::: DRO ::: Tech Section - Twist and Spin

By Mike Bumbeck
4/8/05

CHECK YOUR FLUIDS

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 miracle of fluid dynamics. The classic analogy used to explain the basics of torque converter operation is that of two fans facing each other. With one fan 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. The other fan 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.

PARTS AND PIECES

There are four parts of a torque converter that together function as a whole. Working from the crankshaft back the first part is the cover. The cover houses the torque converter and is connected directly to the crankshaft by way of the flexplate. 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 energy created by the impeller spins the turbine. In between the impeller and the turbine is the thinking part of the torque converter – the mighty stator – or reactor. The stator alters the flow of fluid between impeller and the turbine, and is the key to the infinite flexibility of 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 fluid in two directions at once to achieve this coupling. The fluid flows in a rotary and vortex motion at the same time. Rotary flow forces the fluid to the outside of the impeller and turbine, and creates a centrifugal force that rotates the assembly. Vortex flow, created by the blades and channels fluid in a vortex within the impeller and turbine. 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.

First in line is the converter impeller. The impeller always turns at the same speed as the crankshaft, and also serves to turn the fluid pump in the transmission by way of an indexed gear.

On the inside of the impeller are the blades. Positive, neutral, or negative blade angle and curvature plays an important role in stall, as the way the blades direct the fluid into the turbine influence the point at which it will start to turn.

Note the difference in shape in blades between the two stators. The straight blade deflects more fluid energy – and delivers a softer hit. The curved blades are more efficient at redirection – and deliver greater multiplication, and harder punch to the tires.

An oversized thrust bearing is added to the stator thrust surface to protect against extreme thrust created as it redirects the jet-like flow of fluid exiting the turbine.