Do electric vehicles need multi-speed ratio transmissions?
Why don't today's electric cars need to be equipped with bulky gearboxes? Because the output characteristics of electric motors have inherent advantages over internal combustion engines.
First of all, the maximum speed of the internal combustion engine is low. Because it is equipped with a 4.0-liter V8 naturally aspirated engine code-named S65, the BMW M3 with chassis code name E90 is popular with fans, but the V8 engine also has a top speed of 8250 rpm. In contrast, the maximum speed of the electric motor can easily exceed 10,000, and the Tesla Model S Plaid can exceed 20,000 rpm because of the support of carbon fiber sleeve rotor (NEW CARBON SLEEVED ROTORE) technology.
Secondly, the torque of the internal combustion engine at low speeds is also small. At the same time, the output characteristics of the internal combustion engine determine that the intersection of the working interval of maximum power, maximum efficiency and smooth output is too small. Under the combined influence of relatively low maximum speed, low torque at low speed and undesirable output characteristics, in order to meet the power demand of starting, high speed and other working conditions at the same time, the multi-speed ratio gearbox has become an indispensable partner for the internal combustion engine, while the electric motor is not needed.
Although the complex multi-speed ratio transmission is abandoned, coupled with the advantages of the size of the electric motor itself compared to the internal combustion engine, the powertrain structure of the electric vehicle can be greatly simplified. However, because the speed ratio is fixed, electric vehicles also have to face the problem of choosing between acceleration ability and extreme speed ability.
In order to win both fish and bear paws, electric vehicles with two-speed transmission capabilities came into being. For example, the Porsche Taycan, which uses planetary gear technology to change the speed ratio, and the SAIC Feifan (formerly SAIC R brand) Marvel R, which uses parallel shaft shift technology on the rear axle.
But they are still a minority in the electric market. On the one hand, they only have the ability to switch power in two gears, on the other hand, the contribution of the two-speed transmission to solve the maximum pain point of pure electric vehicles, especially the most criticized high-speed power loss, is not obvious.
Although the high-efficiency working range of the motor covers a relatively wide range, if it can be combined with the multi-speed ratio gearbox to further improve the output efficiency of the motor, it is theoretically possible to improve the endurance performance of the electric vehicle.
In 2020, Bosch announced that it will develop a CVT gearbox for electric vehicles. Recently, based on the electric vehicle e-Golf, auto parts supplier Bosch once again showed the gain brought by the CVT4EV gearbox to the electric vehicle.
In the Bosch demonstration, an interesting point is that the developers artificially limited the torque of the e-Golf to 145N·m, only half of the maximum torque of 290N·m, but the output performance of the e-Golf was close to normal, and the credit behind it was naturally the CVT4EV gearbox.
According to Bosch's calculations, the e-Golf equipped with the CVT4EV gearbox can achieve a 4% increase in range under the WLTP cycle. In the cycle of higher operating conditions at high speeds, the CVT4EV transmission can contribute even 10% to the increase in the range of electric vehicles. The efficiency improvement of the motor is more obvious in high-speed driving, which precisely cuts the pain point of high energy consumption under the high-speed working conditions of electric vehicles.
In addition, the precise control of the power output of the electric motor by the CVT4EV gearbox also enables the electric vehicle to maintain the acceleration ability of the proud fuel vehicle, while having a good speed ability and a certain climbing ability. As Gert-Jan van Spijk, head of engineering at Bosch CVT, says, the CVT4ET pushes the boundaries of electric vehicles.
But the Bosch CVT4EV gearbox is clearly in the early stages of development, and one of the big problems is "big and heavy". The CVT transmission on the e-Golf was not specifically designed for electric vehicles, but was modified on the gearbox developed for the Mitsubishi gasoline engine. The CVT4EV weighs 100Kg, and in the ideal planning of developers, its weight should be limited to 20Kg.
And the structure of this CVT4EV should also be more compact, because its body also retains the bulk of the internal combustion engine transmission. You can imagine how abrupt this CVT4EV gearbox would be when it's in a cramped electric car power compartment, paired with a "slim and dexterous" electric motor.
In addition to the problem of technological maturity, the Bosch CVT4EV project also needs to face cost problems on the way from experiment to mass production. Although the CVT4EV transmission has improved the performance of the electric vehicle in terms of speed and energy consumption, it is still unknown whether it will impress consumers to pay for the additional cost of buying a car. If the integration is too low, or the cost is too high, then the electric car CVT transmission will inevitably be a luxury configuration that belongs to only a few supercars.
The author of this article is kicking car gang Li Bingxue