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AU2351AUTOMOTIVE ENGINE COMPONENTS DESIGN

UNIT I INTRIDUCTION

MECHANICAL PROPERTIES OF MATERIALS & HOOKS LAW

When studying materials and especially when selecting materials for a project /design, it is important to understand key properties. The most important properties areoutlined below.

STRE!T"

The ability of a material to stand up to forces beingapplied without it bending, breaking, shattering ordeforming in any way.

E#$ST%&%T'

The ability of a material to absorb force and fle( indifferent directions, returning to its original position.

)#$ST%&%T'

The ability of a material to be change in shapepermanently.

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*plasticity+ can be demonstrated by pouring moltenaluminium it into a mould. nce the aluminium hascooled down, it can be remo-ed from the castingsand. %t has a new shape.

&T%#%T'

The ability of a metal to change shape 0deform1usually by stretching along its length.

TES%#E STRE!T"

The ability of a material to stretch without breaking orsnapping.

2$##E$3%#%T'

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The ability of a material to be reshaped in alldirections without cracking

T!"ESS

$ characteristic of a material that does not break or

shatter when recei-ing a blow or under a suddenshock.

"$RESS

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The ability of a material to resist scratching, wear and

tear and indentation.

&&T%4%T'

The ability of a material to conduct electricity.

Strain

Strainis the result of the application of forces to solid objects. The forces are defined ina special way described by the general term, stress.

FIGURE 5.12 Tensile and compress ional stress can be defined in terms of forcesapplied to a uniform rod.

$ relationship e(ists between force applied to a solid object and the resultingdeformation of that object. Solids are assemblages of atoms in which the atomic spacinghas been adjusted to render the solid in e5uilibrium with all e(ternal forces acting on the

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object. This spacing determines the physical dimensions of the solid. %f the appliedforces are changed, the object atoms rearrange themsel-es again to come intoe5uilibrium with the new set of forces. This rearrangement results in a change inphysical dimensions that is referred to as a deformationof the solid. The effect of appliedforce is referred to as a stressand the resulting deformation as a strain.

Tensile Stress-Strain

%n 6igure 7.89a, the nature of a tensile force is shown as a force applied to a sample ofmaterial so as to elongate or pull apart the sample. %n this case, the stress is defined as

where F =applied force in A =cross:sectional area of the sample in m9

We see that the units of stress are /m9in the S% units 0or %b/in9in the English units1 andthey are like a pressure.

The strain in this case is defined as the fractional change in lengthof the sample;

where l < change in length in m 0mm0l< original length in m 0mm0

Strain is thus a unitless 5uantity.Compressional Stress-Strain

The only differences between compressional and tensilestress are the direction

of the applied force and the polarity of the change in length. Thus, in a compressionalstress, the force presses in on the sample, as shown in 6igure 7.89b. The compressional

stress is defined as in E5uation 07.9.0

The resulting strain is also defined as the fractional change in length as in E5uation07.=1, but the sample will now decrease in length.

Shear Stress-Strain

6igure 7.8=a shows the nature of the shear stress. %n this case, the force is applied as acouple0that is, notalong the same line1, tending to shear off the solid object thatseparates the force arms. %n this case, the stress is again

where 6 < force in A =cross:sectional area of sheared member in m9

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The strain in this case is denned as the fractional change in dimension of the shearedmember. This is shown in the cross:sectional -iew of 6igure 7.8=b.

FIGURE 5.13 Shear stress is defined in terms of a couple that tends to deform a joiningmember as shown in this figure.

where x < deformation in m 0as shown in 6igure 7.8=b0l < width of a sample in m

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Hooks Law: For elastic materials, the force applied is proportional to the extension of the

sample. A material is said to be elastic if it reains its shape after the applied forces

are remo!ed.

