In
piston engine engineering, a balance shaft
is an eccentric weighted shaft which offsets
vibrations in engine designs that are not
inherently balanced.
Balance shafts are most common in inline
four cylinder engines which, due to the
asymmetry of their design, have an
inherent second order vibration
(vibrating at twice the engine RPM)
which cannot be eliminated no matter how
well the internal components are
balanced.
Flat engines have
their pistons horizontally opposed, so
they are naturally balanced and do not
incur the extra complexity, cost or
power loss associated with balance
shafts.
This vibration is
generated because the movement of the
connecting rods in an inline engine is
not symmetrical throughout the
crankshaft rotation; thus during a given
period of crankshaft rotation, the
descending and ascending pistons are not
always completely opposed in their
acceleration, giving rise to a net
vertical inertial force twice in each
revolution whose intensity increases
quadratically with RPM, no matter how
closely the components are matched for
weight.
The problem increases with
larger engine displacement,
since the only ways to achieve
larger displacement are with a
longer piston stroke, increasing
the difference in acceleration,
or by a larger bore, increasing
the mass of the pistons; either
way, the Magnitude of the
inertial vibration increases.
For many
years, two litres was viewed as
the 'unofficial' displacement
limit for a production inline
four-cylinder engine with
acceptable NVH characteristics.
The basic concept behind balance
shafts has existed since 1904,
when it was invented and
patented by British engineer
Frederick Lanchester.
Two balance
shafts rotate in opposite
directions at twice the engine
speed.
Equally sized
eccentric weights on these
shafts are sized and phased so
that the inertial reaction to
their counter-rotation cancels
out in the horizontal plane, but
adds in the vertical plane,
giving a net force equal to but
180 degrees out of phase with
the undesired second-order
vibration of the basic engine,
thereby cancelling it.
The actual
implementation of the concept,
however, is concrete enough to
be patented.
The basic
problem presented by the concept
is adequately supporting and
lubricating a part rotating at
twice engine speed at the higher
RPMs where the second order
vibration becomes unacceptable.
There is some debate as to how
much power the twin balance
shafts cost the engine. The
basic figure given is usually
around 15 hp (11 kW), but this
may be excessive for pure
friction losses. It is possible
that this is a miscalculation
derived from the common use of
an inertial dynamometer, which
calculates power from angular
acceleration rather than actual
measurement of steady state
torque. The 15 hp (11 kW), then,
includes both the actual
frictional loss as well as the
increase in angular inertia of
the rapidly rotating shafts,
which would not be a factor at
steady speed. Nevertheless, some
owners modify their engines by
removing the balance shafts,
both to reclaim some of this
power and to reduce complexity
and potential areas of breakage
for high performance and racing
use, as it is commonly (but
falsely) believed that the
smoothness provided by the
balance shafts can be attained
after their removal by careful
balancing of the reciprocating
components of the engine.
Mitsubishi Motors
pioneered the design in
the modern era with its
"Silent Shaft" Astron
engines in 1975, with
balance shafts located
low on the side of the
engine block and driven
by chains from the oil
pump, and they
subsequently licensed
the patent to Fiat, Saab
and Porsche.
Saab has further refined
the balance shaft
principle to overcome
second harmonic sideways
vibrations (due to the
same basic asymmetry in
engine design, but much
smaller in magnitude) by
locating the balance
shafts with lateral
symmetry but at
different heights above
the crankshaft, thereby
introducing a torque
which counteracts the
sideways vibrations at
double engine RPM,
resulting in the
exceptionally smooth
B234 engine.
Another balance shaft design is
found in many V6 engines.
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While an optimally
designed V6 engine would have a 60
degree angle between the two banks of
cylinders, many current V6 engines are
derived from older V8 engines, which
have a 90 degree angle between the two
banks of cylinders.
While this provides
for an evenly spaced firing order in an
8 cylinder engine, in a six cylinder
engine this results in a loping rhythm,
where during each rotation of the
crankshaft three cylinders fire at 90
degree intervals, followed by a gap of
90 degrees with no power pulse.
This can be eliminated
by using a more complex, and expensive,
crankshaft which alters the relationship
between the cylinders in the two banks
to give an effective 60 degree
difference, but recently many
manufacturers have found it more
economical to adapt the balance shaft
concept, using a single shaft with
counterweights spaced so as to provide a
vibration which cancels out the shake
inherent in the 90 degree V6.
