Performance and Operating Characteristics of IC Engine
1
Geometric parameter of reciprocating engine
The performance of the internal combustionengine is characterized with several geometricand thermodynamic parameters
The following geometric parameters are ofparticular interest: bore(B), connecting rod length(l), crank radius (a), stroke (S) and crank angle()
For any single cylinder, the cranks shaft,connecting rod, piston, and head assembly can berepresented by the mechanism shown to the left
2
Geometric parameter of reciprocating engine
The top dead center TDC of an engine refers tothe crankshaft being in a position such that =00.
The volume at TDC is minimum and is often calledthe clearance volume Vc
The bottom dead center (BDC) refers to thecrankshaft being at =1800, the volume at BDCis maximum and often denoted by VT
The difference between the VT and Vc is thedisplacement volume Vd
3
Geometric parameter of reciprocating engine
Engine Capacity (Ve)
Where n- is number of cylinders
Vd - cylinder swept volume
Displacement Rate
Stroke VS
Bore
VS VS VS
TDC
BDC( )
==
4
2BnSnVV de
For 4-Stroke Engine
Geometrical Properties of Reciprocating Engines
Compression ratio r,
o r = 8 to 12 for SI engines and
o r = 12 to 24 for CI engines;
Ratio of Cylinder bore to piston Stroke:
B/S = 0.8 to 1.2 for small- and medium-size engines,about 0.5 for large slow-speed CI engines;
5
Geometrical Properties of Reciprocating Engines
Ratio of Connecting rod length to crank radius:
R = 3 to 4 for small- and medium-size engines, increasing to 5 to 9 for large slow-speed CI engines.
The stroke and crank radius are related by
alR =
6
The cylinder volume V at any crank position
The volume of the cylinder can be determined asfunction of crank angle ( ), from the compressionratio, the stroke, bore and connecting rod length.
At TDC the crank shaft is at crank angle of 0o.(Clearance volume, Vc)
At BDC the crank angle is at 180 o. (Maximumcylinder volume, VT )
7
The cylinder volume V at any crank position
Displacement volume = (Maximum -minimum) cylinder volume
The displacement volume can also berepresented as a function of the bore andstroke
At a given crank angle the volume is given by:
)(4
2
xBVV C +=
8
The cylinder volume V at any crank position
Again using geometry, a relationship for x() can be developed:
The compression ratio becomes
Solving for Vc results in:
( )
++= cossin)( 2
1222 aallax
9
The cylinder volume V at any crank position
The cylinder volume at any crank angle becomes:
Since, a=S/2 and setting, , gives:
( )
+++
= cossin
412
12222
aallaBrVV D
+
++
= cossin1
41
21
222
al
alaB
rVV D
alR =
( )
++
= 2
122 sincos121
RRVrVV DD
Non-dimensional form of the aboveequation becomes,
.
( )
++
= 2
122 sincos121
11 RR
rVV
D
10
The cylinder volume V at any crank position 11
+
= 2
2
sin221
2cos1
1V
Sl
Sl
rr
VD
a
VDV
TDCV
BDC
B
l
If crank angle is measured from BDC in CCWdirection
The cylinder volume V at any crank position
The cylinder volume at any crank angle becomes:
Since, a=S/2 and setting, , gives:
( )
+++
= cossin
412
12222
aallaBrVV D
+
++
= cossin1
41
21
222
al
alaB
rVV D
alR =
( )
++
= 2
122 sincos121
RRVrVV DD
Non-dimensional form of the aboveequation becomes,
.
( )
++
= 2
122 sincos121
11 RR
rVV
D
12
Full throttle operation chemically correct mixture (Y=12.5) Fuel C8H18 Speed 4000rpmTm 300k P1 1atmFriction and heat transfer neglected Fuel vaporization neglect
Crank angle Vdisp Pr Crank angle Vdisp Pr(deg) (cc) (bar) (cc) (bar)
360 636.6 10 636.6 1 375 629.8 115 629.8 1 390 609.4 130 609.4 1.1 405 575.3 145 575.3 1.2 420 528.1 160 528.1 1.3 435 469 175 469 1.5 450 400.4 190 400.4 1.9 465 326.4 1105 326.4 2.5 480 252.8 1120 252.8 3.6 495 186 1135 186 5.6 510 132.5 1150 132.5 9 525 98 1165 98 13.7 540 86 1180 86 16.5 540 86 1180 86 98.2 555 98 1195 98 81.9 570 132.5 1210 132.5 53.6 585 186 1225 186 33.4 600 252.8 1240 252.8 21.7 615 326.5 1255 326.5 15.2 630 400.4 1270 400.4 11.4 645 469 1285 469 9.1 660 528.1 1300 528.1 7.7 675 575.3 1315 575.3 6.9 690 609.4 1330 609.4 6.3 705 629.8 1345 629.8 6 720 636.6 1360 636.6 6
0
20
40
60
80
100
120
0 100 200 300 400 500 600 700
volume (cc)
pres
sure
(bar
)
Engine Performance Parameters
The performance of the engine depends on inter-relationship betweenpower developed, speed and the specific fuel consumption at eachoperating condition within the useful range of speed and load.
