View
165
Download
1
Tags:
Embed Size (px)
DESCRIPTION
Nota subjek Applied Thermo
Citation preview
8-Nov-13
1
Internal Combustion Engine
8-Nov-13 Internal Combustion Engine 1
Introduction
8-Nov-13 Internal Combustion Engine 2
What is IC Engine?
An internal combustion engine is a thermal system (power plant) that
converts heat obtained from chemical energy sources (gasoline, natural
gas) into mechanical work.
Where are IC Engines Used?
IC engines are used as the propulsion systems for land transport
vehicles such as automobiles (cars, etc.), marine vehicles (boats, etc.)
and small airplanes.
IC engines are also used in portable electrical generators and as prime
mover in grass cutting machine, etc.
8-Nov-13
2
Introduction
8-Nov-13 Internal Combustion Engine 3
Components of IC Engines
• Cylinder, piston, inlet valve and
exhaust valve.
• Piston moves from the top dead
center (TDC) to the bottom dead
center (BDC).
• Clearance volume, Vc is a spacing
between the top of the piston and
the valve’s heads when the piston is
at the end of the delivery stroke. piston
bore
stro
ke
TDC
BDC
Inlet valve (air)
Exhaust valve
(gas)
Engine Classification
8-Nov-13 Internal Combustion Engine 4
Reciprocating internal combustion (IC) engines are
classified into two general categories:
a) Spark-ignition (SI) engines;
b) Compression-ignition (CI) engines.
Description of SI Engines
• Run on liquid fuel such as gasoline or petrol, which is
mixed with air.
• The air-fuel mixture enters the cylinder and is
compressed to a higher pressure and temperature.
8-Nov-13
3
Engine Classification
8-Nov-13 Internal Combustion Engine 5
Description of SI Engines
• A spark from a spark-plug ignites the combustible air-
fuel mixture.
• It burns and combustion gases is produced.
• The high pressure of the gases pushes the piston
downwards, producing a power stroke of the piston.
• The crankshaft transforms the reciprocating motion into
rotational motion (rpm), which is carried by gears and
drive shaft systems to the wheels, causing the vehicle to
move.
Engine Classification
8-Nov-13 Internal Combustion Engine 6
Description of CI Engines
• Run on diesel liquid fuel.
• The fresh atmospheric air enters the cylinder in which it
is compressed to about 1/22 of its original volume,
causing its temperature to raise to about 540oC (1000 oF)
or higher.
• Diesel fuel is then injected into the compressed air.
• The heat of compression of the air causes the diesel to
burn.
8-Nov-13
4
Engine Classification
8-Nov-13 Internal Combustion Engine 7
Description of CI Engines
• Thus producing high temperature combustion gases.
• The combustion gases pushes the piston downward during the
power stroke of the piston.
• As in the SI engines, the reciprocating motion is transformed into
rotational motion.
IN BOTH ENGINES, THE COMBUSTION GASES ARE EVENTUALLY
EXHAUSTED OUT OF THE CYLINDER SO THAT FRESH-AIR MIXTURE
CAN BE INDUCED INTO THE CYLINDER TO CONTINUE THE
THERMODYNAMICS CYCLES.
Compression Ratio
volumeClearance
volumeMaximum
c
scv
V
VVr
+=
8-Nov-13 Internal Combustion Engine 8
Compression ratio =
ie.
Note: compression ratio is volume ratio and
it is not pressure ratio.
(1)
piston
bore
stro
ke
TDC
BDC
Inlet valve (air)
Exhaust valve
(gas)
8-Nov-13
5
Mean Effective Pressure
8-Nov-13 Internal Combustion Engine 9
• Mean effective pressure is a conceptual pressure or imagination.
• It is defined as the height of a rectangle on a pressure-volume (p-v)
diagram, having an area equals to that of the air-standard cycle drawn
on the same diagram.
∫= dvpW
p
vA D
p changes with
piston
movement
sm xVpdvpW == ∫
pm = mean effective pressure
pmC
B
Mean Effective Pressure
8-Nov-13 Internal Combustion Engine 10
• This pressure can give the same total net work as actual pressure.
• For the same engines size, pm can be used as a criteria or parameter to
compare the engines performance.
∫= dvpW
p
vA D
p changes with
piston
movement
sm xVpdvpW == ∫
pm = mean effective pressure
pmC
B
8-Nov-13
6
Mean Effective Pressure
VxP
ALxP
LxPA
LxF
xWork
=
=
=
=
= distanceforce
smnet VvolumesweptxpW ,=
minmax VV
W
V
Wp net
s
netm −
==∴
8-Nov-13 Internal Combustion Engine 11
ie.
