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Why do electric cars use just a single gear and don’t have traditional transmissions like you’d find paired with internal combustion engines while it might be more appropriate to ask, why do gasoline engines even need gears?
Electric motors can get away without numerous gears because they are high revving, remained fairly efficient across a very broad range and produce a great amount of torque at low RPM.
While a gasoline engine will need to have multiple gears to reach top speed, an electric motor can easily be geared specifically for that top speed and still be useful at lower RPM. For example, in formula E the cars typically won’t go any faster than 225 kilometres per hour. We’re about 62.5 meters per second. The electric motors are capable of spinning at about 20,000 RPM. So we can do some quick math based on the tire data to see what our gear ratio might be. The rear tires have a diameter of 0.68 meters multiplied by PI gives our circumference. We’ll divide 20,000 RPM by 60 to turn revolutions per minute into revolutions per second. And now we can solve for the gear ratio, which turns out to be 11.4. This means that if we have an electric motor that revs to 20 K RPM with a gear reduction of 11.4 to one, we could drive this electric car from zero kilometres per hour at zero RPM to 225 kilometres per hour at 20,000 RPM.
Since many electric road cars have top speeds limited to less than 160 kilometres per hour. You can see how one gear would easily be enough, even if the motor was lower revving or if the gearing was more aggressive. Well, what happens if we were to take this same gear ratio of 11.4 to one and match it with an internal combustion engine with a red line of 6,000 RPM now, yes, in racing you’ll find gasoline engines with much higher red lines, but for road cars, 6,000 RPM is a perfectly normal red line. While road cars with electric motors are still high revving at 6,000 RPM with a gear ratio of 11.4 to one, the gasoline engine would only be able to drive the car up to a top speed of 67.5 kilometres per hour. It would need much taller gearing in order to reach the top speed of 225 kilometres per hour.
|Gas / ICE||6.000||11.4||67.5 km/h|
|Gas / ICE||6.000||3.4||225 km/h|
|Gas / ICE||530||3.4||20 km/h|
In order to reach that top speed, it would need a gear ratio of 3.4. But what if we then wanted to drive at 20 kilometres per hour with a gear ratio of 3.4, between the engine and the wheels, the engine would only be spinning at 530 RPM, far too low for the engine operate. And even if it could, it would have very little torque. Hence internal combustion engines use gears to keep the engine within the appropriate power band and still allow for the car to travel at any of the desired speeds. Since electric motors are so high, revving and efficient, across a much wider rev range, a transmission simply adds complexity, cost, weight, and efficiency losses with a little added benefit. Now, does this mean that there’s never a need for electric cars to have multiple gears? Actually, no. And in formula E there is a wide range of strategies used while this seasons team champion was Reno who used a single speed.
The driver’s champion was Lucas Degrassi who was driving a three-speed Audi. So what are the advantages of adding a few gears? To better understand this we need to look at an electric motors power curve in formula E during the race power is limited to 170 kilowatts at zero RPM because horsepower is a function of torque multiplied by RPM. Power is zero power gradually increases until flat-lining at the limit allowed by the series. Now for super low speeds, you can see that you’re not able to put down the full amount of power that the series allows, which means there’s some opportunity here to gain an edge. As long as traction allows for it. More aggressive gearing means you can spin the motor up faster. You’ll have more wheel torque can accelerate faster, and we’ll be able to get into peak power faster than if the gearing were taller.
Meaning you’re accomplishing more work in scenarios, such as the start of the race or during very low-speed corners. This can play to your advantage. Gearing can also be dependent on the motors. Used smaller motors tend to have less torque. So multiple gears can be used to compensate larger motors or pairs of two motors will have more torque and can easily get away with just a single speed.
With relatively low top speeds needed high rev limits and a wide efficient rev range. All of these features make it possible and beneficial for electric cars to use just a single gear ratio.
Morten has been working with technology, IoT and electronics for over a decade. His passion for technology is reflected into this blog to give you relevant and correct information.Read more...