A dual-mass flywheel is a commonly used type of flywheel that is designed to minimise torsional vibration being transmitted from a car's engine to the rest of its drivetrain.
This vibration causes several unwanted effects, including gearbox noise - usually a pronounced rattling - and resonance in the body, which can result in an annoying droning sound. Both of these issues can be exacerbated further when the engine is working hard. These noises and vibrations reduce comfort and increase driver fatigue, so it's desirable for manufacturers to eliminate them.
These issues arise from the fluctuating torque output of the engine, which is the result of the independent combustion events that occur while it is running - and this fluctuating output can also increase the load on the transmission. This can be countered by using a dual-mass flywheel, which serves to minimise the erratic nature of the forces between the engine and transmission; consequently, excess load on the gearbox is reduced.
These complex flywheels can reduce the forces acting on the crank, too, due to their construction. A conventional solid flywheel and clutch is a typically heavy affair and bolted to the crank, generating significant forces during engine operation. The element of a dual-mass flywheel that is rigidly fixed to the crank, however, is far lighter - reducing stress on it.
Dual-mass flywheels are not a recent innovation, with the first production DMFs arriving in the mid-1980s. There were a plethora of reasons that spurred on their development; for one thing, ever-increasing engine outputs and lighter, more efficient powertrains and bodies were resulting in greater vibration and resonance.
The drive for improved efficiency, however, was also resulting in engines that produced their power at lower RPM and had fewer cylinders - again resulting in increased vibration and necessitating some form of damping.
How does a dual-mass flywheel work?
A four-stroke engine, due to its intermittent combustion cycles, is not the smoothest thing on the planet - and these periodic combustion events cause the torque output to fluctuate, which transmits a vibration through the drivetrain and the rest of the car.
The dual-mass flywheel, which is a type of torsion damper, is designed to reduce the conduction of these vibrations between the engine and transmission - removing the unwanted side-effects.
Unlike a standard solid flywheel, a dual-mass flywheel consists of two separate pieces. These are called the primary and secondary mass and, thanks to a bearing in the middle, the two masses can rotate against each other.
The primary mass, which features the ring gear required for starting the engine, is bolted to the engine's crankshaft. The secondary mass, as you would expect, is coupled to the clutch assembly - which transmits drive to the transmission.
Around the edge of the primary mass is a channel in which a series of large springs are retained - and the ends of these 'arc springs' protrude above the retaining stops in the channels. Attached to the secondary mass is a flange; the wings of this flange sit between the springs, allowing for drive to be transmitted from one mass to the other.
These radially mounted springs dampen the movements between the two masses, meaning that the output to the clutch and gearbox is free from excess vibration. They can also absorb some of the initial surge in torque from an engine, resulting in a smoother delivery of power and more refined clutch action.
Careful design and tuning by the manufacturer is required to phase out the unwanted vibrations, though - and different numbers and configurations of springs may be used to deliver improved damping at various speeds. DMFs are not only used in manual transmissions; you will also find them in dual-clutch and continuously variable transmissions, integrated between the engine and transmission itself to reduce vibrations.
There are some downsides to DMFs, though. Primarily, they are far more complicated than their solid counterparts and, consequently, far more expensive. As they contain moving parts, they can also be prone to failure - particularly in vibration- and torque-heavy diesel applications - and replacements can be comparatively costly. In some cases, owners revert to conventional single-mass flywheels.
Do petrol-engined cars use DMFs?
Yes - and they have done for many years. By 1997, for example, documentation from German technology group Schaeffler indicated that over 75 per cent of new petrol-engined cars with engines displacing more than two litres featured a DMF. Diesel was not far behind, with some 67 per cent reportedly using a DMF.
Due to the higher degree of vibration and larger torques produced by diesel engines, though, DMFs were more common in smaller-displacement engines; in 1997, the figures showed that 17 per cent of petrol engines displacing two litres or below used a DMF - whereas 21 per cent of diesels in that class featured a DMF.
As customer demand for improved refinement has increased, even in smaller and less expensive cars, the use of DMFs has continued to spread. This is in part due to reductions in cost as production has increased and the technology has become more widespread.