thermodynamic,engine types&overview nezar
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P and T sDiagrams of Power Cycles
The area under the heat addition process on a T-s
diagram is a geometric measure of the total heat
supplied during the cycle
q
in
, and the area under the
heat rejection process is a measure of the total heat
rejected q
out
. The difference between these two (the
area enclosed by the cyclic curve) is the net heat
transfer, which is also the net work produced during
the cycle.
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Reversible Heat-Engine Cycles
The second law of thermodynamics states that it is
impossible to construct a heat engine or to develop a
power cycle that has a thermal efficiency of 100%.
This means that at least part of the thermal energy
transferred to a power cycle must be transferred to a
low-temperature sink.
There are four phenomena that render anythermodynamic process irreversible. They are:
Friction
Unrestrained expansion
Mixing of different substances
Transfer of heat across a finite temperature difference
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Thermodynamic cyclescan be divided into twogeneral categories: Powercycles andrefrigeration
cycles.
Thermodynamic cycles can also be categorized asgascycles orvaporcycles, depending upon the phase
of the working fluid.
Thermodynamic cycles can be categorized yetanother way: closedandopencycles.
Heat engines are categorized asinternal or external
combustion engines.
Categorize Cycles
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Air-Standard Assumptions
To reduce the analysis of an actual gas power cycle to a
manageable level, we utilize the following
approximations, commonly know as the air-
standard assumptions:
1. The working fluid is air, which continuously circulates
in a closed loop and always behaves as an ideal gas.
2. All the processes that make up the cycle are internally
reversible.
3. The combustion process is replaced by a heat-
addition process from an external source.
4. The exhaust process is replaced by a heat rejection
process that restores the working fluid to its initial
state.
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Bore and
stroke of acylinder
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Mean Effective Pressure
Notice that the compression ratio is a
volume ratio and should not be
confused with the pressure ratio.
Mean effective pressure(MEP) is a
fictitious pressure that, if it acted
on the piston during the entire
power stroke, would produce the
same amount of net work as that
produced during the actual cycle.
The ratio of the maximum volume formed in the cylinder to
the minimum (clearance) volume is called the compression
ratio
of the engine.
TDC
BDC
min
max
V
V
V
Vr
minmax
net
VV
WMEP
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Three Ideal Power Cycles
Three ideal power cycles are completely reversiblepower cycles, called externally reversible power
cycles. These threeideal cycles are the Carnot cycle,
the Ericsson cycle, and the Stirling Cycle.
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Three Ideal Power Cycles
The Carnot cycle is an externally reversible power cycleand is sometimes referred to as the optimum power
cyclein thermodynamic textbooks. It is composed of
two reversible isothermal processes and two reversible
adiabatic (isentropic) processes.
The Ericsson power cycle is another heat-engine cyclethat is completely reversible or externally reversible. It
is composed of two reversible isothermal processes and
two reversible isobaric processes (with regenerator).
The Stirling cycle is also an externally reversible heat-
engine cycle and is the only one of the three ideal power
cycles that has seen considerable practical application.
It is composed of two reversible isothermal processes
and two reversible isometric (constant volume)
processes.
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Carnot Cycle and Its Value in Engineering
The Carnot cycle is composed
of four totally reversible
processes: isothermal heat
addition, isentropic expansion,
isothermal heat rejection, and
isentropic compression (as
shown in the
P-
diagram at
right). The Carnot cycle can be
executed in a closed system (a
piston-cylinder device) or a
steady-flow system (utilizing
two turbines and two
compressors), and either a gas
or vapor can be used as the
working fluid.
H
LCarnot,th
T
T1
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Internal-Combustion Engine Cycles
Internal-combustion(IC) engines cannot operate onan ideal reversible heat-engine cycle but they can be
approximated by internally reversible cycles in which
all the processes are reversible except the heat-
addition and heat-rejection processes.
In general, IC engines are more polluting thanexternal-combustion (EC) engines because of the
formation of nitrogen oxides, carbon dioxide, and
unburned hydrocarbons.
The Otto cycle is the basic thermodynamic powercycle for the spark-ignition (SI), internal-
combustion engine.
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Otto Cycle: The ideal Cycle for Spark-Ignition Engines
Figures below show the actual and ideal cycles in spark-
ignition (SI) engines and their P- diagrams.
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Ideal Otto Cycle
The thermodynamic analysis of
the actual four-stroke or two-
stroke cycles can be simplified
significantly if the air-standard
assumptions are utilized. The T-
sdiagram of the Otto cycle is
given in the figure at left.
The ideal Otto cycle consists of four internally
reversible processes:
1
2 Isentropic compression
23 Constant volume heat addition
3
4 Isentropic expansion
41 Constant volume heat rejection
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Thermal Efficiency of an Otto Cycle
The Otto cycle is executed in a closed system, and
disregarding the changes in kinetic and potential
energies, we have
1
2
1
232
141
23
14
1414
2323
111
1
11
11
k
in
out
in
netOtto,th
vout
vin
outinoutin
rT
T
T/TT
T/TT
TT
TT
q
q
q
w
TTCuuq
TTCuuq
uwwqq
2
1
2
1
3
4
1
4
3
1
1
2
2
1
V
V
V
Vr
T
T
T
T
min
max
kk
and;Where,
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Example IV-4.1: The Ideal Otto Cycle
determine a) the maximum temperature and pressure
that occur during the cycle,
b
) the net work output,
c
)
the thermal efficiency, and d) the mean effective
pressure for the cycle.
