Electric cars operate very differently from internal combustion engine vehicles. One of the most noticeable differences is that electric cars do not have multiple gears like traditional cars do. Instead, they just have a single speed transmission. This simple design is made possible by the characteristics of electric motors.
How Do Electric Motors Work?
Electric motors provide torque, which is rotational power or twisting force, from a standstill. They have high torque at low speeds, which allows them to accelerate quickly. Unlike gas engines, the maximum torque is available at all speeds. The output speed of an electric motor depends only on the input power, not the load.
Electric motors have a smooth torque curve, meaning the twisting force is consistent across a broad speed range. Gas engines, on the other hand, have a torque curve that varies based on engine speed (rpm). Their torque peaks in a certain rpm range and falls off at lower and higher speeds. That’s why gas engines need multiple gears to keep the engine operating in its optimal rpm range as the vehicle speeds up.
No Gearbox Needed
The broad constant torque output of electric motors eliminates the need for multispeed transmissions. A single gear ratio provides effective acceleration from 0 to 100+ mph. Electric motors can spin at over 10,000 rpm, far higher than gasoline engines. The high rpm allows them to maintain power with a single gear.
Some key advantages of a single speed gearbox:
- Simpler, lighter design
- Lower manufacturing cost
- Less maintenance due to fewer moving parts
- Smooth acceleration without shifting gears
Advantages of High Torque
The high low-end torque is a major factor enabling electric cars to get by with a single gear. Here are some of the benefits:
- Excellent acceleration from 0 mph – electric motors can produce 100% torque instantly
- High climbing power – electric vehicles can handle steep hills without downshifting
- Efficient cruising – the motor doesn’t have to work hard to maintain highway speeds
The motor’s torque delivery also allows regenerative braking, which converts momentum back into electricity when decelerating. This helps recharge the battery and enhances driving range.
Challenges of a Single Speed Design
While a single gear transmission makes sense for most electric passenger vehicles, there are some potential downsides:
- Less top speed – without multiple gears, top speed is limited when the motor spins too fast
- Efficiency – operation may be less efficient at very high or very low vehicle speeds
- Heat management – gearboxes help dissipate motor heat which can be an issue
Some high performance and specialty electric vehicles use a 2-speed manual transmission or have multiple drive modes to provide better efficiency across all speeds. But the added complexity reduces reliability and increases cost.
One Gear is Sufficient for Most Driving
A single speed gearbox meets the performance needs of most electric car owners given the motor characteristics and real world driving. Multiple gears provide little practical benefit for daily commuting and local travel which rarely involves high speeds or steep mountain grades. The simplicity and smooth operation of a single speed is perfectly adequate for normal urban and highway driving.
Differences from Traditional Transmissions
It helps to understand how the single speed gearbox in an electric car differs from a traditional transmission:
| Electric Vehicle | Gasoline Vehicle |
|---|---|
| Single gear ratio | 5-10 gear ratios |
| No shifting between gears | Manual/automatic gear shifting |
| Constant torque | Peak torque at certain RPM |
| Full acceleration at any speed | Gearing affects acceleration |
| Simpler, lighter transmission | Complex, heavy transmission |
This illustrates how the electric motor characteristics allow a single gear to replace a complex multi-gear transmission. The simple design provides smooth consistent power without inefficient gear shifting.
Electric Vehicle Drivetrain
Here are the main components of an electric car drivetrain using a single speed gearbox:
- Electric Motor – Provides torque to the wheels from a standstill. AC induction motors are common.
- Single Speed Gearbox – A single gear ratio to transfer power from the motor to the wheels.
- Differential – Divides torque between the wheels while allowing them to spin at different speeds when turning.
- Drive axles – Transfer torque to spin the wheels.
This simplified drivetrain eliminates unnecessary complexity of multiple gears. The electric motor, inverter, and single speed gearbox are often combined into a compact integrated unit.
Motor Controllers
Sophisticated motor controllers manage the motor’s torque and speed while maximizing efficiency. They regulate electrical current and frequency to vary the motor’s output as needed. Regenerative braking is also handled electronically by the motor controller.
Conclusion
In summary, electric motors provide abundant torque across a wide rpm range which removes the need for multiple gears. A single speed gearbox provides smooth acceleration while reducing weight, mechanical complexity, and manufacturing cost. While a few high end electric sports cars utilize multi-speed gearboxes, a single gear meets the needs of most electric passenger vehicles.