6Stress-Strain C"r!e

%f a specific sample is e(posed to a range of applied stress and the resulting strain ismeasured, a graph similar to shown below results. This graph shows that therelationship between stress and strain is linear o-er some range of stress. %f the stress iskept within the linear region, the material is essentially elasticin that if the stress isremo-ed, the deformation is also gone. 3ut if the elastic limit is e(ceeded, permanentdeformation results. The material may begin to >neck> at some location and finally break.Within the linear region, a specific type of material will always follow the same cur-esdespite different physical dimensions. Thus, we can say that the linearity and slope are aconstant of the type of material only. %n tensile and compressional stress, this constant iscalled the modulus of elasticityor Young's modulus,as gi-en by

where stress = F/Ain /m9strain = l/lunitless

E =2odulus of elasticity in /m9

The modulus of elasticity has units of stress, that is, /m9. Table 7.8 gi-es the modulusof elasticity for se-eral materials. %n an e(actly similar fashion, the shear modulus isdefined for shear stress:strain as

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where xis defined in 6igure 7.8=b and all other units ha-e been denned in E5uation07.?.0

TABLE 5.1 2odulus of elasticity

2aterial 2odulus 0/m90

$luminum&opper

Steel)olyethylene

0plastic0

@.AB( 8C8C

88.?=D 8CF9C.?C D 8C8C

=.G7( 8CA

EXAMPLE 5.56ind the strain that results from a tensile force of 8CCC applied to a 8C:m aluminumbeam ha-ing a G D 8C:Gm9cross:sectional area.

SolutionThe modulus of elasticity of aluminum is found from Table 7.8 to beE < @.AB D 8C8C

/m9. ow we ha-e, from E5uation 07.?,0

so that

Engineeing S!e""#"!$in C%e

The engineering tension test is widely used to pro-ide basic design information on thestrength of materials and as an acceptance test for the specification of materials. %n thetension test a specimen is subjected to a continually increasing unia(ial tensile forcewhile simultaneous obser-ations are made of the elongation of the specimen. Theparameters, which are used to describe the stress:strain cur-e of a metal, are the!en"i'e "!eng!() *ie'+ "!eng!( , *ie'+ -,in!) -een! e',ng$!i,n) $n+ e+%!i,n ,/$e$. The first two are strength parametersH the last two indicate ductility.

Ten"i'e S!eng!(

The tensile strength, or ultimate tensile strength 0TS1, is the ma(imum load di-ided bythe original cross:sectional area of the specimen.

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#lectric $roperties of Solids

Solids may be classified in terms of their resisti-ity or conducti-ity as conductors,

insulators, or semiconductors. &loser e(amination of the microscopic conditions forhms lawin-ol-es free electron densityin solids.

Conductors and Insulators

%n a conductor, electric current can flow freely, in an insulatorit cannot. 2etals such ascopper typify conductors, while most non:metallic solids are said to be good insulators,ha-ing e(tremely high resistance to the flow of charge through them. >&onductor>implies that theouter electronsof the atoms are loosely bound and free to mo-e throughthe material. 2ost atoms hold on to their electrons tightly and are insulators. %n copper,the -alence electrons are essentially free and strongly repel each other. $ny e(ternalinfluence which mo-es one of them will cause a repulsion of other electrons whichpropagates, >domino fashion> through the conductor.

Simply stated, most metalsare good electrical conductors, most nonmetals are not.2etals are also generally good heat conductors while nonmetals are not.

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%hm&s Law

6or many conductorsof electricity, the electric currentwhich will flow through them isdirectly proportional to the -oltageapplied to them. The ratio of -oltage to current iscalled the resistance, and if the ratio is constant o-er a wide range of -oltages, thematerial is said to be an >ohmic> material. %f the material can be characteriIed by such aresistance, then the current can be predicted from the relationship;

'esisti!it( and Cond"cti!it(

The electricalresistanceof a wire would be e(pected to be greater for a longer wire, lessfor a wire of larger cross sectional area, and would be e(pected to depend upon thematerial out of which the wire is made. E(perimentally, the dependence upon theseproperties is a straightforward one for a wide range of conditions, and the resistance of a

wire can be e(pressed as

The factor in the resistance which takes into account the nature of the material is theresisti-ity . $lthough it is temperature dependent, it can be used at a gi-en temperatureto calculate the resistance of a wire of gi-en geometry.