(2)
Classification by Cycles
8-Nov-13 Internal Combustion Engine 12
Reciprocating internal combustion engines operate either on two-
stroke or four-stroke cycle.
Four-stroke Cycle
• Most automotive engines operate on 4-stroke cycle.
• Every forth piston stroke is the power stroke.
• The crankshaft makes two revolutions to complete the cycle.
• The sequence of events in this cycle is as follows:
� Intake stroke: The intake valve opened. The piston moving
downward, allowing the air fuel mixture to enter the cylinder.
� Compression stroke: The intake valve closed. The piston is
moving upward, compressing the mixture.
8-Nov-13
7
Classification by Cycles
8-Nov-13 Internal Combustion Engine 13
Four-stroke Cycle
� Power stroke: The ignition system delivered a spark
to the spark plug the ignites the compressed mixture.
As the mixture burns, it creates high pressure that
pushes the piston down.
� Exhaust stroke: Exhaust stroke: The exhaust valve
opened. The piston moves upward as the burned
gases escape from the cylinder.
Classification by Cycles
8-Nov-13 Internal Combustion Engine 14
Four-stroke Cycle
• The ignition occurs before
the compression process
end, ie. at point x.
• p > patm during exhaust
stroke.
• p < patm during intake stroke.TDC BDC
1
2
4
3
p
V
8-Nov-13
8
Classification by Cycles–4-stroke cycle
The cylinders are
arranged in a line
in a single bank
The cylinders are
arranged in 2
banks set at an
angle to one
another The cylinders are
arranged in 2 banks on
opposite sides of the
engine
Classification by Cycles–4-stroke cycle
8-Nov-13
9
Classification by Cycles
8-Nov-13 Internal Combustion Engine 17
Two-stroke Cycle
BDC
TDC
Exhaust port
Intake port
PistonPower stroke
Piston
PistonCompression stroke
Classification by Cycles
8-Nov-13 Internal Combustion Engine 18
Two-stroke Cycle
• Only two strokes – power stroke
and compression stroke.
• One revolution per cycle.
• The exhaust gases exits from the
cylinder during the end of the
power stroke, while the mixture
of fuel/air enters the cylinder.
• This cycle is simple and cheap –
suitable for low power
consumption machine such as
motorbike, etc.
PistonPower stroke
Piston
PistonCompression stroke
8-Nov-13
10
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 19
Actual Cycle
• The working fluid is gas.
• Energy obtained from the combustion gases.
• This process is complicated since chemical analysis is involved.
As an engineer we need to simplify the analysis for design purposes.
Therefore several standard cycles have been developed. Those
standards are based on several assumptions and are known as ‘Air
Standard Assumption’.
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 20
The Air Standard Assumption
• The working fluid is air. It follows the ideal gas laws and no chemical
reaction.
• Each process is reversible and isentropic.
• The combustion process is replaced with heat supply process.
• The exhaust process is replaced with heat rejection process.
• Air specific heats (cp and cv) are constant.
The cycle uses those assumptions is called ‘The Air Standard Cycles’.
Models developed from those cycles are simple and able to study the
effects of major parameters towards actual engines performance.
8-Nov-13
11
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 21
The Otto Cycle
• Otto cycle is an ideal air-standard
cycle for spark-ignition (SI) or
petrol engine, the gas engine, and
high-speed oil engines.
• The cycle is shown on pressure -
volume (p-v) diagram.
= Vmin = Vmax
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 22
The Otto Cycle
The Process Sequence
The processes making up the air-
standard Otto cycle are,
1-2 Isentropic compression
2-3 Reversible constant volume
heating
3-4 Isentropic expansion
4-1 Reversible constant volume
cooling
= Vmin = Vmax
8-Nov-13
12
The Air Standard Cycles
v
p
c
c=γ
8-Nov-13 Internal Combustion Engine 23
The Otto Cycle Analysis
= Vmin = Vmax
Compression / expansion index
(3)
The Air Standard Cycles
(4) ie.
