start by asking what at first seems to be a daft question. If your car's
engine idles smoothly at 800rpm, what speed is the crankshaft doing? No,
it's not 800rpm, except perhaps for a millisecond or two. The rev counter
only reads 800rpm because it only registers an average value. The real
instantaneous speed fluctuates, in the case of the four-cylinder engine
graphed in on the right, between about 710 and 860rpm, during the course
of one revolution.
Why does this variation happen? One of the reasons is that the flywheel
cannot do a perfect job. It has been designed to smooth the individual
firing pulses and speed variation as much as possible, but can only be
successful within certain limits, as dictated by its dimensions. Another
reason is torsional vibration, the winding and unwinding of the crankshaft
caused by the varying forces acting on it. At certain speeds, the crankshaft
even writhes like a snake in the free space between the main bearings,
causing eventual bell-mouthing of the main bearings.
graphs show the deviations from (1) the theoretically correct angle.
(2) the engine speed (measured on a re counter) and (3) the average
angular acceleration of the crankshaft while a four-cylinder engine
idles at 800rpm.
design of a flywheel starts with a graph of the torque variation during
the course of the two revolutions that it takes to complete one cycle
of events, using the average engine torque at the chosen speed as a base
line. With a four-cylinder engine, there will be four peaks, corresponding
to the four power strokes. At these points the areas between the curves
and the base line are called positive, to denote the fact that the combustion
process inputs energy into the system. Between these peaks the curves
will dip below the base line, sinking lowest during the compression stroke.
These areas are labelled negative because energy is being withdrawn from
the system. Adding up all the positive values and subtracting all the
negative ones from the total will give the work done by the crankshaft
in two revolutions.
At some of the points the graphs cross the base line from positive to
negative - the torque input changes from positive to negative. These points
correspond to maximum crankshaft speeds, whereas other crossover points
where the torque changes from negative to positive, represent minimum
crankshaft speeds. These speeds, and their fluctuation, as well as the
fluctuation of energy, can be calculated from the graph, enabling the
dimensions of a flywheel that will keep the speed fluctuations within
certain limits to be chosen.
way to show how the torsional twist varies along the length of a crankshaft
is to represent each crankpin assembly by a disc that has the same
deflection properties. These curves show the amplitude of relative
twist between the various crankpins during the course of one revolution.
Note that the greater portion of the crank twists from clockwise to
anticlockwise, while the flywheel and small portion of crank at the
rear twist in the opposite direction. The node is the point of zero
twist, ie it is the weakest point, and this is where a crankshaft
the amount of energy a flywheel can absorb - and hence give out - depends
mainly on its diameter or more precisely, on the amount of material near
the outer rim, a large diameter flywheel would seem to be more effective
than a smaller one. Unfortunately, space and weight considerations come
into the picture so that the dimensions of the average flywheel, like
everything else on a car, are largely a matter of compromise. The speed
variation results from the choice of a non-ideal flywheel is not enough
to bother even the most fastidious motorist, but some of the effects,
such as gear rattle, and even body boom, are subjects for continuing study
by clutch and transmission engineers.
The dual-mass flywheel fitted to a number of upmarket engines, introduces
extra mass into the system without increasing the flywheel diameter, in
such a way that it also helps to dampen the torsional vibrations. Furthermore,
the flywheel/starter/alternators that we will soon see on some luxury
models will also help to curb unwanted vibration.
But let's return to torsional vibration, not only because it is such an
interesting subject, but also because it is a phenomenon that can destroy
or shorten the life of components. If we return to our engine idling at
800rpm we find that this speed is equivalent to 13.33r/sec, which means
one revolution takes about 75 milliseconds.
Now for a look at what is happening in cylinder number one. The intake
valves are open for about 240 degrees, or 50 milliseconds, during which
time the flywheel inertia keeps the crank turning, and the major resistance
to motion is the drag of the piston and rings. The compression stroke
lasts for about 28 milliseconds, and the increasing force from the mixture
being compressed slows the crank down considerably.
before top dead centre the spark occurs, initiating a rapid burning of
the mixture that lasts for about 19 crank degrees, or four milliseconds.
