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Engine geometry

AndOperating Characteristics

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engine is really

indicated inside the

work, power, mep etc.

engine is actually

work, power, mep

shaft.

ii- indicated ; is what the

producing, that is

cylinder in terms of

iii- brake ;is what the

delivering, similarly the

measured at the output

power

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inefficiency

iv- the values of the two

parameters is always

given as )indicated l under square )large engines(

d < l over square

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5. Compression ratio r

r =VBDC /VTDC

=) Vd +Vc)/Vc

8 SI 11

12 CI 24

3. Average piston speed

5 Up 15m/s

Up 15 m/s engine high performance automobile

5 Up m/s large diesel engine

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4. Engine speed

200-40012000- higher Up accelerates &decelerates twice in each crankrpm

So N- and t- for each rpmhence, Up- as N -

.Alimit must be set to avoid material strengthfailure in components like piston and con rod.

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.1Clearance volume (Vc)

this is the minimum volume

in the combustion

chamber with the pistonat the TDC

.2 Displacementvolume(Vst)

volume swept by the pistonas it travels through one

stroke

.3Total volume (Vt(

Is the sum of Vc &Vst

Vt =Vc +Vd

.4TDC &BDC

Vc =VTDC

Vt =VBDC

Related volumes

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From which)Vc) can be expressed as:

Compression ratio )Cr(

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P = F/A

P = pressure acting onforce is exerted.

combustion

Pg =gas pressure inside

Work )W) is always the

force )F) exerted over a

distance (x (. When there is no movementof an

object, there will be no work

done,regardless of how much

W =F .X

W =work (kJ/kg(

F =force

x =distance piston moves

But since

F =P .A

A =piston face area

piston face due to

P =Pg Pc

the cylinder due to

combustion process

Pc =pressure in crank case

work

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mass, the expression

Specific work; is the

of AF mixture:

And specific volume is

work )w) is

Sub and rearrangeW =P .A .X For small displacement(dx(

dW =P .A .dx

Where )A. dx =dV) incrementalswept volume

So, dW =P. dV

Therefore;

W =P.dV

Where )P.dV ) is thearea under the PVdiagram

When considering unit

specific is used, i.e

work done per unit mass

w =W/m

v =V/m

Therefore the specific

w =P. dv

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+)ve) whenthegas

expansionstroke(

compressedandthe

**The specificwork is

producingthepressureis

expanding )poweror

**The specificwork is

)- ve) whenthegasis

workisdoneonthegasbytheoutsidesource.

In this equation if )P) is the

pressure inside the cylinder,

then thework is the indicatedwork )w)i.

however, the actual work

delivered by the crank shaft is

the brakework )w)b ,and

)w)i - )w)b ) =w)l

Where )w)l is the specific work

lost

)w)l =] friction +parasitic[

specific workThe units representing work is

)kJ/kg(

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W =mep .Vd

mep =w/Vd =w/ v

TDCBDC

Since )mep) is related to

(w (,

the equation determines the

Due to piston motion andstroke, the pressureinside the cylinder iscontinuously changing

during the cycle, and therefore we can

define )mep) as

the everage pressure

acting on the pistonthrough the power

stroke

So, the work done bymep is:

W =P.dV

Then

v = v -v

then the type of )w) used in

type of (mep.(

Mean effective pressure (mep(

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For SI engines

(w)b bmep(w)i imep(w)pump pmep

(w)frictionfmep if gmep is gross mep and nmep is net mep

nmep =gmep +pmep

bmep =nmep fmepbmep =imep fmep

bmep =m .imep

For typical engines

850kPa bmep 1050kPa

forCI engines

kPa700 bmep 900kPa

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Calculation of engine

power output can be

done in several

methods, since thedefinition of power is

the rate at which

engine is doing work,then power is related

to work and speed.

