In some cases multi-speed gearboxes must be used; for example for extreme performance (acceleration/top-speed), heavy load/steep grades trucking, off-road high load/grade applications.
SOME BENEFITS OF MULTI-SPEED GEARBOXES
– keeps e-motor(s) in higher efficiency range improving overall efficiency of EV powertrain to deliver more range, or alternatively same range with a smaller battery pack, hence cost and weight reduction of the powertrain.
– lower torque motor can be used which results in cost benefits and an overall reduction in the weight of the powertrain or alternatively, with same motor, it can deliver better performance at higher level of efficiency.
– offer better launch acceleration, lower noise and a higher cruising speed, while the hill climbing ability of utility vehicles can be improved.
– simpler, lighter and cheaper power electronics, and because the motor can be kept within its optimum speed range there’s less wear.
– for both high acceleration requirements and extreme top speed requirements, to avoid extremely wide RPM e-motors/controllers, multi-speed is required.
– for extreme grade, terrain and carrying load requirements, multi-speed is required or the motor/controller sizing will not be cost-effective.
HOWEVER, if you are fortunate enough to be able to design your own optimized motors/controllers for specific mission “low duty” usage profiles, as Tesla does, single-speed gearboxes do come out ahead – however in some circles, the debate rages on whether multi-speed gearboxes can be justified to benefit these non-heavy duty applications. Another approach is rather than use multi-speed gearboxes, use front and rear motors with different single-speed ratios, than the control system directs more power to the motor with the better gear ratio for the specific situation.
SOME CASE STUDIES PROVE OUT THESE BENEFITS:
1 – DCT Transmission specialist Vocis’s simulations show that their 2-4SED two-speed configuration could reduce the battery energy consumed by up to 15% (4SED tests reached 15%) over the industry-standard New European Drive Cycle, without any significant increase in overall cost or packaging volume, as well as extending battery life and providing the potential to substantially downsize the powertrain and battery pack.
The 4SED system resembles a dual-clutch transmission configuration, but with two motors instead of twin clutches. There are two input shafts, each driven by its own electric motor. The two motors replace the usual clutches and synchronisers, allowing the next gear to be preselected before the previous one has been disengaged, and can synchronise shaft speeds. Fewer software algorithms needed compared to a normal automated gearbox, and because there are no hydraulics, there is no need for valve control.
2 – Zeroshift’s concept prevents any torque interruptions during ratio changes and does not require a clutch, an issue that may have deterred EV manufacturers from fitting multi-speed gearboxes. Zeroshift studies suggest that by using a compact, multi-speed transmission and a smaller electric motor, manufacturers can gain an operating efficiency of up to 10% or more. You can use that 10% to improve EVs’ range or reduce the size, weight and cost of battery packs. The benefits of Zeroshift’s transmission would be greatest for electric delivery vehicles where the compromises in efficiency are greatest. To be able to pull away fully laden on hills requires low gearing, but the motor then runs too fast and too inefficiently on the highway. Also Zeroshift is less expensive to manufacture than a continuously variable transmission (CVT) or a dual clutch transmission (DCT), says Bill Martin, Zeroshift’s managing director.
There are several motorcycle seamless shift systems available, but the Moto GP teams are keeping tight-lipped about whose technology they are using. Two British companies, Xtrac and Zeroshift, produce the technology, but neither is willing to confirm their involvement with a particular team. Zeroshift though was willing to explain the workings of its system.
3 – Wrightspeed powertrains feature a four-speed clutchless gearbox/e-motor 250hp each per rear wheel. The powertrain is capable of powering vehicles weighing up to 66,000 pounds up grades as steep as 40% plus handle highway speeds, so the Wrightspeed powertrain provides the refuse market with the power-to-weight ratio needed for collection duty cycles. Single speed gearboxes, even with custom motor/controllers, would simply not work to cover the worst-case requirements for this application.
4 – Not clutchless but multi-speed – This case study also backs up the efficiency improvements. The Antonov 3-speed transmission system grew out of the Jaguar Limo-Green project in 2009. Antonov was initially asked to provide a fixed ratio transmission for this range extended series hybrid, but it was decided to look at additional gears with the aim of improving off-the-line performance and top speed. It worked. Unfortunately this design does use synchorizers.
