Combustion engines have a problem: They provide a usable output only in a very specific speed range – just the opposite to electric or steam engines. This means that speed, transferable engine output and gearbox must be optimally synchronised in various drive conditions – this function is provided by the clutch. The clutch separates or connects the engine or power unit to and from the gearbox. When the clutch pedal is depressed, the drive from the engine is interrupted and a different gear can be engaged. The clutch operation, because of its ability to slip when the clutch pedal is slowly released, enables the car to accelerate away smoothly.
Modern engines can be driven at a whole range of engine speeds with a trend towards ever increasing engine torques. Wind-tunnel-optimized bodies are creating less wind noise. New manufacturing methods are helping reduce vehicle weights and weight-saving concepts are boosting engine fuel economyl. The addition of a fifth or a sixth gear will also reduce fuel consumption. What remains is the principle of the internal combustion engine whose rotational combustion processes create torsional vibrations in the drive train, the unpleasant consequences of which can be gear rattles and body booms.
Drivers who are accustomed to increased comfort no longer accept such background noises. The job of the clutch is now more important than ever – in addition to engaging and disengaging, it must effectively insulate the engine’s vibrations. Physically, this is fairly easy to solve, clutch discs are supplied with simple torsion dampers or springs through to variants with multi-stage dampers and on later vehicles, the implementation of the dual mass flywheel.
The clutch disc is the clutch’s central connection element, in combination with the clutch pressure plate, it both separates and links engine and drivetrain, gearbox etc. and helps to smooth out the change point of the connection of the engine to the road wheels
Dual mass, it's all in the name
Dual-mass flywheels are very common on modern vehicles, are in large-scale standard production and are able to realise the physical separation principle. The name says it all, the mass of the conventional flywheel was simply split in two. One part continues to belong to the engine’s mass moment of inertia, while the other part now increases the mass moment of inertia of the transmission. The two decoupled masses are linked by a spring/damping system. One clutch disc, sometimes without a torsion damper, between the secondary mass and the transmission handles the engaging and disengaging functions. A favorable side effect is that the transmission is easier to shift because of the low mass to be synchronized, and there is less synchronization wear.
It is unfortunate that the dual mass flywheel has found not to be as reliable as could be expected and failure, where the two sections of the flywheel are no longer connected by the dampers, can result in accelerated failure of the clutch itself or in the most severe cases, loss of drive to the wheels. Because of its construction, it can be an expensive item to replace and is often found to be an additional cost when replacing a worn clutch. There are kits in the market place to replace the dual mass with a conventional solid and cheaper flywheel. This may sound like a good and lower cost alternative, but a word of caution, the car is designed to be driven with a dual mass flywheel and the result of the conversion, can be a hard clutch change with considerable vibration when engaging.