What Transmissions Do

April 3rd, 2017  Part 1 of 6

Before we get into the specifics of how a manual transmission works,  let’s talk about what transmissions do in general.

As discussed in our primer on how a car engine works, the engine of your vehicle creates rotational power. To move the car, we need to transfer that rotational power to the wheels. That’s what the car’s drivetrain — of which the transmission is a part of — does.

But there are a couple problems with power produced by an internal combustion engine. First, it only delivers usable power, or torque, within     a certain range of engine speed (this range is called an engine’s power band). Go too slow or too fast, and you don’t get the optimal amount of torque to get the car moving. Second, cars often need more or less torque than what the engine can optimally provide within its power band.

To understand the second problem, you need to understand the first problem. And to understand the first problem, you need to understand     the difference between engine speed and engine torque.

Engine speed is the rate at which the engine’s crankshaft spins. This is measured in revolutions per minute (RPMs).

Engine torque is how much twisting force the engine generates at its shaft for a particular speed of rotation.

A car mechanic gave this nice analogy to understand the difference between engine speed and engine torque:

Imagine you were an engine and you’re trying to drive a nail into a wall:

Speed = How many times you hit the nail head in a minute.

Torque = How hard you hit the nail every time.

Think back to the last time you were hammering nails. If you were hammering really fast, you probably noticed that you weren’t striking the nail with much force. What’s more, you probably exhausted yourself from so much frantic swinging.

Conversely, if you took your time between each swing, but made sure that each swing you did make was as hard as possible, you’d drive the nail in with fewer swings, but it might take you a bit longer because you’re not swinging at a steady tempo.

Ideally, you’d find a pace of hammering that allowed you to hit the nail head several times with a good amount of force with each swing without tiring yourself out. Not too fast, not too slow, but just right.

Well, we want our car’s engine to do the same thing. We want it to spin at the speed that allows it to deliver the needed torque without working so hard that it destroys itself. We need the engine to stay within its power band.

If an engine is spinning below its power band, you won’t have the torque you need to move the car forward. If it goes above its power band, torque starts dropping off and your engine starts sounding like it’s about to break due to stress (sort of like what happens when you try hammering too fast — you hit the nail with less power and you get really, really tired). If you’ve revved your engine until the tachometer gets into the red, you understand this concept viscerally. Your engine sounds like it’s about to die, but you’re not moving any faster.

Okay, so you understand the need to keep a vehicle running in its power band so that it’s working effectively.