LOVELY PROFESSIONAL UNIVERSITY

2011

G . T . R O A D , J A L A N D H A R , P U N J A B

Prepared by:-

Pathak Ratnesh Raman

B.tech(ME)

Roll no-RB4912B37

Reg.no-10907708

Submitted to:-

Mr.Jaspreet Singh

Lovely School Of Engineering-B

Deptt. Of Mechanical Engineering

DYNAMICS OF MACHINE

TOPIC

Balancing of crankshaft of four

cylinder and four stroke engine.

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TABLE OF CONTENT

Acknowledgement

Introduction

Various forces

Single cylinder engine

Multi cylinder engine

Components of balancing

Blue printing

Generic balance technique

Balance procedure

Conclusion

Internal vs external engine

Invention background

Advantage of balancing

Refrences

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ACKNOWLEDGEMENT

I take this opportunity to present my votes of thanks to all those guidepost who

really acted as lightening pillars to enlighten our way throughout this project

that has led to successful and satisfactory completion of this study. We are

really grateful to our HOD Mr. Gurpreet singh phull for providing us with an

opportunity to undertake this project in this university and providing us with all

the facilities. We are highly thankful to Mr.jaspreet singh for his active support,

valuable time and advice, whole-hearted guidance, sincere cooperation and

pains-taking involvement during the study and in completing the assignment of

preparing the said project within the time stipulated. Lastly, We are thankful to

all those, particularly the various friends , who have been instrumental in

creating proper, healthy and conductive environment and including new and

fresh innovative ideas for us during the project, their help, it would have been

extremely difficult for us to prepare the project in a time bound framework.

Pathak Ratnesh Raman

02/04/2011

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Introduction

Balanced combustion on reciprocating engines is important for reliable, emission-compliant

operation. Through the years, a number of engine balance techniques have been developed

and utilized. Unfortunately, no clear-cut standard has been established for engine balance

methodology or frequency. This paper will discuss the fundamentals of engine balance,

describe the history of balancing, and present the most current thinking on proper engine

balance technique, equipment, and frequency.

Engine balancing

In 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 engines). They were first invented and patented by British engineer Frederick

Lanchester in 1904.

Balance shafts are most common in inline four-cylinder engines which, due to the asymmetry

of their design, have an inherent second order vibration (vibrating at twice the engine RPM)

which cannot be eliminated no matter how well the internal components are balanced. Four-

cylinder flat engines in the boxer configuration have their pistons horizontally opposed, so

they are naturally balanced and do not incur the extra complexity, cost or power loss

associated with balance shafts (though the slight offset of the pistons introduces a rocking

couple). This vibration is generated because the movement of the connecting rods in an even-

firing four-cylinder inline engine is not symmetrical throughout the crankshaft rotation; thus

during a given period of crankshaft rotation, the descending and ascending pistons are not

always completely opposed in their acceleration, giving rise to a net vertical inertial force

twice in each revolution whose intensity increases quadratically with RPM, no matter how

closely the components are matched for weight.

The problem increases with larger engine displacement, since the only ways to achieve larger

displacement are with a longer piston stroke, increasing the difference in acceleration or by a

larger bore thereby increasing the mass of the pistons; either way, the magnitude of the

inertial vibration increases. For many years, two litres was viewed as the 'unofficial'

displacement limit for a production inline four-cylinder engine with acceptable noise,

vibration, and harshness (NVH) characteristics.

The basic concept has a pair of balance shafts rotating in opposite directions at twice the

engine speed. Equally-sized eccentric weights on these shafts are sized and phased so that

the inertialreaction to their counter-rotation cancels out in the horizontal plane, but adds in

the vertical plane, giving a net force equal to but 180 degrees out-of-phase with the undesired

second-order vibration of the basic engine, thereby canceling it. The actual implementation of

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the concept, however, is concrete enough to be patented. The basic problem presented by the

concept is adequately supporting andlubricating a part rotating at twice engine speed where

the second order vibration becomes unacceptable.

There is some debate as to how much power the twin balance shafts cost the engine. The

basic figure given is usually around 15 hp (11 kW), but this may be excessive for

pure friction losses. It is possible that this is a miscalculation derived from the common use

of an inertial dynamometer, which calculates power from angular acceleration rather than

actual measurement of steady state torque. The 15 hp (11 kW), then, includes both the actual

frictional loss as well as the increase in angular inertia of the rapidly rotating shafts, which

would not be a factor at steady speed. Nevertheless, some owners modify their engines by

removing the balance shafts, both to reclaim some of this power and to reduce complexity

and potential areas of breakage for high performance and racing use, as it is commonly (but

falsely) believed that the smoothness provided by the balance shafts can be attained after their

removal by careful balancing of the reciprocating components of the engine.

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:

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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

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. 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.

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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.

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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 andcrankshaft 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-two (also known as parallel twin).

V-twin.

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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.

More than two cylinders

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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, 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-4 with two geared crankshafts

Flat-6

Flat-12

V12

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

I3 engines have a strong balance induced rocking motion

Straight-4 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 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.

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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.

What is engine balance?

In gas compression applications, reciprocating engines are designed to proportionately

distribute the compressor and auxiliary loads between the engine‘s power cylinders.

Unfortunately, a number of factors introduce variability into the cylinder-to-cylinder and

cycle-to-cycle combustion process.

Practical factors affecting cylinder-to-cylinder variability include:

Mechanical construction - stroke length, head and piston heights, gasket and ring size,

camshaft profile, fuel manifold

wave harmonics, etc.

Engine and component condition - worn rings, weak lifters, leaking fuel valves, spark

plug gap wear, ignition coil

degradation, etc.

Combustion controls – air/fuel ratio, ignition timing, pcc pressure, engine cooling, etc.

To help reduce cylinder-to-cylinder variability, engine operators install fuel balancing valves,

hereafter referred to as a fuel gas modulator valves, in the fuel manifold piping upstream of

the cylinder‘s fuel injection valve. Using these valves, the operator affects combustion by

adjusting the fuel delivery to a given cylinder (thus affecting the cylinder‘s air/fuel ratio and

burn characteristics).

By properly adjusting each cylinder‘s fuel gas modulator valve, the combustion in each

cylinder can be stabilized and controlled for good engine balance.