The crank feels this as a kick, delivered down the con-rod, which results
in a distortion of the big-end journal and crank webs by a tiny fraction
of a millimetre. The kick speeds up the motion of the crank, even before
the steel has had a chance to spring back to its original state. The crank
now accelerates, imparting fresh energy to the flywheel, and as the exhaust
valve opens before bottom dead centre for about 50 milliseconds, the reduced
resistance above the piston helps to shift more energy into the flywheel.
The cycle of events now repeats itself in this cylinder.
is a Lanchester slipper-flywheel vibration damper, similar to the
early Rolls-Royce damper.
half a revolution, or 37.5 milliseconds, after number one cylinder has
started down the intake strike, number three starts the same cycle of
events, followed by cylinders number four and two at the same time intervals
later. If the throttle is now opened wide, the size of the kicks will
increase, as well as the frequency. As stated, at 800rpm the crank gets
a kick every 37.5 milliseconds, but at 600rpm this changes to a kick every
five milliseconds. The crank on a six-cylinder engine gets even less respite,
being jerked every 3.33 milliseconds, at this speed.
But these are not the only shocks the crank has to endure. The crank also
has to absorb the normal now-up now-down inertia loading due to the con-rods
and pistons, which could easily amount to a ton on each cylinder. The
net result is not only a speed variation in the crankshaft, but also the
abovementioned distinction. The size of these crank movements depends
to some extent on the shape of the crank, and the material used, but the
length of the crank has by far the bigger influence on the torsional vibration.
It is therefore no surprise that the phenomenon was first encountered
on six-cylinder engines, and the cure was not easy to find.
The earliest cars had single-cylinder engines, but twins followed soon
afterwards, and by the turn of the century a motorist could choose between
one, two, three or four cylinders. Engine speeds were not much more than
1500rpm and engine smoothness was not a prime consideration. A low price
and simplicity were the main selling points. Many motorists were against
four cylinders, and the first sixes were considered ridiculously complicated,
so much so that even eminent engineers were vociferous in their consideration
of these complicated monsters.
The first six-cylinder car was exhibited in 1902 by the Dutch Spyker firm.
It also had four-wheel drive and four-wheel brakes, but was never put
into production. Soon after that, the sole selling agent for Napier cars,
the never-reticent S F Edge, persuaded Montague Napier to build a six.
Edge was thrilled with the prototype because of its smooth and even flow
of torque, although he noticed a bad vibration at a certain speed, which
he called a power-rattle to placate the customers.
figure shows the traces from a Summer's torsiograph, which records
the angular speed variation as a change in radius; a vibration shows
up as a wavy line. These readings were taken from the nose of a six-cylinder
engine. Note that the bad vibration at 2025rpm is only 25rpm away
from a smooth 2000rpm.
first cars were sold in 1904, but the Napier engineers were not happy.
They experimented with different ways to balance the engine, reduced the
compression on the number six cylinder, and tried bigger diameter crank
journals. Unfortunately, they never really stopped the power-rattle. Within
a year other manufacturers were producing sixes, including Rolls-Royce,
but it faced even worse - initially. One of the first cars produced broke
a crank while out on test followed by two more within days, forcing Henry
Royce to find a quick cure.
These early Rolls-Royce sixes had a normal flywheel at the rear as well
as a smaller flywheel at the front, to curb a disturbing timing-gear rattle.
Royce suspected that the crank was winding and unwinding between the flywheels,
like a torsion bar. He realised that this constant reversing of the twist,
or torsional oscillation, could easily break a crank in half. Royce lightened
the crank by removing the balance masses, and reduced the front flywheel
mass by half. This cured the problem, for the time being, by changing
the critical speed of the assembly to a value greater than the speed the
engine could attain.