volumeClearance
meSwept volu volumeClearance
volumeMinimum
volumeMaximum ration Compressio
2
1
v
vrv =
+=
=
8-Nov-13 Internal Combustion Engine 24
The Otto Cycle Analysis
8-Nov-13
13
The Air Standard Cycles
( )( )23
14
23
41
23
4123
supply
1
1
TTcm
TTcm
Q
Q
Q
Q
W
v
v
netotto
−−
−=
−=
−==η
( )( )23
141TT
TTotto −
−−=∴ η
8-Nov-13Internal Combustion Engine25
The Otto Cycle Analysis
= Vmin = Vmax
Thermal efficiency of the cycle
(5)
The Air Standard Cycles
1
3
4
4
3
−
=
γ
v
v
T
T
1
2
1
1
2
−
=
γ
v
v
T
T
8-Nov-13 Internal Combustion Engine 26
The Otto Cycle Analysis
= Vmin = Vmax
Since processes 1-2 and 3-4 are
both isentropic, then,
and
8-Nov-13
14
The Air Standard Cycles
1
2
1
2
1
4
3
T
T
v
v
T
T=
=
−γ
2314 and vvvv ==
8-Nov-13 Internal Combustion Engine 27
The Otto Cycle Analysis
= Vmin = Vmax
but
The Air Standard Cycles
( )( ) 3
4
23
14
4
14
3
23
4
1
3
2
4
1
3
2
1
2
4
3
or
or
11or
rearrange ie.,
T
T
TT
TT
T
TT
T
TT
T
T
T
T
T
T
T
T
T
T
T
T
=−−
−=
−
−=−
==
8-Nov-13 Internal Combustion Engine 28
The Otto Cycle Analysis
( )( )
( )( )6
11 ie.,
1111
1otto
1
2
13
4
23
14
−
−
−=
−=−=
−−
−=∴
γ
γ
η
η
v
otto
r
v
vT
T
TT
TT
8-Nov-13
15
The Air Standard Cycles
γη
η
η
γ index and ration compressio offunction 1
1
res temperatuoffunction 1
basic
1
23
14
⋯
⋯
⋯
−−=
−−
−=
=
v
otto
otto
in
netotto
r
TT
TT
Q
W
8-Nov-13 Internal Combustion Engine 29
The Otto Cycle Analysis
Summary
Example 1
8-Nov-13 Internal Combustion Engine 30
An Otto cycle has an inlet pressure and temperature of 100
kN/m2 and 17 oC respectively. The compression ratio is
8/1. If 800 kJ/kg heat is supplied to the system at constant
volume calculate,
a) The maximum cycle temperature;
b) The maximum cycle pressure;
c) The net work;
d) The engine thermal efficiency;
e) The mean effective pressure.
For air, cv = 0.718 kJ/kgK and γ = 1.4.
8-Nov-13
16
Example 2
8-Nov-13 Internal Combustion Engine 31
When working on the Otto with air as the working fluid, an
engine has an air standard efficiency of 54.5% and rejects
heat at the rate of 520 kJ/kg of air used. The engine has a
single cylinder with a bore of 72 mm and a stroke of 85 mm.
The pressure and temperature at the beginning of
compression are 0.98 bar and 66oC respectively. Determine,
a) The compression ratio of the engine;
b) The net work done per kg of air;
c) The pressure and temperature at the end of compression;
d) The maximum pressure and temperature in the cycle;
e) The mean effective pressure.
For air, cv = 0.718 kJ/kgK and γ = 1.4.
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 32
The Diesel Cycle
Diesel cycle is an ideal air
standard cycle for compression-
ignition (CI) engines.
p
vv1v2
p3 = p2
1
4
32
pvγ = const
8-Nov-13
17
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 33
The Diesel Cycle
The Process Sequence
The processes making up the air-
standard Diesel cycle are,
1-2 Isentropic compression
2-3 Reversible constant pressure
heating
3-4 Isentropic expansion
4-1 Reversible constant volume
cooling
p
vv1v2
p3 = p2
1
4
32
pvγ = const
The Air Standard Cycles
v
p
c
c=γ
8-Nov-13 Internal Combustion Engine 34
The Diesel Cycle Analysis
Compression / expansion index
(3)
p
vv1v2
p3 = p2
1
4
32
pvγ = const
8-Nov-13
18
The Air Standard Cycles
( )1441 . TTcmQ v −=
( )2323 . TTcmQ p −=
8-Nov-13 Internal Combustion Engine 35
The Diesel Cycle Analysis
Heat added to the engine
(7)
p
vv1v2
p3 = p2
1
4
32
pvγ = const
Heat rejected from the engine
(8)
The Air Standard Cycles
( )( )
( )( )
( )( )
( ) res temperatuoffunction 9 1
ie.,
1.
