13 October 1995

Careless torque costs drives

Aided by Dave Bramwell, Product support engineer for ZF Great Britain, Andrew Pearce slips through torque converter operation

THE torque converter falls into that technological grey area occupied by refrigerators, microwave ovens and MPs. Most people know where to find one, but arent quite sure how it works.

Materials handlers would be a lot harder to drive without a converter. It takes the place of a conventional clutch, smoothly connecting the engine and transmission as an agricultural boot floors the throttle.

Geographically, the converter is usually behind the crankshaft and either above or in front of the gearbox, but sometimes may connect to both by separate shaft(s). Here we explore at ZFs Trilok unit, though others work in much the same way.

Fluid clutch

In the beginning was the fluid clutch, which shows most of the torque converters principles without its complications. To see how one works, imagine a pair of fans sitting face to face. One is driven and the other is free to turn (diag 1). As the driven fan starts up, so its mate slowly begins to spin and finally the two run at much the same speed.

Whats happening? The first fans mechanical energy passes to the air, which starts to flow. Flow energy is picked up by the second fan, which reconverts it to mechanical energy and so turns. The fans are coupled hydrokinetically – by the movement of a fluid.

Trouble is, you dont get far on air. Whats needed are more efficient fans and a better fluid. So in a fluid clutch the first fan becomes a circular impeller, driven by the engine and carrying a forest of curved vanes (2). The second fan is a matching circular turbine, only this time connected to the gearbox.

The unit is closed and full of oil, which arrives by pump and filter from the transmission sump. As engine revs increase, oil is flung outward between the impeller vanes. With nowhere else to go, it hits the curved turbine vanes; these react to the thrust and the turbine starts to turn. Power passes into the gearbox, out to the wheels and the vehicle moves off. The turbine vanes constantly circulate oil back into the impeller for another go.

But connection between the engine and box is never complete. Think of a stationary materials handler: The impeller is idling along at tickover, the turbine is not moving. As engine revs pick up, the impeller flings oil increasingly hard at the turbine. For a while nothing happens, as there is slip in the system. But soon the turbine collects enough torque to turn against the vehicles resistance, and starts to move. The vehicle gets underway and accelerates to a stable speed, at which point impeller and turbine are spinning at roughly the same rate.

All through this the turbine always lags behind its impeller. The lag (or slip) is 100% with the turbine stationary, dropping steadily to 2%-5% in stable load conditions. Energy used in slip just heats up the oil rather than driving the vehicle, so a simple fluid clutch is not particularly efficient.

Torque converter

A torque converter goes one step beyond the fluid clutch and reclaims some of the lost energy. It too uses an impeller and turbine, but between them adds a stator. Smaller than both, this third disc sits on its own fixed shaft with vanes curving the opposite way to those on either side of it (3).

Oil leaving the turbine now cant circle passively back to the impeller. Instead, the stators vanes force it into a U-turn and shoot it back into the impeller, where its energy boosts the oil thats already streaming out into the turbine. Thus more than just engine torque drives the turbine and hence the gearbox – the unit has converted engine torque into something bigger, and thus earned its name.

The greater the speed difference between the impeller and turbine, the bigger is the torque multiplication. When a materials handler is standing still, the turbine is stationary (the drive-off point). Working against a motionless turbine, slip is 100% and the converter multiplies engine torque 2-3 times. As the turbine begins to turn, multiplication progressively and smoothly drops until, at a steady load, there is none at all. At this point, slip is in the 2%-5% range and the unit mirrors a fluid clutch.

There is one complication. As turbine speed picks up, the stator vanes would increasingly disrupt smooth oil flow back into the impeller and hurt efficiency. Crafty designers get round that one by giving the stator a one-way clutch (3 and 4).

All the time the turbine is spinning substantially slower than its impeller, the stator feels significant side thrust from the passing oil.

But as speeds even out, that thrust drops. The clutch then lets the stator rotate along with the other two components. By the time the converter has reached fluid clutch mode, all three are going round together and the stator no longer disturbs flow.

Lockup and finish

Even after torque multiplication there is still room for improvement. Converter slip wastes energy and hits performance, so enter the lockup clutch (3 ).

This provides complete mechanical connection between the engine and gearbox. Once the converter is operating as a fluid clutch – ie input and output torques are equal – and a pre-set forward speed is reached, the lockup clutch is triggered. Connection between the impeller and turbine is now purely mechanical, so slip disappears, fuel economy and performance go up and the vehicle drives as though the clutch were conventional.

Oil again does the business. Behind the turbine is a chamber: In it are clutch plates, sitting behind a piston plate and between the turbine and its surrounding impeller housing.

When the transmission control system senses that conditions are right, a valve lets oil in behind the piston. The clutch closes, locking turbine and impeller together. When speed falls below a pre-set value (always lower than the engagement point), valving lets oil pressure drop and the clutch opens.