“Having More Gears in the Drivetrain of an Electric Vehicle Can Offer Better Performance and Efficiency, As Well As Optimizing
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BEV TRANSMISSIONS e numbers “Having more gears in the drivetrain of an electric vehicle can offer better performance and efficiency, as well as optimizing the size of the electric power system” Alexander Moser, supervisor for advanced product engineering, BorgWarner 28 // September 2019 // Transmission Technology International BEV TRANSMISSIONS As electric vehicles cater for an increasingly wide spectrum of consumer needs, could multispeed transmissions become as prevalent as they are in IC engine applications? WORDS:WORDS: ALEXALEX GGRANTRANT rom the often-crude fiberglass city cars of 10 years ago, electric vehicles have evolved quickly as legislation, technology, engineering and consumer attitudes have shifted in their Above: GKN experts believe that favor. Yet, while even mainstream gasoline high-performance EV applications Fand diesel cars now offer eight or more gear ratios, will be among the first to adopt multispeed transmission solutions, their electric counterparts – now including some of as they are more likely to lack the fastest-accelerating production models ever made – mechanically limited top speeds often make do with just one. At least for now. Above left: Mercedes-Benz is the “The number of gears [for combustion engines] first car maker to combine innovative has increased in step with the demands with regard fuel cell and battery technology into a plug-in hybrid application to driving performance, comfort and efficiency,” says Alexander Moser, supervisor for advanced product engineering at BorgWarner. “A similar trend could be expected for electric vehicles. Having more gears in the drivetrain of an electric vehicle can offer better Transmission Technology International // September 2019 // 29 BEV TRANSMISSIONS performance and efficiency, as well as optimizing the size of the electric power system. This might also reduce the use of natural resources for the electric system.” BMW goes alone So far, no manufacturer has brought an electric vehicle to market with more than two gears. Tesla experimented with, and then abandoned, two gears for the original Roadster, but BMW was the first to reach production with this setup, utilizing a GKN Automotive eAxle in the i8 plug-in hybrid supercar. Dr Rainer Link, GKN Automotive’s senior vice president of engineering, expects high-performance applications to be the earliest home for multispeed units, in part because they avoid a mechanically limited top speed. “We did some market studies with our customers, 1. The I-Pace features and this showed there is a limited interest in multispeed JaguarDrive, which transmissions – just 10% of the total P4 market until 2030 delivers acceleration will have more than one speed,” he reveals. “For us, it’s a from 0-12,000rpm niche market for high-performance EVs and PHEVs where with no shift interruptions you have this extreme torque and speed requirement.” 2. GKN’s multimode hybrid Prototypes of mainstream cars back up this thinking. transmission helps offset the added battery and And despite differing takes on electric mobility, even power electronics costs of startup brands are following the established names in hybrid powertrain systems using single-speed transmissions, reserving multispeed applications for the ultra-high-performance creations. The 3. The requirements of mass-market EVs such Gumpert Aiways RG Nathalie methanol fuel cell supercar, as the Renault Zoe can for example, utilizes a twin-motor eAxle front and rear, be met by a single speed each with a 2-speed transmission. These sequentially 4. After the standardized engage their higher gear at 160km/h (99mph), avoiding an transmission requirements interruption in acceleration, and enabling the car to reach were opened up, Formula E its 300km/h (186mph) top speed. teams have opted for a range of speed gearboxes 3 “Germany is the only country that has no speed limit on the motorway, and I’m sure that will change at some point. In China people drive very smoothly, and a speed up to 150km/h would be sufficient, so it would not make sense to have a 2-speed transmission” Dr Rainer Link, senior vice president of engineering, GKN Automotive 30 // September 2019 // Transmission Technology International BEV TRANSMISSIONS LESSONS FROM MOTORSPORT Following a familiar process for IC engine drivetrains, engineering developments in electric vehicle transmissions could come to light first in the motorsport arena. Although there are few dedicated e-powertrain series at present, even the earliest racers are making very different uses of multiple gears. Formula E drivetrains, for example, were standardized during the first season, including a 5-speed sequential gearbox from Hewland Transmissions, but teams have since been able to develop their own. Most don’t reveal drivetrain 1 2 specifications, short of drawing links with roadgoing models, but differing single motor specifications have fragmented the grid. Hewland claims to still be producing 2-, 3- and 