.11
or
basic or
23
14
23
14
23
14
23
41
23
4123
⋯
⋯
TT
TT
TT
TT
TTcm
TTcm
Q
Q
Q
Q
W
Diesel
p
vDiesel
Diesel
in
netDiesel
−−
−=
−−
−=−/
−/−=−=
−==
γη
γη
ηη
8-Nov-13 Internal Combustion Engine 36
The Diesel Cycle Analysis
Thermal efficiency
8-Nov-13
19
The Air Standard Cycles
β
β
γγ
γ
23
2
3
2
3
1
211
2
1
21
1
2
1
1
2
ie
3 2 process isobaricfor ratio off-cut
TT
v
v
T
T
r
TT
v
v
TT
v
v
T
T
v
=
→==
=→
=→
= −−
−
⋯
8-Nov-13 Internal Combustion Engine 37
The Diesel Cycle Analysis
Thermal efficiency in terms of compression ratio rv
and cut-off
ratio, b p
vv1v2
p3 = p2
1
4
32
pvγ = const
The Air Standard Cycles
[ ]( )[ ]
( )101
11
ie
Also,
1
12
1
2
1
34
11
4
2
2
3
1
4
3
3
4
−
−−=∴
=
=
=
=
=
=
−
−
−−
−−−
βγβ
η
βββ
β
β
γ
γ
γ
γγγ
γγγ
v
Diesel
vvv
v
r
rT
rT
rTT
rv
vx
v
v
v
v
T
T
8-Nov-13 Internal Combustion Engine 38
The Diesel Cycle Analysis
Thermal efficiency in terms of compression ratio rv
and cut-off
ratio, b
p
vv1v2
p3 = p2
1
4
32
pvγ = const
8-Nov-13
20
Example 3
8-Nov-13 Internal Combustion Engine 39
A Diesel cycle has an inlet pressure
and temperature of 0.1 MPa and 300
K respectively. The compression ratio
is 18 and the cut-off ratio is 2.
Calculate,
a) The cycle thermal efficiency;
b) The mean effective pressure, MPa.
For air, cp = 1.005 kJ/kgK , cv = 0.718
kJ/kgK and γ = 1.4.
p
vv1v2
p3 = p2
1
4
32
pvγ = const
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 40
The Dual-Combustion Cycle
The air-standard dual-combustion
cycle is a basis for comparing the
performance of modern oil engines
(vegetables oil).
p
vv1v2 = v3
p3 = p4
1
43
2
pvγ = const
5
8-Nov-13
21
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 41
The Dual-Combustion Cycle
The Process Sequence
The processes making up the air-
standard Dual-combustion cycle are,
1-2 Isentropic compression
2-3 Reversible constant volume
heating
3-4 Reversible constant pressure
heating
4-5 Isentropic expansion
5-1 Reversible constant volume
cooling
p
vv1v2 = v3
p3 = p4
1
43
2
pvγ = const
5
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 42
The Dual-Combustion Cycle
Note:
The heat is supplied to the air in
two parts; the first part at constant
volume and the remainder at
constant pressure, hence the name ‘
dual-combustion’.
p
vv1v2 = v3
p3 = p4
1
43
2
pvγ = const
5
8-Nov-13
22
The Air Standard Cycles
8-Nov-13 Internal Combustion Engine 43
The Dual-combustion Cycle Analysis
Thermal efficiency
To fix the thermal efficiency completely, three factors are necessary, and
these are:
3
4
v
v=β
2
1
v
vrv =Compression ratio,
Ratio of pressures,
Ratio of volumes,
(11)
(12)
2
3
p
p=κ
(13)
The Air Standard Cycles
( ) ( )[ ] 111
11 −−+−
−−= γ
γ
βγκκκβ
ηvr
8-Nov-13 Internal Combustion Engine 44
The Dual-combustion Cycle Analysis
Thermal efficiency
Then it can be shown that the thermal efficiency of the dual-combustion
cycle is,
(14)
8-Nov-13
23
The Air Standard Cycles
in
outinth
Q
QQ −=η
8-Nov-13 Internal Combustion Engine 45
The Dual-combustion Cycle Analysis
Thermal efficiency
Equation (14) is too cumbersome to use. The best method to evaluate
the thermal efficiency of dual-combustion cycle is to use the relation,
(15)
The Air Standard Cycles
( ) ( )3423 TTcTTcQ pvin −⋅+−⋅=
( )15 TTcQ vout −⋅=
8-Nov-13 Internal Combustion Engine 46
The Dual-combustion Cycle Analysis
Thermal efficiency
where Qin is the total amount of heat added to the air and Qout is the heat
rejected from the air, which are given by
(16)
(17)
8-Nov-13
24
Example 4
8-Nov-13 Internal Combustion Engine 47
A dual-combustion cycle has an
inlet pressure and temperature
of 0.1 MPa and 300 K respectively. The
compression ratio is 18.5, ratio of
pressures is 1.5/1 and the cut-off ratio is
1.2. Calculate,
a) The cycle thermal efficiency;
b) The mean effective pressure.