During the experimentation, Royce had a wooden distance-piece made to
fit between the crank and flywheel, which cured the timing gear rattle,
but the steel production piece did not work. When Royce examined the wooden
piece, he saw that the wood was charred on one side, which meant it was
moving against the hold-down bolts. He immediately realised why it worked,
and devised the slipper-flywheel vibration dumper, which was a feature
of Rolls royce engines for many years. This incorporated two small flywheels
that could move relative to a hub, mounted on the nose of the crank. Frictional
material was introduced between the mating surfaces, so that any vibration
of the crank would set the flywheels vibrating in opposition, forming
an effective damper.
section through a modern Boge hydraulic vibration damper, which prevents
the transmission of all kinds of vibrations. It is fitted between
the engine-transmission unit and the chassis or floorpan.
vibration may have seemed mysterious to the pioneers, but it has been
studied extensively since then. The preceding description details how
the crank gets excited, but that is only a part of the picture. This excitation
is harmful because of two other considerations. Firstly, the presence
of a flywheel, so necessary to curb the normal speed fluctuation, actually
accentuates the torsional vibration. The flywheel's inertia literally
acts like a millstone towards the vibrating crank. The result can be imagined
if you picture the crank as torsion bar, twisting hither and thither against
the unyielding flywheel. It is no wonder that on most crankshafts the
vicinity of the rear main bearing is the weakest point. When passenger
car engines are adapted for trucks, the fitting of a heavier flywheel
sometimes results in the crank twisting off just in front of the flywheel.
The second accentuating factor is that every object has a natural vibration
frequency that depends on the material itself and shape, ie every object
behaves like a tuning fork if a sudden force is applied to it. Most crankshafts
are strong enough to withstand the torsional vibration, considered as
a lone phenomenon. However, as soon as the frequency of the exciting force
approaches the natural frequency of the crankshaft, the amplitudes - the
magnitudes of the displacements - grow to such an extent that a breakage
becomes almost inevitable. The phenomenon is known as resonance, and the
engine speed where this occurs is known as the major critical speed.
curves show how effective a good damper can be, by contrasting an
undamped vibration (1) with the effect of a rubber damper (2) and
a viscous damper (3).
can be avoided by either damping the system, or by making sure that the
engine cannot be run at the major critical speed. The shortness of four-cylinder
cranks results in a major critical speed of well over 9000rpm, so that
a torsional damper is not essential on a four. It is still sometimes fitted
to take care of the minor critical speeds. These are the speeds at which
resonance of a lower amplitude occurs because the forces involved are
smaller, or are damped by other induced vibrations.
The straight-eight engine, used extensively for racing between the world
wars, resonates at about 3100rpm, where the forces are low enough to be
dealt with, either by making sure the crank is robust, or fitting a damper,
or having the power take-off in the centre of the crankshaft. In the latter
case, the crankshaft behaves like two four-cylinder cranks. V8s and V6s
have shorter cranks, but modern designs are fitted with dampers. The six-cylinder
engine is the difficult one, with a major critical near 5000rpm, where
the forces are high enough to render a good damper essential.
Modern vibration dampers belong to one of four basic designs.
The Lanchester-type damper, uses the friction in two sets of multiple
disc plates to curb the vibration. One set of discs is internally splined
to the hub, and the other set is splined externally to a heavy inertia
ring. The whole design looks very much like a multiple-plate motor cycle
clutch. The friction discs are lightly squeezed together by six evenly
spaced coil springs, and the assembly is submerged in oil. The relative
slip between the many pairs of surfaces dissipates the vibrational energy.
The solid friction damper is very much like the Rolls-Royce damper. It
uses the friction between two flywheels and the hub to dampen the vibration.
The viscous fluid damper uses a silicon fluid that changes very little
with temperature, enclosed in a steel casing, to drag an annular ring
with it when there is no vibration. When a vibration occurs, the shearing
forces in the fluid damp it.
tuned rubber vibration damper has a rubber mass interposed between an
outer inertia ring and a central hub. The action is similar to what would
happen were another mass attached to the crank by means of a rubber band.
That is, the original vibration frequency is replaced by another (much
smaller) vibration frequency, above and below the original frequency.
The critical aspects may cause a rumbling noise in an engine due to backlash
in meshing gear teeth, or due to pistons pounding from side to side in
their clearances. The noise may be suppressed when the engine is cold
and the viscosity of the oil is high. When the oil is hot the reduced
damping capacity of the oil permits the noise to become audible. If the
engine is run up to a high speed and the fuel supply cut off, so that
the engine has to run down over its speed range, amy vibration period
it goes through may be due to resonant torsional vibration.
In the USA there are a number of aftermarket vibration dampers. Engine
tuners are as fussy about choosing the correct damper for their particular
application as they are about choosing the correct manifolds. A good damper
will not only prolong engine life, but will also help the engine to deliver