While conventional internal combustion engine vehicles have a multi-speed transmission, electric vehicles are generally equipped with only a single-speed transmission, eliminating the need to shift gears during acceleration, a pairing that has to do with the operating principles and limitations of electric motors versus internal combustion engines.

Audi’s Formula E car used a 3-speed gearbox in the past to improve starting power, but eventually found that the complexity of the gearbox did not provide a significant advantage, and the latest FE05 car has switched to a single-speed gearbox.

For a car to move, the most important thing needed for acceleration is torque, and how fast it travels depends on how fast the axles of the drive wheels turn. Therefore, the torque of a car’s internal combustion engine or motor must reach a certain RPM level in order to provide the desired acceleration.

The way torque is generated varies greatly

In a car with an internal combustion engine, torque is generated by the combustion of a mixture of air and fuel compressed by a piston in the engine cylinder. The combustion process pushes the piston downward, which then pushes the crankshaft through the connecting rods to rotate the engine’s output shaft. However, the internal combustion engine this generates torque and the way, the need for the piston constantly up and down the reciprocating motion can be sustained, and this reciprocating process involves a large number of mechanical parts, such as pistons, camshafts and intake and exhaust valve can produce a lot of resistance, greatly limiting the engine speed, the general civilian automobile engines, the upper limit of the speed of only about 6,000 rpm, diesel engines are generally more only about 4000rpm.

The single-ratio gearbox used in electric cars is much simpler in structure.

The motor used in electric vehicles, the electric current passes through the metal coil around the center, the coil will generate a magnetic field, and the magnet on the inner wall of the motor generated by the magnetic field line cut, will let the coil rotate around the center of the axle, the process of converting electric energy into kinetic energy, does not involve any reciprocating mechanical operation, and is the moment the electric current passes through the moment can generate a large torque, and the upper limit of the motor’s axle speed, basically depends on the axle assembly and the coil related components structure can withstand how much centrifugal force, the higher the voltage through the coil, the faster it will be. The upper speed limit of the motor shaft basically depends on how much centrifugal force the structure of the shaft assembly and coil-related components can withstand. The higher the voltage through the coil, the faster the rotational speed is, and it is common for civilian motors to operate at more than 10,000 rpm, while motors for electric formula cars even operate at 20,000 rpm.

In addition to the high rotational speed of electric motors, they also have ideal torque output throughout the entire operating area of more than 10,000 revolutions per minute. Due to the wide range of available torque, engineers only need to equip electric vehicles with a set of reduction gears, and only need to consider whether or not the starting acceleration and top speed performance meets the needs of daily driving.

Reduction Gear Operation Principle

The so-called reduction gears, just like the electric model cars we used to play with when we were kids, are connected to a small gear on the motor output shaft and a large gear on the axle. Assuming that the number of teeth on the large gear is 10 times the number of teeth on the small gear, the gear ratio of this single-ratio reduction gear is 10 to 1, which means that the motor has to make 10 revolutions for the axle or the wheels to make one revolution, and the axle rotation speed is 10 times slower than that of the motor output shaft in the process. In the process, the axle speed is 10 times slower than the motor’s output shaft, which is why this type of single-ratio gearbox is also called a reduction gear.

Taking the Kia Niro EV as an example, the gearbox has a single ratio of 8.206 to 1, which means that the motor has to make 8.206 revolutions before the wheels will make one revolution. The top speed of the Niro EV is 167km/h, and with 215/55 R17 tires, the tires will travel about 2.1 meters for each revolution, and with the top speed of 167km/h, the speed is about 46.39 meters per second, which is equivalent to the wheel’s speed of 46.39 meters per second. At a top speed of 167km/h, that’s about 46.39 meters per second, which equates to 22 wheel revolutions per second, or 1,325 revolutions per minute. Multiply that number by 8.206, and you can calculate that the Niro EV’s motor is running at about 10,880rpm at top speed.

What if the Niro EV wasn’t an electric car, but was instead equipped with an internal combustion engine running at a maximum of 6,000rpm and a single-speed gearbox with a fixed gear ratio of 8.206 to 1? Even if the engine operated at a maximum speed of 6000rpm, the wheels would make 731 revolutions per minute, or 12.2 revolutions per second, and would only travel 25.6 meters per second, giving a top speed of 92.1km/h. To maintain the top speed at 167km/h, the theoretical single-speed gear ratio would have to be changed from 8.206 to 4.527 ratio 1. However, a ratio of 4.527 to 1 is equivalent to the ratio of a 6-speed internal combustion engine car with 5 or 6 gears. Imagine a car starting with 5 gears, even if it doesn’t die, it can only move forward at a turtle speed! It doesn’t meet the needs of real-world driving.

Reasons why internal combustion engines can not be used with a single-speed transmission

Internal combustion engine cars generally have two sets of reduction gears, the first group is the gearbox, 6-speed gearbox of the first group of reduction gears and then subdivided into 6 different gears, gear ratio range is generally from about 4.X than 0.X than 1, the second group of reduction gears commonly known as the “tail gear”, usually a single-ratio, the tail of the gear ratio of more than 4 to 1 or so, the ratio depends on the overall performance of the vehicle and the characteristics of the parts, the tail gear’s role, is to transfer the gearbox to a single-speed gearbox. The role of the tail gear is to further slow down the speed of the output shaft of the gearbox before it is output to the axle. Therefore, when calculating the actual transmission ratio of an internal combustion engine gearbox, it is necessary to multiply the gear ratio of each gearbox by the ratio of the tail gear before it can be directly compared to the ratio of a single-ratio gearbox in an electric vehicle.

A diagram of Volvo’s 6-speed automatic transmission, which is very complex.

An internal combustion engine consists of multiple reciprocating components that create a lot of resistance during operation, limiting the engine’s speed.

Why would a conventional engine car need a multi-ratio gearbox? This is exactly related to the fact that the internal combustion engine runs about twice as slow as an electric motor, and the optimal torque output range of the engine is relatively narrower. When starting out, the first gear with the largest transmission ratio transmits the strongest torque to the axle, which is the most powerful at the beginning, but when the engine speed reaches the top, the car’s speed is only about 60km/h. Therefore, it is necessary to switch to the gears with decreasing ratios, such as 2, 3, 4, etc. to continue to accelerate, and when it goes to the 6th gear, the engine will be in the 6th gear, even if it is in a higher speed cruise. When you go to 6th gear, even if you are cruising at a higher speed, the engine will operate at a lower speed due to the smallest transmission ratio, which can reduce fuel consumption as well as engine and exhaust noise. In the final analysis, the function of a multi-gear transmission in an internal combustion engine vehicle is to maintain the engine in the optimal operating area through the transmission ratio of different gears to enhance the overall operating efficiency, so the simplest single-ratio reduction gear can already meet the needs of daily driving.