Why should I balance my engine?

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There are five reasons to perform an engine balance.

1. Control peak combustion pressures – assuring safe operation within manufacturer‘s

specifications

2. Proportionately distribute the horsepower load across the power cylinders –

minimizing unbalanced crankshaft torsion forces

3. Reduce misfires and cycle-to-cycle variability - minimizing fuel consumption

4. Minimize excessive stresses on engine components created by high peak firing

pressures and detonation – maximizing

reliability and availability

5. Control combustion temperatures – stabilizing exhaust emissions

Generic Balance Technique

The engine balance process can be divided into four simple steps.

1. Direct pressure readings are taken for each power cylinder.

2. The pressure readings are analyzed and compared to determine which cylinders are

firing high and which cylinders are firing low (compared to the average pressure).

3. Using this information, the fuel gas modulator valves are adjusted to make the high

pressures lower and/or the low pressures higher (see example below for more details).

4. After the adjustments are made, pressure readings are retaken on all cylinders to

assure that the pressures are balanced within the balance criteria. If the balance is not

achieved, Steps 2, 3, and 4 are repeated.

Engine Balance Example

Imagine that the fuel system of an engine is a line of sprinklers in a sprinkler system (Figure

1). The system has one control valve (the governor) and six adjustable sprinkler heads

(modulator valves).

The yard needs 10 gallons of water per minute (or so many BTU‘s of fuel to generate the

required horsepower) and the water should be evenly distributed to the grass around each

sprinkler head (or the same pressures in every cylinder).

To do this, the individual sprinkler heads (or modulator valves) are adjusted. By pinching

valves #2 and #5 on sprinklers (modulator valves) the spray will be lower. If the flow through

the control valve (governor) stays constant, then sprinklers 1, 3, 4, & 6 would increase. Why?

To get the same flow through a smaller orifice requires a higher pressure.

Alternately, opening #4 sprinkler head would decrease the height of 1, 2, 3, 5 & 6. By

continuing to adjust the individual sprinkler heads, the spray on all the sprinklers will be the

same height. The same is true of a fuel system on an engine.

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History of Balance Equipment and Methodologies

The key to the balance technique is choosing the ―best‖ pressure. There are two thoughts

related to choosing the ―best‖ pressure. They are:

Mean Effective Pressure (Horsepower) ?

or

Peak Firing Pressure ?

Controversy has surrounded engine balance in gas compression service from the beginning.

Two issues lead to this controversy.

First, some engine manufacturers recommended against engine balance for years. Concerns

about unqualified personnel changing the air/fuel ratio weighed heavily on there minds.

Secondly, many of the early instruments used for engine balance came from diesel

applications. However, as balancing tools and training improved so did the practice of engine

balance.

The history of the engine balance controversy was strongly influenced by technological

advances in pressure indicating equipment. The BMEP gaugeTM and Pi meterTM

(mechanical devices used to measure Mean Effective Pressure) and the BacarachTM (Figure

2) and MaihakTM (mechanical devices used to measure Peak Pressure) were initially used on

diesel engines.

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With the invention of electronic pressure indicators a clearer picture was drawn inside the

power cylinder. Electronic tools include the Beta-TrapTM (Figure 3) and Enspec 1000TM

(peak pressure indicators), GET 1000TM (maintenance analyzer which graphically displays

pressure-time and pressure-volume patterns) and Recip-TrapTM, WindrockTM, PFMTM and

CarmaTM (performance analyzers with both mean effective calculations and peak firing

pressure traces and analysis).

Interestingly, in diesel engine applications, both mean effective and peak firing pressure

readings are good balance indicators due to the stability and repeatability of combustion. The

diesel is a compression-ignited (CI) internal combustion engine that uses the heat from highly

compressed air to ignite a spray of fuel introduced after the start of the compression stroke.

Extremely consistent peak firing and mean effective pressures characterize compression

ignition. As such, balancing by either pressure

generates similar engine balance results.

Conversely, a spark-ignited engine (SI) propagates the flame from one or two points across

the cylinder. The peak firing and mean effective pressures are very susceptible to flame front

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velocity changes due to poor homogenization. Adjustments to the fuel gas modulators don‘t

usually result in proportionally similar changes to peak firing and mean effective pressures.

As such, balancing by mean effective pressure and peak firing pressure generate different

engine balance results. Thus leading to the

question of which pressure is the ―best‖ or ―right‖ one to use for engine balance.

Mean Effective Pressure (Horsepower)

One horsepower is the energy required to lift 33,000 lbs of weight one foot in one minute.

It is numerically expressed using the formula:

Horsepower = P L A N / 33,000

Where: P = Mean Effective Pressure (MEP)

L = Piston stroke in feet

A = Area of piston in square inches

N = Number of power strokes per minute

Once installed in an engine, the piston stroke and area are fixed. Leaving horsepower as a

function of mean effective pressure (hereafter referred to as MEP) and engine speed only.

MEP is defined as the average (mean) theoretical pressure throughout the power stroke. It is

measured primarily and most accurately with some type of performance analyzer. Figures 4

& 5 are pressure-volume traces of 2-stroke and 4-stroke cycle engines.

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The area inside each pressure-volume trace determines the MEP of the power cylinder.

Imagine that the line representing the pressure volume is a string. To determine the MEP, the

odd-shaped power cylinder pressure volume is stretched into the shape of a rectangle from

TDC to BDC. The height of that rectangle would approximate the MEP (See Figure 6).

Pressure

Volume

MEP is equal to the average

theoretical pressure throughout

the power stroke, represented

by the rectangle stretched from

TDC to BDC on the power stroke.

Engine speed affects horsepower by increasing or decreasing the number of cycles per

minute. Higher operating speeds result in

more pressure-volume cycles per minute and consequently more horsepower.

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What Controls MEP?

