rear end. Even when they ran the two-speed gearbox in Season2, the entire powertrain could not only sit inside the rear case, but all the components sat at floor level. So, the inverter lies flat behind the motor, the motor is rotated around the differential to also sit at floor level.
The silver coloured ballast is evident when looking through the back of the carbon case.
The weight saving of the carbon case and lack of gearbox, allows the entire rear end to be well under the 120kg required to ensure the entire car meets the minimum weight limit for the series. This frees up Renault to add ballast to meet the weight limit. With the rear impact structure removed, the open end of the carbon case shows that there is a significant amount of ballast low down in the back of the case. This will further lower the centre of gravity height at the rear of the car.
One odd piece of packaging is the placement of the McLaren ECU on the side of the radiator duct, while technically this is no problem, but operationally it leaves it vulnerable from incidents, if you were a rival driver, you should choose to make any risky overtakes up the left side of the Renault, there being a greater chance of any contact causing a retirement from damage to the ECU!
The ECU is precariously mounted to the side of the radiator duct.
A mystery is the large electric control unit under the left-hand radiator duct. Initially I thought this might be the inverter, but this is still within the carbon case as in previous seasons. It has what appears to be a DC power outlet, but the unit appears too large to be the DCDC converter, but as its not the ECU or Inverter this leaves little else the unit could be, but the size is confusing compared to other team’s units.
In Season3 Renault appeared struggled with handling through chicanes and over kerbs, suggesting the rear suspension wasn’t coping with the large wheel displacements and not recovering sufficiently for traction out of the corner. Rivals noted the rear suspension worked much better in Season4, with the car having great traction in all circumstances. Part of this was down to clever geometry with the rear wishbones and track rod. This is another area where other teams are looking to make gains this year. Again, this year the car runs Sachs dampers and Eibach springs.
Over the past three seasons Audi with its team Abt have trailed behind other teams in regard to its power train layout. With the Audi supported, but Schaeffler developed motor, the car still ran three gears last season. Although the car’s performance was far more adaptable as a result, the gears giving good electrical efficiency and a range of speeds with which to run the car in both race/qualifying power modes and especially useful on longer straights. The offset was the weight and mechanical inefficiency the gear train incurred. Over the course of a season this penalty offset Renault’s performance advantage.
For Season4 Audi have stepped up and now headline the team, the WEC LMP1 operation being closed and some (but not all) of the engineers, switched to the FE project. This all being based from the Audi Motorsport headquarters in Neuberg. There’s still the Schaeffler connection, but the new Motor mates the Season2-3 Schaeffler tech with Audi’s LMP1 hybrid knowhow. Thus, the Season4 motor is all new, bearing no direct relationship to its parent motors. Mentioned by Di Grassi at the car’s launch there is no gearbox, merely a reduction gear to the differential, so from this was can assume the motor is a high torque device.
It’s also clear the motor is transverse mounted and sits with the inverter within a rear carbon casing. Talking to Audi engineers the Inverter semi-conductor technology remains a secret, having not seen anything of the powertrain, it’s hard to make a judgement, but the cooling radiator is of reasonable size, so Audi may not yet have jumped to Silicon Carbide for the switching.
As with their LMP1 carbon gearcase, the FE casing is carbon fibre laid around the outside of a male mould for strength
The carbon casing itself is interesting, as it follows some of the team’s LMP1 practices. Usually a carbon fibre moulding the size of the rear case is made by laying the carbon fibre inside a multipart female mould (a mould that is hollow) and the accurate finished-surface is on the outside of the casing. In contrast the Audi case is made with a male mould (a mould that is convex) and the accurately finished surface is on the inside. This is evident as the external surface of the Audi carbon case has an unfinished surface, where the breather and release films have left their mark, creating a rough surface, rather than the neat flat shiny A1 finish on female moulded parts. The advantage for Audi with this approach to the moulding process, could be in several areas, firstly as the inner surface of the of the case exactly matches the mould, the dimensionally accuracy is much better than the outside, this means any parts mounted to inside of the case, such as the motor and transmission can be positioned much more accurately. Another benefit is the male mould allows the composite laminators to lay up the carbon fibre layers continuously, so rather than make up the case in several parts and bond them together, or struggle to lay up the carbon deep inside a female mould. Making parts easily on the outer surface of the mould, in one monolithic piece, makes for a much more structurally efficient part, thus can be stiffer for a given weight.
