Automatic transmission basics

Automatic transmission basics
The basic purpose of an automatic transmission is to provide a forward and reverse driving range that increases the torque between the engine and the drive wheels.
Rather than having a solid conventional clutch as in a manual transmission, automatic transmissions utilize a fluid coupling device called a torque converter to transmit power from the engine to the transmission. Hydraulic pressure in the converter allows power flow from the torque converter to the transmission’s input shaft. The only time a torque converter is mechanically connected to the transmission is during torque converter lockup, generally at relatively high speeds (highway speeds) and no load.
The input shaft of the automatic transmission drives a planetary gearset which provides different forward gears, a neutral position, and reverse. Power flow through the gears is controlled by multiple-disc clutches, one way valves and friction bands. These friction elements either hold or turn the gear sets to provide different gear ratios. The control valve assembly (valve body) controls the hydraulic pressure required to operate the clutches and bands that shift the gears automatically. By holding different parts of the planetary gearset, different gear ratios are obtained. When the gear selector is moved to a given forward driving range, the “drive” position for example, the transmission shifts itself up or down depending on vehicle speed, throttle position and engine load.
Electronically controlled automatic transmissions were first introduced in the mid-1980s, with electronics used to control the timing and length of shifts, as well as to engage and release the clutches. Newer advances in automatic transmissions include continuously variable transmissions (CVT), which do not use specific gear ratios, rather an infinite number of ratios based on a belt moving on a variable diameter spool. Continuously slipping torque converters are also gaining popularity as they improve fuel economy. As automatic transmissions employ a fluid coupling between the engine and transmission, they have always been at a fuel economy disadvantage to manual transmissions, which employ a solid clutch coupling.
Planetary Gear System
Automatic transmissions use a gear system that does not require gear shifts to change gear ratios. A simple planetary gearset is made up of three elements, the sun gear, the planet carrier assembly and the internal ring gear.
The sun gear is so called because it is at the center of the gearset. The other gears revolve around it like planets around the sun. The planet carrier assembly holds the planet pinions within a cage, or carrier. The pinions rotate on pins and are meshed with the sun gear at all times. The internal ring gear has its teeth cut on the inside of the gear, and these teeth are constantly in mesh with the pinions. This planetary system rotates on the same axis, with input and output power on this same axis, so that the gears are never disengaged to change ratios.
By causing one of the planetary gearset members to be stationary, and the other two still turning, a different output ratio is obtained. The holding of one member is done by a band or clutch pack using hydraulic pressure routed by the valve body. Each gearset member is attached to an input or output shaft in order to transfer power when the gear ratio is changed.
Holding Devices
There are two elements used to hold members of the planetary gearset, so another member can provide the output drive. These devices are transmission bands and multiple-disc clutches, also called clutch packs.
Transmission bands tighten around the outside of a drum and keep the drum from turning. The drum is engaged with a member of the gearset, so if the drum does not turn, that member of the gearset does not turn. The band is anchored at one end and mechanical action against the other end is activated by hydraulic pressure via the valve body, which tightens the band.
Multiple-disc clutches hold one or more members of the planetary gearset, via a hydraulically activated piston that locks the clutches together when hydraulic pressure is applied to the piston. The inside diameter of the clutch pack is splined to an input or output shaft. Multiple-disc clutches are used more than any other type of holding device in an automatic transmission. There may be as many as six different clutch packs in an overdrive transmission. Overdrive is a gear ratio in which the output shaft turns faster than the input shaft, which provides very low torque, but very good high speed fuel economy.
Torque Converters
 The torque converter couples and uncouples the engine and the transmission. Converters use impellers and turbines, with vanes, to develop fluid flow and the transfer of torque.
In addition to an impeller and turbine, torque converters use a stator to accelerate the flow of fluid from the impeller to the turbine and back to the impeller, thereby multiplying torque.
Whenever the impeller moves faster than the turbine, torque is multiplied.
A torque converter consists of:
• An impeller (pump, or driven member)
• The turbine (driven member)
• The stator (reaction member)

The torque converter housing is filled with fluid by the transmission pump. The impeller is the driving member with its curved vanes picking up fluid in the converter housing and directing it toward the turbine.
The turbine is the driven member, having vanes that receive fluid from the rotating impeller.
With sufficient fluid flow from the impeller, the turbine rotates to turn the transmission input shaft.
The stator is the reaction member whose curved vanes multiply the fluid force being sentback to the impeller by the turbine.
The impeller is attached to the crankcase via a flexplate or flywheel, and the turbine is attached to the front of the transmission input shaft.
Fluid is pumped into the torque converter housing by the transmission pump. As the impeller is turned by the engine, its vanes pick up fluid in the housing and throw the fluid outward toward the turbine.
As the fluid moves, it follows two flow paths:
• Rotary flow: The oil flow path, in a torque converter, that is in the same circular direction as the rotation of the impeller (engine crankshaft rotation).
• Vortex flow: The oil flow path, in a torque converter, that is at a right angle to the rotation of the impeller and to rotary flow.
Three elements of a torque converter are curved backward to boost the acceleration of the oil flow as fluid leaves the impeller. The turbine vanes are curved on their inlet sides, back toward the impeller to absorb as much energy as possible from fluid moving through the turbine.
Turbine vanes also curve to absorb shock and a loss of power whenever there is a sudden change in oil flow between the impeller and the turbine.
Turbine vanes are curved to take advantage of the basic hydraulic principle that the more the direction of a moving fluid is diverted, the greater the force exerted by the fluid on the diverting surface.
Stator vanes curve in the opposite direction from impeller and turbine vanes. Since the stator is located between the impeller and the turbine, its curved vanes multiply the force returning to the impeller from the turbine.
Fluid that leaves a moving impeller to enter a turbine does not exit as rotary flow or vortex flow. It exits as a combination of both. Fluid from the impeller strikes the vanes of the turbine. When the speed and force of the fluid hitting the turbine vanes is great enough, the turbine rotates.
Rotary flow and vortex flow moving through the turbine vanes at the same time create resultant force, which is the combined force and flow direction in the torque converter.
The stator is the reaction member whose curved vanes multiply fluid force being sent back to the impeller by the turbine. The stator is mounted on a one-way overrunning clutch that is splined to a stationary extension of the oil pump assembly. This extension is called the reaction shaft or stator support.
The hub at the rear of the torque converter housing, which is the back of the impeller passes over the stator support and through the front seal into the oil pump. The converter engages the drive gear or rotor of the pump to drive it. Because the converter hub is part of the housing that is bolted to the engine, the oil pump is turning and pumping fluid whenever the engine is running.
Because of the complexity of fluid flow through the converter, there is a great deal of stress on the fluid in the automatic transmission. The converter serves as a direct link between the engine and drive wheels of the vehicle.
Typically, the converter has no means of draining the fluid it contains, which is why less than 50 percent of the transmission fluid is changed during a conventional drain and refill service.
Valve Body
The valve body is where many of the hydraulic valves are housed. This is where fluid is channeled and directed throughout the transmission to determine when shifting of gear ratios occurs. This is accomplished in relation to engine load and throttle position


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