The first rule of thermodynamics generally states:

Net Work Output = Net Heat Input When applied to gas compressor applications and

reordered, this rule can generally be restated as:

Brake HP (Engine) =

Indicated HP (Compressor) + Parasitic HP + Mechanical Loss HP

As a rule of thumb, the gas industry roughly approximates the lost mechanical horsepower as

5% of the indicated compressor horsepower. The parasitic load (auxiliary pumps, blowers,

fans, etc) is typically fixed at a given engine speed. As such, it can be said that:

Brake HP =

c1 (Indicated Compressor HP) + c2 where c1 = 1/0.95 and c2 = Parasitic HP

Then removing the constant values c1 and c2:

Brake HP (engine) µ Indicated HP (compressor)

Then, substituting the PLAN/33,000 formula for Indicated Engine HP, assuming a fixed

piston stroke and piston area, and lumping all constants together, the formula is further

simplified to state:

Mean Effective Pressure x Speed (engine) µ Indicated HP (compressor) Indicated horsepower

for the compressor is also calculated with the PLAN/33,000 formula (using compressor

dimensions and pressures).

In most cases, the compressor speed is equal to the engine speed thus leaving the simplest

form of the equation:

Mean Effective Pressure (engine) µ Mean Effective Pressure (compressor)

Answering the question of ―How is MEP controlled?‖ is now quite simple.

The engine‘s average MEP is controlled exclusively by the average compressor MEP (or

compressor load). With a fixed speed, the average compressor MEP (compressor load) is

affected operational by changing clearance volume (bottles or pockets), valve spacers, lifters,

or unloaders, or by varying suction and/or discharge pressure. Engine operating variables

such as air/fuel ratio, ignition timing and air manifold temperature don‘t change compressor

load – and subsequently don‘t change the engine‘s average MEP.

Can Air/Fuel Ratio Affect MEP?

Overall adjustments to the air/fuel ratio controller won‘t change the engine‘s average MEP.

However, these changes will affect the combustion properties within the power cylinder.

Assuming a ―normal‖ air/fuel mixture, leaning or richening the mixture will affect the burn

rate (flame front velocity), peak firing pressures, and temperatures. In general, the average

pressure-volume graph for the power cylinders will change in shape – but remain equal in

terms of area (or MEP). The pressure-volume chart and table in Figure 4 show the effects of

changing the engine‘s overall air/fuel mixture.

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If the mixture is further richened, the high peak firing pressures and advanced firing angle

will result in unstable combustion and eventually lead to cylinder detonation. On the other

hand, as the mixture is further leaned, partial and complete misfires will begin to occur. In

both cases, severe detonation and misfires will reduce the cylinder‘s MEP.

When spark plugs start to misfire or the air/fuel mixture doesn‘t burn properly, engine

performance decreases and fuel consumption increases. Most experts agree that a typical two-

cycle spark-ignited engine misfires approximately 25% of the time.

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A misfire is incomplete combustion as opposed to a dead or laying out cylinder that fails to

burn. Some misfires look very similar to a lean mixture (low peaks with high exhaust

pressures) but are able to maintain MEP. Some misfires are so severe that peak firing

pressure is lower than running compression also resulting in a lower MEP.

Severe detonation or misfires that results in lower MEP for a given cylinder will

consequently deliver less horsepower to the crankshaft. Assuming the compressor load has

not changed, the engine will begin to slow down. In turn, the governor will open up and

delivery more fuel to all cylinders. With the increased fuel, the good cylinders will pick up

load (increase MEP) for the

cylinder where MEP decreased. The pressure-volume chart and table in Figure 8 show the

effects of detonation and misfires on a cylinder‘s MEP.

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Can an Individual Cylinder‘s MEP Change by Adjusting the Fuel Modulator Valve?

During the engine balance process, adjustments to individual fuel gas modulator valves

change the air/fuel ratio within a given power cylinder. Generally MEP is not very responsive

to small changes in air/fuel ratio. To significantly change the cylinder‘s MEP, the fuel gas

modulator valve must be adjusted until the cylinder begins to misfire or detonate. In this case,

MEP is changed by adversely impacting the cylinder‘s combustion characteristics.

Peak Firing Pressure

Peak Firing Pressure (hereafter referred to as PFP) is much simpler to describe. It is the

highest pressure in the power cylinder due to combustion (See Figures 4 & 5). This pressure

is easily measured with mechanical and electronic devices.

In addition to PFP, another useful measurement is Peak Firing Angle. Peak firing angle

represents the crank position when peak firing pressure is achieved. The location of the peak

firing angle has a dramatic impact on the transfer of energy from the piston to the crankshaft.

When peak firing pressure occurs too close to TDC, some of the combustion occurs before

TDC - resulting in wasted energy and excessive wear on bearing surfaces.

What Affects PFP?

Unlike MEP, Peak Firing Pressure is affected by most changes to the engine and compressor.

The primary variables affecting PFP include compressor load, engine speed, air/fuel ratio,

ignition timing, fuel BTU, and mechanical condition.

How Does Air/Fuel Ratio Affect PFP?

As described in the MEP section, air/fuel ratio affects the burn rate and combustibility of the

mixture. In richer air/fuel mixtures, the rate of burn is accelerated causing the burn to occur

more quickly. The faster burn occurs closer to TDC and generally results in higher peak

firing pressures (Review Figure 7). Conversely, a lean air/fuel mixture slows the burn and

results in later and lower peak firing pressures.

Comparison of Balance Techniques

With a basic understanding of Mean Effective Pressure (MEP) and Peak Firing Pressure

(PFP), a logical comparison can be made to determine the ―best‖ pressure for balancing.

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Comparing each balance technique against the desired objectives results in the following

table:

MEP (Horsepower) Technique PFP Technique

Operate safely by

controlling peak

combustion pressures.

Maybe – Balancing by MEP does not

assure that peak firing ressures will

not exceed the manufacturer‘s rating.

This can be avoided by using analyzer

equipment that also measures PFP.

Yes – Balancing by PFP

inherently

allows the operator to

control peak pressures.

Minimize unbalanced

torsion forces applied to

the crankshaft by

proportionally distributing

horsepower

across the power

cylinders.

Yes – assuming that mathematical

corrections are made to account for

differences in stroke length and

cylinder liner geometry.

Somewhat – this

technique errs to the

safe side by limiting peak

firing pressures. Note:

Balanced horsepower

distribution is not

guaranteed with this

method.

Optimize fuel

consumption by

minimizing misfires and

cycle-to-cycle variability.

