ENGINE BALANCE

An engine has many moving parts. Some of them have reciprocating motion such as piston and some of them rotating motion such as crankshaft. If the moving parts are not in complete balance, inertia forces are set up which may cause excessive noise, vibration and cause wear and tear of the system. The purpose of balancing is to design the parts so that unbalance is reduced up to an acceptable level or eliminated completely. Improving engine balance reduces vibration and other stresses and, within a respectable percentage, improves the overall performance, efficiency ownership and reliability of the engine, as well as reducing the stress on other machinery near the engine.

Theory behind engine balancingRotating massesWhen a mass moves in a circular path, it experiences a centripetal acceleration which generates a force and acts inward. An equal and opposite force acts outward radially on the axis of rotation. This force is called centrifugal force which is a disturbing force for the system. The magnitude of this force remains constant but direction changes with the rotation of mass. The centrifugal disturbing force F is given by F c=m w2 r NewtonsWhere m= mass of the rotating component in kg w = angular speed of the component in rad/sec r = distance of center of gravity of mass from the axis of rotation For the balance of rotating masses, it is the centrifugal force which is to be balanced. This type of problem is very common in engine crankshafts.When several masses rotate in a single plane the system is said to be statically balanced if the resultant of all centrifugal forces is zero. For dynamic balancing, the resultant of couples is zero. For a system to be dynamically balanced the condition of static balance is automatically satisfied.

Reciprocating Masses

The Inertia Force of a rotating mass M is

The Inertia Force can be separated into two parts:-

Is called The Primary Force.

which is called The Secondary Force

It is clear that the primary force is equivalent to the component along the line of stroke of the centrifugal force due to an equal mass M rotating with the crank and at crank radius. Consequently, in the case of a single-cylinder engine, the primary reciprocating force could be balanced by a rotating mass on the other side of the crank pin. However this would introduce an unbalanced component of the centrifugal force of magnitude perpendicular to the line of stroke. A compromise solution (partial balance) is usually applied, the inertia force being reduced to a minimum when 50% of the reciprocating mass is balanced. The secondary force is similarly equivalent to the component of the centrifugal force of mass M at radius r/4n rotating at being coincident with the crank at inner dead-center.

Multi-cylinder In-line Engines

The usual arrangement for multi-cylinder engines is to have the cylinder center line all in the same plane and on the same side of the crankshaft center line. This constitutes an "In-line" engine. Notable exceptions to this rule are "Vee" engines in which there are in effect two banks of in-line cylinders and "Flat" engines in which half the cylinders are arranged on opposite sides of the crankshaft.

Assuming that and n are the same for all cranks, they can be omitted from all considerations of engine balance but they must be included when actual values are required.

(a) Primary Balance:For couples and forces to be in balance:-

(8)

And

(9)

The equations can be solved analytically or by polygons drawn in the relative crank directions. This is similar to those used for rotating balance.

Any gap remaining in the force or couple polygons represents ( to a certain scale) the maximum out-of-balance value. This occurs twice per revolution of the crank-shaft when its direction lies along the line of stroke.

(b) Secondary Balance : For complete balance:-

(10)

And

(11)

To solve graphically it is only necessary to draw vectors in the directions (i.e. relative to any one crank taken as zero) and repeat af for primary balance.

The Partial Balance of Two-cylinder LocomotivesIt is normal for the cranks to be at right angles and as a result the secondary forces are small and in opposite directions. As a result they are usually neglected and only the primary forces and couples are considered.

It is usual to balance about two-thirds of the reciprocating parts with masses fixed to the wheels. The unbalanced vertical components of the reciprocating masses give rise to a variation of rail pressure known as Hammer Blow and a Rocking Couple about a fore and aft horizontal axis.

Radial Engines - Direct and Reverse Cranks

The primary force for a reciprocating mass M is equivalent to the resultant of the centrifugal forces of two masses M/2 rotating at a crank radius r and at a speed , one in the forward direction of motion and the other in the reverse direction. Note that the "direct" and "reverse" cranks are equally inclined to the dead center position,

Similarly the secondary force can be represented by direct and reverse cranks inclined at to the inner dead center and each carrying a mass M/2 at a radius r/4n rotating at a speed of .

This method is particularly useful for examining the balance of radial engines with a number of connecting rods attached to the same crank. It is usually assumed that the crank and connecting rod lengths are the same for each cylinder, though from a practical consideration of design this is not generally true

The balancing of engine gives the following advantages Reduced need for a heavy flywheel or similar devices. Reduced wear. The opportunity to reduce the size and weight of components as a result of reduced

stress and wear. Reduced vibration transmitted to the surroundings of the engine. Lower Chance of Fluid Leaks Save on Fuel Increase Longevity Lessens Component Failure

What Incorrect Balancing DoesAny engine that is not balanced or balanced incorrectly will hurt internal engine components. The engine is a unique device. Being able to sustain the speeds in which the internal parts reciprocate is a feat within itself. The stress loads, the friction, the heat all are trying to tear the engine apart. The smoother the engine runs, the less these negatives have an effect. The parts most affected by this inept balancing are the ones that the engine needs the most to survive.