The in-erse of resisti-ity is called conducti-ity. There are conte(ts where the use ofconducti-ity is more con-enient.

Electrical conducti-ity < J < 8/K

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C(e0i$' -,-e!ie"

&hemical properties of matter describes its >potential> to undergo some chemical

change or reaction by -irtue of its composition. What elements, electrons, and bondingare present to gi-e the potential for chemical change.

6or e(ample hydrogen has the potential to ignite and e(plode gi-en the right conditions.This is a chemical property.

2etals in general ha-e they chemical property of reacting with an acid. Linc reacts withhydrochloric acid to produce hydrogen gas. This is a chemical property.

&hemical change results in one or more substances of entirely different compositionfrom the original substances. The elements and/or compounds at the start of thereaction are rearranged into new product compounds or elements.

$ &"E2%&$# &"$!E alters the composition of the original matter. ifferent elementsor compounds are present at the end of the chemical change. The atoms in compoundsare rearranged to make new and different compounds.

2agnesium reacts with o(ygen from the air producing an e(tremely bright flame. This isa chemical change since magnesium o(ide has completely different properties thanmagnesium metal shown on the left.

6or e(ample iron has the potential to rust gi-en the right conditions. This is a chemicalproperty.

%f iron does rust, this is a slow chemical change since rust is an iron o(ide with differentproperties than iron metal.%n the element iron only atoms of iron are in contact with eachother. %n the element o(ygen each o(ygen is joined with one other to make a diatomicmolecule. These atoms and molecules are rearranged so that two iron atoms combinewith three atoms of o(ygen to form a new compoun

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UNIT II

DESIGN OF SAFT AND E!ICA! SPRINGS"a#t d$s%&n'

)t consists of the determination of the correct shaft diameter to ens"re satisfactor(

strenth and riidit( when the shaft is transmittin power "nder !ario"s operatin and

loadin conditions. Shafts are "s"all( circ"lar in cross-section, and ma( be either hollow

or solid.

D$s%&n o# s"a#ts'

For d"ctile materials, based on strenth, is controlled b( the maxim"m-shear theor(.

Shafts of brittle materials wo"ld be desined on the basis of the maxim"m-normal-stresstheor(. Shafts are "s"all( s"b*ected to torsion, bendin, and axial loads.

1()For tors%onal loads'

The torsional stress, x( is: *+ , -Mtr(./ , -10Mt(.d3 For solid shafts or *+ , -10Mtdo(.-do)d%( For hollow shafts2()For $nd%n& loads'

The bendin stress +btension or compression is:

,-Mr(.I , 32M.d3 For solid shafts, -32Mdo(.-do)d%( For hollow hafts

3()For a*%al load'

The tensile or compressi!e stress +ais:

a, Fa.d2 For solid shaftsa,Fa.-do2)d%2( Forhollow shafts

The ASMEcode e"ation for hollow shaft combines torsion, bendin, and axial loads b(

appl(in the maxim"m-shear e"ation modified b( introd"cin shock, fati"e, and

col"mn factor as follows:

( )

//

0 /

1

2 -23 -

42

a oo b b t t

s

F d Kd k M k M

K

+= + +

For solid shaft ha!in little or no axial loadin, the e"ation is:

( ) ( )/ /0 23

t t b b

s

d k M k M

= +

5here:

67di8dos7 allowable shear stress, 98m/7 0; of the elastic limit b"t not o!er 24; of the

"ltimate strenth in tension for shafts witho"t ke(wa(s. These !al"es are to be red"cedb( /

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For stat%onar+ s"a#ts kb ktLoad rad"all( applied 2. 2.

Load s"ddenl( applied 2.< to /. 2.< to /.

For rotat%n& s"a#tsLoad rad"all( applied 2.< 2.

Load s"ddenl( applied minor shock 2.< to /. 2. to 2.SThe inset of Fi. 2-22 shows a coned-disc sprin, commonl( called a Delle!ille sprin.