For air, cp = 1.005 kJ/kgK , cv = 0.718
kJ/kgK, g = 1.4 and R = 0.287 kJ/kgK.
p
vv1v2 = v3
p3 = p4
1
43
2
pvγ = const
5
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 48
In order that different types of engines or different
engines of the same type may be compared, certain
performance criteria must be defined.
They are obtained by measurement of the quantities
concerned during bench tests, and calculation is by
standard procedures.
8-Nov-13
25
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 49
Indicated Power (ip)
Indicated power (ip) is
defined as the rate of work
done by the combustion gases
on the piston.
It is evaluated based on the
indicator (p-V) diagram
obtained during engine
testing, typically shown as in
the figure. V
p
3
Criteria of Performance for ICE
nNLApip i ⋅⋅⋅⋅=
2
nNLApip i ⋅⋅⋅⋅=
8-Nov-13 Internal Combustion Engine 50
Indicated Power (ip)
a) Two-stroke engines
For two-stroke engines, the
indicated power is given by
(18)
b) Four-stroke engines
For the 4-stroke engines, the indicated
power is given by
(19)
V
p
3
l
8-Nov-13
26
Criteria of Performance for ICE
pressure. effectivemean indicated
cylinders; ofnumber
(rpm); engine theof speed rotational
piston; theof stroke oflenght
piston; theof area where,
=
=
=
=
=
ip
n
N
L
A
8-Nov-13 Internal Combustion Engine 51
Indicated Power (ip)
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 52
Indicated Power (ip)
-Indicated mean effective pressure
When testing an engine to determine its
performance, it is necessary to measure
the power which is actually produced
inside the cylinder. This is done using a
device known as an indicator.
The purpose of any indicator is to
reproduce the relationship between the
pressure and the volume of the working
fluid as the piston moves through a
complete cycle in the cylinder. V
p
3
l
8-Nov-13
27
8-Nov-13 Internal Combustion Engine 53
Mechanical engine indicator
Criteria of Performance for ICE
sxl
api
=
8-Nov-13 Internal Combustion Engine 54
Indicated Power (ip)
The indicator produces a pressure-
volume diagram for an actual cycle.
From the diagram, the indicated mean
effective pressure, pi, can be obtained.
(20)
V
p
3
l
8-Nov-13
28
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 55
Indicated Power (ip)
where, a = net area of the indicator
diagram;
l = length of the indicator
diagram;
s = a recorder constant.
V
p
3
l
Example 5
8-Nov-13 Internal Combustion Engine 56
In a test on a 4-stroke, 4-cylinder automobile engine an
indicator diagram is taken and found to have an area of
670 mm2 and a length of 82 mm. The spring in the
indicator has a stiffness of 0.9 bar/mm. Determine the
indicated power of the engine at a crankshaft speed of
3200 rpm if the cylinders have a bore of 80 mm and the
piston stroke is 105 mm. What is the capacity of the
engine?
8-Nov-13
29
Example 5
kW 4.41
4x60
3200x0.105x10x03.5x10x35.7x
2
1
2
1
m 10x03.508.0x4
x4
piston, of Area
bar 35782
670x90x
m 0.105 mm 105 m; 0.08 mm 80
bar/mm 0.9 mm; 82 ;mm 670
4 engine; stroke-4 :Data
3-2
2322
2
=
=
⋅⋅⋅⋅⋅=∴
===
===
====
===
=
−
nNLApip
πd
π A
. .
l
as p
Ld
sla
n
i
i
8-Nov-13 Internal Combustion Engine 57
Example 5
3
63-
3
33-
3-
cm 2113
10 x 10 x 2.113 capacity
hence ;cmin capacity engine express topracticecommon isIt
m 10 x 2.113
0.105 x 10 x 5.03 x 4
xx
cylinders all of meswept volu Total capacity Engine
=
=
=
=
=
=
LAn
8-Nov-13 Internal Combustion Engine 58
8-Nov-13
30
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 59
Brake Power (bp)
The power measured at the output shaft of the engine is known as the
brake power of the engine.
