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CATEGORIES (articles) > Engine, Gearbox > Technical > Engine balance

Engine balance


Engine balance is the design, construction and tuning of an engine to run smoothly. Engine balance reduces vibration and other stresses, and may improve the performance, efficiency, cost of ownership and reliability of the engine, as well as reducing the stress both on other machinery and on the people near the engine.

These benefits are produced by:

  • Reduced need for a heavy flywheel or similar devices.
  • Reduced wear.
  • The opportunity to reduce the size and weight of components (other than the obvious one of the flywheel) as a result of reduced stress and wear.
  • Reduced vibration transmitted to the surroundings of the engine.
  • The opportunity to extract more power from a given engine by:
    • Higher maximum operating speeds made possible by reduced stress.
    • Spreading loads equally over multiple components, for example if multiple carburetors are poorly balanced, the maximum available throttle will be reduced.
Even a single cylinder engine can be balanced in many aspects. Multiple cylinder engines offer far more opportunities for balancing, with each cylinder configuration offering its own advantages and disadvantages so far as balance is concerned.


Inherent mechanical balance

The mechanical balance of a piston engine is one of the key considerations in choosing an engine configuration.


Primary and secondary balance

Historically, engine designers have spoken of primary balance and secondary balance. These terms generally refer to the order in which the problems of engine balance were addressed as piston engines developed. Because of this they also to some degree reflect the importance of these aspects of balance, but not absolutely, nor do they cover all aspects of mechanical balance.

The definitions used of primary and secondary balance also vary. In general, primary balance is the balance achieved by compensating for the varying momentum (but not the varying kinetic energy) of the pistons during rotation of the crankshaft. Secondary balance can include compensating (or being unable to compensate) for:

  • The kinetic energy of the pistons.
  • The non-sinusoidal motion of the pistons (which may otherwise be regarded as part of primary balance).
  • The sideways motion of crankshaft and balance shaft weights.
  • Various rocking motions produced by displacement of balancing masses and not included as primary balance (such as the unwanted offset of opposing cylinders in the boxer engine necessitated by the crankshaft design).
Despite claims by designers and manufacturers, no configuration is perfectly balanced. However by adopting particular definitions for primary and secondary balance, particular configurations can be correctly claimed to be perfectly balanced in these restricted senses. That is not to say that there is no substance to these claims. In particular, the straight six, the V12 and the crossplane V8 configurations offer exceptional inherent mechanical balance, with the boxer engine being perfectly balanced, as the movement on one side of the engine is completely compensated for and balanced by the mirrored movement on the other side of the engine.

Vibrations not normally included in either primary or secondary balance include the uneven firing patterns inherent in some configurations. Many definitions of secondary balance also exclude some aspects of mechanical balance.


Single cylinder engines

A single cylinder engine produces three main vibrations. In describing them we will assume that the cylinder is vertical.

Firstly, in an engine with no balancing counterweights, there would be an enormous vibration produced by the change in momentum of the piston, connecting rod and crankshaft once every revolution. Nearly all single-cylinder crankshafts incorporate balancing weights to reduce this.

While these weights can balance the crankshaft completely, they cannot completely balance the motion of the piston, for two reasons. The first reason is that the balancing weights have horizontal motion as well as vertical motion, so balancing the purely vertical motion of the piston by a crankshaft weight adds a horizontal vibration. The second reason is that, considering now the vertical motion only, the little-end is closer to the big-end in mid-stroke than it is at the top or bottom of the stroke, because of the angle of the connecting-rod. The piston therefore travels faster in the top half of the cylinder than it does in the bottom half, while the motion of the crankshaft weights is sinusoidal. The vertical motion of the piston is therefore not quite the same as that of the balancing weight, so they can't be made to cancel out completely.

Secondly, there is a vibration produced by the change in speed and therefore kinetic energy of the piston. The crankshaft will tend to slow down as the piston speeds up and absorbs energy, and to speed up again as the piston gives up energy in slowing down at the top and bottom of the stroke. This vibration has twice the frequency of the first vibration, and absorbing it is one function of the flywheel.

Thirdly, there is a vibration produced by the fact that the engine is only producing power during the power stroke. In a four-stroke engine this vibration will have half the frequency of the first vibration, as the cylinder fires once every two revolutions. In a two-stroke engine, it will have the same frequency as the first vibration. This vibration is also absorbed by the flywheel.


Two cylinder engines

Even a two cylinder engine has three common configurations:

  • Straight-twin.
  • V-twin.
  • Boxer twin.
Each of the three has advantages and disadvantages so far as balance is concerned.

