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The vast majority of e-cars get by with one gear, in individual cases the transmission offers a second stage. But there is much more that can be done using a well-known technique.
Six-speed manual gearbox? Eight-stage DSG? Ten-speed automatic? When it comes to electric cars, you will look in vain for these fine transmission solutions. For electric vehicles, a simple and inexpensive gear with a fixed ratio is usually sufficient for almost all areas of application. But at Bosch you also see opportunities for transmissions beyond this simple diet. Specifically, the supplier wants to make the continuously variable CVT transmission for future electric cars more attractive to automakers. The automatic system without fixed gear ratios could expand the range of uses of electric vehicles and at the same time ensure more comfort, better performance and lower consumption. But it will be a few more years before that happens.
First try in golf
The driver behind the CVT project is the Dutch Bosch subsidiary Transmission Technology BV, which developed the “CVT4EV” concept and implemented it in a first drivable demonstration car. The Tilburg-based company specializes in the development and series production of push-link belts, which are responsible for the power transmission within a continuously variable transmission. For their first electric test vehicle, the Dutch have relied on tried-and-tested series technology, because the original input of the vehicle base, a VW e-Golf, was replaced by a CVT transmission from the Mitsubishi Eclipse Cross. In some points, the vehicle technology was adapted to the new transmission, for example the previously mechanical one was replaced by two electric oil pumps.
The engine compartment of the converted e-Golf is pretty tight. The operation also added a good 100 kilograms of weight. "A series version will only weigh around 20 kilograms," says Marketing Manager Caro Roeffen. She wants to dispel further prejudices and points out that although CVT transmissions have a rather bad reputation in Europe and especially in Germany, the opposite is the case in China, the USA and Japan. "Worldwide, one in four transmissions is a CVT - and the trend is rising," says Roeffen. Bosch Transmission Technology has long been supplying some key components of this transmission technology. “Last year we reached an important milestone with the 80 millionth push link belt,” says the manager. "It is a tried and tested technology."
Slight savings in energy
This is known above all for its efficiency, which should also help electric cars to be more economical. The continuously variable transmission allows the transmission ratio to be adapted to the optimum operating point of the electric motor as often and for as long as possible. According to Dirk Van den Heuvel, Head of Business Development at Bosch Transmission Technology, the efficiency advantages depend on a number of different factors and thus on the individual case, but overall, up to four percent less energy would be possible in the passenger car sector and over six percent less in light commercial vehicles. Accordingly, the use of a CVT could either provide a longer range or smaller batteries.
Another advantage relates to comfort, because a variable transmission ratio allows lower engine speeds in many speed ranges compared to the input solution. In the case of the already very quiet electric drives, this is a negligible figure, but during a test drive on the Bosch test site in Boxberg, the test vehicle gave the impression that it was doing its propulsion work acoustically more discreetly and smoothly than with the single-speed gearbox. And although the CVT transmission can vary the gear ratios, no load change reactions or gear changes are noticeable. The drive characteristics of the electric car are therefore retained despite the transmission technology (with an infinite number of gear ratios). Also pleasing: The rubber band effect, which is often criticized with CVT transmissions, was not noticeable in the demonstration Golf, because while a combustion engine with this transmission technology increases the engine speed significantly when the power is requested, the torque of the electric motor can be called up directly without annoying engine roar.
A few km / h faster
Perhaps the most important pro argument, however, is likely to be the performance advantages that can be achieved with the CVT. Simulations show that for vehicles in the C-segment in the 150 kW class, the top speed can increase by 11 percent. In the case of the test vehicle, for example, it is 170 instead of 150 km / h. Bosch also promises advantages when it comes to pulling through. The sprint from standstill to 100 km / h can be shortened by 3 percent, from 100 to 150 km / h it is even 20 percent faster. In order to make these performance advantages tangible, we drove in the test Golf with a full 290 Newton meters and a simulated input gearbox as well as with a power reduced to 203 Newton meters in CVT mode. Despite significantly less torque, the e-Golf drove noticeably more lively in the power-reduced CVT mode. The installation of the continuously variable transmission would allow the car manufacturer to either opt for better performance or for smaller, lighter and cheaper engines. The latter in turn offer potential for weight and cost savings.
In terms of costs, however, the CVT transmission solution does comparatively poorly. In the case of electric vehicles, they are slightly above the cost of a dual clutch transmission and are even two and a half times higher than for a single-clutch transmission. With the rightsizing of the battery and motor, at least part of the additional costs could be absorbed, but the system costs will be even higher than with the single-speed gearbox, admits Van den Heuvel.
For premium vehicles only
Correspondingly, Bosch sees the opportunity for CVT electric cars primarily in higher-priced segments. Van den Heuvel excludes micro and small cars. Premium vehicles from the C / D segment or higher are the target group. The transmission would then be an option that the customer would want to afford because it promises better driving characteristics. In addition, according to Van den Heuvel, various driving modes can be better represented using the CVT drive train. Drivers could then choose between caravan, high-speed or energy-saving modes in a vehicle. Thanks to the CVT, a higher tensile load could also be achieved in trailer operation, while the climbing ability of off-road vehicles could be improved.
Sports cars have more torque than the transmission can withstand
The CVT solution could also offer advantages for car manufacturers from an economic point of view. Instead of developing different drive units for different models, a single drive train could be sufficient for many models and vehicle segments, because the CVT offers the possibility of adapting one drive to the different requirements of an SUV, van or sports car and also to give each one a brand-specific character . Speaking of sports cars: their electric drives usually offer more total torque than the maximum of 600 Newton meters that the CVT can withstand. In the case of sporty electric vehicles, however, the power is usually generated by several motors. In an all-wheel drive vehicle with one motor per axle, the CVT transmission would only be used on one axle, while the other electric motor drives the second axle without a variable ratio. The transmission would therefore also be conceivable in vehicles with very high torques.
It remains to be seen whether the many arguments put forward by Bosch will catch on with automakers. At the moment there are many discussions with OEMs and transmission manufacturers who are definitely interested in future projects. But there will be no market-ready solution before 2025, says Van den Heuvel.