Multi-speed Transmissions Mean Better Efficiency for Electric Cars
Could you imagine comparing a 1955 Chevrolet with a brand new Tesla Roadster, and finding something technically better in the old Chevy? Here's one thing: The old Chevrolet will likely have a 2-speed automatic Powerglide transmission. Lo and Hi. The Tesla Roadster has only one gear, just like the Nissan LEAF, and most other EVs. That sounds strange when you can buy a gas car with an 8-speed transmission from BMW or Lexus. Is there an unwritten law stating that all gas cars must have a transmission, and that electric cars should never have one?
There's no such law, but there are horsepower and torque characteristics that reduce the need for a choice of gears on an electric motor. Controllers have improved over the years. On old EVs, with DC motors, there was a need of a transmission to allow reverse driving, but there's no need for that anymore. On today's AC motors, the controller can just command the motor to turn the other way around. So that mechanically, the car could drive in reverse at the same speed as it drives forward. Of course, the manufacturer will prevent this with some software tricks applied to the controller, and that means that a car like a Nissan LEAF doesn't have anything that a mechanic would call a transmission. All it has is a reduction gear.
Electric motors have a wider power range than an internal combustion engine. Their torque curve is flatter, but their efficiency curve is far from flat. There would be a lot to gain from a choice of gears. Imagine a Nissan LEAF with a 5-liter V8 engine and a 5-speed manual blocked in third gear. Performance would be the same as with the standard electric motor, but the efficiency would be terrible. It would be much better with a 2-liter and a choice of gears, and the same reasoning also applies to an electric drive, though to a significantly lesser degree. With a transmission offering several speeds, the Nissan LEAF could do with a smaller motor, and it would be more efficient because it would be possible to keep that motor within the most efficient RPM range.
Do EV drivers want better performance and more range? Then just add gears. I'm pleased to report that several manufacturers are looking seriously into this.
The latest is Morgan, which is building an electric Aero Supersport with the help of Zytek, a British specialist in electric drives. Morgan will use a modified 5-speed manual. Then there is the Furtive eGT from Exagon Motors, which will arrive with an innovative 4-speed. That transmission will be totally new, and designed from scratch to work with an electric motor. Fisker Automotive also recently said it was looking into transmissions as a way to dramatically improve performance on EVs.
Last but not the least, a car company can already buy a gearbox specifically designed for EVs from a transmission manufacturer. That's Antonov, which is proposing a 3-speed transmission. The company already won several awards for it, and I bet there are more to come. Electric motors don't readily accept sudden rev changes the way an ICE does. Some fine tuning software will be required, but it's on the way.
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