A straight twin engine may have a simple single-throw crankshaft, with both pistons at top dead centre similtaneously. For a four-stroke engine, this gives the best possible firing sequence, with one cylinder firing per revolution, equally spaced. But it also gives the worst possible mechanical balance, no better than a single cylinder engine. Many straight twin engines therefore have an offset angle crankshaft, that is, two throws at an angle of up to 180°, with the result that the pistons reach top dead centre at different times. This produces better mechanical balance, but at the cost of uneven firing.

The first vibration noted above for the single cylinder is minimised for a crank offset angle of 180°, but balance is still far from perfect. There is still a rocking moment produced by the displacement of the cylinders one from the other, and there is still the second vibration noted for the single cylinder owing to the kinetic energy of motion of the pistons. This second vibration is minimised by a crank offset of 90°. See external links below for a detailed analysis of the effect of different crankshaft offset angles.

A "true" V-twin, like all true V engines, has only one crank throw for each pair of cylinders, so the crankshaft is a simple one like that of a single cylinder engine, and unlike any other V engine no crankshaft offset is possible. However there is still the question of the angle of the V. An angle of 90° gives a very good mechanical balance, but the firing is uneven. Smaller angles give poorer mechanical balance, but more even firing for a four-stroke (but, even less even firing for a two-stroke). Many classic V-twin motorcycles use narrow V angles as a compromise. See external links for a detailed analysis of the 90° V twin mechanical balance.

Other engines with two cylinders in a V configuration have a small offset between the cylinders in order to allow two separate crank pins, set at whatever angle the engine designer may specify in similar fashion to a straight twin. Although the characteristics of such engines are similar to those of a straight twin rather than a V, they are almost always called V engines. These engines include the Suzuki VX800 and Honda Transalp, which although called V-twins have a two-pin crankshaft, and an offset angle between the two crank throws.

The boxer engine is a type of flat engine in which each of a pair of opposing cylinders is on a separate crank throw, offset at 180° to its partner, so both cylinders of the pair reach top dead centre together. Any boxer therefore is inherently balanced so far as the momentum of the pistons is concerned, except that corresponding cylinders cannot exactly line up owing to the crankshaft design, and this produces a rocking motion. The four-stroke boxer twin has an even firing pattern, but the worst possible balance so far as the kinetic energy goes, as both pistons accelerate and deccelerate together. See external links for a detailed analysis of the boxer twin mechanical balance.


More than two cylinders

The number of possible configurations with more than two cylinders is enormous. See articles on individual configurations listed in Piston engine configurations for detailed discussions of particular configurations.

Engines with particular balance advantages include:

  • Straight-6.
  • V12.
  • Crossplane (90° V angle, 90° offset crankshaft) V8 (but see also below for problems).
Engines with characteristic problems include:
  • Flat-4 boxer and straight-4 have no better kinetic energy balance than a single, and require a relatively large flywheel.
  • Crossplane V8, which requires a very heavily weighted crankshaft, and has unbalanced firing between the cylinder banks (producing the distinctive and much-loved V8 "burble").
  • Flatplane (180° offset crankshaft) V8.
In modern multi-cylinder engines, many inherent balance problems are addressed by use of balance shafts.


Steam engines

The question of mechanical balance was addressed on steam engines long before the invention of the internal combustion engine. Steam locomotives commonly have balancing weights on the driving wheels to control wheel hammer caused by the up and down motion of the tie rods and to some degree the connecting rods. Again, the balance is a compromise, and some main line locomotives such as the Australian 38 class have no such weights.


Component balancing

In order to achieve the inherent balance of any engine configuration, the balancing masses must be matched. In most engines, some individual components are matched as a set. Exactly which components are matched is part of the design of the engine.

For example, pistons are often matched, and must be replaced as a set to preserve the engine balance. Less commonly, a piston may be matched to its connecting rod, the two being machined as an assembly to tighter tolerances than either alone.

Component balancing is not restricted to considerations of mechanical balance. It is vital, for example, that the compression ratio and valve timing of each cylinder should be closely matched, for optimum balance and performance. Many components affect this balance.


Blueprinting

Blueprinting is the remachining of components to tighter tolerances to achieve better balance.

Ideally, blueprinting is performed on components removed from the production line before normal balancing and finishing. If finished components are blueprinted, there is the risk that the further removal of material will weaken the component. However, lightening components is generally an advantage in itself provided balance and adequate strength are both maintained, and more precise machining will in general strengthen a part by removing stress points, so in many cases performance tuners are able to work with finished components.


Carburetor balance

In engines with multiple carburetors, balancing the carburetors is a vital part of engine tuning. Imbalance will not only mean that the carburetors are operating at less than ideal, but will also unbalance the cylinders that they serve.




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