Note the filament wound carbon fibre around the driveshaft
Along with the use of carbon fibre on the rear casing, the driveshafts also have a carbon fibre filaments wound around the metal driveshaft. This is allowed as the rules define the envelope that teams can develop within, extends laterally to include the driveshafts, as far the outer CV joint. Audi will gain torsional stiffness with this carbon wrapped solution. Some of the other teams employ this solution, although some engineers will prefer a driveshaft that winds up a little under load, to prevent sudden spikes of torque to the wheels causing a loss of traction.
Audi's angled the axis of the rear wishbone mounts to provide a an effect to counter power oversteer
As well as stepping up the motor and casing design, the rear suspension has also been the subject of close attention too, in both its geometry ad damper layout. Firstly the rear suspension kinematics are very different to other teams, but perhaps aiming to achieve what Renault and DS Virgin were already exploiting last season. In plan view the rear wishbones do not mount to the gearcase parallel to the car’s centerline, as most teams do. Rather they are angled to the centerline, with the front legs offset and the rear legs nearer the centerline. In turns this will tend to steer the rear of the car, this helps overcome the power oversteer that the high torque motors naturally create on corner exit. With the compliant elements (dampers and springs) are reversed to the normal layout, so that the rocker is ahead of the coil over dampers, the anti-roll then ahead of the rockers mounted between the battery and gearcase. While this set up may have some subtle compliant effect, it’s more likely is simply puts the weight further forwards along the wheelbase.
Not ones to miss an opportunity, Audi have been running in the Sauber wind tunnel, by the same aerodynamicists that ran their LMP1 programme. While there isn’t much to be changed on the car to improve aero, every little helps and runs will have considered radiator duct design, wheels and mapping aero performance over a range of front and rear ride heights, roll, yaw etc. While not new for Season4, Audi run flanged wheels, that close off as much of the large 18” wheels as possible, while meeting the regulations. This will help reduce drag on the straights, for better energy efficiency.
DS VIRGIN RACING
After Season2 saw the team race with an unconventional and heavy powertrain running twin axial flux motors, Season3 saw a more conventional powertrain developed with Citroen\DS. So, there was a single motor and it returned to radial flux layout, driving through a two-speed gearbox with a pneumatic shifting mechanism. This year the layout has not changed much, there’s still a single motor, that now appears to be a single speed transmission, although some reports say its still a multispeed gearbox, but I doubt this from hearing the unit on track.
DSVR's powertrain is long and needs the inverter mounted over the motor and gearbox. The DCDC converter provides 12V power to the car's other low voltage systems
While it looks like a single speed set up, the gearing from motor to differential appears to be a stepped gear arrangement. So rather than the drive from the motor output being geared directly to differential ring gear, there’s is an intermediate gear. This is easier to package, but conflicts powertrain length, weight distribution and mechanical losses with the direct geared set up. The new DSVR powertrain is a neater package this year, the inner gear casing houses the differential, reduction gears, then the motor and inverter bolt to the case, before sliding into the carbon outer case. The inverter sits over the motor and the DCDC converter sits in the small space left beneath the motor. As with Season2 the powertrain was heavy in Season3, so the development in the new drivetrain should have helped bring weight and distribution down to a competitive level. But compared to the Renault set up, the hardware is mounted high up, this is probably due to the gearing package, forcing the high inverter placement. It’s clear that the car still exhibits a lot of oversteer, this may be the result of the CofG height rather overall weight.
Also tidying the package, the cooling set up uses a heat exchanger to use the water cooling circuit, to cool another. reducing the pipe work exiting the carbon case. DSVR run two quite small radiators in the left-hand sidepod, albeit not the diminutive size of the Renault package.
Curiously DSVR do not run the McLaren ECU, rather using a Marelli unit, there aren’t any obvious pro’s or con’s to doing this, most likely their experience and commercial deals with Magnetti Marelli being the factor.
One of the teams to be understood to be running an advantageous rear suspension geometry, the wishbone and rocker arrange appears to be conventional. Although the team do run Ohlin’s dampers, with Eibach springs.