The indicated power developed by the engine is the power available at
the piston.
This mechanical power, in the form of linear motion of the piston, is
transmitted through the connecting rod and the crankshaft, to be
transformed into rotary power at the output shaft.
However, during this process some of the power is used to overcome
the frictional resistance between the moving parts.
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 60
Brake Power (bp)
Hence, the power available at the
output shaft is less than the
indicated power ……..that is the
brake power.
The brake power is measured using
a dynamometer which provides
resistance to engine torque by
opposing the rotation of the shaft.
Water-cooled brake
drum coupled to the
engine shaft
W = force due to weights, N
S = spring balance load, N
Resisting torque, T = (W- S)R Nm
8-Nov-13
31
Criteria of Performance for ICE
TNbp ⋅⋅⋅= π2
( )
length. arm torque arm, brakebetween forces andwith
x
x i.e.
r;dynamomete by the measured torque
(rpm); speed rotational engine where,
==
=
−=
=
=
RSW
RF
RSWT
T
N
8-Nov-13 Internal Combustion Engine 61
Brake Power (bp)
(21)
Criteria of Performance for ICE
bpipfp −=
8-Nov-13 Internal Combustion Engine 62
Friction Power (fp)
Friction power (fp) is the amount of power needed to
overcome friction resistance in many moving parts of the
engine. We have,
(22)
8-Nov-13
32
Criteria of Performance for ICE
ip
bpm =η
8-Nov-13 Internal Combustion Engine 63
Mechanical Efficiency (ηηηηm)
The mechanical efficiency of an IC engine is defined as
(23)
Typical IC engines have mechanical efficiency of 80 to 90 %.
Example 6
8-Nov-13 Internal Combustion Engine 64
The specification for a 4-stroke, single cylinder internal combustion engine is
as follows:
Bore = 146 mm
Stroke = 280 mm
Speed at full load = 475 rpm
Not brake load = 433 N
Torque arm length = 0.45 m
Area of indicator diagram = 578 mm2
Length of indicator diagram = 70 mm
Recorder spring rating = 0.815 bar/mm
Determine for the engine:
a) Indicated power
b) Brake power
c) Mechanical efficiency
8-Nov-13
33
Example 6
kW 2.461
1x60
475x280.0x0167.0x10x73.6x
2
1
2
1
m 0167.0146.0x4
x4
piston, of Area
bar 73.670
578x8150x
1 engine; stroke-4 :Data
2
222
=
=
⋅⋅⋅⋅⋅=∴
===
===
=
nNLApip
πd
π A
.
l
as p
n
i
i
8-Nov-13 Internal Combustion Engine 65
Example 6
kW .699
85.194x60
475xx2
2
85.19445.0x433x Torque,
=
=
⋅⋅⋅=∴
===
π
π TNbp
Nm R FT
% 77.8
778.046.12
69.9
=
=== ip
bp mη
8-Nov-13 Internal Combustion Engine 66
8-Nov-13
34
Example 7
8-Nov-13 Internal Combustion Engine 67
The engine in Example 5 is connected to a rope brake in
order to measure the brake power. The brake drum
diameter is 0.9 meter and the rope 20 mm in diameter. At
3200 rpm the dead load is 250 N and the spring balance
reads 18 N. Neglecting the weight of the rope, what is the
brake power of the engine?
