As electric vehicles (EVs) move from niche offerings to mainstream roles in the product portfolios of the world’s traditional automakers, research and development to enhance and improve all aspects of EV functionality and efficiency is accelerating. Mega-supplier Bosch recently revealed results of an effort to improve one of the most crucial aspects of EV development – traction motor performance – by fusing electric motors with a transmission technology familiar to the internal-combustion world: continuously variable transmission (CVT).
Bosch revealed that coupling a lightly modified conventional pushbelt CVT it has dubbed CVT4EV with a typical EV traction motor can both improve performance and reduce energy consumption compared with today’s typical setup of a traction motor integrated with a single-speed transmission. Fitted to a C-segment-sized demonstration EV, Bosch said the CVT4EV improves acceleration and top speed and reduces torque demand on the traction motor, making it more efficient in many driving situations.
Perhaps best of all, the company said using the CVT4EV could allow an automaker to fit essentially the same electric powertrain across a range of vehicle sizes from the C segment to F-segment large cars and SUVs – and even G-segment light-commercial vehicles (LCVs). It also can be used in dual-motor vehicle layouts that power both axles to impart all-wheel drive (AWD).
Enhanced ratio coverage
The CVT4EV’s chief advantage is a wider ratio spread than even the 2-speed step transmissions available today for some higher-end EVs. The CVT4EV also offers the ability to consistently maintain the traction motor’s speed in a more-efficient range that’s less tied to road speed or load, said Gert-Jan van Spijk, transmission director for Bosch Transmission Technology.
“Fixed-ratio transmissions have to deal with a compromise between the required wheel torque to perform with high loads and gradients, against maximum-speed requirements,” said van Spijk in an interview with SAE International. “At the same time, fixed-ratio powertrains have to deal with the efficiency of the electric motor ‘as it is,’ following the load and speed conditions.
“A 2-speed transmission offers the first stage of improvement on this, but with a limited ratio coverage – typically up to 2.0 per step – and with disruptions in torque and speed, influencing acceleration, efficiency and comfort,” he said. “If a larger ratio spread is required, typically, more steps have to be added; CVT is a more logical pairing, as it has the ability to improve the electric motor’s efficiency while keeping the ‘fluent’” power characteristic of an electric machine, van Spijk asserted.
Due to the wider flexibility of electric motors, the CVT4EV ratio spread need only be between 3-4 to one, he said, as opposed to a ratio spread of 6 to 8 when coupled with IC engines. This is one reason that CVT-equipped EVs will not evince the sometimes harsh and strained-sounding NVH characteristics that have made them open to criticism in ICE vehicles, particularly higher-powered premium models, van Spijk said.
“The flexibility of the electric motor puts different requirements to the control of the CVT, meaning that shift speed is much lower than in ICE applications,” he explained. “With this, the inertia effect of rotating masses is hardly noticeable. It has become one of the bigger benefits of CVT compared to other multi-speed solutions for EVs.
“Demonstration of a CVT4EV prototype to Bosch specialists, customers and press resulted in positive feedback on the driving behavior, including remarks like ‘not distinguishable from single-speed,’’’ he added. “Because CVT4EV results in lower torque/speed requirements for the electric motor, the sound of [the traction motor’s] rpm also is reduced, improving comfort even more.”
Modeled for optimized performance
The CVT4EV concept uses “numerous Bosch components” and was developed by Bosch Transmission Technology BV, the supplier said in a release about the new EV transmission concept. Bosch worked with Eindhoven University of Technology (TU/e) Ph.D candidate Caiyang Wei on design and optimization of the electric CVT. Wei and Bosch used an optimization method that considers not only the control of the car’s speed, energy consumption and heat production, but also the physical design of the car’s electric motor and transmission system.
With this method – a combination of machine learning and a thorough analysis of the measurement data – the researchers said they were able to design a system that is optimal in terms of both performance and efficiency, while at the same time reducing the total costs for both the manufacturer and the user.
