different type of air standard ( otto and diesel cycle)
Post on 04-Apr-2018
219 Views
Preview:
TRANSCRIPT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
1/23
Air Standard Cycle
The accurate analysis of IC process is very completed. In order to
Because of the complex chemical reactions that take place when
fuel burns and friction and heat transfer between the gases and
the cylinder. It is therefore a usual practice to analyze the cycleMaking some simplifying assumptions.
The assumptions are as follows:
The working fluid is a perfect/ideal gas through out i.e. it
follows the PV= mRTm = mass (Kg), V= volume occupied(m3),
= Universal gas constant (8.314 KJ/Kg oK)
M = Molecular weight of gas
M
R__
M
R__
M
RR
__
__R
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
2/23
Air Standard Cycle
The working fluid is a fixed mass of air either contained in a closed
system or flowing at a constant rate round a closed circuit. The intake
and exhaust process are not considered.
The physical constants of the working medium are the same as those
of air at standard atmospheric conditions, i.e molecular weight=29
Specific heat at constant pressure (CP)= 1.005 KJ/kgoK
Specific heat at constant volume ( Cv)= 0.718 KJ/kgo
K
Adiabatic index, 4.1718.0
005.1
CV
Cp
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
3/23
Air Standard Cycle
The working medium has a constant specific heat having following
relation:
Cp- Cv = R and CV=
The working medium does not undergo any chemical change throughout the cycle
The combustion process is replaced by a heat transfer process from
an external source.
The cycle is completed by heat rejection to the surrounding until theair temperature corresponds to initial condition
All process that constitute the cycle are reversible i.e no loss or gain
of entropy
1)(
R
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
4/23
Air Standard Cycle
The compression and expansion processes are reversible adiabatic
The operation of the engine is frictionless.
Because of the simplified assumptions, the peak temperature, the
pressure, the work output and the thermal efficiency calculated by the
Analysis of an air standard cycle are higher than those found in anActual engine. However, the analysis shows the relative effects of the
principal variable such as compression ratio, inlet pressure, inlet
temperature effect on the engine performance
The main use of ideal cycle is that it is amenable to simple
mathematical calculation based on perfect gas law and can give
rough answer on the effect on principal values such as r, inlet t, P
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
5/23
Otto Cycle or constant volume cycle
Process 1-2 : Reversible adiabaticor isentropic process. There is no
heat transfer
Process 2-3: Reversible constantvolume heating
Process 3-4: Reversible adiabaticor isentropic expansion. There is no
heat transfer
Process 4-1: Reversible constantvolume heat rejection.
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
6/23
Otto Cycle or constant volume cycleDerivation of Thermal
efficiency and mean effectivepressure:
Vs = Swept volume
Vc = Clearance volume
V1= total cylinder volume = Vs + Vc
Compression ration
SuppliedHeat
rejectedHeatSuppliedHeat
SuppliedHeat
doneWorkEf f iciencyThermalThe
V
V
Vc
VcVs
volumeClearnce
volumecylinderTotalrrationCompressio
,
2
1,
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
7/23
Thermal Efficiency of Otto Cycle
--------------------------------------------------------------
---------------------------------------------------------------
1
2
1pp
1Q
Q
liedsuHeat
rejectedHeat
r1
1
1
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
8/23
The effect of compression ratio on thermal efficiency
The thermal efficiency of
the cycle increase with increaseof compression ratio
At a higher value of adiabatic
exponent thermal
efficiency() is also increasesFor a given value of
compression ratio and adiabatic
exponent , the thermal value is
constant and does not depend onamount heat supplied
Heat supplied is directly
proportional to the load and hence
the thermal efficiency does not
depend on the engine load
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
9/23
Mean Effective Pressure
Mean effective Pressure,
VolumeSwept
DoneWorkPpressureEffectiveMean m )(
Vs
W
diagramtheofLength
diagramindicatortheofArea
r
PPpressureEffectiveMean rm
)1)(1(
)1()(
1
additionheatatratiopressure
barpressureInitialpasWhere
)(1
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
10/23
Mean effective pressure
)1)(1(1
1
rTmcW v
The work output of the cycle increases with pressure
ratio() and compression ratio (r) The increase of the pressure ratio result in increase of
amount of heat received
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
11/23
ANALYSIS
Variation of Pm versus Pressure ratio()
Pm increase with Pressure ratio() and
compression ratio (r)
All the analysis of Pm and of the cycleshows that using higher compression
ratio (r) is the effective way of improving
the performance of the engine
> the value of r is restricted to ensure of avoiding auto ignitionand detonation
)1)(1(
)1()(
1
r
PPpressureEffectiveMean rm
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
12/23
Problem and solution A gas engine working on the otto cycle has cylinder bore of
200mm and stroke length of 250 mm. The clearance volume is
1570 cm3. The pressure and temperature at the beginning ofcompression are 1 bar and 27 oC respectively. The maximumtemperature of the cycle is 1400 oC. Determine the pressure andtemperature at the salient points, the air standard efficiency,the work done and mean effective pressure . ( For air , take Cv=
0.718 KJ/Kgo
K) and R =0.287 KJ/Kgo
K).Also calculate the idealpower developed by the engine, if the number of workingcycles per minute is 500.
