Automatic gearbox (Description): Description and operation

DESCRIPTION

The automatic transmission which is used with the ’K’ series engine is a continuously variable transmission (CVT) and different to conventional automatic transmissions. This compact transmission has been specifically designed for front wheel drive vehicles with transverse engines.

Unlike conventional automatic transmissions that provide a limited number of gear ratios, usually three or four which change in steps, the continuously variable transmission has an infinite number of gear ratios within it’s operating range. The stepless shifting pattern of the transmission, provides a smoother transfer of power to the road wheels as well as having full vehicle performance available at all times.

OPERATION

The transmission is driven from the engine via a torsion damper bolted to the flywheel. The torsion damper drives the input shaft which in turn drives the planet carrier. Depending upon whether forward or reverse gear is selected the primary pulley will rotate transferring torque to the secondary pulley which causes the vehicle to move in the desired direction.

At the heart of the transmission is a steel belt consisting of two steel bands made of 10 steel strips. These two bands carry approximately 350 steel segments which butt against each other to allow the belt to transmit torque by compression. The belt has several different thicknesses of steel segments intermingled which reduce the noise of the segments hitting the pulleys by changing the harmonic frequencies.

The steel belt is fitted between the primary and secondary pulleys. Each pulley consists of one fixed half and one axially movable half. The moving pulley halves are situated diagonally opposite to each other to reduce misalignment of the belt during shifting. Each moving pulley half is connected to a hydraulic cylinder controlled by hydraulic pressure generated by an integral fluid pump running at engine speed. By moving the pulley halves together their effective diameter is increased and by moving them apart it is decreased thus altering the gearing of the unit due to the conical faces of the pulleys. The belt is lubricated by fluid sprayed from a nozzle mounted in the casing.

To pull away a low ratio is required. To provide for this, the primary pulley is open, allowing the belt to sit down into the pulley and forcing it to run on the larger diameter of the closed secondary pulley.

As vehicle speed increases, a higher ratio is required. As engine speed increases the fluid pressure increases and the primary pulley gradually closes increasing its effective diameter as it does so the secondary pulley opens decreasing its effective diameter giving a higher ratio. An overdrive ratio is obtained when the primary pulley is fully closed and the secondary pulley is fully open. The secondary pulley is now forced to rotate approximately two and a half times that of the primary pulley.