PERFORMANCEOF ENGINE
POWER
13
Engine performance
Internal combustion engine should generally operate within a usefulrange of speed.
Some engines are made to run at fixed speed by means of a speedgovernor which is its rated speed
At each speed within the useful range, the power output varies and it hasa maximum usable value.
The specific fuel consumption varies with load and speed
14
Engine performance definition
Absolute Rated Power: The highest power which the engine coulddevelop at sea level with no arbitrary limitation on speed, fuel-air ratioor throttle opening
Maximum rated power: The highest power an engine is allowed todevelop for short periods of operation.
Normal rated power: The highest power an engine is allowed todevelop in continuous operation.
Rated speed: The crankshaft rotational speed at which rated power isdeveloped
15
Engine Performance Parameters
The performance an engine is judged by quantifying its
efficiencies
Five important engine efficiencies are
Indicated thermal efficiency (ith) Indicated Power
Brake thermal efficiency (bth) Brake Power
Mechanical efficiency (m)
Volumetric efficiency (v)
Relative efficiency or Efficiency ratio (rel)
16
Engine Performance Parameters
Other Engine performance Parameters Mean effective pressure (MEP or Pm)
Mean piston speed (sp)
Specific power output (Ps)
Specific fuel consumption (sfc)
Inlet-valve Mach Index (Z)
Fuel-air or air-fuel ratio (F/A or AI F)
Calorific value of the fuel (CV)
17
The Energy Flow
The energy flow through the engine is expressed in 3
distinct terms
Indicated Power
Brake Power
Friction Power
18
The Energy Flow
Expansion Force
The Energy Flow
Indicated work
The Engine cycle on a P-V coordinates, is often called an indicatordiagram.
The indicated work per cycle Wc,i is obtained by integrating around thecurve to obtain the area enclosed on the diagram
= PdVW ic,
21
Gross Indicated Work
The upper loop of the engine cycle of the indicator diagram, thecompression and power strokes, where output work is generated iscalled the gross indicated work.
CAW igc +=,
22
Pump work
The lower loop, which includes the intake and exhaust is called Pump workand absorbs work from the engine.
Wide-Open Throttle (WOT) Engine operated with throttle valve fully open when maximum power and/or speed is desired.
Pumpigcinetc
pump
WWWCBW=
+=
,,
Net indicated work is
23
Indicated Work at Part Throttle
At WOT the pressure at the intake valve is just below atmosphericpressure, however at part throttle the pressure is much lower thanatmospheric
Therefore at part throttle the
pump work (area B+C) can
be significant compared to
gross indicated work (area
A+C)
24
Indicated Work with Supercharging/Turbocharged
Engines with superchargers or turbochargers can have intakepressures greater than the exhaust pressure, giving a positive pumpwork
( ) ( )BAreaAAreaWnet +=
Supercharges increase the netindicated work but is a parasiticload since they are driven by thecrankshaft
25
Work during engine cycle26
Indicated Power (ip) or (Pi)
Gross indicated work
p = imep (N/m2)A (m2)
F= P.A (N)
L (m)
F (N)
Work (W) = F.L (N m)
Time (t) = 60 / (Ne /k) (s)
Indicated power (Pi) cylinder = W/t = F.L .Ne/(k*60) (W)
(Pi) cylinder = (imep.A.L.N) / (n R . 60)
(Pi) engine = imep. (A.L.n) N) / (n R . 60)
(Pi) engine = [imep. Ve . N)/ (n R . 60)] (W)
a
b
c
n R = 2 (four stroke)n R = 1 (two stoke)n = number of cylinder
Indicated, brake and frictional power
The indicated power per engine can also be given in terms ofindicated work per cycle :
where Ncrankshaft speed in rev/s
nR - number of crank r