The initial information released about the CVT4EV development stressed that Bosch does not make transmissions – rather, it develops and produces the steel pushbelts at the heart of any CVT. The company stated that it “supports customers with transmission concepts for ICE, hybrid, and EV applications,” adding, “Full CVT4EV solutions are available through Tier-1 suppliers.”
The CVT4EV’s belt is “based upon the same belt design as for the ICE application,” van Spijk said. “In terms of durability, pushbelts are known to be robust and typically they outlive the lifespan of the vehicle. It is up to the CVT manufacturer to select the belt that best suits their transmissions/applications and determine the KPI [key performance indicator] they want to focus on – power density, efficiency, etc.”
Performance and efficiency gains
Bosch’s testing of the CVT4EV in a 150-kW (201-hp) C-segment vehicle showed a 3% reduction in 0-100 km/h (0-62 mph) acceleration, a 13% drop in 80-120 km/h (50-75 mph) acceleration and an 11% increase in top speed. Meanwhile, an efficiency gain of up to 4% is projected for a D-segment vehicle. The efficiency-gain potential is particularly attractive for EVs. Even minor overall efficiency improvements can translate to the potential to fit a smaller, less-expensive battery pack and/or a smaller traction motor.
“The advantage of reduced energy consumption is valid at all speeds,” said van Spijk. “Over a reference cycle like WLTP, this means an overall advantage in the magnitude of 2% to 4% depending on vehicle type. At constant-speed driving, this advantage is even larger since the adaptation to the best [motor] efficiency can be held closer. At speeds from 40 to 80 km/h, this leads to an advantage in the magnitude of up to 8% – again, depending on vehicle type.”
He said the CVT4EV can handle up to 4,000 Nm (2,950 lb-ft) of wheel torque, which helps make the transmission viable into the LCV segment. For ICE vehicles, pushbelt CVTs typically are limited in the amount of engine torque they can reliably transmit, which has capped their applicability to vehicles with engines with torque output broadly less than 300 lb-ft (407 Nm), but van Spijk said the reduced ratio spread required for coupling with a traction motor will allow it to handle more torque than CVTs coupled to IC engines.
“The maximum torque capacity of CVT4EV is higher than in ICE applications because, technically, ratio-coverage and torque capacity are exchangeable parameters. Since ratio coverage is only about half the value of ICE applications, it enables significantly higher torques for EV,” he explained. “This makes the pushbelt CVT suitable for applications from C-segment (compact) cars to light commercial vehicles.
“Often for a C-segment vehicle,” he continued, “a single speed [transmission] can be sufficient, but when a vehicle needs the following requirements, it makes sense to use a CVT4EV: towing capacity and improved gradeability; possibility for increased driving range or smaller battery, which lowers weight and cost; higher top speed, lower motor revs, motor downsizing and optimized regeneration.”
Counting on cost offsets
Compared with the single-speed transmission typical for most current EVs, van Spijk concedes the CVT4EV represents increased cost. But using the CVT4EV offers potential for a variety of “system” cost savings that can offset the higher component cost of the new Bosch transmission concept.
“CVT4EV is a more complex system compared to the single-speed systems, but compensation for the add-on cost is found in the reduced torque and speed requirements for the electric motor, resulting in the need for less magnet and copper – and with that, lower cost,” he asserted. “Further compensation is in the smaller battery package for the same range and performance compared to a single-speed transmission.
“The system is still in development at Tier-1 suppliers; the exact prices have yet to be defined and communicated by these suppliers,” he said. “CVT represents additional value in performance – but also in modularity for these suppliers and the OEMs. Transmission builders can put together one platform design, using one CVT4EV, one inverter and one [traction motor], which can be applied in a range of vehicles from compact to light commercial vehicles. For OEMs, this reduces the costs for powertrain investments and spending in the fields of development, manufacturing, purchasing, logistics and aftermarket.”Continue reading »