The Single Gear Conundrum: Understanding Electric Cars' Unique Transmission Requirements

As we explored during our recent visit to the New York CityePrix, electric cars use only one gear for transmission, which may seem puzzling given that gasoline engines typically require multiple gears. However, this is where the fundamental difference between internal combustion engines and electric motors lies.

Electric motors are high-revving, remain efficient across a wide rev range, and produce a great amount of torque at low RPMs. This unique combination allows them to achieve their maximum performance without the need for numerous gears. In contrast, gasoline engines require multiple gears to reach their top speed because they are not as efficient or powerful at lower RPMs. For instance, in Formula E, the cars typically won't exceed 225 Km/h or about 62.5 m/s, while electric motors can easily reach speeds of up to 20,000 RPM.

To understand why electric cars don't need multiple gears, let's take a closer look at an example. In Formula E, we calculated that with an electric motor revving at 20,000 RPM and a gear reduction of 11.4:1, the car could accelerate from 0 Km/h to 225 Km/h. This is because the rear tires have a diameter of 0.68 meters, which gives us our circumference. When we divide the motor's revolutions per minute by 60 to get revolutions per second, we can solve for the gear ratio, resulting in an impressive 11.4:1 reduction. This means that even if the motor is lower revving or has more aggressive gearing, one gear would be enough to achieve the desired top speed.

Now, let's consider what happens when we apply this same gear ratio to an internal combustion engine with a redline of 6,000 RPM. As it turns out, with a gear reduction of 11.4:1, the gasoline engine could only reach a top speed of 67.5 Km/h, which is significantly lower than the electric car's achievable 225 Km/h. To reach this top speed, the engine would need an even taller gearing ratio of 3.4. However, if we wanted to drive 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, which is far too low for optimal performance.

This illustrates why internal combustion engines require multiple gears: they need to keep the engine within its power band to achieve any desired speed. Electric motors, on the other hand, are so efficient across a wide rev range that adding complexity, cost, weight, and efficiency losses through a transmission would provide little benefit. In fact, many electric cars have top speeds limited to less than 160 Km/h, making one gear even more than sufficient.

While it's true that there may never be a need for electric cars with multiple gears, there are indeed strategies employed in Formula E to overcome this limitation. For instance, the drivers' champion Lucas De Grassi was driving a three-speed Audi during the season, while Renault used a single speed as their team's strategy. So, what are the advantages of adding multiple gears to an electric car? Let's dive deeper into the world of electric motors and explore their power curve to understand this better.

In Formula E, power is limited to 170 kW at 0 RPM because horsepower is a function of torque multiplied by RPM. At low speeds, you can see that not all of this power can be utilized due to traction limitations. This creates an opportunity for more aggressive gearing to gain an edge. By spinning the motor up faster and achieving more wheel torque, electric cars can accelerate faster and get into peak power mode quicker than if they had taller gearing.

Gearing can also be dependent on the motors used, with smaller motors having less torque and requiring multiple gears for optimal performance. Larger motors or pairs of two motors will have more torque, making a single-speed transmission possible. Given the relatively low top speeds needed in electric cars, high rev limits, and a wide efficient rev range, it's indeed beneficial to use just one gear ratio.

In conclusion, understanding the unique requirements of electric cars' transmissions is crucial for grasping their advantages over traditional gasoline engines. By leveraging their high-revving capabilities, efficiency across a wide rev range, and low-torque outputs, electric motors can achieve remarkable performance without the need for multiple gears. As we continue to explore the world of sustainable transportation, it's essential to appreciate the intricacies of electric car design and its implications on the road ahead.

"WEBVTTKind: captionsLanguage: enHello 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 CityePrix 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 traditionaltransmissions 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 withoutnumerous gears because they are high reving, remain fairly efficient across a very broad rev range, and produce a great amount of torque at low RPMwhile a gasoline engine will need to have multiple gears to reach a top speed anelectric motor can easily be geared specifically for that top speed, and still be useful at lower RPM. For example, inFormula 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 minuteinto revolutions per secondand now we can solve for the gear ratio, which turns out to be 11.4. This means that if we have anelectric motor that revs to 20k RPM with a gear reduction of 11.4:1 we could drive this electric car fromzero Km/h at zero RPM to225 Km/h at20,000 RPM. And since many electric road cars have top speeds limited to less than 160 Km/hYou can see how one gear would easily be enougheven 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 aninternal combustion engine with a redline of 6,000 RPM. Now yes, in racing you'll find gasoline engines with much higher red linesbut 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 toa top speed of 67.5 km/hIt would need much taller gearing in order to reach the top speed of 225 km/hIn order to reach that top speed, it would need a gear ratio of 3.4But 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 ofthe desired speeds.Since electric motors are so high revving, and efficient across a much wider rev range, a transmission simply adds complexity, cost, weight, andefficiency 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 Ethere 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 thatyou're not able to put down the full amount of power that the series allows which means there's someopportunity 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 possibleand 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.\n"