The previous article in the RET-Monitor section on crankshafts introduced the concept of damping the torsional vibration of the crankshaft. We discussed the most common form of commercially available damper currently supplied for racing – the inertia damper. Dampers with multiple damping masses can offer a further improvement by damping vibrations across a wider frequency range, but the advantage of the multi-mass dampers (usually two masses) is really confined to roadcars, where the operating range of the engine is wide and the performance loss due to lugging extra mass around is negligible.
Race engines, with their relatively narrow power bands and narrow operating ranges (especially for series running wide-open throttle on ovals), tend to run into the lower speed resonances only as a short-term transient, as the engine increases or decreases speed. Where engines run for extended periods outside their designed operating range, for example during safety car periods, it is common for there to be certain engine speeds which are avoided completely through gear selection.
In the ‘good old days’, when ships were powered by reciprocating internal combustion engines, it was a legal requirement to have calculated all torsional resonances for each engine installation throughout the engine running range, precisely in order that damaging torsional resonances could be avoided.
It was during that period, when crankshaft vibration really could be a matter of life and death (imagine losing drive due to a broken propshaft halfway across the Pacific), that many of the damping devices that have been used with varying degrees of success were first developed. Some of these are still used in racing, while others have been adapted to other uses in engines, for controlling torsional vibrations in the cam gear drive for example.
One such device (and there are many) is the viscous damper, of which there are variations. One of the more popular is similar in principle to the inertia damper described previously. An inertia ring is supported on either its inner or outer diameter in an enclosed volume containing a special damping fluid. During normal operation, the ring rotates along with the damper housing with minimal losses. Only when vibrations begin is there a significant speed difference between the inertia ring and the housing. The oil in the carefully controlled gap between the housing and the inertia ring is therefore subjected to shear, which in turn exerts a torque on the housing and hence on the component to which it is fastened, in this case the crankshaft.
As with the rubber-damped inertia damper, the viscous damper has operating limits – the amount of energy being put into the damping fluid needs to be controlled. Just as a rubber damping element will rapidly degrade if too much energy is dissipated through it, so the damping fluid can also be damaged. The effect is one of a large change in viscosity, and hence the damping characteristics are changed.
The damper can be tuned by dimensional changes to the inertia ring and housing, but the parameters that show the greatest sensitivity to changes are the fluid inertia and the gap between the housing and inertia ring which, along with the speed difference between the housing and ring, dictates the shear rate of the fluid.
Written by Wayne Ward