Altho"h the mathematical treatment is be(ond the scope, one sho"ld at least be familiar

with the remarkable characteristics of these sprins.

=)SC#LLA9#%@S S$')9>S.

Flat stocks are "se for a reat !ariet( of sprins, s"ch as clock sprins, power sprins,

torsion sprins, cantile!er sprins and hair sprinsI fre"entl( it is speciall( shaped tocreate certain sprin actions for f"se clips, rela( sprins, sprin washers, snap rins and

retainers. The( ma( be anal(sed and desined b( "sin the abo!e and other f"ndamental

concepts disc"ssed earlier.

UNIT III

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ESIGN OF PISTON ANCLINER.

.

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PISTON

)iston is considered to be one of the most important parts in a

reciprocating engine in which it helps to con-ert the chemical energy obtained by

the combustion of fuel into useful 0work1 mechanical power. The purpose of the

piston is to pro-ide a means of con-eying the e(pansion of gases to the

crankshaft -ia connecting rod, without loss of gas from abo-e or oil from below.

)iston is essentially a cylindrical plug that mo-es up M down in the

cylinder. %t is e5uipped with piston rings to pro-ide a good seal between the

cylinder wall M piston.

FUNCTIONS;

8. To reciprocate in the cylinder as a gas tight plug causing suction,

compression, e(pansion and e(haust strokes.

9. To recei-e the thrust generated by the e(plosion of the gas in the cylinder

and transmit it to the connecting rod.

=. To form a guide and bearing to the small end of the connecting rod and to

take the side thrust due to obli5uity of the rod.

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The top of the piston is called head or crown and parts below the ring groo-es

is called skirt. Ring groo-es are cut on the circumference of the upper portion of

the piston. The portions of the piston that separate the groo-es are called lands.

Some pistons ha-e a groo-e in the top land called as a heat dam which reduces

heat transfer to the rings.

The piston bosses are those reinforced sections of the piston designed to

hold the piston pin or wrist pin.

MATERIALS;

The materials used for piston is mainly $lluminium alloy. &ast %ron is also

used for piston as it possesses e(cellent wearing 5ualities, co:efficient of

e(pansion. 3ut due to the reduction of weight, the use of alluminium for piston

was essential. To get e5ual strength a greater thickness of metal is essential.

Thus some of the ad-antage of the light metal is lost. $lluminium is inferior to

&ast iron in strength and wearing 5ualities and hence re5uires greater clearance

in the cylinder to a-oid the risk of seiIure.

The piston made by the alloy of alluminium produces less inertia forces

there by rotating the crankshaft more smoothly. The heat conducti-ity of

alluminium is three:times that of cast iron and this combined with a greaterthickness necessary for strength, enables an alluminium piston alloy to run at

much lower temperatures than cast iron. $s a result carbonised oil does not form

on the under side of the piston and the crank case keeps always clean. S$E has

recommended the following composition.

SAE 3;"eat resistant aluminum alloy with the composition, &u 7.7 to ?.7 N,

6e 8.7 N, Si 7.C to @.C N, 2g C.9 to C.@ N, Ln C.A N, Ti C.9 N, other Elements

C.A.N

$d-antages;

8. 2aintain mechanical properties at ele-ated temperature

9. "eat conducti-ity about G.G times cast iron

=. Specific gra-ity 9.AB

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SAE 321;#ow e(pansion $lloy ha-ing the composition, &u C.7 to 8.7 N, 6e 8.=

N, Si 88 to 8= N, 2n C.8 N, 2g C.? to 8.= N, Ln C.8 N, Ti C.9 N, i 9 to = N,

other Elements C.C7.N

4 A'',*0e-eloped by ational )hysical #aboratory, #ondon.1 it is also called

alluminium alloy 99A7. This alloy is noted for its strength at ele-ated

temperatures. $lso used for cylinder heads. &omposition of &u GN, i 9N, and

2g 8.7N.

