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==Production implementations==
==Production implementations==
Other manufacturers producing engines with one or two balance shafts include:
Other manufacturers producing engines with one or two balance shafts include:
* [[Porsche]]
* [[Porsche]] 2.4 liter
* [[General Motors]] [[GM Quad-4 engine|Quad 4]]
* [[General Motors]] [[GM Quad-4 engine|Quad 4]]
* [[General Motors]] [[GM Atlas engine|Atlas]] four- and five-cylinder engines (two balance shafts)
* [[General Motors]] [[GM Atlas engine|Atlas]] four- and five-cylinder engines (two balance shafts)

Revision as of 15:12, 5 January 2005

In piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets the vibrations of unbalanced engines.

Four cylinder applications

Balance shafts are most common in inline four cylinder (straight-4) engines which, due to the asymmetry of their design, have an inherent second order vibration (vibrating at twice the engine RPM) which, contrary to popular belief, cannot be eliminated no matter how well the internal components are balanced. 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 pistons 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 exponentially 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 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 liters was viewed as the 'unofficial' displacement limit for a production inline four cylinder engine with acceptable Noise/Vibration/Harshness characteristics; the development of the General Motors 2.3 liter Quad 4 engine in 1987, described as "rough as a cob" by one automotive reviewer, and its subsequent development into the 2.4 liter version with balance shafts confirms the wisdom of this assessment.

The basic concept behind balance shafts has existed for nearly a century and is no longer patentable. Two balance shafts rotate in opposite directions at twice 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 canceling it. (Some motorcycle enthusiasts believe that Honda's application of this technology to their original V twin motorcycle engine overly damped out the vibration, giving an excessively 'dead' feel, so that they later reduced the size of the balance shafts in order to furnish the rider with some feedback as to engine speed).

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. 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, driven by chains from the oil pump, and subsequently licensed the patent to Porsche, then to other manufacturers. Since then, other manufacturers have adapted the same basic layout to their needs.

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 an exceptionally smooth four cylinder engine.

There is some debate as to how much power the twin balance shafts cost the engine. The basic figure given is usually around 15 horsepower, but this seems excessive for pure friction losses. It is likely that this is a miscalculation derived from the common use of an inertial dynomometer, which calculates horsepower from angular acceleration rather than actual measurement of steady state torque. The 15 horsepower, 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, many owners modify their engines by removing the balance shafts, both to reclaim some of this power, but also to reduce complexity and potential areas of breakage for high performance and racing use. As mentioned above, it is commonly 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, but that stems from a basic misunderstanding of their operation.

Six cylinder applications

Another balance shaft design is found in many V-6 engines. While an optimally designed V-6 engine would have a 60 degree angle between the two banks of cylinders, many current V-6 engines are derived from older V-8 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 V-6.

Production implementations

Other manufacturers producing engines with one or two balance shafts include:

as well as numerous motorcycle engines, particularly vertical twins, and even some small single cylinder engines.

Engine balancing