In testing, the 3-speed transmission improved efficiency by 14.7% while delivering the same performance of the baseline vehicle, which translates to greater range.
5 – Not clutchless but multi-speed – Evolute Drive, a new company from the UK, presents its 3-speed MSYS electric vehicle transmission – and a paper entitled ‘Next Generation Development – MSYS 3-speed EV Transmission’. According to the press release, prototype of MSYS tested independently in a B-class demonstrator vehicle at the MIRA facility achieved up to 18% reduction in energy consumption over the NEDC test cycle compared to a single-speed gearbox. 18% is a significant improvement (even more than the expected 10-15%). But we believe that real world range will increase by less value as the energy savings concerns only the drivetrain specifically.
James Potter, controls manager for ZF Powertrain Technology predicts, “For passenger cars, I would say we will probably eventually rise up to 3- or 4-speed … two-speeds are coming out, and that will be the next generation,” he says. Four speeds are the maximum gears that ZF foresee for EVs. The first benefit of more speeds for electric vehicles is an improvement in driving range by up to 20%.
6 – GKN is not only readying 2-speed gearboxes for EV OEMs but integrating them with motor and torque vectoring control differential as a complete axle. https://www.gkn.com/en/newsroom/news-releases/driveline/2018/gkns-next-generation-electric-vehicle-driveline-now-in-prototype-test-phase/
7 – Punch Powertrain Twinspeed 2-speed gearbox now ready for EV OEMs. They report the performance, efficiency and sizing advantages and 4 to 7% drive range benefit of their gearbox (page 22).
In addition to designing multi-speed gearboxes, eliminating clutches and synchronizers, to simplify these multi-speed gearboxes and minimize torque interruptions, are the objectives in application to EVs. Dog engagement and other clutchless methods, with proper software control, allows e-motor speed matching to output drive during split second disengagement to minimize torque interruptions. Multiple motor sets slightly offset in shift timing ensure torque to wheels stays more constant during shifting. (In the engineering design and analysis, QWC was involved in for the Nikola Series PHEV Gas Turbine Semi truck long haul prototype, where 6 motor/gearboxes were required for 6×6 AWD, these types of method were employed to ensure minimal torque interruptions. Also 2-speed clutchless design per gearbox was mandatory however, a 3-speed per gearbox would have reduced the motor/controller sizing, if the resulting physical size of the integrated motor/gearbox unit could be maintained)
8 – Automated 2-speed transmission with built-in limited-slip differential, production ready they claim, from startup Austria based Kreisel Electric hails benefits of 2-speed for electric vehicle powertrains.
9 – Eaton Develops Clutchless 4 speed AMT – Automated Manual Transmission for medium duty applications, such as for buses and medium duty trucks. The 4-speed EV automated transmission is based on traditional, robust and efficient lay shaft architecture typical of AMTs, but is designed specifically for EV applications. Unlike traditional commercial vehicle transmissions, Eaton’s 4-speed EV gearbox does not have a clutch, and shifts are synchronized using the traction motor. It also operates at higher speeds than its traditional counterparts, and gears are optimized for typical electric motor performance and power curves for maximum efficiency. The compact and lightweight transmission enables a significant reduction in rated motor torque for the same application, enabling a steep reduction in motor costs and overall powertrain cost and weight.
Eaton’s 4-speed EV transmission is designed with a deep-ratio first gear for launches under difficult conditions that would stress the driveline, while the second gear is used for smooth launches under normal road or load conditions. The new design allows for a smooth launch on grades up to 30%, compared with the approximately 10% grade limit of direct drives. On grades of 5-7%, the transmission can maintain speeds of 80 km/h (50 mph), and at grades around 3%, the vehicles can drive steadily at 95 km/h (60 mph).
The 4-speed EV transmission also provides higher output speed capability and torque range than a direct-drive system. This enables the usage of a smaller, lighter and less expensive electric motor for large vehicles.
This unit is ideally suited for 7- to 18-ton applications and provides 5,000 rpm input speeds and up to 9:1 overall ratio coverage. The transmission improves acceleration by keeping the vehicle in lower gears, which provides maximum motor power while maintaining efficient operation.
More to come on this subject …