CONSTUCTION;

$ piston is a cylindrical plug which mo-es up and down in the engine

cylinder. %t is attached to the small end of the connecting rod by means of a

piston pin. %ts diameter is slightly smaller than that of cylinder bore. The space

between the piston and the cylinder wall is called the piston clearance. The

purpose of this clearance is to a-oid seiIing of the piston in the cylinder and to

pro-ide a film of lubricant between the piston and the cylinder wall. The amount

of this clearance depends upon the siIe of the cylinder bore and the piston

material because the different metals ha-e different rates of contraction and

e(pansion when cooled and heat.

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8. &rown,

9. ish 0or bowl1,

=. 3owl lip,

G. Top land,

7. 9nd and =rd ring lands

@. &ompression ring groo-es,

?. il ring groo-e,

A. )in retainer ring groo-e

B. )in boss,

8C. &rown thickness,

88. under crown surface,

89. il return or drain holes,

8=. Skirt, M8G. Skirt tail,

87. 3oss spacing, 8@. )in bore diameter,

8?. Skirt length, 8A. #ower skirt length, 8B. &ompression height, 9C. Total

length

PISTON CLEARANCE;

The two different metals ha-ing une5ual coefficient of e(pansion which

causes engine slap 0piston slap1. The space between the piston and the cylinder

wall is called the piston clearance. This clearance is essential to pro-ide a space

for a film of lubricant between the piston and cylinder wall to reduce friction. The

piston clearance is re5uired for the piston to reciprocate in the cylinder. There are

different methods to maintain the proper clearance to dissipate the heat from the

piston. They are e(plained as below,8.P,i+ing He$! +$0;

To keep the heat away from the piston skirt or lower part of the piston a

groo-e is cut near the top of the piston as shown in fig. This reduce the path of

heat transfer 0tra-el1 from the piston head to the piston skirt, there by cooling the

skirt and pre-enting it from e(panding in e(cess.

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)iston with heat dam

9.P,i+ing "',!";

This method is used to control the piston e(pansion that is by pro-iding

slots in the lower portion of the piston. These slots may be horiIontal, -ertical or

T:type

as shown in fig. These slots reduce the path for the heat tra-eling from the pistonhead to the skirt. Thus the skirt does not become much hot and e(pands with in

limit.

)iston with T:slot

=.C$0 4 G,%n+ -i"!,n;

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The pistons are finished so that they are slightly o-al when cold. These

pistons are called &am O !round pistons. When a cam ground piston warms up,

it assumes a round shape. %ts area of contact with the cylinder wall increases.

The minor a(is of the ellipse lie-in the direction of the piston pin a(is. ue to

pro-iding the bosses for mounting the piston pin in the wall of the piston these is

une5ual thickness or amount of material with the piston wall. When heated there

will be une5ual e(pansion in the piston diameter which gi-es engine knocks. To

o-ercome this difficulty the pistons are made cam ground in elliptical section

instead of circular.

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G.Wie 6,%n+ -i"!,n";

Some of the pistons such as split or cam ground type are pro-ided with

the bonds of steel wire between the piston pin and the oil control ring as shown in

fig. There by controlling the e(pansion of the piston skirt to a certain limit.

7.A%!,!(e0i Pi"!,n";

This type of pistons contains steel inserts at the piston pin bosses as

shown in the fig. 2ostly this piston is cam ground type and the low e(pansion

steel inserts control the e(pansion of the bosses which are pro-iding along the

major diameter of the piston.

$utothermic )istons

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@.Bi 4 Me!$' -i"!,n";

This piston is made from two metals alluminium and steel as shown in fig.

The skirt is made of steel in which alluminium is casted to form the bosses and

the piston of the head. The steel has -ery small e(pansion when heated thereby

obtaining a smaller cold clearance of the piston.

?.S-ei$' -i"!,n";

The surface of the modern piston are anodiIed or treated with a coating ofIinc o(ide or tin. $nodiIing is a treatment gi-en to the surfaces of the pistons to

resist wear in which the pistons also increase their diameters slightly thereby

obtaining a close cold clearance. The special constructions control, the clearance

as well as e(pansion of the pistons in addition to their own ad-antages. These

pistons are oil cooled pistons, pistons with inserted ring carrier, cast steel

pistons, tinned pistons etc.

