Speed game

A move to three-speed transmissions could help improve electric vehicle efficiency considerably, and smaller engineering firms are taking the lead in developing the technology

While single-speed transmissions are the norm for electric vehicles, and two-speed devices have also been used, increasing the number of ratios still further could have huge efficiency benefits.

Tests of multi-speed systems have shown that efficiency can be increased by up to 15%, which could then be used to increase the range of vehicles or decrease the size of other powertrain components such as the motor or battery pack.

Drive System Design (DSD) is one of the firms developing the technology. It has designed a three-speed system, and its technical director Alex Tylee-Birdsall says that more companies will follow: “I guarantee most people are working on some sort of multi-speed system because, no matter what you do with a single-speed, theoretically with a high-speed motor you can get a reasonable speed range up to a certain v-max speed, but it takes a lot more energy.”

The logic behind the move to a greater number of transmission speeds is simple. It takes more power to get motors to high speed because the magnetic field needs to be weakened, and in order to achieve that more energy is required.

“And running transmissions at higher speeds means more losses in the bearings, the gears, and churning the oil around,” says Tylee-Birdsall.

In DSD’s research vehicle, engineers achieved up to an 18% efficiency improvement over a single-speed system.

“The majority of that is power electronics because you aren’t having to run a field-weakening system, so you don’t have to run a second stage on the power electronics,” says Tylee-Birdsall.

Although the improvement in efficiency might be substantial, there are still numerous challenges that DSD had to overcome, not least shift feel. Many combustion engine vehicles use automated manual transmissions, but these are hindered by the torque being interrupted during shifts, decelerating the car every time you change gear.

tags: December 2015 Drive System Design Transmissions