Electric Cars: Why Do They Only Have 1 Gear?
When you think of a car, you probably think of an engine with two or three gears at most. Electric cars are widely seen as the future of transport because they’re much cleaner than petrol or diesel engines. That being said, many people are still hesitant to buy one because they only have one gear. So why do electric cars only have one gear? Let’s take a look…
The first thing to understand is what a car’s gear does. There are two main parts to a car; the engine, and the transmission. The transmission is the system that changes the speed at which the engine is spinning and how the car uses that energy to drive.
A car can have two, three, four, or even five different gears that allow it to drive at the different speeds that you need it to. If a car has only one gear, it means that it can only drive at one speed.
If you’re thinking, “why would anyone design a car like that?”, you’re not alone. So let’s take a look at what an electric car is and see why they only have one gear.
What does a car’s gear do?
Gear is the system inside a car that allows the engine to drive the wheels. This means that it helps to turn the energy inside the engine into the energy that moves the car. In a car with an electric motor, the transmission and the gear don’t do anything.
They are totally unnecessary. This is because an electric motor turns the energy from the battery into energy that moves the car automatically. A petrol (gasoline) engine, on the other hand, needs to be turned into rotational energy by a gear system.
This is why petrol cars have two or three gears because a different gear is needed for each different speed. This is also why car engines are always making a noise; it’s the sound of the gears inside the transmission.
What is an electric car?
An electric car is a vehicle that is powered by a battery. The battery is charged by plugging it into the mains electricity like you would charge your phone or laptop. The battery then turns the car’s wheels through a motor, which makes them go.
Many electric cars also have a petrol engine, which can be used to charge the battery if the car isn’t near a power source. Because the battery turns the car’s wheels automatically there is no need to have a gearbox.
There is no need to change speed, so the car can simply drive at one speed. This is the reason why electric cars only have one gear. This gear is often referred to as first gear, second gear, or even reverse gear, but it’s all one gear.
Benefits of having only one gear in an EV
Less moving parts: Electric cars don’t have gearboxes because they don’t need them. This means that there are a lot fewer moving parts in an electric car. This makes servicing the vehicle easier and there is less that can go wrong.
Less noise: As with the above, there is no gearbox in an electric car. This means that there is no need for noises, like the grinding of gears. This also means that there is no need to change gears, which is why all-electric cars have one gear.
No transmission fluid: A gearbox is filled with transmission fluid, which keeps the gears cool and lubricated. This fluid needs to be changed from time to time, and it can leak out, causing damage to the car’s floor. Electric cars don’t have transmission fluid, so there is no need to check or change it. This means that electric cars are much more reliable.
Drawbacks of only having one gear in an EV
No revving: This is probably the biggest issue for petrolheads. There is no need to shift gears so the car is always in first or second gear. This means that the car doesn’t have the engine noise that petrol heads like. It doesn’t have the engine noise because it doesn’t have the revving action that comes with shifting through the gears.
A constant noise: Another issue with only having one gear is that it makes the car’s noise constant. This is because there is no shifting action, so the car’s noise is always there.
Let’s go deeper into why electric cars only have one gear.
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 kilometers 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 kilometers per hour at zero RPM to 225 kilometers per hour at 20,000 RPM.
Since many electric road cars have top speeds limited to less than 160 kilometers 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 kilometers per hour. It would need much taller gearing to reach the top speed of 225 kilometers 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|
To reach that top speed, it would need a gear ratio of 3.4. But what if we then wanted to drive at 20 kilometers 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 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 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 season’s 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 motor’s 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.
This means 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 for 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 in this blog to give you relevant and correct information.Read more...