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Pi"!,n !e0-e$!%e +i"!i7%!i,n;

Pi"!,n P,+%!i,n

The first machining operations on a piston of con-entional design consists

in center drilling the little boss generally pro-ided on the piston head, facing the

open end, and boring and chamfering that end. 2ost of the following opens are

located from the center hole in the head end and the finished face and flange of

the open end. $ no. of turning facing and chamfering operations usually areperformed in an automatic lathe in a single setting.

The piston is located from the inside chamfer at the open end and

supported by a re-ol-ing center mounted in a G in air operated tail stock ram. $

locating spindle fi(ture stands e(tends in to the piston and dri-es it through the

piston bosses. The skirt is cam turned to an elliptic form, from the center of the oil

rings groo-es to the open end, by carbide tipped tool in a cam turning attachment

mounted on the carriage. This tool is mounted in a cam oscillated holder, the

mo-ement of which is synchroniIed with that of the spindle .

Pi"!,n 0%"! ($e ",0e +e"i$7'e 8-,-e!ie"9 ($$!ei"!i"

8. %t should be silent in operation both during warm:up and the normal

running.

9. The design should be such that the seiIure does not occur.

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=. %t should offer sufficient resistance to corrosion due to some properties of

combustion E( ; Sulphur dio(ide.

G. %t should ha-e the shortest possible length so as the decrease o-erall

engine siIe.

7. %t should be lighten in weight so that inertia forces created by its

reciprocating motion are minimum.

@. %ts material should ha-e a high thermal conducti-ity for efficient heat

transfer so that higher compression ratios may be used with out the

occurrence of detonation.

?. %t must ha-e a long life.

PISTON RINGS

)iston rings are fitted into the groo-es of the piston to maintain good seal

between the piston and the cylinder wall.

F%n!i,n";

8. To pre-ent the leakage of the compressed and e(panding gases abo-e

the piston into the crankcase.

9. To control and pro-ide the lubricating oil between piston skirt and cylinder

walls.=. To pre-ent the entry of lubricating oil from crankcase to the combustion

chamber abo-e the piston head.

G. To pre-ent the deposit of carbon and other materials 0matter1 on the

piston head caused by burning of lubricant.

7. To pro-ide easy transmission of heat from piston to cylinder walls.

M$!ei$'")iston rings are made of fine grained alloy cast iron. This material

possesses e(cellent heat and wears resisting 5uantities. The elasticity of this

material is also sufficient to impact radial e(pansion and compression which is

necessary for assembly and remo-al of the ring.

T*-e" ,/ Pi"!,n Ring"There are two types of piston rings.

8. &ompression rings or !as rings.

9. il control rings or il regulating rings.

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8.C,0-e""i,n Ring";

&ompression rings seal in the air fuel mi(ture as it is compressed and also

the combustion pressure as the mi(ture burns. The top two rings are called

compression rings 6ig 0a1. They pre-ent the leakage of gases which are under

pressure, from the combustion chamber to the crankcase. 6igure shows the

nomenclature of piston ring 0compression ring.0

)iston ring nomenclatureThe outer diameter of the ring is some what longer than the cylinder bore

and the split joint is open.

6ig 0a1 6unction of compression ring

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&ompression rings may ha-e tapered, chamfered, counter bored, scraper,

plain or center groo-ed cross sections as shown in fig.

Types of cross sections of compression rings.

%n modern engines there are two or three compression rings fitted into top

groo-es. The number of compression rings tends to increase the compression

ratio. !enerally the second and third rings are taper faced and supplied to

impro-e oil sealing. %n many engines, counter bored and scraper rings.

