Whenever we sit in our road cars we are faced with two large dials displaying the vehicle’s speed and the engine speed (or the revs). Usually, unless you are trying to prove a point to the car next to you, or are on a race track, we very rarely accelerate the needle into the red zone near the rev limiter. This is because, despite internal combustion engines having a large range of operating speeds, there is only a very narrow window in which the engine can operate at its highest efficiency, where it is achieving the maximum output power for the minimum amount of fuel (brake specific fuel consumption, or BSFC).
Therefore, a transmission is used to effectively vary the speed of the driven wheels, to ensure that the engine is always operating within that efficiency band as the vehicle accelerates or decelerates. So when you accelerate, you are demanding more torque from the engine so the revs increase up to a point where you change to a higher gear and the revs drop back down before gradually increasing up through the rev range once again.
In a manual transmission, to enable a smooth transition between the gear changes, the flow of torque from the engine to the transmission has to be disconnected. Of course, it is not practical to turn the engine off every time you want to make a gear change. Therefore, a clutch is used to transfer the torque from the powerplant to the gearbox, and this can also be used to disconnect the engine from the gearbox during gear changes without having to turn off the engine.
A clutch is also necessary to get a car in motion. When you turn your keys in the ignition, the engine starts to tick over and after an initial increase in revs it will settle down at idle. The engine is providing a small amount of torque, but the transmission is stationary. The torque of the engine can’t be stopped or decreased below idle otherwise it will stall, so to increase the torque of the transmission up to that of the engine, the clutch needs to slip. This is essentially a balancing act between the clutch pedal and the throttle pedal where the clutch starts to engage, or bite, up until when the clutch is fully engaged and the speeds of the engine and the transmission are matched.
To achieve this, a clutch is made of several parts. Firstly, there is a clutch disc, also known as a driven plate (friction plate) which, as the name suggests, is coated with a high friction material. This disc sits on a flywheel and when an external force is applied, the friction between the two results in the friction plate rotating at the same speed as the flywheel. To apply this external force, a diaphragm spring is used which exerts the necessary force on to the driven plate via a pressure plate. The input shaft of the transmission is splined to the driven plate, and so this is how the clutch transmits the torque from the engine to the transmission, and under this condition the clutch is engaged.
However, to enable the clutch to disengage, a diaphragm spring is incorporated between the clutch cover and the pressure plate. Diaphragm springs are circular steel discs that have a hole in the centre. The inner portion of the disc consists of a series of radial slots, which essentially creates a set of actuating fingers. When a force is applied to these fingers, the outer section of the spring moves in the opposite direction. This spring lies between the pressure plate and the clutch cover and as the cover is fastened, the diaphragm spring is slightly flattened, and therefore loaded, exerting a force onto the pressure plate, which is then transmitted to the friction disc. When the fingers of the inner portion of the spring are pushed inwards, the outer portion of the spring reacts in the opposite direction, moving the pressure plate away from the friction disc, removing the external force and disengaging the clutch.
This is exactly what happens when the clutch pedal is pressed. A hydraulic system is used to translate the movement of the clutch pedal to the centre of the diaphragm spring. So when you press the clutch pedal, the centre of the diaphragm spring moves inwards, the outside of the diaphragm spring moves outwards, removing the load from the pressure plate, which disconnects the drive from the flywheel. When you release the pedal, the centre of the diaphragm spring moves back outwards, the outside of the diaphragm spring moves inwards, re-applying load via the pressure plate and thereby connecting the driven plate back to the flywheel, resuming drive.
‘A clutch is actually very simple to explain in principle, the complications come in the details,’ says Marco Trautmann, at ZF Race Engineering. ‘On one side you have a diaphragm spring within the pressure plate which basically creates the clamp load. The work is done by the friction stack which is in the middle and on the other side you have the flywheel. If the car is standing still the flywheel on the engine side is rotating and the clutch discs on the gearbox input shaft are static so as you engage the clutch, the surface of the pressure plate which is in the clutch cover, contacts the friction stack and this is pushed towards the flywheel and the friction between the three transmits torque from the engine to the gearbox.’
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