- What Does a Transmission Do?
- A Conventional Transmission
- How Does an EV Motor Work?
- Can EVs Have Multiple Gears?
- Multiple Motors Rather Than Multiple Gears
When you drive an electric vehicle for the first time, your car might not even have a gear lever or gear stick allowing you to shift gears. Step on the accelerator (not the “gas pedal”), and you won’t feel any transmission shifting gears. So do electric vehicles even have transmissions?
The answer depends on who you ask. Scan reputable automotive websites and you will find different answers, from “no transmission” to “a transmission of sorts” and “a single-speed transmission.” Here, we’ll straighten out the facts and get to the bottom of what powers your electric vehicle.
What Does a Transmission Do?
It helps to define what a transmission does first. A transmission is a machine that transmits power, so in that strictest, mechanical-engineering sense, every car has a transmission.
A vehicle transmission transmits the rotating power of the energy source, whether an electric motor or an internal combustion engine (ICE), through a set of gears to a differential, the unit that spins the wheels. But in common parlance, many people think of the transmission as the part of an engine that shifts gears from low-speed to high-speed or from forward to reverse, as in “manual transmission” and “automatic transmission.” That’s where things get cloudy.
A Conventional Transmission
In a gas-powered car, an internal combustion engine must rotate at a range of speeds so that it neither stalls (because it’s rotating too slowly) or overheats (because it’s rotating too quickly). That range is roughly between 500 and 7,000 revolutions per minute (RPM). To compensate for that limitation, the transmission adjusts the ratio between the rotation of the engine and the rotation of the wheels by shifting between lower and higher gears.
The rotation of the lowest gear is slower than the engine, allowing the engine to run at high enough RPMs so as not to stall. The lowest gear rotates slowly because it is the biggest gear in size, which transfers more force but less speed to the wheels since the gear needs to move the vehicle forward from a dead stop.
The highest gear, by contrast, is the smallest and runs in “overdrive,” meaning it rotates more quickly than the engine, allowing the car to travel at high speed without the engine overheating. In a manual transmission vehicle, engaging the clutch disengages one gear so that you can shift to another. An automatic transmission does the same thing, but without driver intervention.
How Does an EV Motor Work?
Among the distinguishing features of electric vehicles is the quiet, instantaneous, and smooth acceleration. That’s because propulsion in an electric vehicle works differently. As their names suggest, the main difference between electric vehicles and gasoline-powered cars is the fuel source. When you step on the accelerator of an electric vehicle, electricity is sent from the battery to the electric motor, sending it rapidly spinning.
What Is Horsepower?
A motor’s horsepower is defined by speed and torque. The speed is defined as the rate at which the motor rotates, while the torque is the amount of rotational force that the motor puts out. When a motor with a steady supply of power rotates quickly, it loses torque. When it spins slower, torque is increased.
Most EVs have a single AC (alternating current) motor connected to a gearbox. What is in the gearbox is what some people call a transmission since it is indeed a set of gears that transmits the rotation of the motor to the rotation of the wheels. But it is more accurately called a single-speed gear reduction unit since the multiple gears in the gearbox are always connected to each other and thus all spin at the same time.
The gear reduction unit reduces the RPMs of the motor to the more reasonable RPMs of the wheels at a ratio of roughly 10 to 1. So there is no clutch, no disengagement of gears, and no shifting between differently sized gears depending on the needs of the vehicle—in other words, no transmission.
An AC motor can rotate anywhere from zero to 10,000 RPM or more. (The motor in a 2021 Tesla Model S Plaid can rotate up to 23,308 RPM, one of the reasons it can accelerate up to 200 miles per hour.) This gives EVs plenty of torque at a wide range of speeds, with a “sweet spot” between enough torque and enough speed in the 30-40 mph range. Energy passes directly and nearly instantaneously from the motor through the gearbox to the wheels rather than through a transmission, and the transition from one speed to the next does not have to shift from one gear to the next, making for smooth and quiet acceleration.
Is There a Reverse Gear?
Because the motor in an electric vehicle usually uses alternating current, there is no need for a reverse gear. The motor merely spins in the opposite direction.
The lack of transmission reduces the friction (and thus wear-and-tear) that comes with engaging and disengaging multiple gears. The fluid transition also better maintains a vehicle’s forward momentum than does gear shifting, which is one of the reasons why electric vehicles are far more efficient at using energy.
Overall, an electric vehicle on average converts 77% of the electricity stored in its battery toward moving the car forward, while a gas-powered car converts from 12% to 30% of the energy stored in the gasoline in its tank. Much of the rest is wasted as heat. The transmitting power from an EV’s motor to its wheels is 89% to 98% efficient, depending on the vehicle, whereas in an ICE car, the same process from engine to wheels is only 14% to 26% efficient.
Can EVs Have Multiple Gears?
Any vehicle, ICE or EV, needs more torque than speed to propel the car from a dead stop, and more speed than torque once the vehicle already has forward momentum. So wouldn’t EVs benefit from multiple gears? Yes, but at the cost of a more complex system requiring more parts, more weight, more labor, and a larger supply chain—in other words, more cost to the consumer both upfront and on maintenance.
Some newer EVs, including the Audi e-tron GT and the Porsche Taycan, do have multiple gears, which allow them to deliver more torque to the wheels to increase acceleration. The planned Jeep Magneto will even have a manual transmission with multiple gears. Race cars such as those in the all-electric Formula E have transmissions as well.
When electric trucks, especially 18-wheelers, come on the market, it is possible they will have multiple gears and transmission, but given the wide range of RPMs possible with an electric motor, they may have as few as two: one for torque, the other for cruising speed, with the shift from one to the other at around 30 mph. (The forthcoming Tesla Semi will have only a single-speed gear reduction.) The same applies to smaller vehicles where the ability to tow or carry heavy loads is important.
Continuously Variable Transmission
Some ICE and hybrid vehicles have continuously variable transmissions (CVT), a form of automatic transmission which accelerates seamlessly from speed to speed, using pulleys rather than gears. CVT systems have recently been introduced for electric vehicles, which can increase the torque at lower speeds to accommodate heavier vehicles and loads. This eliminates the need for EV engineers to find a “sweet spot” compromise between torque and speed.
Promising greater efficiency, CVT systems may allow electric vehicles to increase their range—a key concern of potential EV buyers.
Multiple Motors Rather Than Multiple Gears
Some EVs solve this problem by having multiple motors with different gear ratios to deliver more or less torque, depending on the needs of the vehicle, with electronics more efficiently shifting electrons to different motors rather than a transmission less efficiently shifting gears. The high-performance Lucid Air comes in either dual or tri-motor versions, for example, as do many Tesla vehicles.
And unlike gears in an ICE vehicle, the multiple motors in an EV can be used simultaneously, giving the vehicle both speed and torque, increased traction, or greater agility. The Rivian electric pickup trucks even have independent motors attached to each wheel, allowing the truck to perform “tank turns.”