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
13/23
Problem and solution
In an ideal otto cycle the air at the beginning of isentropic
compression is at 1 bar at 15oC. The compression ratio is 8. Ifthe heat rejected during the constant volume process is 1000KJ/Kg, determine
The maximum temperature in the cycle
The air standard cycle efficiency
The work done per kg of air
The heat rejected
Considering Cv=0.718 and =1.4
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
14/23
Problem and solution In a petrol engine the swept volume ( Stroke volume) is 0.13 m3.
The temperature T1= 2000
o
K, T2= 977
o
K, T3= 333
o
K and T4= 681oK. The engine produces power stroke one in 2 revolution. The
Engine runs at 1000 rpm. Calculate
Heat supplied
Heat rejected Work done ( all per cycle and per minute)
If CV of fuel is 4500 J/Kg, what amount of fuel is required per minute?
Use Cv = 713 J/Kg oK , m= 0.1615 Kg.
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
15/23
Diesel Cycle or constant pressure cycle In SI engine the upper limit of compression ratio is
limited by self ignition temperature of the fuel This limitation of compression ration is circumvented if
air and fuel is compressed separately and bought
together at the time of combustion
The fuel is injected in to the cylinder which containscompressed air at a high temperature than self ignition
temperature of the fuel
The fuel is ignite on its own accord and requires no
special device for ignition
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
16/23
Diesel Cycle or constant pressure cycle
Process 1-2 : Reversible adiabaticor isentropic process. There is noheat transfer
Process 2-3: Reversible constantpressure heating
Process 3-4: Reversible adiabaticor isentropic expansion. There is no
heat transfer
Process 4-1: Reversible constantvolume heat rejection.
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
17/23
Diesel Cycle or constant pressure cycle
Derivation of Thermalefficiency and mean effectivepressure:
Vs = Swept volume
Vc = Clearance volume
V1= total cylinder volume = Vs + Vc
Compression ration
SuppliedHeat
rejectedHeatSuppliedHeat
SuppliedHeat
doneWorkEff iciencyThermalThe
V
V
Vc
VcVs
volumeClearnce
volumecylinderTotalrrationCompressio
,
2
1,
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
)(
)(int)(
2
3
VvolumeClearance
VcutoffofpotheatVolumeratiooffCut
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
18/23
Thermal Efficiency of Diesel Cycle
--------------------------------------------------------------
---------------------------------------------------------------
1
2
1pp
1Q
Q
liedsuHeat
rejectedHeat
])1(
1
[
1
1 1
r
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
19/23
Mean Effective Pressure
Mean effective Pressure,
Pm increases with initial pressure p1, adiabatic index , r. The cut off
ratio increases when large amount of heat received, which increases Pm,
and reduces
VolumeSwept
DoneWorkPpressureEffectiveMean m )(
Vs
W
diagramtheofLengthdiagramindicatortheofArea
r
PPpressureEffectiveMean rm
)1)(1(
)1()(
1
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
)()( 142321 TTmcTTmcQQW vp
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
20/23
Diesel Cycle or constant pressure cycle The thermal efficiency of a diesel cycle depends on the
compression ration r, adiabatic exponent & the cut offratio .
The air standard thermal efficiency of otto cycle is
The air standard thermal efficiency of Diesel cycle
Diesel cycle differs from otto cycle by terms which
is always greater than unity. Therefore the air standard
efficiency of diesel cycle is less than otto cycle for same
compression ratio.Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
r1
11
])1(
1[
11
1
r]
)1(
1[
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
21/23
Graphical Analysis
The following figure shows that the thermal efficiency of a Diesel
cycle depends on the compression ratio( r), the adiabatic index For a given value of the cut- off ration , the efficiency of the diesel
cycle increases with increase of cut off ratio
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
22/23
Thermal efficiency
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
-
7/29/2019 Different type of air standard ( otto and diesel cycle)
23/23
Problem and solution
An ideal diesel engine has a diameter of 15 cm and stroke 20
cm. The clearance volume is 10 percent of the swept volume.Determine the compression ratio and the air standardefficiency of the engine if the cut- off takes place at 6 percentof the stroke.
In an engine working on Diesel cycle, the air fuel is 30:1. The
temp of the air at the beginning of compression is 27oC., thecompression ratio is 16:1. What is the ideal efficiency of theengine? The calorific value of the fuel is 42000Kj/Kg
In a engine working on Diesel cycle, the inlet pressure is 1 bar.The pressure at the end of isentropic compression is 32.425
bar. The ratio of expansion is 6. Calculate the air standardefficiency and the mean effective pressure of the cycle.
Engr. Md. Irteza Hossain, Faculty,Mechanical Engineering, IUBAT
top related