When you get into the details about how electric cars work, you learn that they are considered “one speed transmissions,” “single gear,” or sometimes “direct drive.”
What exactly does this mean? How does that compare to an automatic transmission gas car, or a six speed? Hopefully we can clear this up for you.
Let’s talk torque
There are several words that you hear when it comes to electric vehicle performance and one of the most frequently heard is torque. EV reviews talk about electric car’s instant torque and lightning fast acceleration - but what is torque? It is a measurement of twisting, or rotational force. When it comes to cars, torque measures the force produced by a motor to turn the wheels - and thus to make the car move. In this way, it can be considered a measure of the motor’s strength.
EV Torque vs. ICE Torque
EV’s have a lot of torque because the electric motor which drives the car generates a lot of power which can be sent to turn the wheels. Power is a measure of force per time, when it comes to cars, we measured it in RPMs (rotations per minute). An electric motor can rotate up to a maximum of around 20,000 RPMs, and most can hit 10,000 RPM very quickly. More importantly, EV motors produce this power with the same efficiency across a wide range of RPM’s.
On the other hand, with normal driving, internal combustion cars accelerate from about 1200 RPM and to as much as 4000 or 4500 RPM. Their maximum output (or, “red line”) is around 5,000 - 6,000 RPM, but they cannot maintain that output for extended periods without damaging the engine. Generally, the combustion engines do not generate enough torque to move the car smoothly or efficiently. They need to rely on multiple gears to ensure that the car runs efficiently over a wide range of speeds. What gears do is amplify the force of the engine so that more torque makes it to the wheels. In contrast, EVs don’t use gears to amplify the power to the wheels, so they are often listed as “direct drive.”
Gas Car Transmissions
When it comes to gas cars, amplifying torque is where the transmission comes in. The transmission of a gas engine transmits the power from the motor (along with multiple other parts) to the wheels through the drivetrain, using gears. When you put the transmission into drive, the motor engages in first gear. Since the torque (and RPMs) are lowest when you first start accelerating, first gear provides the most amplification. Each subsequent gear is smaller and amplifies the engine output less, since the motor can produce more RPMs once it picks up speed.
What are the gears? They are essentially wheels with teeth that interlock with the engine to amplify the torque that goes to the wheels. The gear ratio refers to the number of turns the input gear (the engine) will make in relation to the output gear (the transmission). First gear has the largest wheels and more teeth to engage with the engine. The next gear is smaller and has fewer teeth, and so on. A typical 5 speed transmission has the following gear ratios:
· Gear 1, 3.35:1 Ratio
· Gear 2, 1.93:1 Ratio
· Gear 3, 1.29:1 Ratio
· Gear 4, 1.00:1 Ratio
· Gear 5, 0.68:1 Ratio (Overdrive)
The gears allow the engine to send more force to the wheels with less energy, allowing the ICE motor to run more efficiently than it otherwise would across a wide range of speeds.
Electric Motor Transmissions
So why don’t electric motors need multiple gears? There are two main reasons for this. First is that electric motors don’t idle. If the car is not moving, the motor is not turning. In a gas car, the motor cannot produce any torque when it’s not moving. It needs to engage a gear (first gear, or drive) in order to start moving. But in an EV, whether the motor is turning at 1 RPM or at 1000 RPMs, the same amount of torque is reaching the wheels. Even at a low RPM, it is enough torque to move the car.
The second reason an EV doesn’t need multiple gears has to do with the gear ratios and performance efficiency.
By amplifying the motor with different gear ratios, you can keep an ICE vehicle running at its most efficient RPM over a wide range of speeds. When the upper end of the speed range is reached, the transmission will change to a new gear ratio automatically so that the engine will run at the optimal RPM given the speed of the vehicle.
Electric motors only need one gear because they perform the same over the entire RPM spectrum. When you touch the accelerator, the motor begins to turn and is powerful enough (has enough torque) to turn the wheels. As you increase the electric charge to the motor it turns faster, and so the wheels turn faster. There is no change in efficiency along the spectrum of speed. When you understand that you will feel, in the way the car drives, why an EV only needs one gear.
Calculating the maximum RPM of a car is based on the projected top speed, max gear ratio, and tire diameter:
Max RPM = (gear ratio * speed (mph) * 336)/tire diameter (inches)
In this formula, 336 is a constant that converts all the units to be uniform. Using this formula, we can see why an EV only needs one speed, while a gas car needs different gears and ratios.
Worked Example: Finding RPM in an EV
Hyundai says the Ioniq 5 EV has a top speed of 186 mph and a final gear ratio of 4.706. They do not give the RPM of the electric motor, but we can figure it out. The diameter of a typical tire for a vehicle this size is about 20 inches. We plug those numbers into the formula and we can solve for RPM.
Max RPM = (186 mph * 4.706 * 336)/ 20 in.
This gives a maximum RPM of over 14,000! That’s a very fast motor and a lot of torque to the wheels.
Remember: a regular ICE engine will redline at about 6,000 RPM. An electric motor can turn a lot faster.
What happens if an ICE car has the same single gear ratio? It’s top speed becomes
(6000 RPM * 20 inches) /(4.706 *336) = 76 miles per hour
The top speed, running flat-out at max RPM, would only be 76 miles an hour. Even for a short period of time, this could do a lot of damage to an internal combustion engine, but it would hardly keep up on a highway. Multiple gears allow ICE cars to go faster without burning out the motor or burning up too much fuel.
Once you understand why an EV only needs one gear, it follows that the transmissions on electric cars are less complex than in a standard automatic transmission. Automatic transmissions are incredibly complex and are made up of seven major components and 800 different parts, many unique to the make and model. It's no wonder they can cost between $1,800 and $3,400 (plus labor) to replace.
On the other hand, EV transmissions have fewer moving parts and less that can break down. This generally translates to fewer maintenance costs compared to ICE vehicles. Tesla Motors says that the entire drivetrain of a Model S has 20 moving parts and Chevrolet maintains that the Bolt has 80% fewer moving parts than a comparable ICE vehicle. With fewer moving parts it is less likely that one will break down which gives EVs a big upper hand in reliability over time with these single-speed transmissions.
So the reason that an electric vehicle only needs a single gear is really quite simple. The electric motor is more powerful and efficient than an internal combustion engine, so it does not need to be amplified by different gears at various speeds and RPMs. Electric motors and the single gear transmission system is a tremendous technological advancement that is possible by switching from gas to electricity. It kind of makes you wonder what took so long.