Piston Rings ... fail to seal Bearings ... early and irregular wear on connecting rod and main bearings

Damper ... the device that tries to assist in controlling harmonic shock gets overworked and begins to deteriorate.

Oil Pumps ... Chatter and bounce, which can also create spark chatter and early ignition part failures (oil pumps driven off same drive as distributor)

Timing Sets ... Early chain stretching as chain has to make up for damper failures Valve Springs ... Valve instability, spring harmonic failures (worse with gear drives) Transmission ... Front Pump failures in automatics, early pressure plate and clutch spring

failures

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.

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

Primary and secondary balancePrimary balance is the balance achieved by compensating for the eccentricities of the masses in the rotating system, including the connecting rods. 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. In theory, any conventional engine design can be balanced perfectly for primary balance.

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.

PARTS NEED TO BE BALANCED1) Rotating weights i) Housing bore end of rods ii) Rod journals iii) Rod bearings2) Reciprocating weights Pistons, pins, rings, locks

BALANCING CONNECTING RODS

Balancing a set of connecting rods is the process of modifying the rods so that they all weigh the same. Balancing and or lightening a motor's internal components are a time-honored technique in the building of performance engines. An engine's internal components include parts like the crankshaft, pistons and connecting rods. The more balanced and light these components are, the better your engine will perform, with smoother revving and more horsepower.

1) The connecting rods are cleaned with degreaser. They can be-tanked (chemically cleaned) by a machine shop, which is a much more effective way to get them completely clean.2) The length of the rods is examined. A casting seam should be seen running down the side of the rod.A dremel is used to grind this seam off of all of the rods. The rod should not be gouged or any material should not be removed besides the casting seam itself.3) The grinding marks are smoothed out with a round sanding tool on a dremel or a high-speed die grinder. The point here is not to completely polish the rod, but to eliminate the grinding marks and smooth out the metal.4) The rods are placed on a balancing fixture (see Resources) to weigh both ends of the rod. A balancing fixture is a device that holds one end of the rod free, and places the other end on the scale so that the weight of each end of the rod can be determined.5) The weight of both ends of the connecting rods on the scale is measured. The heaviest rods are lightened down to the weight of the lightest rods with the die grinder or dremel.6) Excess material from the both ends of the rod, are grinded off using a die grinder or dremel, until they all weigh the same as the lightest rod in the set. It is then Stop and measure frequently to avoid removing excess material. To avoid weakening the rod, do not grind off material in the middle of the rod or material that is very close to where the rod attaches to the crankshaft or to the piston. Instead, remove material from the outer ends of the rods. There should be extra mater

Fig: Record weight of rod Fig: Equalize the pin end of the rods

Balancing Piston Assembly1) All the pistons and rod assemblies are weighted and sorted the group from the lightest to heaviest. The lightest assembly is the benchmark weight that the others will match upon completion of the balancing process.2) The heaviest piston and rod assembly are clamped in a vise and small amounts of metal from the casting seam of the rod are removed with a file or grinder. The piston and rod assembly are weighted periodically, being careful not to remove too much metal.3) The casting seam of each rod is grinded or filed until all piston and rod assemblies meet the benchmark weight. Each rod has two casting seams -- one on either side -- so equal amounts of metal are tried to remove by switching from side to side during the metal removal process.

4) The weight of each piston and rod assembly are verified by a final cleaning and reweighing. Filings or grindings will lodge inside the piston crown. Each one is cleaned thoroughly, and then a final weight comparison is made. Any weight variances are adjusted as needed.

Fig: Weighing piston assemblies Fig: Reducing piston assembly weights

Balancing enginesBalancing procedure

1) Record piston weights and lighten heavy pistons to match lighter pistons2) Equalize rotating weights3) Equalize reciprocating weights4) Crankshafts are dynamically balanced in 2 planes5) to eliminate wobble6) Corrections are made to the end counterweights

DIMENSIONS REQUIRED

i) Radius from center to counterweights

ii) Distance between counterweights

iii Distance between counterweight and support

7) Balancing equipment locates point of correction

8) Weight is either added to one side or removed from the other

9) Amount varies with the radius

10) In-line engines do not require bob weights

11)V-block engines use bob weights

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, pin, connecting 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-center 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-center 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-two (also known as 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 center 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 center 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 minimized for a crank offset angle of 180°, but balance is still far from perfect. There is still a rocking moment produced by the no concentricity 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 minimized 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.

Balancing of inertial forces in the multi-cylinder engine

In multi-cylinder engines the mutual counteractions of the various components in the crankshaft assembly are one of the essential factors determining the selection of the crankshaft's configuration, and with it the design of the engine itself. The inertial forces arebalanced if the common center of gravity for all moving crankshaft-assembly components lies at the crankshaft's midpoint, i.e. if the crankshaft is symmetrical (as viewed from the front). The crankshaft's symmetry level can be defined using geometrical representations of 1st- and 2nd-order forces (star diagrams). The 2nd order star diagram for the four-cylinder in-line engine is asymmetrical, meaning that this order is characterized by substantial free inertial forces. These forces can be balanced using two countershafts rotating in opposite directions at double the rate of the crankshaft.

