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Torque Converters and Fluid Dynamics

The twist and spin of power conversion

While not at all new technology, the automatic transmission is the stuff of miracles. Step on the gas and engine energy is converted into forward motion, just like that. No clutch. No fuss.

In between the engine and the transmission is the mighty torque converter. Think of a torque converter as a miniature transmission with an infinite number of gears between idle and near 1:1 efficiency at full throttle. The torque converter accomplishes this task through the wonder of fluid dynamics.

The Basics

The classic analogy used to explain the basics of torque converter operation is that of two fans facing each other. With one fan turned on and the other off, both sets of fan blades will spin. The blades of the turned off fan are being driven by the energy created by the one turned on.

The transmission fluid inside a torque converter behaves the same way as the air between the fans. One fan is connected to the engine and the other is connected to the transmission. Between the two fans is transmission fluid. While the two fans analogy goes far to explain the operation of a fluid coupling, it is really only two-thirds of the story of a torque converter.

Working from the engine crankshaft back, the first part is the cover. The cover houses the torque converter and is connected directly to the engine crankshaft by way of a flexplate and bolts. Welded to the back of the cover is the driving member of the converter, or the impeller. Since the impeller is physically connected to the cover, it always spins in direct relation to the crankshaft, and also drives the fluid pump in the transmission.

The driven part of the converter is the turbine. The turbine spins inside the cover, and is connected directly to the input shaft of the transmission. The fluid coupling energy-created impeller rotation spins the turbine. In between the impeller and the turbine is the thinking part of the torque converter—the stator—or reactor. The stator redirects the flow of fluid between the impeller and the turbine, and is the key to smooth converter operation. The stator creates the multiplication of torque by redirecting fluid as it flows from the center of the turbine.

Fluid Coupling

Power applied to a direct fluid coupling, such as a turbine and impeller with no stator, would quickly bring the coupling to the point where the two parts and the fluid are rotating as a solid mass. This is known as the coupling phase, or the point where the turbine is turning 9/10ths as fast as the impeller. The fins inside the impeller and turbine force the transmission fluid in two directions at once to achieve this coupling. The transmission fluid flows in both a rotary and vortex motion at the same time.

To visualize these forces imagine spinning a half empty bucket of water around and around over your head. Rotary flow and centrifugal force holds the water in the bottom of the bucket. Vortex flow is taking place as the water circulates inside the bucket as you spin it around.

Inside the torque converter, rotary flow forces the fluid to the outside of the impeller and turbine assembly, and creates a centrifugal force that makes it spin ever faster. Vortex flow created by the blades channels fluid in a vortex within the impeller and turbine and helps this situation occur. This forces the fluid to circulate from the outside and back through the centers of the turbine and impeller. At the coupling stage, at 9/10ths, rotary flow overcomes vortex flow and the two halves spin essentially as one. Vortex flow is crucial to get things going with the help of the stator.

Multiplying Reactor

At idle, the stator locks on its one-way clutch, stops turning, and redirects the flow of fluid exiting the center of the turbine. On throttle application, the fluid exiting the center of the turbine hits the blades of the stationary stator. The reason this is key is that as the fluid jets out of the center of the turbine, the vortex is turning the wrong way—against engine rotation. The stator, or reactor, corrects the flow direction of the fluid and multiplies the energy contained within. This helps the impeller to spin the turbine faster by redirecting the fluid from the center of the turbine. The fluid then enters the center of the impeller with engine rotation, and then back out through the outside blades at a higher flow rate.

In this way the stator allows the converter to multiply torque as the turbine assembly spins up on its way to the coupling phase of 9/10ths. The one-way clutch in the stator will release as the 9/10ths is achieved and the stator will freewheel, and go with the flow. The stator is crucial to efficiently and seamlessly transferring twisting power of the engine, or torque, to the transmission.

Quit Stalling

While the stator controls the multiplication and delivery of torque as the torque converter reaches the desired 9/10ths, other factors control when the converter will stall, or actually start on its way to 9/10ths. Stall is the condition where the impeller is spinning around with engine rotation, but the turbine is stationary, such as when the vehicle is stopped in drive, with the brake applied. This is why a vehicle with an automatic transmission can effortlessly idle away at the stoplight with no complaint. Maximum stall is the point where this situation is overcome and the turbine starts to spin, or when the vehicle starts to move as your foot hits the gas.

Some torque converters have another key component at the very end of the 9/10ths condition known as the lock-up clutch. Performing the opposite function as stall, the lockup clutch quite simply locks up the converter assembly solid during cruising conditions for improved economy. It turns 9/10ths into 10/10ths, or 1:1. The fluid coupling becomes a mechanical coupling. The lockup clutch automatically disengages when power is applied to the converter.

All the parts of the torque converter work together with the transmission fluid for effortless driving thanks to the wonder of fluid dynamics. The next time you're stirring a cold drink on a hot summer day and watching the ice spin around in the glass, think of the mighty torque converter.