4-speed units for the series. Electric saloon car racing is taking a similar route. The Electric GT Championship had used cars based on the Tesla Model S P100D, including its single-speed, dual-motor setup. For its fourth season it’s open to other manufacturers, and teams will also be able to develop their own drivetrains – likely to be just as varied as in Formula E. BorgWarner, meanwhile, has begun providing 2-speed transmissions to OEMs, with a view to near-future commercialization of the technology. Moser says it’s an engineering challenge to develop a smooth shift given the required high ratio step and faster rotational speeds of an electric motor, adding that the Tier 1 has a wet dual clutch setup capable of doing so. “The usual expectations with regard to shift changes in EVs are that they should be smooth, quiet and imperceptible by the driver. If it is desired, however, the shift feel can be replicated,” he explains. “Since electric motors have much higher rotational speeds than internal combustion engines, the transmission should be robust and efficient enough to handle high rotational speeds and high ratio steps between two gears. Wet dual clutch transmissions can meet this challenge.” Limited benefits Of course, not all markets share the same taste for high speeds and sporty driving characteristics as Europe, and even the EU is mandating speed warning systems from 2022. Dr Jörg Gindele, senior director of engineering at Magna Powertrain, says that the company’s simulations have shown limited benefits for a second gear in mainstream vehicles. “There is a break-even at around 180km/h [112mph], where you have to consider introducing a second speed,” 4 he states. “Then the propulsion spread of the e-motor is Transmission Technology International // September 2019 // 31 BEV TRANSMISSIONS not enough to have enough torque from zero. That’s why Multispeed transmissions, today we see a lot of vehicle manufacturers limit the [top] such as this development used by Volvo Trucks, speed of pure EVs – the driving range is limited, and could prove invaluable in power consumption at these speeds is very high.” heavy-duty commercial And, Gindele adds, applications could be limited: vehicle applications “Germany is the only country that has no speed limit on the motorway, and I’m sure that will change at some LIGHTENING THE LOAD point. In China people drive very smoothly, and a speed up to 150km/h [93mph] would be sufficient, so it would The first mainstream applications of multispeed not make sense to have a 2-speed transmission.” electric transmissions might not be in passenger cars. Efficiency and cost of ownership are just two aspects Multiple ratios could enable heavy-duty vehicles, of this EV transmissions trend. Link from GKN says a particularly trucks, to balance low-speed torque for second gear adds around 10% to the weight of an eAxle trailer-towing with high-speed economy for long-haul transportation. In turn, this could mitigate some of the and makes it harder to package – an area of particular need for the heavy, high-capacity batteries that would engineering focus for the company’s forthcoming limit the payload and make electric trucks unviable. eTwinsterX 2-speed unit. “We are still integrating e-drives Solutions are fast forthcoming. Volvo Trucks will into existing platforms, and there is a given space,” he bring its own 2-speed transmission to market with explains further. “If you make it bigger and add more electric versions of the FL and FE, which begin series parts, it’s another issue meeting packaging requirements.” production in 2020. And not everyone sees a market yet anyway. Mercedes- Gindele from Magna says the business case is Benz, for example, avoids using extra gears in the EQC perhaps clearer than it is in lighter vehicles: “It is possible to introduce a more expensive technology by calibrating the front axle for efficient low- to medium- as commercial vehicle operators have a completely speed driving, augmented by the rear axle for heavier different cashback behavior in terms of driving loads. Renault, meanwhile, believes that 2-speed efficiency and driving range. With all BEVs, efficiency transmissions offer minimal efficiency gains and add is king. But it’s even more important for commercial needless complexity for most users. vehicles, as charging time is money, and it’s all about What’s more, driver preferences change quickly, driving range.” continues Gindele: “All of us have come from conventional Transmission Technology International // September 2019 // 33 BEV TRANSMISSIONS 1 1. GKN’s GTD19 tech demonstrator vehicle is based on a standard Jeep Renegade, and features a 2-speed transmission and torque vectoring powertrains with combustion engines and six to eight speeds, and we are used to the interruption of shifts during acceleration and the change of motor speed. But once you’ve sat in an EV, you quickly get used to the lack of interruption.” Evolution not revolution 2 So near-future EV engineering development is likely to evolve current single-speed units rather than a rapid move to multiple ratios, as seen in IC engine vehicles.