A device that directly drives the wheels with an electric motor is called a hub motor, which Porsche used more than 100 years ago. At that time, an internal combustion engine generated electricity to drive the hub motor, but it was later abandoned due to high cost, low energy efficiency, and troublesome maintenance. In the end, an internal combustion engine and a transmission were used to drive the vehicle.
In-wheel motors offer numerous advantages. Vehicles using in-wheel motors can eliminate the need for transmission components such as clutch assemblies, gearboxes, differentials, rear axles, and drive shafts. The most typical examples are electric motorcycles and regular motorcycles. Electric motorcycles rely on the motor to directly drive the wheels, eliminating both the internal combustion engine and gearbox, replacing the chain-driven wheel reduction mechanism with direct motor drive.
In-wheel motors eliminate the need for some transmission components, saving significant space and reducing vehicle weight, while also greatly improving transmission efficiency! Furthermore, each of the four wheels is independently controlled, allowing for easy implementation of inter-wheel differential functionality via electronic control, eliminating the need for a mechanical differential. However, given these numerous advantages, why haven't in-wheel motors been widely adopted in electric vehicles?
First, hub motors are heavy. As you can see in the picture above, hub motors have many more components than ordinary car hubs, with the motor itself being the heaviest.
Finished wheel hub motors weigh over 30kg, and the excessive weight of the wheels places higher demands on the suspension system. Engineers at traditional automakers have racked their brains to reduce unsprung mass by using lightweight materials; research shows that reducing unsprung mass by 1kg is equivalent to reducing sprung mass by 15kg.
Unsprung mass is closely related to a car's acceleration, handling, and stability, so engineers are always trying to reduce it. However, using a hub motor in an electric vehicle adds tens of kilograms to the unsprung mass, which is the opposite of what engineers intend. But for everyday use of family cars, the handling requirements can be lower, since we're not racing.
Lifespan is an issue. Permanent magnet materials may demagnetize at high temperatures, inevitably leading to a decrease in motor performance. Brake pad temperatures can reach over 200 degrees Celsius during driving, making perfect heat dissipation a challenge. Furthermore, costs are relatively high, and mature, stable electronic control technology requires time to develop.
Integrating the motor, controller, and cooling system into a confined space, while the wheels operate in harsh environments, places high demands on the in-wheel motor's airtightness, heat dissipation, corrosion resistance, and stability. These seemingly simple technologies are not easy to apply successfully. Currently, BYD's K9 (electric bus) already uses in-wheel motors, and FAW has also developed a passenger vehicle in-wheel motor chassis. It is believed that in-wheel motors will shine brightly in the near future!
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