In modern multi-cylinder engines, many inherent balance problems are addressed by use of balance shafts. Wear-and-tear is reduced only when the crankshaft is partly balanced before it touches any bearing as it is done in the flat and the V-engines. A balancer shaft transfers its force via bearings onto the crankshaft and rather increases wear-and-tear.

3-cylinder 4-cylinder 5-cylinder 6-cylinderCrank sequence

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 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

Balancing with heavy metals

In this method of engine balancing holes are generally filled with Tungsten alloy.it is used for external to internal change. This is expensive for conventional balancing.

Special Balancing ProcessesThe most common special Balancing Process includes the addition of Mallory Metal to the crankshaft. Mallory Metal is an extremely heavy metal used in extreme "out of balance" situations or when using ultra-light crankshafts, whereas normal welding material weight is not enough weight to correct imbalance. Other use is in certain racing applications there is the

need to "neutral balance" the crank. This is usually done when no damper is used, as in the Alcohol Burning Sprint Cars classes. Mallory Metal is quite expensive and it is only used as necessary.

The high speeds of rotation at which modern machines and engines are required to operate, has made it increasingly important that all revolving and reciprocating parts are as completely balanced as possible. Not only are the bearing loads and stresses in components increased by out of balance dynamic forces but there is also the possibility of causing significant harmful vibrations. On the railways these were called "Hammer Blows" and they were reduced by casting Balancing weights into the driving wheels. These can be clearly seen in the photograph at the bottom of the wheel

Balance ShaftIn 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 engine.Automotive designers are constantly striving to improve the level of comfort in the car for both driver and passengers. Two key factors here are engine vibration and engine noise, both of which are a product of the basic design of the engine. Through combustion, where chemical energy is converted into mechanical energy, gas forces are generated which act on the piston crown. The reciprocating movement of the pistons and connecting rods, combined with the rotation of the crankshaft, generate inertial forces that act on the engine block and cause it to vibrate in various ways. At low engine speeds the gas forces are greater than the inertial forces but at high engine speeds the converse is true. The most significant forces arise periodically once or twice per crankshaft revolution. They are known as first-order and second-order forces. The first-order inertial forces are completely unbalanced since the crankshaft is balanced and the two piston pairs, 1-4 and 2-3, change direction simultaneously when they reach top dead center and bottom dead center.

Second-order forces acting verticallyInertial forces are generated because both descending pistons in a four-cylinder engine travel further at a given crankshaft angle than the two ascending pistons (the lateral movement of the connecting rods accelerates the descending pistons but delays the ascending pistons). The common center of gravity of the ascending and descending masses therefore varies, giving rise to forces moving upwards and downwards which vary periodically twice per crankshaft revolution and cause the engine to vibrate in a vertical direction. Second-order forces acting laterallyDuring the power stroke the piston is pressed against the cylinder wall due to the angle of the connecting rod relative to the cylinder. At higher engine speeds, however, the inertial force is much greater. It can then be said that the crankshaft pulls the piston down and due to the angle

of the connecting rod relative to the cylinder the piston is pressed against the cylinder wall, but this time on the opposite side. Regardless of engine speed, the gas and inertial forces acting sideways vary periodically twice per crankshaft revolution and cause the engine to vibrate in a lateral direction.

Balance-shafts are used to overcome the second-order inertial forces. Two balance shafts located with lateral symmetry on the sides of the engine block but at different heights above the crankshaft centerline incorporate eccentrically mounted balance weights. The shafts are driven by a chain and rotate in opposite directions to each other at twice the crankshaft speed.

The balance weights on the shafts are positioned so as to eliminate the upward and downward moving forces generated by the movement of the pistons, as described on the preceding page.

Since the balance shafts are situated at different heights above the crankshaft centerline, they also counteract lateral forces. The torque generated by the balance shafts is designed to counteract the gas and inertial forces acting in a sideways direction.

The balance shafts are of identical design and supported by aluminum bearing shells in the The bearing shells are a press fit in the block and lubricated by special oilways.

For the balance shafts to perform as intended, it is imperative that they are aligned precisely on fitting. Sprocket assemblies of different design for the exhaust and inlet sides are therefore used on the shafts and marked with identifying text.

The balance shaft, sprocket and bearing housing are fitted together as an assembly before being mounted in the correct side of the cylinder block as indicated by the marking on the bearing housing.

Balance shaft driveThe balance shafts are driven by a chain and rotate at twice the crankshaft speed.

A special idler sprocket over which the chain passes causes the balance shaft on the exhaust side to rotate in the opposite direction to the other balance shaft. The chain is located by two fixed chain guides and a pivoted chain guide on which a chain tensioner acts.

When the engine is running, pressurized engine oil acts on the chain tensioner in the opposite direction to control the force applied to the chain and so reduce both chains wear and chain noise to a minimum.

Both the balance-shaft sprockets and the idler sprocket incorporate thrust rings to reduce chain noise still further.

The thrust rings absorb the force of the chain where it engages the sprocket teeth, ensuring smoother operation and quieter running.

Due to the smaller surface contact angle, the thrust rings on the exhaust side are larger than those on the inlet side.