After Season2 saw a privately developed Season1 powertrain (McLaren Motor/Inverter and Hewland gearbox), Season3 brought a bespoke powertrain, developed in house with Magneti Marelli motors and inverter, mated to an Xtrac two-speed gearbox. In this guise the team claimed its first pole position and race win. With this success, the focus in Season4 is to optimise the package, with new Marelli hardware and a weight reduction programme. The new Marelli motor is of the high torque type and thus needs no multi ratio gearbox, while the inverters adopt Silicone Carbide MOSFETs over Silicon IGBTs. This along with a partially painted, rather fully vinyl wrapped livery has saved some 10kg from the car.
Marelli's six phase electric motor
A new step for Marelli with its six-phase motor, which is effectively two motors running in the same case, is the inverter package. Last season there were two physically separate inverters synchronized to work in unison to power the motor. Now the inverters are packaged within one casing, saving some weight and consolidating the synchronization connections.
ECU and PDM mounted below the radiator duc, tyre temprature/pressure (TPMS) controller mounted on the side
Not only weight, has been saved by the new Marelli tech, but also cooling which is reduced, the smaller radiator duct allowing the McLaren ECU and Motec PDU to be fitted underneath. Unusually the DCDC converter sits on top of the battery, which seems an odd location given there being space lower down and nearer the main low voltage hardware (ECU, PDU, coolant pumps).
Cooling duct to feed air to the motors electrical connections
A sign of the tight packaging is the need to feed cooling air through a corrugated pipe into a 3D printed duct bonded to the carbon casing. This appears to be in the area where the power cables would connect to the motor, with the current running through the cables they are liable to get hot, so this set up helps manage temperatures.
The new gearbox does away with the multi ratio gearbox, the size of the casing around the differential suggests that the reduction gearing is largely done with the large final drive gear, rather than a stepped reduction.
Mahindra's neat carbon case, which makes extensive use of aluminium inserts for the suspension mounts
Externally the carbon case follows that of Season3, a well finished unit with aluminum hardpoints for the rocker and damper mounts. A neat touch is Mahindra followed Renault, by having a fixed connector on the gearcase for the battery DC connection, rather than a fly lead, as is used on most other cars.
Like Mahindra, Venturi ran a developed Season1 package in Season2, then switched to a bespoke design for Season3. A transverse McLaren motor, driving through a multi speed gearbox to the differential, all bolted up in a neat aluminum housing, sitting within a carbon rear case. Technology advanced with Rohm supplying Silicon carbide for the Inverter diodes, reducing the inverter size and cooling requirement, ahead of a full switch to Silicone Carbide MOSFETs for Season4. But the inhouse gearbox was a weak point and led to issue sin the early part of the season.
Rohm's image shows just how much the Inverter has been downsized by use of Silicon Carbide
For Season4 partner ZF have designed a new gearbox, still with two speeds, as the team feel that’s the best set up to suit the torque and electrical efficiency of the McLaren motor. While Rohm have delivered on the Silicon carbide upgrade to the McLaren inverter which is smaller still. From the Season1 McLaren Inverter, weight has been reduced from 15.8kg to 13kg with a 19% smaller unit for season3, then with the full SiC set up this year weight has reduced to 9kg with a huge 30% decrease in volume. This makes for a tiny unit compared to most other inverters in the formula, now some 900mm3 in volume compared to 1600mm3 in season one.
No sign of the cleverly low mounted inboard suspension, which allow a clear view of the large carbon case
Along with this progression the carbon rear case and rear suspension have been worked on too. At first the casing appears to be oversized, especially around the differential, but the team say this is improve its stiffness. The new shape and reduction in openings helping to make the rear end structurally more efficient.
Even with a high mounted pushrod rocker, the coilover damper is mounted below, to lower the CofG
A clever piece of work is the rear suspension rocker arrangement. The rules demand the car follows the Season1 layout with pushrod operated rockers working on coil over dampers. Venturi have maintained this layout with a twist, as the rocker is still pushrod operated, but has the coil over damper slung below it. This lowers the centre of gravity a little aiding handling. To allow this low damper position, the anti roll bar has been repositioned and sits over the top of the middle of the case and arms reach down to meet the rockers. Like many teams Sachs dampers and Eibach springs are used.
Even in Season2 NextEV were unconventional with its powertrain, albeit the twin axial flux motors in their spaceframe subframe were overweight and energy hungry. Most of the hardware was bought in through partners, Yasa for Motors, TM4 for Inverter and Campos made the tubular subframe.