No – In fact, in order to reduce a

cylinder‘s MEP, fuel gas modulator

adjustment frequently result in

increased

misfires.

Yes – Most PFP

adjustments can be

made without creating

misfires.

Maximize reliability and

availability by reducing

excessive stresses on

engine

components created by

high peak firing pressures

and detonation.

Maybe – Since MEP is constant over

a wide range of pressure-volume

settings,

excessive peak firing pressures can

and do occur. This can be minimized

by also

measuring PFP.

Yes – By directly

controlling PFP, the

maximum forces created

are controlled

within the safe operating

limits of the

equipment.

Comply with exhaust

emission standards by

controlling combustion

temperatures.

No – Unless misfires or detonation

occur MEP is relatively independent

of

temperature and therefore unable to

control emissions.

Yes – Combustion

temperature is

directly related to PFP.

As such, NOx

exhaust emissions can be

controlled by

limiting PFP.

Which Pressure is “Best”?

Peak Firing Pressure is clearly the best choice based on the engine balance objectives. For

every objective, except horsepower distribution, peak firing pressure balance control is

superior. And, for the objective of distributing horsepower along the crankshaft, the peak

firing pressure technique errs to the safe side by limiting combustion pressures within the

manufacturer‘s

safety guidelines. In addition, PFP is much more responsive to modulator valve adjustments

than MEP in the stable combustion range.

Dynamics of Machine

22

When is the Engine Considered Balanced?

The most widely accepted technique for measuring engine balance is to compare each

individual cylinder‘s average peak firing pressure against the average peak firing pressure of

the entire engine. The largest individual cylinder pressure difference from the average is

divided by the engine average. The resulting number is the ―balance percent‖. Most

companies agree that 5% is a good target for balance criteria.

Interestingly a number of studies have shown that tighter balance standards (i.e. 4%, 3%, etc.)

have negligible benefits to engine performance and efficiency.

How Often Should an Engine Be Balanced?

Engines should be balanced every time maintenance is performed, after any change in

operating condition (load, weather conditions, etc.), or on a periodic basis (weekly). This is

important because engine balance directly impacts reliability, fuel cost, emissions, and

maintenance cost.

Note: The balance frequency often varies by engine, but weekly is a good rule of thumb –

especially if balance is also used as a maintenance and troubleshooting tool.

When Should Engines Be Balanced?

Anytime of the day is acceptable, but to protect the engine from detonation, balancing in the

heat of the day is preferable (except to the person balancing the unit). If the engine is

balanced during the heat of the day, when ambient temperatures are the highest, the peaks

will be at the highest level. But if the engine is balanced in the cool of the morning, the peaks

will rise as ambient temperatures increase – potential creating unacceptably high peaks. .

Basic Engine Balance Procedure Regardless of the instrument used to balance engines, the

principles are the same. Below is a basic engine balance procdure.

Note: Refinements to this procedure should be made to conform to the operating instruction

of the specific balancing instrument.

Preliminary Tasks

1. Check ignition timing and ensure it is correct. If possible, have someone with an

ignition analyzer inspect the condition of the primary and secondary of the ignition

system.

2. Check the mechanical condition of the injection, intake, and/or exhaust valves to

insure proper clearances.

3. Inspect the rocker arm bushing for wear and repair as needed.

4. Load the engine as near as possible to 100% speed and torque. If an engine begins to

detonate, unload the engine.

NEVER PERMIT AN ENGINE TO DETONATE!!!!! Detonation is usually caused

by one or more cylinders not carrying their share of the load. It is also important to

remember detonation is the symptom of the problem. The real problem is somewhere

else.

Dynamics of Machine

23

5. If automatic unloaders are used, lock into manual so load changes don‘t occur while

balancing.

6. Ensure the engine and compressors are up to normal operating temperatures.

7. On engines with mixture controls, check the set points and calibrate to known

performance grids.

8. Ensure the balancing device is in good operating condition and calibrated.

9. Record the following information (where available):

1. Engine Number

2. Suction Press

3. Discharge Press

4. Pocket Setting

5. Horsepower

6. Engine Speed

7. Eng Oil Press

8. Eng Oil Temp

9. Exhaust Temps

10. Engine Water Temp

11. Ignition Timing

12. Air Manifold Temp

13. Date & Time

14. Ambient Temp

15. Name of Person

16. Air Manifold Press

17. Fuel Manifold Press

18. Turbo Bypass Position

Balance Procedure

1. Collect peak firing pressures for each individual cylinder. If a mechanical indicator is

being used, collect a minimum of 10 peak firing pressures in a readable manner. The

greater the number of peaks collected, the better the sample (one of the advantages of

electronic engine balancers).

2. Analyze the pressure data and determine the average peak firing pressure for each

cylinder and the overall engine average peak firing pressure.

3. Note the high and low cylinders. Begin adjusting the individual fuel valves by

opening low cylinders or closing high cylinders. Determine whether to open or close

the fuel gas modulator valve based on governor position. Pinching the modulator

valves causes the governor to open; opening the modulator valves causes the governor

to close. Governor position should be approximately 75-80% open. This permits

enough range for the governor to compensate for load

changes without giving it enough range to compensate for major problems such as

dead cylinders.

4. After making any adjustment wait at least 20 minutes to allow cylinder conditions to

stabilize.

5. Retake the peak firing pressure readings on all cylinders (not just the cylinders that

were adjusted).

6. Calculate the engine balance percent and compare it against the desired balance

criteria. If the balance criteria is not met, steps 3, 4, and 5 should be repeated until the

balance is achieved.

Dynamics of Machine

24

7. Evaluate the balance data by looking for high cycle-to-cycle pressure deviations on a

given cycle to identify problem cylinders.

8. Clean and put away the balancing instrument.

9. Balance the engine once a week, anytime maintenance is performed, or when

significant changes in operations occur.

Engine Balance As a Troubleshooting Tool

The more sophisticated, electronic balance equipment provides local operating and

maintenance personnel with an effective troubleshooting and predictive maintenance tool.

Problems such as dead cylinders, detonation, excessive misfires, leaky fuel

injectors, worn spark plugs, and failing PCC check valves can be identified by carefully

reviewing the engine balance data. The following table identifies some of the common

problems that may be identified through engine balance.

