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.
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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.
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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.
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.