Pi"!,n Ring M$!ei$';

6or piston ring we re5uire a material which must be elastic 0or resilient1,

ha-e high ultimate strength, and ha-e pro-ided resistance to wear. &ast iron is

the material which meets the re5uirements. Earlier some &% as used for cylinder

blocks, but due to de-elopment and continued research special grades of %ronare de-eloped. The typical specification is gi-en for &.% piston rings

Silicon : 9.7 to 9.A, Sulphur : ot o-er C.8C, )rosperous : C.7 to C.?, 2anganese

:C.@ to C.A, &ombined carbon : C.@ to C.A, Total &arbon : =.7 to =.A

Elastic property is re5uired to impart radial e(pansion and compression which is

necessary for assembly and remo-al of the rings. ltimate strength necessarily

the amount with which it can e(ert necessary strength against the cylinder wall.

Resistance to wear so that it may ha-e satisfactory life.

S!e""e" in Pi"!,n Ring;

When a ring is inserted in the cylinder it is compressed to a radius which

is, of course, the radius of the cylinder bore. %f the ring is subjected to plain

bending stresses, the compression on the inner fibers e5uals the tension on the

outer fibers and is gi-en by the relationship.

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=$llowable stress for cast iron,E < 'oung+s modulus of elasticity for

the ring material, tr< Radialthickness of the ring, < 3ore

siIe or cylinder bore dia or$(ial thickness of piston rings( ) ra ttot% :.2K.:=

The e(pression for appro(imate no. of ringsi

D%2:=

%

Di

=2:

i

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il control rings scrape off e(cessi-e oil from the cylinder wall and return it

to the oil pan. Some connecting rods will ha-e an oil split hole which splits oil

from the oil pan on to the cylinder wall during each re-olution of the crankpin, for

more oil reaches on the cylinder wall than is needed. This must be scraped off

and returned to oil pan. therwise it will go the combustion chamber and burn.

This burned oil would foul the sparkplug and increase the possibility of knock.

ne piece slotted cast iron type oil control ring has slots between the upper and

lower faces that bear on the cylinder wall. The oil scraped off the cylinder wall

passes through those slots in the back of the oil ring groo-es in the piston and

from there it returns to the oil pan.

W(* !6, C,0-e""i,n ing" $n+ One Oi' C,n!,' ingP

sually two compression rings are fitted on the piston. uring the power

stroke the pressure increases and would be difficult for a single compression

rings to hold this pressure. %f there are two rings, this pressure will be di-ided

between two rings. The loss of pressure past the upper ring is reduced. The load

on the upper ring is also reduced so that it doesn+t press 5uite so hard on the

cylinder wall. Wearing of ring and cylinder is also reduced.

3ecause of two compression rings are necessary to withstand the high

combustion pressure, hence these remains only one oil control ring. %t is 5uitepossible to use one oil control ring because of engineering and manufacturing

impro-ements and the more effecti-e action of the modern oil control ring.

Pi"!,n Ring G$-;

)iston rings ha-e gap so that they may be installed into the piston groo-es

and remo-ed when worn out by e(panding them. The gap ensures radial

pressure against the cylinder wall thus ha-ing effecti-e seal to pre-ent leakage of

hea-y combustion pressure. This gap must be checked because if it is too great

due to cylinder bore wear, the radial pressure will be reduced. To check this gap

clean the carbon from the ends of the ring and then check it with feeler gauge.

This gap is C.8?A O C.7C mm go-erned by the dia of the bore but if it e(ceeds 8

mm per 8CC mm of bore then, new rings must be fitted.

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The gap between the ring and the groo-e in the piston should also be

checked by feeler gauge. This gap is usually C.C=A O C.8C9 mm for compression

rings and a little less for the oil compression rings. Wear in the piston ring

groo-es causes the rings to rise and fall during mo-ement of the piston, so

causing a pumping action and resulting in hea-y oil consumption. E(cessi-e gas

blow by, loss of compression will also take place if this gap is too much.

Pi"!,n ing 0$n%/$!%e

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PISTON PIN

)iston pin or gudgeon pin or wrist pin connects the piston and the small

end of the connecting rod. )iston pin is generally hollow and made from case

hardening steel heat treated to produce a hard wear resisting surface.

