Hello everyone, and welcome. In this video we will be answering the question why do electric cars only have one gear.
This will be the first of a five-part series brought to you by Formula E, who I've partnered with to talk about the engineering behind electric cars.
I had the opportunity to get behind the scenes at the New York City
ePrix and was able to chat with team principals, hang out in engineering rooms during qualifying,
and even learn from this Year's champion Lucas De Grassi.
Now the topic at hand is why 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 need gears,
electric motors can get away without
numerous gears because they are high reving, remain fairly efficient across a very broad rev range, and produce a great amount of torque at low RPM
while a gasoline engine will need to have multiple gears to reach a 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 Km/h or about 62.5 m/s.
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 20k RPM with a gear reduction of 11.4:1 we could drive this electric car from
zero Km/h at zero RPM to
225 Km/h at
20,000 RPM. And since many electric road cars have top speeds limited to less than 160 Km/h
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 11.4:1, and match it with an
internal combustion engine with a redline 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 6000 RPM with a gear ratio of 11.4:1 the gasoline engine would only be able to drive the car up to
a top speed of 67.5 km/h
It would need much taller gearing in order to reach the top speed of 225 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 Km/h? At 20 Km/h,
with a gear ratio of three point four between the engine and the wheels the engine would only be spinning at 530 RPM.
Far too low for the engine to 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 little added benefit. Now does this mean that there's never a need for electric cars that have multiple gears?
Actually no, and in Formula E
there are a wide range of strategies used. While this season's team champion was Renault, who used a single speed,
the drivers Champion was Lucas De Grassi, 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 kW.
At 0 RPM, because horsepower is a function of torque multiplied by RPM,
power is 0. Power gradually increases until flatlining 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 will 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.
A big thanks to formula E, and thank you all for watching. If you have any questions or comments, feel free to leave them below.