Power ((

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1. Related to work

= W . (N/n)W : work (kJ/kg)

N : engine speed (rpm x 1/60) rps

n = 2 for 4 stroke engine

= 1 for 2 stroke engine

= power output (kW)

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The conversion factor )C (

C =4500 for )metrichp(

C =6120 for (kW(

2c. Related to (mep(

=k[PLAN/n] /CWhere:

C =conversion factork =number of cylinders

=power output )kW, metric hp(P =mep )kgf/cm (

L =piston stroke )m)A =piston cross section area (cm (N =engine speed (rpm(

n =rev/cycle

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get

3. Related to mean Up

= [PLAN/n] / CSince

L= stroke (S)N= engine speed

And

Up = 2LN = 2SN

ThenUp/2 = LN

Sub in equation to

=] P. A.Up] /n C

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swept volume (m (

)N/60n) :rev/s

a =is the air mass rate

a =v .Vd .a .) N/60n(

a : air mass rate kg/s

v : volumetric efficiency

Vd =/ 4d l

a =Pin/(Rg.Tin)

inlet air density

Pin (kPa (

Rg =0.287 kJ/kg.k

Tin )deg. K(

4. Related to combustion

=f .CV .th.i

Also since

FA =f/a

or AF =a /f

then f =FA. a

)=FA.a) CV .th.i

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From above equation, we can notice the followings:

If either )mf) or )AF) is given then, the power can be evaluated

engine geometry is known.

Considering the term )CV .th.i(

if

CV - is the fuel calorific value which is the fuel heating

value (Q).

- is the thermal efficiency of the engine cycle, which

means the fraction of heat liberated from fuel(CV . th.i ) that could be converted to mechanical

energy.

th.i

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In general power output =heat energy in fuel x abilityof conversion

What is the engine ability of conversion

In this respect, ability of conversion is best looked at through terms of efficiencies:

c -combustionefficiency, which is the ability of how much of the fuel that can really

be burned.

f - fuel conversion efficiency, which is the ability of how much heat energy canreally be converted to power.

So [f . CV] determines the total heat that can possibly be produced by combustionprocess.

Using the above efficiencies :Heat input to the engine

Qin= f . CV . c

Poweroutput from engine

=f .CV. f

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th = f .CV. f /f. CV . c

Heat input to the engine

Qin =f .CV . c Power output from

engine

=f .CV. f

th. -is the thermalefficiency

th = /Qin

th =f / c

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From all of the previous equations, power

correlation can be summarized as follows:

1. =W .) N/n(

2. =] PLAN/n] /C

3. =] P. A.Up] /n C

4. =f

.CV .th.

5. =2 N

summary

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2 =W/nAnd

obtain torque-powercorrelation

Torque is a force acting ata distance, measured in(N.m.(

Or more specifically

torque is a force that ,when applied, results in

twisting an object rather

than its physical

movement .Analytically, torque is

2 = W/n =bmep.Vd/n =bmep.Vd/ 2 n

Since =W ) .N/n(

Using the common factor )W)to

=2 N

Engine Torque

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correlation depends on the power used

i.e

units(gm/kW.hr(

sfc =fuel mass rate /engine power

sfc =f/The indicated or brake (sfc) in this

isfc =f / i

bsfc =f

/bThe )sfc) is usually measured in the

Specific fuel consumption

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are more commonly used, i.e

AF =a /f )mass rates kg/s(

Fuel/air ratio =fuel/air =FA

FA =mf /ma )mass kg()combustion) safe and

rates are set in such a values

Oxygen, from ambient air, isneeded in the engine cylinder

to chemically react with fuel

HC constituents.

In order to control suchchemical reaction

)combustion), there must be a

limit set upon both reactants

to keep the reaction

economical.

Therefore air and fuel mass

that satisfy such requirements

The ratios of both mass rates

Air/fuel ratio =air/fuel =AF

AF =ma /mf )mass kg(

FA =f /a )mass rates kg/s(

It is useful to acknowledge that

AF =1/ FAand

FA =1/ AF

Air /fuel mixture

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process (AF)a differ from

(to be discussed later).the engine to completely react

appears called the

stoichiometric)AF)stor )FA)st .

) =FA)a /) FA)st

In actual combustion

)AF)st due to many causes

So new expressionequivalence ratio

) =AF)st ) /AF)a

=1 stoichiometric >1 lean )O2 in exh(.