There are three methods of connecting piston and connecting rod by the

piston pin.

8. The piston pin is fastened to the piston by set screws through the piston boss

and has a bearing in the connecting rod, thus permitting the connecting rod

end to swi-el as re5uired by the combined reciprocal and rotary motion of the

piston and crank shaft.

9. The pin is fastened to the connecting rod with a clamp screw. %n this case the

piston bosses from the bearing. $ screw slot is made on the circumference of

the piston pin in which the clamp screw is fitted as shown in fig.

=. The pin floats in both the piston bosses and the small end of the connecting

rod. %t is pre-ented from coming in contact with the cylinder wall by two lock

rings fitted in groo-es in the outer end of the piston bosses and these rings

are called &%R%)S as shown in the fig. This method is widely used. %n this

case a burning of )hosper 3ronIe or alluminium is used in the small end of

the connecting rod. The bush de-elops -ery little wear and re5uires replacingonly at long inter-als. %n -ery hea-y loading of -ehicles of &% engines, special

care is taken to a-oid risk of fatigue failure cracks. The e(ternal bearing

surface is finished to a -ery high degree of accuracy to ensure correct fit in

the piston and connecting rod.

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)iston pin and connecting rod arrangement

e"ign ,/ $ Pi"!,n /, I.C Engine"

In!,+%!i,n;The design of %.& engine piston is probably more subject to contro-ersy

than any other machine part or engine mechanism, and any attempt to adhere to

rigid rules of design may lead to failure in the first instance.

The shape of the combustion chamber will fi( the profile of the piston

crown,

While, the amount of distortion to be e(pected and the stresses due to gas

pressure will be affected by the shape re5uired.

The rating of the engine and efficiency of combustion will affect the

thermal stresses.

The ratio of the connecting rod length to the crank radius will determining

the amount of side thrust on the cylinder wallH While, 2any factors including the

bottom end design 0the presence of balance weights on the crank and so on, will

influence the no. piston rings and their type.0

P,e+%e;

Pi"!,n He$+ , C,6n;

The piston head or crown is designed keeping in -iew the following two

main considerations, i.e.

8. %t should ha-e ade5uate strength to withstand the straining action due to

pressure of e(plosion inside the engine cylinder.

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9. %t should dissipate the heat of combustion to the cylinder walls as 5uickly

as possible.

The top of the piston may be considered as a flat, fi(ed on the cylindrical portion

of the piston crown and subjected to uniformly distributed load of ma(imum

intensity of gas pressure.

The thickness of the piston top 0head1 based on the straining action due to

fluid pressure is gi-en by 08stcondition0

e5. 8A.8A 0a1 QQQ..pgQQ.=@8

t8

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)iston rings are pro-ided at the head of piston. %t is ad-isable to use many

narrow rings than using few wide shallow rings.

The radial thickness of the piston rings is gi-en by,

r

r

P

Dt

0

=QQQQQQQ..mm QQQQ.. e5. 8A.9? QQ... pgQQQ.. =@=

)r< 2agnitude of radial pressure on the piston rings QQQQQ. 2)a

6rom T 8A.@ QQQQ. )g. QQQQQ.. =@@

2)

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1P,7'e0 N,.10

esign a cast iron piston for a G:stroke single acting engine from the following

data;

&ylinder bore dia < 8CC mm 01, Stroke length < 89C mm 0#1, !as pressure < 7

2)a, 32E) < C.7 2)a, 6uel consumption < C.87 g / 3) 0W1, Speed < 99CC

rpm. 00

S,'%!i,n;

Step 8;

3rake power 03)1 .in W

KWPLAN

'P3:2:::

=

3:

//::12::

2:::2/:

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V%f either of or both of & or W are gi-en, need to calculate *5 or Take 5 directly

from mentioned -alues in "3U

3) < 8?.9?A QQQ. W

$

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""Dt

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( )

02:24.A44

2::2:::/K.0A

=

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$ < 88.=8 ( 8C:=m9