2 operating parameters

7/25/2019 2 Operating Parameters

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22TOPICTOPIC

Engine design andEngine design andoperating parametersoperating parameters


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Satisfactory performance over its operating range

Low fuel consumption

Low noise and emissions Low initial cost

High reliability and durability

What do we need from engine?What do we need from engine?


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Maximum rated power The highest power an engine is

allowed to develop for short periods of operation

Normal or nominal rated power.The highest power an

engine is allowed to develop in continuous operation

Rated speed The cran!shaft rotational speed at which

rated power is developed

Engine performance definitionsEngine performance definitions


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Geometrical propertiesGeometrical properties


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d cc

c

V Vr

V

+=

Compression ratio

" # $% for SI

$% # %& for CI


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d cc

c

V Vr

V

+=

bsR B L=

Compression ratio

'atio of cylinder bore to piston stro!e

" # $% for SI

$% # %& for CI

(" # $% Small

() Large* slow speed


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d cc

c

V Vr

V

+=

bsR B L=

R l a=

2L a=

Compression ratio

'atio of cylinder bore to piston stro!e

'atio of connecting rod length to cran! radius

" # $% for SI

$% # %& for CI

(" # $% Small

() Large* slow speed

+ # & Small

) # , Large* slow speed


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Cylinder volume at any crank positionCylinder volume at any crank position


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Cylinder volume at any crank positionCylinder volume at any crank position

cV V=

cV V V= +

c dV V V= +

( )2

4

BV l a s

= +

( )1 22 2 2cos sins a l a = +

( )( )2 211 1 1 cos sin2

c

c

Vr R R

V

= + +

Piston at T-C

Piston at .-C

/t any cran! angle

where sis the distance between the cran! a0is

and the piston pin a0is and is given by

Cylinder volume at any cran! position


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Comustion chamer surface areaComustion chamer surface area

( )ch pA A A B l a s= + + +

( )2 21 cos sin2

ch p

BLA A A R R

= + + +

chA

2

4p

BA

=

Cylinder head surface area

Piston crown surface area


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!iston speed!iston speed


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!iston speed!iston speed

pds ds d Sdt d dt

= =

2d

N

dt

= =

2pds

S Nd

=

2 2

cossin 1

sin

pS LN

R

= +

Instantaneous piston speed

/fter differentiation

( )1 2

2 2 2cos sins a l a = +


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!iston speed vs crank angle!iston speed vs crank angle


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Mean piston speedMean piston speed


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.etter than using rotational speed 1'P23

4as flow velocities in the inta!e manifold and cylinder all scale with

mean piston speed

5low resistance and stresses limit ma0imum values to within therange " to $) m6s

Lower end is typical for large industrial engines

High end is typical of automotive engines

Mean piston speedMean piston speed

2pS LN=


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"rake tor#ue and power"rake tor#ue and power

Tor7ue is a wor! Power is a rate at which wor! is done

.ra!e power is a usable power delivered by engine to consumer


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FbT=

2P T NT = =

( ) ( ) ( )

3.kW 2 rev/s N m 10P N T =


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$ndicated work per cycle$ndicated work per cycle

,c iW pdV =


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,c iW pdV =

Gross indicated work per cycle is the wor! delivered to the piston

over the compression and e0pansion stro!es only,c igW

area + areac igW A C=,


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,c iW pdV =


over the compression and e0pansion stro!es only

Net indicated work per cycle is the wor! delivered to the pistonover the entire four8stro!e cycle

,c igW

,c inW


area area,c inW A B=


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,c iW pdV =


over the compression and e0pansion stro!es only

Net indicated work per cycle is the wor! delivered to the pistonover the entire four8stro!e cycle

!umping work is the wor! transfer between piston and cylinder

gases during inlet and e0haust stro!es

,c igW

,c inW


pW

area area,c inW A B=

area areapW B C= +


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!umping work!umping work

Pumping wor! transfer will be

8 from piston to gases if the pressure during the inta!e stro!e is less

than the pressure during the e0haust stro!e 1naturally aspirated

engines3

8 from gases to piston if the e0haust stro!e pressure is lower than

inta!e pressure 1turbocharged engines3


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$ndicated power$ndicated power


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$ndicated power$ndicated power

The power per cylinder is related to the indicated wor! per cycle by

,c i

i

R

W NP

n=

where nRis the number of cran! revolutions for each power stro!e per

cylinder

5or four8stro!e cycles* nR =%* for two8stro!e cycles* nR=$

Indicated power differs from bra!e power by the power absorbed inovercoming engine friction* driving engine accessories* and 1in the case of

gross indicated power3 the pumping power

9e will use mostly gross indicated wor! and power

Terms brakeand indicatedare used to describe other parameters


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Mechanical efficiencyMechanical efficiency


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Mechanical efficiencyMechanical efficiency

.ra!e power is the gross indicated power diminished by the friction power

b ig f P P P=

5riction power is defined as power re7uired to overcome the flow friction*

friction of the bearings* pistons* and other mechanical components of theengine* and to drive the engine accessories

5riction power may be determine by driving unfired engine with a

dynamometer

1 fb

m

ig ig

PP

P P = = ,( to :); under load

( at idling

2echanical

efficiency


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Road-load poweris the power re7uired to drive a vehicle on a level road

at a steady speed

Coefficient of rolling resistance

Drag coefficient

Road%load powerRoad%load power

212

r R v a D v v vP C M g C A S S = +

0 012 0 015RC<


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Mean effective pressureMean effective pressure


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Mean effective pressureMean effective pressure

Mean effective pressure# the ratio of wor! per cycle and

swept volume can be increased without increase in mean

piston speed and* conse7uently* without increase of frictional

losses* both flow and rubbing

# Increased volumetric efficiency due to lower flow velocity and

increased valve area

# -ecreased heat transfer to cooling system

Cons

# Increased mass 1length and width increases* height is thesame3

# Higher probability of !noc! combustion for SI Fngines

# -esign consideration 1balancing counterweights can meet

piston at .-C3

2igher ",3 ratio4 pros and cons2igher ",3 ratio4 pros and cons


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2 operating parameters

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