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 on other machinery and 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:
o Higher maximum operating speeds made possible by reduced stress.
o 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.


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 smaller piston end of the connecting rod is closer to the larger crankshaft end of the connecting rod in mid-stroke than it is at the top or bottom of the stroke, because of the connecting-rod's angle. 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 simultaneously. 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 Category:Piston engine configurations for detailed discussions of particular configurations.

There are four different forces and moments of vibration that can occur in an engine design: free forces of the first order, free forces of the second order, free moments of the first order, and free moments of the second order. The straight-6, flat-6, and V12 designs have none of these forces or moments of vibration, and hence are the naturally smoothest engine designs. (See the Bosch Automotive Handbook, Sixth Edition, pages 459-463 for details.)

Engines with particular balance advantages include:

* Straight-6
* Flat-6
* Flat-12
* V12

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.


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.


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.