Example 7
kW 6.543
04.109x60
3200xx2
2
04.10947.0x232x motion, opposing torqueHence,
47.002.02
9.0
2
radius,mean aat applied is force This
23218250
rpm 3200 N; 18 N; 250
0.02m mm 20 d m; 0.9 D
4 engine; stroke-4 :Data
=
=
⋅⋅⋅=∴
===
=+=+=
=−=−=
===
===
=
π
π TNbp
NmR FT
mdD
R
N S WF
NSW
n
8-Nov-13 Internal Combustion Engine 68
8-Nov-13
35
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 69
( )24 2
1
where,
2
1x
engine, stroke-4for Therefore
x
nNALpbp
pp
nNALpbp
ipbp
b
imb
im
m
⋅⋅⋅⋅⋅=∴
⋅=
⋅⋅⋅⋅⋅=
=
η
η
η
Brake Mean Effective Pressure (bmep)
From the definition of mechanical efficiency, we can write the
expression for the engine’s brake power (bp) as
Criteria of Performance for ICE
pressure effectivemean brake
pressure effectivemean indicated
pressure effectivemean standard
=
=
=
b
i
m
p
p
p
8-Nov-13 Internal Combustion Engine 70
Note
8-Nov-13
36
Criteria of Performance for ICE
•==E
bpbt
suppliedenergy of Rate
outputpower brakeη
8-Nov-13 Internal Combustion Engine 71
Brake Thermal Efficiency (ηηηηbt)
The brake thermal efficiency, ηbt is the measure of the
overall efficiency of the engine. It is defined as
(25)
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 72
Brake Thermal Efficiency (ηηηηbt)
The rate of thermal energy supplied to the engine, is given
by
•
E
( )
fuel theof valuecalorificnet
fuel of rate flow mass where,
26.
=
=
=
•
••
C.V
m
VCxmE
f
f
8-Nov-13
37
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 73
Brake Thermal Efficiency (ηηηηbt)
( )
( ) ( )28 kg/kW.hr ,
where,
27
.x
1
.x
Therefore,
bp
msfcptionuel consumspecific f
VCbp
mVCm
bp
f
ff
bt
•
••
=
==η
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 74
Brake Thermal Efficiency (ηηηηbt)
VCsfc .x
1
Therefore,
bt =η
8-Nov-13
38
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 75
Indicated Thermal Efficiency (ηηηηit)
Based on the definition of the brake thermal efficiency, the
indicated thermal efficiency is defined as
VCm
ipη
f
it
.x•= (29)
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 76
Indicated Thermal Efficiency (ηηηηit)
Dividing equation (27) by equation (29), we obtain
itmbtη ηη x= (30)
8-Nov-13
39
Example 8
8-Nov-13 Internal Combustion Engine 77
An engine produces 50 kW of brake power. A test shows
that the time for the engine to consume 4 liter of fuel was
recorded as 12 minutes. If the fuel density is 1.1 kg/liter,
determine,
a) the sfc of the engine when the C.V is 40400 kJ/kg;
b) the brake thermal efficiency.
Example 8
203.040400x44.0
3600
.x
1 ,efficiency thermalBrake (b)
kg/kW.hr44.060x50
3667.0 (a)
Therefore,
kg/min 3667.012
4x1.1x
, fuel, of rate flow Mass
kg/ 1.1
minutes 12;liter 4
kJ/kg 40400 kW; 50 :Data
===
===
===
=
==
==
•
•
•
VCsfc η
bp
msfc
t
V m
m
l
tV
C.Vbp
bt
f
fff
f
f
f
ρ
ρ
8-Nov-13 Internal Combustion Engine 78
8-Nov-13
40
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 79
Volumetric Efficiency (ηηηηv)
Is defined as the ratio of the actual volume of air drawn in
during the suction stroke to the swept volume of the cylinder at
atmospheric pressure and temperature or
( )31
s
o
s
ov
V
V
V
Vη •
•
==
Criteria of Performance for ICE
f
a
m
m
fuelofrateflowmass
airofrateflowmassRatioFuelAir •
•
==/
8-Nov-13 Internal Combustion Engine 80
Air/Fuel Ratio
(32)
8-Nov-13
41
Example 9
8-Nov-13 Internal Combustion Engine 81
A 4-stroke, 4 cylinder engine has a bore of 57 mm and a
stroke of 90 mm. When tested at 2800 rpm, the engine fuel
consumption is 0.001376 kg/s and the air fuel/ratio is
14.5/1. If atmospheric conditions in the test room is 1.013
bar and 15oC, determine the volumetric efficiency.
Criteria of Performance for ICE
## Energy DistributionEnergy Distribution kJ/skJ/s %%
1.1. Fuel energyFuel energy XX 100100
2.2. Brake powerBrake power XX11 XX11/X/X * 100* 100
3.3. Heat to cooling water Heat to cooling water XX22 XX22/X/X * 100* 100
4. 4. Heat to exhaustHeat to exhaust XX33 XX33/X/X * 100* 100
5. 5. Other reductionsOther reductions XX44 = X = X -- ((XX11 + X+ X22 + X+ X33)) XX44/X /X * 100* 100
8-Nov-13 Internal Combustion Engine 82
Energy Balance of an Engine
8-Nov-13
42
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 83
The Morse Test
Morse test is an experimental procedure for determining
the engine’s indicated power without having to compute
the mean effective pressure, pi of the engine.