Balance Indicator Engine Symptom Potential Equipment Cause

Extremely low peak

firing pressures

(similar to running

compression

pressure)

Dead Cylinder Fuel injector problem, faulty ignition

components,

worn spark plugs, collapsed lifters, or fuel

modulator

closed

Sporadic low peak

firing pressures

Misfires Fuel injector problem, faulty ignition

components, worn spark plugs, or collapsed

lifters

Extremely high peak

firing pressures

Detonation or Pre-

Ignition

Typically caused by misfires on other cylinders

or hot

spots in the power cylinder

Erratic peak pressure /

high pressure

deviations

Unstable exhaust

emissions

(NOx and CO)

Air/fuel ratio, PCC check valves, leaking fuel

injector,

improper balance, or defective ignition drive

Conclusions

This paper covers a lot of ground in an effort to fairly and objectively evaluate the engine

balance process. As a result of that discussion, the following conclusions and

recommendations were made:

Engine balance is ―best‖ achieved using Peak Firing Pressure.

Ideal engine balance equipment would have the following attributes:

1. Accurate peak firing pressure measurements

2. Calculated cylinder and engine average peak firing pressures

3. Calculated pressure deviations for each cylinder

4. Measured peak firing angle (typically only available on performance analyzers)

5. Capability to measure performance data over at least 32 cycles per cylinder

General guidelines for engine balance frequency are:

1. Each time maintenance is performed,

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25

2. After any change in operating conditions, and

3. At least once per week.

A good rule of thumb for balance criteria is to assure that the average peak firing

pressure for each cylinder is within 5% of the engine‘s average peak firing pressure.

A system balancing an internal combustion engine, particularly a four-stroke engine with five

cylinders in line, in which two of the counterweights for balancing the crankshaft have

asymmetric geometry defined by an additional mass in each case. The respective centers of

gravity of the two additional masses are situated in the plane containing the resultant moment

of the rotary components of the first-order alternating forces and the centrifugal forces of the

rotating masses.

BACKGROUND OF THE INVENTION

The present invention relates in general to internal combustion engines, particularly four-

stroke motor-vehicle engines with five cylinders in line, each including a crankshaft with

crank pins which are equi-angularly spaced around the axis of the shaft and provided with

balancing counterweights.

In engines of this type, there is a problem in balancing the resultant moments of the

centrifugal forces of the rotating masses and the rotary components of the first-order

alternating forces.

Balancing systems are known for this purpose which have counterweights designed so that

each has a static moment opposed to that of the respective crank pin. This solution, however,

involves the use of very large counterweights which are sometimes incompatible with the

space available for the installation of the engine in the vehicle. In fact, in order to balance the

said moments, which are indicated M c below, it is necessary for the counterweights to

achieve a static moment m s given by the formula: M c=aim s ω 2

where a is a coefficient which depends on the engine type (for example a=0.449 for a five-

cylinder, in-line engine), and i is the interaxial spacing of the cylinders.

m s is therefore large since a and i are small.

SUMMARY OF THE INVENTION

The object of the present invention is to avoid the aforesaid problem and to provide a system

for balancing an engine of the type defined above which enables the resultant moment of the

centrifugal forces and the rotary components of the first order alternating forces to be

balanced by means of light-weight drive shaft counterweights of low bulk, without the need

for separate balancing of the flywheel and the pulley associated with the engine.

According to the invention, this object is achieved by virtue of the fact that two of the

counterweights for balancing the crankshaft have assymetric geometry defined by an

additional lateral mass in each case, the respective centers of gravity of the two additional

Dynamics of Machine

26

masses being situated in the plane containing the resultant moment of the torques generated

by the rotary components of first-order alternating forces and the centrifugal forces of the

rotating masses.

As is well known, this plane can be identified in precise geometric terms by calculation and

its position is characteristic of the number of cylinders in the engine. In the case of an engine

with five cylinders, the plane intersects the vertical plane passing through the axis of the

crankshaft at an angle of 54°.

To advantage, the two additional masses are carried by the balancing counterweights

associated with the ends of the shaft.

This leads to the two additional masses having very low static moments: in fact the moment

to be balanced thus becomes: M c =(ni)m' s ω 2

where n i is the lever arm expressed as a multiple of the interaxial spacing: clearly ##EQU1##

Moreover by calibrating the values of m' s one can easily adapt the same drive shaft to

different alternating masses, that is, for example to Otto-cycle and Diesel-cycle versions of

the same engine: it is in fact sufficient to provide corresponding cut lines at different depths

for the different versions on the additional masses m' s .

how to balance an engine?

'The performance world is full of cool stroker engine combinations these days that fill more

pages in crankshaft catalogs than ever before. We're talkin' about long-arm 460 Fords, wild

Cleveland combos, oddball inline-six cylinders, and stroke bumps for the new generation of

GM Gen III engines. All of this is in the service of the "mine is bigger than yours" approach

to horsepower heroism. But all this stroker hype can also cause confusion.

Car crafters are famous for buying parts through nontraditional means such as swap meets,

shop closeouts, and clandestine good-guy deals. While the price is usually right, this can also

lead to hidden costs when it comes time to balance the rotating assembly. Balancing sounds

simple, but as we found, there are many shops out there still drilling holes the traditional way

when applying a few simple tricks may make life easier and less expensive. We ran across a

typical weight issue when it came time to balance a small-block 331ci stroker Ford. The gruff

old guy at the local balance shop said, "I'm not gonna balance this. I'd have to turn it into a

piece of Swiss cheese. Take it someplace else. . . . " So we did and learned a little about

balancing engines in the process.

Before digging into the custom stuff, we figured we'd better brush up on exactly how OE

engines are balanced and look into the internal/external-balance issue. To start us on our

journey, we decided to talk to Scat Crankshaft's Tom Lieb, who has a strong background in

the area of balancing. Lieb is very opinionated about this issue because he has seen just about

every imaginable variation on crankshaft failure. Most of these are not due to poor quality, as

many think. Usually, the engine has either been abused with excessive rpm, balanced

improperly, or often suffered some parts-abusing combination of the two.

