MAIN FUNCTIONAL REQUIREMENT: Convert power from engine (T x w) and output a wider range of w without manual switching.
DESIGN PARAMETER: Automatic Transmission
EXPLANATION OF HOW IT WORKS/ IS USED:
An automatic transmission can be categorized into two main parts; the torque converter and the gearbox.
The torque converter is driven by the engines crankshaft. That in turn drives the rest of the transmission. The torque converter is not a direct drive mechanism. It transfers power from mechanical to fluid and back to mechanical. This allows for slip so that the vehicle can come to a stop when the brakes are applied, even though the gears in the gearbox are still engaged. It also absorbs shocks either from the engine to the drive train or from the drive train to the engine. Sudden jerks are far less common than with a manual transmission. A more detailed description of how a torque converter works is available here.
The gearbox is a series of clutches, planetary gears, and brakes. By engaging these components in different combinations, the angular velocity of the drive shaft can be varied much more than by just varying the angular velocity of the crankshaft. For instance, when the transmission modeled in the preceding diagram is in first gear, the Forward Drive Clutch and the Second Planet Carrier Brake Band are engaged. The Sun Gear Brake Band and the Reverse High Clutch are not engaged however. By following the power flow chart in the diagram one can see how the parts would move in the transmission.
The engagement and disengagement of the gearbox components are controlled by another subsystem. This subsystem consists of shift valves, the valve body, the oil pump and a governor. This governor is linked to the output shaft and to the throttle valve in the automobile. The faster the drive shaft spins, the faster the governor spins. The governor uses centrifugal force to direct oil from the oil pump through the shift valves to the appropriate clutches and brake bands. As you accelerate, the shift valves move out directing the oil through the valve body to the gear shifting mechanisms in the transmission. When you slow down, the opposite thing happens.
Pcrank = Tcrank x wcrank As a function of time
Using the impeller, it transfers the power to the Transmission Fluid. The fluid then transfers the power back through the turbine. At this point, the power is transferred mechanically through the combinations of clutches and planetary gears and eventually out to the drive shaft. Some of the power is again transferred to the transmission fluid by a hydraulic pump. This power is used to "run" the automatic transmission. That is to say, it is used to shift the gears.
Power is also dissipated in the transmission through coulomb friction and viscous dissipation. This power will be denoted at Ploss.
Ploss = f(friction, viscous effects, gear changes......)
The power, which can then be derived is:
Pout = (Tout x wout) = Pin - Ploss = (Tin x win) - Ploss
LIMITING PHYSICS:The performance/use of the transmission is limited by its:
The efficiency of the transmission is defined as Pout / Pin =h. The efficiency decreases over the lifetime of the transmission as parts wear and as the transmission fluid collects dirt. Efficiency also varies during each operation. As the transmission fluid heats up, the viscosity goes down. It becomes more efficient in that there is less drag on the gears and on the fluid flows to the clutches and brakes. It also means that less power is transferred through the torque converter, and this leads to less efficiency. The overall change in efficiency is the sum of the two affects.
The transmission fluid is the key to why the automatic transmission works. As with all fluids, transmission fluid has certain characteristics which limit/determine the transfer of power in the transmission.
The automatic transmission must fit into a certain specified place. Originally this was the same volume as needed for a manual transmission. This volume restriction constrains the size and number of parts inside the transmission, and thus constrains the number and/or size of gears and the mechanisms that are used.
WHERE TO FIND AUTOMATIC TRANSMISSIONS:
You can find automatic transmissions primarily in automobiles, although some buses and other larger vehicles use them too.
General Info | Syllabus