This indicated power is evaluated outside the cylinders.
This test is only applicable to multi-cylinder engines, is
carried out at constant speed (rpm).
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 84
The Morse Test
The steps in conducting the test is described as follows:
Assume that n = 4, engine speed = N rpm, torque = T, force = F
and torque arm length = R.
1. The engine is run at desired constant speed (rpm),
combustion occurred in all cylinders, and the brake power
(bp) is measured using a dynamometer.
2. The first cylinder is being cut-off by disconnecting the cable
to the spark plug (in a SI engine) or fuel-injector line (in a CI
engine) for that cylinder. The engine is still run at constant
speed (rpm). The new value of the brake power is measured.
8-Nov-13
43
Criteria of Performance for ICE
8-Nov-13 Internal Combustion Engine 85
The Morse Test
3. Step number 2 is repeated but now cylinder number 2 is
being cut-off. While cylinder number 1, 3 and 4 are firing.
Another new value of the brake power is measured.
4. Step number 2 is repeated but cylinder number 3 is being
cut-off. While cylinder number 1, 2 and 4 are firing. The new
value of the engine’s brake power is measured.
5. Finally, cylinder number 4 is being cut-off. Cylinder number
1, 2 and 3 are firing. Another new value of the brake power
is measured.
Criteria of Performance for ICE
123443211234
123460
2fpipipipip
TNbp −+++=
⋅⋅⋅=
π
8-Nov-13 Internal Combustion Engine 86
The Morse Test
During the 1st step, that all the cylinders are firing, the brake
power can be expressed as follows,
8-Nov-13
44
Criteria of Performance for ICE
1234432234
234 060
2fpipipip
TNbp −+++=
⋅⋅⋅=
π
8-Nov-13 Internal Combustion Engine 87
The Morse TestThe Morse TestThe Morse TestThe Morse Test
During the 2nd step, cylinder number 1 is not functioning, the
brake power is,
Since the engine is still run at constant speed, N rpm, the
frictional power fp is still the same.
Criteria of Performance for ICE
12341234 ipbpbp =−
8-Nov-13 Internal Combustion Engine 88
The Morse Test
When we subtract both equations, we will get,
8-Nov-13
45
Criteria of Performance for ICE
41231234
31241234
21341234
ipbpbp
ipbpbp
ipbpbp
=−
=−
=−
8-Nov-13 Internal Combustion Engine 89
The Morse Test
By continuing this procedure, we will get,
Criteria of Performance for ICE
( ) ( )( ) ( )12312341241234
134123423412344321
bpbpbpbp
bpbpbpbpipipipip
−+−+
−+−=+++
8-Nov-13 Internal Combustion Engine 90
The Morse Test
Therefore, the sum of all the indicated powers for 4 cylinders
is,
8-Nov-13
46
Criteria of Performance for ICE
( )F
RNRFNTNbp
bpbpbpbpbpip
x60
2
60
x2
60
2
that,know also We
4 12312413423412341234
⋅⋅⋅=
⋅⋅⋅=
⋅⋅⋅=
−−−−⋅=
πππ
8-Nov-13 Internal Combustion Engine 91
The Morse Test
i.e.,
Criteria of Performance for ICE
[ ]12312413423412341234 460
2FFFFF
RNip −−−−⋅
⋅⋅⋅=
π
8-Nov-13 Internal Combustion Engine 92
The Morse Test
Therefore,
(34)
8-Nov-13
47
Example 10
8-Nov-13 Internal Combustion Engine 93
A 4-cylinder petrol engine has a bore and a stroke of 57 mm and 90 mm
respectively. At 2800 rpm the net load on the friction brake is 155 N and
the torque arm is 0.356 m. The engine consumes 6.74 liter of fuel/hour.
The relative density of the fuel is 0.735 and the lower calorific value of the
fuel is 44200 kJ/kg. A Morse test is carried out on the engine. The engine
cylinder is cut out one after another following the order of 1, 2, 3 and 4
and producing the brake loads of 111, 106.5, 104.2 and 111 N
respectively. Calculate,
i. the engine torque;
ii. the brake mean effective pressure;
iii. the brake thermal efficiency;
iv. the specific fuel consumption;
v. the mechanical efficiency;
vi. the indicated mean effective pressure.