How do Automatic Transmissions Work

Since the Thirties auto transmissions have provided convenience for drivers, and still play a part in development.

An automatic transmission makes a car easier to drive. Most executive cars have automatic gearboxes, and while there are various types of ‘self-shifting’ transmissions on the market, the most well-established is the conventional torque converter automatic.

The key component is the torque converter itself. This provides the coupling between the engine and the transmission, converting engine motion into vehicular movement. This is not a direct connection, though – instead, it is a hydraulic connection. Automatic cars are said to use a hydraulic coupling or fluid coupling.

There are three main components in a torque converter – the impeller, turbine and stator. The impeller and turbine face one another in a round metal case that looks like a doughnut: the impeller is connected to the engine and the turbine is connected to the transmission. The case is sealed and filled with transmission fluid, and both impeller and turbine are fitted with blades that catch the fluid when rotated.

When the engine rotates, the impeller begins to circulate. As this also swirls the transmission fluid, the turbine thus begins to move in the same direction – this is how engine drive is sent to the transmission. Due to centrifugal force, the fluid is forced to the outside edge of the blades. The stator redirects this back to the side of the turbine – this flow of fluid is what multiplies the engine power.

The torque converter is connected to a planetary gear set. This consists of a central sun gear, around which several planetary gears and an outside ring gear can rotate. Ratios are altered by locking and unlocking different combinations of gears. Most automatic transmissions consist of compound planetary gears, where two gear sets are connected in series.

Meaning of letters in automatic transmission

P for Park – In this mode, the output shaft is locked by a device called a ‘parking pawl’: a pin locks the output shaft to the transmission casing. It is the first position on all automatic transmission shift patterns: the ordering layout was standardized in the Sixties.

R for Reverse – The planetary gear set is locked, so it can’t move. Power is applied to the ring gear, which causes the sun gear to turn in the opposite direction. This enables the vehicle to move backwards. Most gearboxes have a shift lock, so you have to press a button on the shifter to engage reverse: electronic computers mean it doesn’t engage when you’re going forwards.

N for Neutral – This mode disengages the transmission from the road wheels – it enables the car to be pushed freely without disturbing the gearbox. Most cars cannot be started unless they are either in ‘N’ or ‘P’.

D for Drive – In this mode, the transmission can engage the full set of gear ratios automatically. The torque converter allows the vehicle to stop with the transmission in gear, with the engine still running. All the driver does is engage the ‘D’ setting and drive.