How transmissions work

In my humble opinion, there are some important machines and technology that have made a huge difference in our lives, like washing machines and GPS navigation technology.

This month, we look at one more that never ceases to amaze me – automatic vehicle transmissions. But first, how do transmission systems work?

In layman’s terms, the transmission is the power train that converts the engine’s force into a controlled source of power. It acts as a mediator between the engine and the wheels, and converts the high power the engine produces into torque (rotational force), which is then transferred to the axles to rotate the wheels.

A transmission system is necessary because a vehicle’s engine creates power and sends it to the crankshaft, but the power produced is often too high and too variable to produce a usable speed for the driver. The engine operates at a high rotational speed (anywhere from 600 to 7000 RPMs), while the wheels rotate at a slower rate (from 0 to about 1800 RPMs).

One could try to slap some wheels on the crankshaft but not only will the car be subject to high speeds, you also won’t be able to control the speed or bring the car to a complete stop.

This is where the transmission comes in. The transmission is able to keep both the engine’s RPM and the RPM of the wheels at optimal rates, sending power to the differentials which turn the wheels through the use of gear ratios.

Gear ratios?

A transmission uses toothed gears that interact with each other in order to produce torque, and the gear ratio refers to the gears’ relation to each other. For example, with an input gear with 20 teeth that interacts with an output gear that has 10 teeth, this 10-teeth gear must make two full rotations in order to spin the gear with 20 teeth once.

A gear ratio is calculated by taking the number of teeth on the output gear and dividing it by the input gear. Thus the gear ratio in this example is 1:2 but it is usually simplified to 0.5:1 in order to tell how many times the output gear must rotate for the input gear to make one full rotation.

Gear ratios must be changed in relation to the speed of the car and that’s why there are multiple gears which are able to switch, and these gears working together is exactly what allows a vehicle to achieve different speeds while in motion.

Without a transmission, cars would be limited to one gear ratio, and that ratio would have to be selected to allow the car to travel at the desired top speed. If you wanted a top speed of 80 mph, then the gear ratio would be similar to third gear in most manual transmission cars.

Imagine trying to drive a manual transmission car using only third gear. If you did, you'd quickly find out that you had almost no acceleration when starting out, and at high speeds, the engine would be screaming along near the redline on the dashboard tachometer.

A car like this would wear out very quickly and would be nearly “undriveable.” The transmission therefore, uses gears to make more effective use of the engine's torque, and to keep the engine operating at an appropriate speed and sound level.

The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission, the same set of gears produces all of the different gear ratios.