Engine balance

Engine balance is the design, construction and tuning of an engine to run smoothly. Improving engine balance reduces vibration and other stresses and can improve the overall performance, efficiency, cost of ownership and reliability of the engine, as well as reducing the stress on other machinery 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. They are so-called because they refer to vibration at the first and second harmonic of the crank's rotational frequency, respectively. These excitations can produce both couples and forces. Higher-order harmonics also exist but, as the orders increase, the magnitudes decrease, thus orders higher than the second are typically neglected. The source of the higher orders is in the motion equation for a slider-crank mechanism, which forms the basis for common reciprocating piston engines. Evaluation of the motion equation reveals an infinite sinusoidal series, meaning there is actually no limit to the balancing orders.

Primary balance is the balance achieved by compensating for the eccentricities of the masses in the rotating system, including the connecting rods. At the design stage primary balance is improved by considering and adjusting the eccentricity of each mass along the crankshaft. In theory, any conventional engine design can be balanced perfectly for primary balance. Once the engine is built primary balance is controlled by adding or removing mass to or from the crankshaft, typically at each end, at the required radius and angle, which varies both due to design and manufacturing tolerances.

Secondary balance can include compensating (or being unable to compensate) for:

• The kinetic energy of the pistons.
• The non-sinusoidal motion of the pistons.
• The motion of the connecting rods.
• The sideways motion of balance shaft weights.

The second of these is the main consideration for secondary balance. There are two main control mechanisms for secondary balance—matching the phasing of pistons along the crank, so that their second order contributions cancel and the use of Lanchester balance shafts, which run at twice engine speed and so can provide a counteracting force.

No widely used engine configuration is perfectly balanced for secondary excitation. However, by adopting particular definitions for secondary balance, particular configurations can be correctly claimed to be reasonably balanced in these restricted senses. In particular, the straight six, the flat six and the V12 configurations offer exceptional inherent mechanical balance. Boxer eights with an appropriate configuration can eliminate all primary and secondary balance problems, without the use of balancing shafts.

Vibrations not normally included in either primary or secondary balance include the uneven firing patterns inherent in some configurations.

The above definitions exclude the dynamic effects due to flexure of the crankshaft and block and ignores the loads in the bearings, which are one of the main considerations when designing a crankshaft.

Single-cylinder engines

A single-cylinder engine produces three main vibrations. In describing them, it will be assumed 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, gudgeon pin(wrist pin, US), 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 (little end) is closer to the larger crankshaft end (big 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. So during the 180° rotation from mid-stroke through top-dead-centre and back to mid-stroke the minor contribution to the piston's up/down movement from the connecting rod's change of angle has the same direction as the major contribution to the piston's up/down movement from the up/down movement of the crank pin. By contrast, during the 180° rotation from mid-stroke through bottom-dead-centre and back to mid-stroke the minor contribution to the piston's up/down movement from the connecting rod's change of angle has the opposite direction of the major contribution to the piston's up/down movement from the up/down movement of the crank pin. 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 cannot 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

There are three common configurations in two-cylinder engines:

• Parallel-twin
• V-twin
• Boxer twin (a common form of flat engine)

Each of the three has advantages and disadvantages so far as balance is concerned.

A straight two engine may have a simple single-throw crankshaft, with both pistons at top dead centre simultaneously (parallel twin). 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. While this causes uneven firing, it produces better mechanical balance. It does not however produce perfect mechanical balance since the piston at the top half of the cylinder moves faster than the one at the bottom half of the cylinder. (See Single-cylinder engines above for a more detailed explanation).

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 nonconcentricity of the cylinders relative to each 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.

Most V-twins, like V engines in general, have 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 to allow two separate crank pins, set at the angle the engine designer specifies, similarly to a straight two. These engines include the Suzuki VX800 and Honda Transalp, which 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 as far as the momentum of the pistons is concerned. That corresponding cylinders do not lie in the same plane owing to the crankshaft design, a reciprocating torque also known as a rocking couple results. 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.

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, certain straight-8, flat-6, flat-8 with 180 degree firing, flat-12, flat-16 with 90 degree firing, V12, V16, and W16 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
• Straight-8
• Flat-4 with two geared crankshafts
• Flat-6
• Flat-8
• Flat-12
• Flat-16
• V12
• V16
• W16

Engines with more than two cylinders with characteristic balance problems include:

• Inline-triple engines have a strong balance induced rocking motion
• Inline-four engine using a single crankshaft has no better kinetic energy balance than a single cylinder engine, and requires a flywheel with a relatively large mass or diameter.
• 60 degree V6s
• 90 degree V6s

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 coupling rods and, to some degree, the connecting rods. Again, the balance is a compromise.

Component balancing

To improve inherent dynamic balance of any engine configuration, the balancing masses can 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's dynamic 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 re-machining 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.

See also

• Balancing machine
• Engine tuning
• Noise, vibration, and harshness

External links

Referred to in the text

• A detailed analysis of the effect of different crankshaft offset angles for a straight twin engine.
• A detailed analysis of the 90° V twin mechanical balance.
• A detailed analysis of the boxer twin mechanical balance.

General

• Engine smoothness extensive article.
• Shaking forces of twin engines.
• Overview of balance, related to the straight 4.
• Primary and secondary balance.