Season3 was an improvement, there still being twin pancake motors, again running without multi ratio gears, but in a neater in-house package, Omnigear created a stepped reduction gear train to the differential, Magelec creating their own motors and inverter. The car was fast but struggled with energy consumption, slowing the car in the race.
Nio's season4 layout is a low mounted row of; Inverter, Twin Motors and Differential
Now the powertrain is again in house, designed in the UK and Italy by the NIO NextEV team. While there’s no official word on the supply and specification of the powertrain, paddock rumour has filled the gaps. Again, there’s the Magelec twin Axial flux motors, driving the differential ring gear directly without an intermediate reduction gear, this is a shorter and mechanically more efficient set up. Employing Silicon Carbide MOSFETs, the units are much smaller further aiding the packaging of the rear end, so the inverters now sit inside the carbon casing for the first time, mounted low down on the floor of the case.
Separate radiators for inverter and motors
The semi-conductor switch to SiC and with the inherently better cooling for the pancake motor, means the two radiators are much smaller this year, with them both being angled within the radiator duct to slow the air through the cores for a better heat exchanger effect.
Jaguar racing arrived into FE two seasons later than the rest of the field, they also signed very late to join Season3, so the in the rush to meet the crash test and homologation deadlines, there was obvious compromise in what could be achieved with the powertrain in the time available, Williams Advanced Engineering’s support notwithstanding. To their credit the rear end package acquitted itself well through Season3. It largely followed the Season1 layout of longitudinal motor, driving a multispeed gearbox via a bevel gear set, then having the inverter sitting atop the battery.
With a proper lead time, Jaguar have tackled the Season4 powertrain with a fresh approach, so there’s now a current generation powertrain in the back of the car.
The team’s management were reticent to talk about any of the technology, layout or materials of the new powertrain. This further hampered by the team being very sensitive with photography around the car during the Valencia test, which is unusual in Formula E, as it prides itself on being an open category.
Detail of the carbon case, the pushrod rocker is mounted on a small carbon tower.
With close observation at the test and at the first ePrix double header, we can make some assumptions on the Jaguar technology. It’s clear the team have gone conventional with the powertrain, with a transverse motor packaged with its inverter within carbon case. I haven’t seen any evidence of gearshifts, but reports have suggested there’s a multi-speed gearbox on the car. From the size of the carbon casing around the differential its apparent there isn’t a huge ring gear, so there could be a multi speed or stepped reduction gearbox between motor and differential. In fact, the carbon case appears to be very neat, as one would expect with Williams Advanced Engineering as a technical partner. The casing keeps a low profile, such that the mounting for the push rod rockers are raised on small carbon fibre towers, this helps keep the pushrod at a steep angle, which is good for the motion ratio of the rocker and a stiff installation. Similar carbon extensions support the upper wishbone to offset it from the lower wishbone.
Jaguar run a carbon rear casing for Season4, the upper wishbone mount is offset by an carbon extension projecting from the case.
The road to Season4 has been tough for Andretti, the adventurous Season2 powertrain project failed before the homologation deadline, leading the team to run the same powertrain from Season1 into Season2. For Season3 there was the arrival of Magnetti Marelli hardware, the so called six-phase motor and inverter package being incorporated into the Season1/Season2 layout, albeit with a carbon bellhousing around the motor.
The aluminium gearbox case and the carbon bellhousing remain, but the Marelli motor and Inverter are new.
With the advent of a full BMW powertrain in Season5, the development to the Season4 set up has been held back. There is the new specification Magnetti Marelli motor and combined SiC inverters, but these sit within the Season3 casings, in the Season1 layout (longitudinal motor, bevel gear, multi speed gearbox, aluminium casing, inverter above the battery). So, while there’s still a three-speed gearbox, but this will superfluous with the high torque motor, so no gears shifts should be required. But weight and mechanical efficiency is penalized by the retention of the gearbox, bevel gear and aluminium case.
Dragon run a modified version of the Mahindra powertrain. The only visible different I have seen is different suspension mounting clevices for the rear wishbones.
2017-18 sees another season for Techeetah as a powertrain customer, again taking the complete Renault rear end package. This is clearly a cheap way of running a cutting edge FE powertrain, certainly comparable in costs to running customer cars in similar series