Engine balance
Encyclopedia
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:
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.
.
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 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 shaft
s, 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.
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.
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-twin
s, like V engine
s 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.
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:
Engines with more than two cylinders with characteristic balance problems include:
In modern multi-cylinder engines, many inherent balance problems are addressed by use of balance shaft
s.
s long before the invention of the internal combustion engine
. Steam locomotive
s 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 rod
s. Again, the balance is a compromise.
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.
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.
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
to run smoothly. Improving engine balance reduces vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
and other stresses
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
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 flywheelFlywheelA flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
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
Engine configuration
Engine configuration is an engineering term for the layout of the major components of a reciprocating piston internal combustion engine. These components are the cylinders and crankshafts in particular but also, sometimes, the camshaft....
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 configurationEngine configuration
Engine configuration is an engineering term for the layout of the major components of a reciprocating piston internal combustion engine. These components are the cylinders and crankshafts in particular but also, sometimes, the camshaft....
.
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 harmonicHarmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
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 shaftBalance shaftIn piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets vibrations in engine designs that are not inherently balanced...
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 shaft
Balance shaft
In piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets vibrations in engine designs that are not inherently balanced...
s, 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
Straight-6
The straight-six engine or inline-six engine is a six-cylinder internal combustion engine with all six cylinders mounted in a straight line along the crankcase...
, the flat six
Flat-6
A flat-6 or horizontally opposed-6 is a flat engine with six cylinders arranged horizontally in two banks of three cylinders on each side of a central crankcase...
and the V12
V12 engine
A V12 engine is a V engine with 12 cylinders mounted on the crankcase in two banks of six cylinders, usually but not always at a 60° angle to each other, with all 12 pistons driving a common crankshaft....
configurations offer exceptional inherent mechanical balance. Boxer eights
Flat engine
A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896...
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
Firing order
The firing order is the sequence of power delivery of each cylinder in a multi-cylinder reciprocating engine.This is achieved by sparking of the spark plugs in a gasoline engine in the correct order, or by the sequence of fuel injection in a Diesel engine...
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
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
of the piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
, gudgeon pin(wrist pin, US), connecting rod
Connecting rod
In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts linear motion into rotating motion....
and crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
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:- Straight-twoStraight-twoA straight-two engine, is a two-cylinder piston engine that has its cylinders arranged side by side....
(also known as parallel twin). - V-twinV-twinA V-twin engine is a two-cylinder internal combustion engine where the cylinders are arranged in a V configuration.- Crankshaft configuration :Most V-twin engines have a single crankpin, which is shared by both connecting rods...
. - Boxer twinFlat-twinA flat-twin is a two cylinder internal combustion engine with the cylinders arranged on opposite sides of the crankshaft. It is part of the class of flat engines, sub-type "boxer", and shares most characteristics of those engines.-Motorcycle use:...
(a common form of flat engineFlat engineA flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896...
).
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-twin
V-twin
A V-twin engine is a two-cylinder internal combustion engine where the cylinders are arranged in a V configuration.- Crankshaft configuration :Most V-twin engines have a single crankpin, which is shared by both connecting rods...
s, like V engine
V engine
A V engine, or Vee engine is a common configuration for an internal combustion engine. The cylinders and pistons are aligned, in two separate planes or 'banks', so that they appear to be in a "V" when viewed along the axis of the crankshaft...
s 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
Suzuki
is a Japanese multinational corporation headquartered in Hamamatsu, Japan that specializes in manufacturing compact automobiles and 4x4 vehicles, a full range of motorcycles, all-terrain vehicles , outboard marine engines, wheelchairs and a variety of other small internal combustion engines...
VX800
Suzuki VX 800
The Suzuki VX800 is a road motorcycle shaft-drive V-twin bike from Suzuki.The VX800 was designed at U.S. Suzuki's Design Studio in Brea, California from 1986 to 1989, and produced in years 1990 to 1997...
and Honda Transalp
Honda Transalp
The Honda Transalp is the name given to the XL600V, XL650V, and XL700V series of dual-sport motorcycles manufactured in Japan by Honda since 1987. The Transalp bikes all feature a liquid-cooled, four-stroke 52° V-twin engine.- History :...
