Ever wonder how your transmission knows to shift gears? Why is it that when you stop, the engine doesn’t die? We’re here to show you how cars work. We recently looked at . This week it’s regular ol’ slushbox time.

This story was originally published on July 1, 2013

Welcome to Sunday Matinee, where we highlight classic car reviews or other longer videos I find on…

Automatic transmissions – they’re pretty much black magic. The sheer number of moving parts makes them very difficult to comprehend. Let’s simplify it a bit to get a basic understanding of how it all works in a traditional, torque converter-based system.

Your engine connects to your transmission at a place called a bell housing. The bell housing contains a torque converter for automatic transmission-equipped vehicles as opposed to a clutch on manual vehicles. The torque converter is a fluid coupling whose job it is to connect your engine to your transmission and thus to your driven wheels. The transmission contains planetary gearsets which are in charge of providing different gear ratios. To get a good understanding of how the whole automatic transmission system works, let’s have a look at torque converters and planetary gearsets.

First and foremost, your engine’s flex plate (basically a flywheel for an automatic) connects directly to a torque converter. So when the crankshaft rotates, so does the torque converter housing. The goal of the torque converter is to provide a means by which to connect and disconnect the engine’s power to the driven load. The torque converter takes the place of a clutch on a conventional manual transmission. How does the torque converter work? Well, have a look at the video above. It explains the basic principles behind a fluid coupling. Once you’ve watched that, continue reading to see how a torque converter differs from a standard fluid coupling.

The major components of a torque converter are: the impeller, the turbine, the stator, and the lock-up clutch. The impeller is part of the torque converter housing, which is connected to the engine. It drives the turbine via viscous forces. The turbine is connected to the transmission input shaft. In essence, the engine turns the impeller which imparts forces on a fluid, which then rotates the turbine, sending torque to the transmission.

The transmission fluid flows in a loop between the impeller to the turbine. The fluid coupling in the video above suffers from severe churning losses (and consequent heat buildup) as the fluid returning from the turbine has a component of its velocity that opposes the rotation of the impeller. That is, the fluid returning from the turbine works against the impeller’s rotation and thus against the engine.

The stator sits between the impeller and turbine. Its goal is to minimize churning losses and to increase torque output by redirecting the fluid as it returns from the turbine to the impeller. The stator directs the fluid so that the majority of its velocity is in the direction of the impeller, helping the impeller move, and thus adding to the torque produced by the motor. This ability to multiply torque is why we call them torque converters, not fluid couplings.

The stator sits on a one-way clutch. It can rotate in one direction only when the turbine and impeller are moving at approximately the same speed (like during highway driving). The stator either rotates with the impeller or not at all. Stators don’t always multiply torque, though. They provide you with more torque when you’re either at stall (applying the brakes at a stop light, for example) or while accelerating, but not during highway cruising.

In addition to the one-way clutch in the stator, some torque converters contain a lock-up clutch whose job it is to lock the turbine with the torque converter housing so that the turbine and impeller are mechanically connected. Eliminating the fluid coupling and replacing it with a mechanical connection ensures that all of the engine’s torque is transmitted to the transmission input shaft.

So, now that we’ve figured out how the engine sends power to the transmission, it’s time to figure out how in tarnation it changes gears. On a conventional transmission, changing gears is the job of a compound planetary gear set. Understanding how planetary gear sets work is a bit tricky, so let’s have a look at a basic planetary gear set.

A planetary gearset (also known as an epicyclic gear set) consists of a sun gear in the center, planet gears that rotate around the sun gear, a planet carrier that connects the planet gears, and a ring gear on the outside that meshes with the planet gears. The basic idea behind a planetary gear set is this: using clutches and brakes, you can prevent certain components from moving. In doing so, you can alter the input and output of the system and thus change the overall gear ratio. Think of it this way: a planetary gear set lets you change gear ratios without having to engage different gears. They’re all already engaged. All you have to do is use clutches and brakes to change which components rotate and which stay stationary.

The final gear ratio depends on which component is fixed. For example, if the ring gear is fixed, the gear ratio will be much shorter than if the sun gear is fixed. Knowing full well the risks associated with ploppin’ an equation on here, I’m gonna put one in anyway. The following equation will tell you your gear ratios depending on which component is fixed and which are in motion. R, C, and S represent the ring gear, carrier, and sun gear. Omega simply represents the angular speed of the gears, and N is the tooth count.

The way it works is thus: let’s say we decided to keep the planet carrier stationary and make the sun gear our input (thus the ring gear is our output). The planets are able to rotate, but they cannot move since the carrier cannot move. Omega_c is zero, so the left side of the equation above is gone. This means that when we rotate the sun gear, it sends torque through the planet gears to the ring gear. To figure out what the gear ratio would be, we simply solve the above equation for Omega_r/Omega_s. We end up with -N_s/N_R, that is, the gear ratio when we fix the carrier and make the ring gear our output and the sun gear our input is simply the ratio of the number of teeth between the sun gear and ring gear. This is negative, since the ring spins in the opposite direction of the sun gear.

You can also lock the ring gear and make the sun gear your input and you can lock the sun gear and make the carrier your input. Depending on what you lock, you’ll get different gear ratios, i.e. you’ll get different “gears.” To obtain a 1:1 gear ratio, you simply lock the components together (you only have to lock two to do this) so that the crankshaft spins at the same speed as the transmission output shaft.

So how do the brakes and clutches move to change gears? Well, the torque converter is also in charge of driving the transmission fluid pump. The fluid pressure is what activates clutches and brakes in the planetary gearset. The pump is often a (a gear pump) meaning that a rotor spins in a pump housing and as it spins, it “meshes” with the housing. This “meshing” creates chambers that change in volume. When the volume increases, a vacuum is created- this is the pump inlet. When the volume decreases, the fluid is compressed or pumped by the meshing of the gears- this is the pump exit. A hydraulic control unit sends hydraulic signals to change gears (via band brakes and clutches) and to lock the torque converter.

Note that most modern automatic transmissions use a . This gearset has two sun gears (a small and a large), two sets of planets (inner and outer), and one planet carrier. This is essentially two simple planetary gearsets in one.

So now that we looked at torque converters and planetary gears, let’s have a look at the video below to see how it all goes together: