Balance shaft
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In piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets vibrations in engine designs that are not inherently balanced (for example, most four-cylinder engines).
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[edit] Four cylinder applications
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, 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 quadratically with RPM, no matter how closely the components are matched for weight. See the 0 degree cylinder angle, 180 degree crankshaft angle animated example here for a very clear depiction of this sometimes hard to visualize vibration (as well as the mathematical equation which describes it).
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 development of the General Motors 2.3 litre Quad 4 engine in 1987, described as "rough as a cob" by one automotive reviewer, and its subsequent development into the much more positively received 2.4 L 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 original application of this technology to their 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 (11 kW), but this seems excessive for pure friction losses. It is likely 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, 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.
[edit] Six cylinder applications
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.
[edit] Production implementations
Other manufacturers producing engines with one or two balance shafts include(d):
- Alfa Romeo 2.0L four-cylinder, as fitted to the Alfa Romeo 156
- BMW 1200GS motorcycle
- Chrysler 2.4 L and 2.5 L Neon engine
- Ford Modular engine V10
- Ford Taunus V4 engine
- General Motors Corporation Quad 4 and Ecotec
- GM Atlas engine four- and five-cylinder engines (two balance shafts)
- GM Vortec engine V-6 (single balance shaft)
- Honda 2.2 L four cylinder engine
- Mazda's 2.3L MZR engine (two balance shafts)
- Mitsubishi 'Astron' engine
- Nissan 2.5L (QR25DE) four-cylinder engine
- Porsche 2.5L and 3.0L
- Saab H engine
- Volvo B234F, B204GT and B204FT (four cylinder, two balance shafts, 16V-head, used in 700 and 900 series)
as well as numerous motorcycle engines, particularly vertical twins, and even some small single cylinder engines.