, which have a two-pin crankshaft, and an offset angle between the two crank throws.
The boxer engine is a type of flat engine
Flat engine
A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896...
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-6Straight-6The straight-six engine or inline-six engine is a six-cylinder internal combustion engine with all six cylinders mounted in a straight line along the crankcase...
- Straight-8Straight-8The straight-eight engine or inline-eight engine is an eight-cylinder internal combustion engine with all eight cylinders mounted in a straight line along the crankcase...
- Flat-4Flat-4A flat-4 or horizontally opposed-4 is a flat engine with four cylinders arranged horizontally in two banks of two cylinders on each side of a central crankcase...
with two geared crankshafts - Flat-6Flat-6A flat-6 or horizontally opposed-6 is a flat engine with six cylinders arranged horizontally in two banks of three cylinders on each side of a central crankcase...
- Flat-8Flat-8A flat-8 or horizontally opposed-8 is an internal combustion engine in flat configuration, having 8 cylinders.The most famous engine of this type is the Porsche air-cooled flat-8 engine introduced in a 1.5 L version for Formula One that grew up to a 3 L version used in the Porsche 908.Another...
- Flat-12Flat-12A flat-12 is an internal combustion engine in a flat configuration, having 12 cylinders.The flat-12 is wider than a V12...
- Flat-16Flat-16A flat-16 is an internal combustion engine in flat configuration, having 16 cylinders.Few examples are known. The British engine manufacturing company Coventry Climax developed a racing flat-16 unit for the 1.5 litre Formula One between the 1963 to 1965 Formula One seasons,the FWMW, but,...
- V12V12 engineA V12 engine is a V engine with 12 cylinders mounted on the crankcase in two banks of six cylinders, usually but not always at a 60° angle to each other, with all 12 pistons driving a common crankshaft....
- V16V16 engineA V16 engine is a V engine with 16 cylinders. Engines of this number of cylinders are uncommon in automotive use.A V16 engine is perfectly balanced regardless of the V angle without requiring counter-rotating balancing shafts which are necessary to balance Straight-4 and odd number of cylinder...
- W16W16 engineA W16 engine is a sixteen cylinder piston internal combustion engine in a four-bank W configuration. All W16 engines consist of two 'offset double-row' banks of eight cylinders, coupled to a single crankshaft....
Engines with more than two cylinders with characteristic balance problems include:
- I3 engines have a strong balance induced rocking motion
- Straight-4Straight-4The inline-four engine or straight-four engine is an internal combustion engine with all four cylinders mounted in a straight line, or plane along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft....
using a single crankshaft has no better kinetic energy balance than a single, and requires a relatively large flywheel. - 60 degree V6s
- 90 degree V6s
In modern multi-cylinder engines, many inherent balance problems are addressed by use of balance shaft
Balance shaft
In piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets vibrations in engine designs that are not inherently balanced...
s.
Steam engines
The question of mechanical balance was addressed on steam engineSteam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
s long before the invention of the internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
. Steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
s commonly have balancing weights on the driving wheels to control wheel hammer
Hammer blow
Hammer blow, in rail terminology, refers to the vertical forces transferred to the track by the driving wheels of a steam locomotive and some diesel locomotives. The largest proportion of this is due to the unbalanced reciprocating motion, although the piston thrusts also contribute a portion to it...
caused by the up and down motion of the coupling rods and, to some degree, the connecting rod
Connecting rod
In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts linear motion into rotating motion....
s. 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
Compression ratio
The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
and valve timing
Valve timing
In a piston engine, the valve timing is the precise timing of the opening and closing of the valves.In four-stroke cycle engines and some two-stroke cycle engines, the valve timing is controlled by the camshaft. It can be varied by modifying the camshaft, or it can be varied during engine operation...
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.
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.