Topic 1
The torque converter
The torque converter is attached to the engine flywheel (also called a ‘flexiplate’ – which, ironically, can crack) and acts as an automatic clutch, torque multiplier and a fluid flywheel. Pictures of which are shown on pages 16/17.
While this sounds complex, its operating principles are fairly straightforward. It’s best explained by this analogy: if you place two desk fans face-to-face and switch-on one of them, the powered fan blades will force air into the blades of the other fan, causing it to rotate as well, albeit not at the same speed. In an automatic transmission, an oil pump forces transmission fluid into the torque converter housing. The engine’s crankshaft rotates an impellor and the turbine sits opposite it, which is connected to the gearbox input shaft. As the transmission fluid is forced outwards from the impellor into the turbine’s blades, it encourages the turbine to rotate and power the gearbox.
However, the fluid needs to return to the impellor. It does this by flowing from the outer blades of the turbine, into the centre, and back towards the impellor. Unfortunately, because it is travelling in the opposite direction, it threatens to restrict the impellor’s movement and so its flow has to be diverted. To resolve this, a stator is sandwiched between the middle of the impellor and the turbine. This redirects the fluid so that it strikes the impellor in the same direction of rotation, thus providing an additional turning force, and creates a torque multiplication effect. This is why automatics can accelerate away relatively quickly from a standstill, compared to a manual, until a specific (albeit relatively low) road speed is reached. A one-way clutch – referred to as a sprag – is fitted to the stator, to enable fluid flow to be reversed. At higher turbine speeds, the sprag allows the stator to turn slightly, to enhance efficiency by not restricting the fluid flow as much. In theory, just as with modern turbocharger technology, it would be possible to create a variable vane torque converter, but designers have preferred to add extra gears within the transmission instead.
As with the desk fans analogy, natural inefficiencies mean that the impellor and turbines never rotate at the same speed, unless under deceleration. To improve fuel consumption at higher cruising speeds, a lock-up clutch is fitted that ensures a direct-drive between the stator and impellor. Lock-up operation tends to be operated hydraulically by a piston inside the torque converter, but it is controlled electronically.
The torque converter, therefore, has three operations. Stall is when the turbine is locked solid, which may be caused by the driver having applied the brakes with the car stationary and in gear. During acceleration, there is a significant difference between the impellor and turbine speeds, so an infinitely variable torque multiplication takes place. Last, when cruising, the difference in speeds between the impellor and turbine tends to be less than 10% and this phase sees the torque converter act more like a fluid coupling – the inefficiencies of which are negated when the lock-up clutch is operational.
Unfortunately for home mechanics, the torque converter is a sealed unit that requires special equipment to allow dismantling. Unlike the clutches in a manual, or automated, manual vehicle, the torque converter will not be damaged by holding the car on a hill using just the throttle, although this will increase the temperature of the transmission fluid markedly. While it is immensely reliable, faults can still occur, with any contamination in the gearbox damaging the torque converter. To avoid ‘infecting’ a replacement transmission, never reuse the torque converter from a failed unit.