Let's start with a short lesson on crankshaft design. Crankshaft counterweights are designed

to offset (or balance, if you will) the inertia effect of a relatively heavy piston

and connecting rod moving in both a rotational and reciprocating (up-and-down) fashion at

Dynamics of Machine

27

speed. The weight of the piston-and-rod combination affects the size and placement of the

counterweight. A longer stroke combined with a heavy piston, pin, and ring package requires

a larger counterweight (more mass) to balance the greater reciprocating weight. Most V-8

engines use large counterweights toward the front and rear of the crankshaft, leaving the

center portion without counterweights. That splits the engine into front and rear halves. The

positions of the counterweights on all V-8 90-degree crankshafts are the same. The height of

the counterweight as measured outward from the crankshaft centerline is limited by both the

cylinder block and by the placement of the bottom of the cylinders. A counterweight placed

farther away from the crank centerline has more balance effect, but it is limited by the width

of the block crankcase. Weights placed toward both ends of the crank also have a greater

effect and therefore don't need to be as large to effectively balance the engine. This makes the

overall crank lighter.

Internal vs. External Balance

Packaging is also an important issue. During the design of the 400ci small-block, a major

engineering hurdle was insufficient real estate inside the small-block crankcase for the larger

counterweights demanded by the 400's supersized 4.125-inch piston. This was especially

difficult in the rear of the engine because the rear crankcase area on a small-block Chevy is

restricted by the placement of the oilfilter. The solution called for external balance weights

placed on the harmonic balancer and flywheel/flexplate. One advantage to external weights is

that they are generally lighter because they are positioned at the extreme ends of

thecrankshaft. The disadvantage is that these offset weights impart their own twisting forces

back into the crankshaft, which is not good. This same situation occurs with the 454ci big-

block Chevy, which is also the only production big-block that uses external weights to

balance the engine.

Small-block Fords have always been externally balanced, but because Ford is a name

synonymous with change, the Blue Oval engineers altered the amount of the external balance

when the engines morphed to a one-piece rear main seal. Early small-block Fords used 28

ounce-inches as the external weight amount, changing in 1981 to 50 ounce-inches. Like

small-block Chevys, parts can interchange between early and late engines, but to guarantee

smooth engine operation, the crankshaft, balancer, and flexplate/flywheel must all be kept

within the same balance family.

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:

Rotating weight (gm.) Reciprocating weight (gm.)

Rod bearing 50

Rod big end 420

Rod small end

180

Piston

450

Wristpin

80

Rings

38

Oil

2

Subtotal 470 750

Before we go further, remember we have a pair of reciprocating weights (two pistons) per

crank journal. The math looks like this:

Half of reciprocating weight is 750/2 = 375 x 2 pistons per journal = 750 grams, while total

rotating weight is 470 x 2 = 940 grams. Thus 750 + 940 = 1,690 grams.

Dynamics of Machine

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Dynamics of Machine

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Balancing goes hand-in-hand with performance engine building. Balancing reduces internal

loads and vibrations that stress metal and may eventually lead to component failure. But is it

worth the time and effort for mild performance applications, everyday passenger car engines

or low-buck rebuilds?

From a technical point of view, every engine regardless of the application or its selling price

can benefit from balancing. A smoother-running engine is also a more powerful engine. Less

energy is wasted by the crank as it thrashes about in its bearings, which translates into a little

more usable power at the flywheel. Reducing engine vibration also reduces stress on motor

mounts and external accessories, and in big over-the-road trucks, the noise and vibration the

driver has to endure mile after mile.

Though all engines are balanced from the factory (some to a better degree than others), the

original balance is lost when the pistons, connecting rods or crankshaft are replaced or

interchanged with those from other engines. The factory balance job is based on the

reciprocating weight of the OE pistons and rods. If any replacements or substitutions are

made, there‘s no guarantee the new or reconditioned parts will match the weights of the

original parts closely enough to retain the original balance. Most aftermarket replacement

parts are "balanced" to the average weight of the OEM parts, which may or may not be close

enough to maintain a reasonable degree of balance inside the engine. Aftermarket crank kits

are even worse and can vary considerably because of variations within engine families.

If the cylinders are worn and a block needs to be bored to oversize, the larger replacement

pistons may be heavier than the original ones. Some piston manufacturers take such

differences into account when engineering replacement pistons and try to match "average"

OE weights. But others do not. Most high performance pistons are designed to be lighter than

the OE pistons to reduce reciprocating weight for faster acceleration and higher rpm.

Consequently, when pistons and rods are replaced there‘s no way of knowing if balance is

still within acceptable limits unless you check it.

If you‘re building a stock engine for a passenger car or light truck that will spend most of its

life loafing along at low rpm, your customer might question the value of balancing such an

engine. But if a customer values durability and smooth operation, selling them a balance job

shouldn‘t be too difficult – and it will add some extra profit, too.

On the other hand, if you‘re building a performance motor, a stroker motor or an engine

that‘s expected to turn a lot of rpms or run a lot of miles, balancing is an absolute must. No

engine is going to survive long at high rpms if it‘s out of balance. And no engine is going to

last in a high mileage application if the crank is bending and flexing because of static or

dynamic imbalances.

Forces In Action

To better understand the mechanics of balancing, let‘s look at the theory behind it. As

everybody knows, a rotating object generates "centripetal force." Centripetal force is an

actual force or load generated perpendicular to the direction of rotation. Tie a rope to a brick

and twirl it around and you‘ll feel the pull of centripetal force generated by the "unbalanced"

weight of the brick. The faster you spin it, the harder it pulls. In fact, the magnitude of the

force increases exponentially with speed. Double the speed and you quadruple the force.

Dynamics of Machine

31

The centripetal force created by a crankshaft imbalance will depend upon the amount of

imbalance and distance from the axis of rotation (which is expressed in units of grams,

ounces or ounce-inches). A crankshaft with only two ounce-inches of imbalance at 2,000 rpm

will be subjected to a force of 14.2 lbs. At 4,000 rpm, the force grows to 56.8 lbs.! Double the

speed again to 8,000 rpm and the force becomes 227.2 lbs.

This may not sound like much when you consider the torque loads placed upon the crankshaft

by the forces of combustion. But centripetal imbalance is not torque twisting the crank. It is a

sideways deflection force that tries to bend the crank with every revolution. Depending on the

magnitude of the force, the back and forth flexing can eventually pound out the main bearings

or induce stress cracks that can cause the crank to snap.

