| A straight spin on torque converters |
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| Written by Chris Stryker | |
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For upper level mechanical engineers (which I am not yet), automatic transmissions may seem to be simple devices. To many mechanics, however, automatic transmissions are sort of an enigma; their operation almost seems like a sort of black magic. This article is targeted at those who fall into the second category, though corrections or clarifications are certainly welcomed. Admittedly, some generalizations have been made in the hopes of making things easier to understand. At their heart, there are only a handful of pieces that the mechanic needs to understand in order to tackle an automatic transmission rebuild. This week will hopefully serve as the first in a series of upcoming articles detailing the rebuild of a GM automatic transmission. Don’t let that throw you if you’re a Ford or Mopar guy, however, because really, they are all quite similar in their basic operating principles. Inside, a converter contains three main elements: the stator, impeller, and turbine. Essentially these are three separate discs with curved fins, through which fluid passes. While whole books have been written on converter design, a simplified view is this: the impeller is affixed to the input side of the drive line, and forces fluid towards the turbine with great force. This fluid then hits the turbine which turns the flow energy back into rotational energy, much like blowing on a pinwheel causes it to spin. When sitting at a stop sign, for example, the impeller is turning and pumping fluid, but the turbine is held stationary by the weight of the vehicle and the braking force applied to the wheels. At this stage it is acting as a simple fluid coupling, and allowing the two elements to slip. As you begin to accelerate, the stator comes in to play. The stator is the third element, which is sandwiched in between the impeller and turbine, and is what distinguishes a torque converter from a simple fluid coupling. This small, yet critical, piece is attached to a one-way clutch that then slides over a splined shaft on the transmission case. Keep in mind, there are two distinct types of flow occurring at all times: rotational and vortex. Rotational flow is, as the name implies, around the circumference of the unit. Vortex flow, however, is the flow of fluid from the impeller to turbine. As you accelerate from a stop, the fluid pressure due to vortex flow attempts to rotate the stator backwards, but the one-way clutch exerts a reaction force and prevents it from doing so. As a result, the fluid strikes the turbine with substantially greater force than it left the impeller and multiplies the torque. As the turbine speed picks up, however, the fluid gains more rotational flow than vortex flow between the driving and driven elements, and the stator begins to turn in the same direction as the turbine. It gradually accelerates (and thus yields less torque multiplication), until all three elements are spinning at nearly the same speed (after efficiency losses are accounted for). The speed at which this occurs, is known as the “stall speed,” and should ideally occur at the same speed at which the engine has peak torque. In most production cars, however, the engine has enough torque in the lower RPM range to tolerate a stall speed a fair bit below maximum torque output. The upshot of this is an overall more efficient converter (less slippage), as well as far less heat being created. In cars designed for drag racing, however, this becomes much more critical; most of these cars have engines designed for maximum peak horsepower at the expense of low end torque. Custom, higher stall converters do a very good job of making up for that. Finally, some newer (post 1980) converters have a “lock-up” system. This system is usually actuated either hydraulically or electrically and physically locks the impeller and turbine together after stall speed has been reached. This results in higher efficiency and lower operating temperature. For a street driven car, this is very desirable and is a direct cause of improved gas mileage. Hopefully this article has eliminated some of the voodoo that surrounds torque converters. For further reading, “Automatic Transmissions,” by Mathias F. Brejcha is valuable resource which offers far more detail, and has some terrific illustrations to go along with it. Understanding converter operation is not an easy task, but once understood, the rest of a transmission’s operational principles seem simple in comparison. Next in the series will be an overview of basic internal operating principles, which are fortunately much more transparent. |
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