A familiar feature common to all manual transmission cars is the clutch.
It transmits the power and torque developed by the engine to the gearbox, where the shaft speed is reduced to facilitate transmission of the power to the final drive and then the driving wheels.
The clutch essentially consists of a friction plate connected to the gearbox input shaft that is forced into contact with the engine’s flywheel face by a pressure plate, the contact pressure of which can be controlled by the driver.
This facilitates a progressive take-up of power transmission, the driver using the clutch pedal to finely control the process of gradually reducing the slippage between the friction material of the clutch plate and the flywheel until both are travelling at the same speed.
The default state of the clutch is one of it being fully engaged, with the drive to the gearbox connected. Clutch operation or declutching is generally cable or hydraulically operated, with today’s cars generally using a diaphragm spring clutch where the clutch drive plate bearing the friction material is kept in contact with the flywheel by a pressure plate connected to a flexible dished diaphragm.
Clutch pedal depression causes the clutch release bearing to apply pressure to the convex diaphragm centre so that, pivoting on what’s called the fulcrum ring, it draws the outer friction pad area of the clutch plate away from its contact with the flywheel face, with a minimum of force.
The fine controlability of clutch plate pressure allows a car to be held stably on a hill by balancing torque transmission against gravitational forces. However, clutch friction material wears, and regular clutch adjustment often used to be necessary to maintain consistent pedal operation, particularly on cable-operated clutches.
In the mid 1990s LUK self-adjusting clutches appeared, where a load sensor detects wear and automatically operates an adjuster ring, eliminating service requirements.
The direct connection between engine and gearbox through the clutch has traditionally been insulated by the employment of heavy springs within the clutch drive plate, to absorb some of the engine pulses and torsional vibrations of the drive train.
For many diesel cars, the absorption of such shock and low engine speed vibration is now further refined by employing what is termed a dual mass flywheel or DMF for short. The objective of this unit is to obtain the shock-damping effects of a heavier flywheel without incurring the usual shortcomings.
A dual mass flywheel is composed of two basic rotating components, a primary mass and secondary mass, linked by a damping mechanism composed of long arc-shaped springs installed in the secondary mass.
The damping mechanism allows considerable rotational movement between the two parts to absorb the torque output variation from the crankshaft and deliver a smooth transfer of drive through the clutch and drivetrain to the wheels.
A further benefit is a reduction in the rotational mass of the gearbox, allowing easier and faster gear changing and reduced stress on the gearbox synchromesh, of which more in a future issue.
During the lifetime of the dual mass flywheel, which first appeared in the 1980s, DMFs have been improved in many stages, although they are still apparently somewhat vulnerable to unexplained failure and vulnerable to driver mistreatment.
Since DMFs were introduced, further improvements have been made to normal single mass flywheel clutch units, and the experiences of one of our readers of replacing a troublesome DMF unit with a solid mass flywheel conversion kit are reported in this issue’s Doctor Diesel column.