Question

Describe how Automatic transmission works

Answer 1

Automatic Transmission is a very complex mechanism due to many moving parts. But lets try to understand it in some simple steps. It consists Input Shaft > Flywheel > Torque Converter > Planetary Gears > Output Shaft

Input shaft and flywheel are mounted on crank shaft which is output shaft of engine but we are taking it as input shaft for our transmission system. Input shaft gets fluctuated energy from engine and flywheel stores it as rotational kinetic energy to reduce fluctuation.

Torque converter in automatic transmission is a replacement of clutch used in conventional manual transmission. Torque converter is a fluid coupling which consists of various parts such as impeller, turbine and stator. Impeller is cnnected to input shaft takes energy from flywheel and drives the turbine through viscous fluid drag which then transmit torque to the transmission.The stator sits between the impeller and turbine. Its aim is to decrease churning losses and to increase torque output by redirecting the fluid as it returns from the turbine to the impeller. The stator let the fluid flow 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. For complete understanding, you can watch video in the link https://youtu.be/leCEmJA0WsI

As of now we know how torque reaches the transmission shaft. Now we have to know how gear changes in automatic transmission system. In conventional manual transmission gears are changed by compound planetary gears. But in automatic transmission there is  Ravigneaux compound planetary gearset.  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. A planetary gearset (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.

So in automatic transmission system torque converter drives a transmission fluid pump which changes the fluid pressure which activates the clutches and brakes of planetary gears. The pump is often a geroter type pump (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.

For more information you can also see this video https://youtu.be/szu-0IqMygA