Centripetal force should not be confused with "centrifugal" force, which is the tendency of an

object to continue in a straight trajectory when released while rotating. Let go of the rope

while you‘re twirling the brick and the brick will fly off in a straight line (we don‘t

recommend trying this because its difficult to control the trajectory of the brick).

Back to centripetal force. As long as the amount of centripetal force is offset by an equal

force in the opposite direction, an object will rotate with no vibration. Tie a brick on each end

of a yardstick and you can twirl it like a baton because the weight of one brick balances the

other. If we‘re talking about a flywheel, the flywheel will spin without wobbling as long as

the weight is evenly distributed about the circumference. A heavy spot at any one point,

however, will create a vibration because there‘s no offsetting weight to balance out the

centripetal force.

This brings us to another law of physics. Every object wants to rotate about its own center of

gravity. Toss a chunk of irregular shaped metal into the air while giving it a spin and it will

automatically rotate about its exact center of gravity. If the chunk of metal happens to be a

flywheel, the center of gravity should be the the flywheel‘s axis. As long as the center of

gravity for the flywheel and the center of rotation on the crankshaft coincide, the flywheel

will spin without vibrating.

But if there‘s a heavy spot on the flywheel, or if the flywheel isn‘t mounted dead center on

the crank, the center of gravity and axis of rotation will be misaligned and the resulting

imbalance will create a vibration.

Applied Physics

Okay, so how does all this scientific mumbo jumbo translate into the real world dynamics of

a spinning crankshaft? A crankshaft, like a flywheel, is a heavy rotating object. What‘s more,

it also has a bunch of piston and rod assemblies reciprocating back and forth along its axis

that greatly complicate the problem of keeping everything in balance.

With inline four and six cylinder engines, and flat horizontally opposed fours and sixes (like

Porsche and Subaru), all pistons move back and forth in the same plane and are typically

phased 180° apart so crankshaft counterweights are not needed to balance the reciprocating

components. Balance can be achieved by carefully weighing all the pistons, rods, wrist pins,

rings and bearings, then equalizing them to the lightest weight.

Dynamics of Machine

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On V6, V8, V10 and V12 engines, it‘s a different story because the pistons are moving in

different planes. This requires crankshaft counterweights to offset the reciprocating weight of

the pistons, rings, wrist pins and upper half of the connecting rods.

With "internally balanced" engines, the counterweights themselves handle the job of

offsetting the reciprocating mass of the pistons and rods. "Externally balanced" engines, on

the other hand, have additional counterweights on the flywheel and/or harmonic damper to

assist the crankshaft in maintaining balance. Some engines have to be externally balanced

because there isn‘t enough clearance inside the crankcase to handle counterweights of

sufficient size to balance the engine. This is true of engines with longer strokes and/or large

displacements.

If you‘re rebuilding an engine that is internally balanced, the flywheel and damper have no

effect on engine balance and can be balanced separately. But with externally balanced

engines, the flywheel and damper must be mounted on the crank prior to balancing.

Customers should be told what type of engine balance they have (internal or external), and

warned about indexing the position of the flywheel if they have to remove it later for

resurfacing. Owners of externally balanced engines should also be warned about installing

different flywheels or harmonic dampers and how it can upset balance.

Balance Shafts

In recent years, the auto makers have added balance shafts to many four and six cylinder

engines to help cancel out crankshaft harmonics. The counter-rotating balance shaft helps

offset vibrations in the crank created by the firing sequence of the engine.

On these motors, make sure the balance shaft is correctly "phased" or timed to the rotation of

the crank. If the shaft is out of sync, it will amplify rather than diminish engine vibrations.

Balance shafts are not a substitute for normal engine balancing, nor do they reduce the

vibration and stress the crankshaft itself experiences as it turns.

Balancing Act

The process of balancing begins by equalizing the reciprocating mass in each of the engine‘s

cylinders. This is done by weighing each piston on a sensitive digital scale to determine the

lightest one in a set. The other pistons are then lightened to match that weight by milling or

grinding metal off a non-stressed area such as the wrist pin boss. The degree of precision to

which the pistons are balanced will vary from one engine builder to another, and depends to

some extent on the application. But generally speaking pistons are balanced to within plus or

minus 0.5 grams of one another.

Next the rods are weighed, but only one end at a time. A special support is used so that the

big ends of all the rods can be weighed and compared, then the little ends. As with the

pistons, weights are equalized by grinding away metal to within 0.5 grams. It‘s important to

note that the direction of grinding is important. Rods should always be ground in a direction

perpendicular to the crankshaft and wrist pin, never parallel. If the grinding scratches are

parallel to the crank, they may concentrate stress causing hairline cracks to form.

On V6 and V8 engines, the 60 or 90 degree angle between the cylinder banks requires the use

of "bobweights" on the rod journals to simulate the reciprocating mass of the piston and rod

Dynamics of Machine

33

assemblies. Inline four and six cylinder crankshafts do not require bobweights. To determine

the correct weight for the bobweights, the full weight of a pair of rod bearings and the big end

of the connecting rod, plus half the weight of the little end of the rod, piston, rings, wrist pin

(and locks if full floating) plus a little oil are added together (100 percent of the rotating

weight plus 50 percent of the reciprocating weight). The correct bobweights are then

assembled and mounted on the crankshaft rod journals.

The crankshaft is then placed on the balancer and spun to determine the points where metal

needs to be added or removed. The balancer indexes the crank and shows the exact position

and weight to be added or subtracted. The electronic brain inside the balancer head does the

calculations and displays the results. The latest machines have graphical displays that make it

easy to see exactly where the corrections are needed.

If the crank is heavy, metal is removed by drilling or grinding the counterweights. Drilling is

usually the preferred means of lightening counterweights, and a balancer that allows the

crank to be drilled while still on the machine can be a real time saver.

If the crank is too light, which is usually the case on engines with stroker cranks or those that

are being converted from externally balanced to internally balanced, heavy metal (a tungsten

alloy that is 1.5 times as heavy as lead) is added to the counterweights. This is usually done

by drilling the counterweights, then press fitting and welding the heavy metal plugs in place.

An alternate technique is to tap the hole and thread a plug into place. Drilling the holes

sideways through the counterweights parallel to the crank rather than perpendicular to the

crank is a technique many prefer because it prevents the metal from being flung out at high

rpm.

After drilling, the crankshaft is again spun on the balancer to determine if additional

corrections are required. If the crank is for an externally balanced engine (such as a big block

Chevy), the balancing will be done with the flywheel and damper installed. On internally

balanced engines, the flywheel and damper can be balanced separately, or installed on the

crank and balanced as an assembly once the crank itself has been balanced.

New machinery has been introduced that both simplifies the balancing process and increases

the accuracy of the job. Electronic equipment that allows accurate measurement of not only

the amount of unbalance force, but also accurately reports the unbalanced vector position is

now available to engine rebuilders. Typically, balancing machines have assumed that the

unbalance force was equally opposed, so they would require the technician to correct the

excessive amounts of unbalance on the excess side to the point of making them equal.

Technicians have had to ‗stair-step‘ the corrections equally until the final tolerance was

attained.

Technology such as that in the Multi-Bal 3000 HMV eliminates this requirement, according

to Randy Neal of Cast Welding Technologies. Software and hardware are combined to allow

radical differences to be reported at each end of the crankshaft (including any rotational

positioning or vector position of the unbalanced force). Because the position and unbalance

amounts are reported correctly the technician can make changes to the crankshaft with

confidence that he will not over shoot the correction. In most cases the required cycles of

analysis and correction are reduced by 80 to 90 percent, according to Neal.

Dynamics of Machine

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The unbalance amount and position are imported into a special computer program called

"Heavy Metal Analysis" (HMA). This program allows the technician to plot the position and

amount of material that will be required to correct the crankshaft. The program lets rebuilders

create multiple scenarios based on rotation and radius position, weight amounts and sizes of

Mallory – all of which can be simulated without having to cut the first chip.

How Long?

How long does it take to actually balance an engine? A typical Chevy smallblock V8 might

take anywhere from 45 minutes to an hour-and-a-half depending on how much work is

needed and the degree of accuracy you‘re trying to achieve. You‘re obviously going to spend

more time on a motor that‘s going into a NASCAR Winston Cup racer than one that‘s going

into Grandma‘s grocery getter.

Though a balancing accuracy of plus or minus one gram is typically good enough for most

production street engines, many balancers today can achieve balancing accuracies in the

tenths or even hundredths of a gram!

The most time-consuming part of the job is weighing and matching the pistons and rods. A

four cylinder engine takes half as much time for this step as a V8. The next most time

consuming part is making up the bobweights for a V6 or V8. This step isn‘t needed with a

straight six or four. The actual setup on the machine takes only a few minutes, and the initial

spin and readings take only a couple of minutes more. The time required to perform the

necessary weight corrections will depend on the crank (weight removal goes much faster than

adding weight). And if you‘ve done your work carefully, the final spin will require no further

corrections because the balance will be right on the mark.

Most shops charge $150 to $225 to balance a V8. If heavy metal is required, add $40 to $75

per slug. Some shops charge less to balance engines, but these tend to be shops that are trying

to compete in the budget rebuild market, not the performance market.

The most profitable applications for balancing include small high revving engines such as

those in motorcycles, boats and go-carts. Some shops get $100 or more to balance a single

cylinder Briggs & Stratton engine, which requires a lot less time and effort than a V8.

Other profitable applications include balancing turbocharger impellers and blower rotors,

flywheels, driveshafts and even brake rotors and drums. Such jobs can be billed by the piece

or by the hour.

Like any other piece of shop equipment, you can spend as much or as little as you can afford

on a balancer. The higher end units will have more bells and whistles, and generally be

equipped with a drill stand as part of the work station. The less expensive units will have

fewer features and be "portable" so they can be mounted on an existing bench or mill

platform.

An entry level balancer will require an initial investment of around $11,000 to $15,000,

which may or may not include bobweights and stand. The electronics will be fairly basic and

probably have a numeric display rather than a graphic display. If you want to add a drill, add

another $4,000 to $5,000 to the cost of the basic unit.

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For $17,000 to $28,000 you can step up to a fully equipped balancer that includes a color

graphic display, upgraded computer electronics and a drill stand. The computer software on

some of the high end units is Windows 98 based and gives you the ability to store and recall

specifications and other technical information about specific crankshafts.

Though most balancers can handle a wide range of crankshaft sizes, some manufacturers

have equipment that is designed for small engine applications or large diesel cranks. At the

recent Performance Racing Industry Show in Indianapolis, IN, Hines introduced a new mini-

balancer called the "Eliminator 10" which is specifically designed for small parts under 10

lbs. The balancer, which sells for $13,999, can balance turbo impellers and blower rotors to

within 0.0001 ounce/inches accuracy.

How fast can a balancer pay for itself? The payback obviously depends on the volume of

work you do and how much you charge. If you finance a $15,000 balancer over 5 years and

the monthly payments are around $300, one-and-a-half jobs a month at $200 each would

cover your payment

References

Heywood, J.B.: Internal Combustion Engine Fundamentals, McGraw-Hill, New York, 1988.

Jones, J.B. and Hawkins, G.A.: Engineering Thermodynamics – An Introductory Textbook

(2nd Edition), John Wiley & Sons, Inc., New York, 1986.

Wylen, G.J.V. and Sonntag, R.J.: Fundamentals of Classical Thermodynamics (2nd Edition),

John Wiley & Sons, Inc., New York, 1973.

Anderson, E.P.: Gas Engine Manual (Revised by Charles G. Fackman), Theodore Audel &

Co., Boston, 1985.

John Deere Service Training: Fundamentals of Service: Engines (6th Edition), Deere &

Company, Moline, IL, 1986.

Refrences:-

1. http://en.wikipedia.org/wiki/Engine_balance 2. http://www.universal-balancing.com/en/balancing-machine-range/crankshaft-balancing-

machines 3. http://www.enginebuildermag.com/Article/48121/understanding_crankshaft_balancing.asp

x 4. http://www.engineersedge.com/wwwboard/posts/2589.html

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The End