Solution:
An ideal Otto cycle has a
compression ratio of 8. At the
beginning of the compression
process, the air is at 100 kPa and
17
o
C, and 800 kJ/kg of heat is
transferred to air during the
constant-volume heat-addition
process. Accounting for the variation
of specific heats of air with
temperature,
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Diesel Cycle: The Ideal Cycle for Compression-Ignition Engines
The diesel cycle is the ideal cycle for CI(Compression-
Ignition) reciprocating engines. The CI engine first
proposed by Rudolph Dieselin the 1890s, is very
similar to the SIengine, differing mainly in the method
of initiating combustion. In SI engines (also known as
gasoline engines), the air-fuel mixture is compressed
to a temperature that is below the autoignition
temperature of the fuel, and the combustion process is
initiated by firing a spark plug. In CI engines (also
known as diesel engines), the air is compressed to a
temperature that is above the autoignition temperature
of the fuel, and combustion starts on contact as the
fuel is injected into this hot air. Therefore, the spark
plug and carburetor are replaced by a fuel injector in
diesel engines.
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The I deal Air Standard Diesel Cycle
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Ideal Cycle for CI Engines continued)
In diesel engines, ONLYair is compressed during the
compression stroke, eliminating the possibility of
autoignition. Therefore, diesel engines can be designed
to operate at much higher compression ratios, typically
between 12 and 24.
The fuel injection process in diesel engines starts when
the piston approaches TDC and continues during the
first part of the power stroke. Therefore, the
combustion process in these engines takes place over a
longer interval. Because of this longer duration, the
combustion process in the ideal Diesel cycle is
approximated as a constant-pressure heat-addition
process. In fact, this is the ONLYprocess where the
Otto and the Diesel cycles differ.
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Ideal Cycle for CI Engines continued)
1
11111
123
14
1414
232323
c
kc
kin
out
in
netDiesel,th
vout
pinout,bin
rk
r
rTTk
TT
q
q
q
wTTCuuq
TTChhquuwq
2
3
2
1
cr
r
and
Where,
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Thermal efficiency of Ideal Diesel Cycle
Under the cold-air-standard assumptions, the
efficiency of a Diesel cycle differs from the efficiency of
Otto cycle by the quantity in the brackets. (See Slide
26)
The quantity in the
brackets is always greater
than 1. Therefore,
th,Otto
>
th, Diesel
whenboth
cycles operate on the
same compression ratio.
Also the
cuttoff ratio, r
c
decreases, the efficiency
of the Diesel cycle
increases. (See figure at
right)
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Cylinder Arrangements for Reciprocating Engines
Figure below shows schematic diagrams of some of the
different cylinder arrangements for reciprocating
engines.
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Verticalin-line engine is commonly used today infour- and six-cylinder automobile engines.
TheV-engineis commonly employed in eight-cylinder (V-8) and some six-cylinder (V-6) automobile
engines.
Thehorizontalengine is essentially a V-engine with
180
o
between the opposed cylinders. This system was
used as the four-cylinder, air-cooled engine that
powered the Volkswagon bug.
Theopposed-pistonengine consists of two pistons,
two crankshafts, and one cylinder. The two crankshafts
are geared together to assure synchronization. These
opposed-piston systems are often employed in large
diesel engines.
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Thedelta engine is composed of three opposed-piston cylinders connected in a delta arrangement.
These systems have found application in the petroleum
industry.
Theradialengine is composed of a ring of cylindersin one plane. One piston rod the master rod is
connected to the single crank on the crankshaft and all
the other piston rods are connected to the master rod.
Radial engines have a high power-to-weight ratio and
were commonly employed in large aircraft before the
advent of the turbojet engine.
When the term rotary engine is used today itimplies something other than a radial engine with a
stationary crank.
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Engine Performance
There are several performance factors that are common
to all engines and prime movers. One of the main
operating parameters of interest is the actual output of
the engine. The brake horsepower(Bhp) is the power
delivered to the driveshaft dynamometer.
The brake horsepower is usually measured by
determining the reaction force on the dynamometer
and using the following equation:
00033
2
,
FRNBhp d
Where
F
is the net reaction force of the dynamometer,
in lbf, Ris the radius arm, in ft, andN
d
is the angular
velocity of the dynamometer, in rpm.
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Horsepower
For a particular engine, the relationship between the
mean effective pressure (mep) and the power is:
minute.perstrokespowerofnumbertheisand
w here
ep
dis
minmax
net
pdis
CNN
strokeboreV
VV
Wmep
,
NVmepBhp
4
00033
2
Where Cis the number of cylinders in the engine,N
e
is
the rpm of the engine, and is equal to 1 for a two-
stroke-cycle engine and 2 for a four-stroke-cycle
engine.
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Brake Thermal Efficiency
The brake thermal efficiency of an engine,
th
, unlike
power plants, is usually based on the lower heating
value (LHV) of the fuel. The relationship between
efficiency and the brake specific fuel consumption
(Bsfc) is:
Bhp
Bsfc
LHVBsfcth
lbm/hrate,fuel
w here
2545
Note that the brake specific fuel consumption (Bsfc) of
an engine is a measure of the fuel economy and is
normally expressed in units of mass of fuel consumed
per unit energy output.
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External-Combustion Systems
External-combustion power systems have several
advantages
over internal-combustion systems. In
general, they are less polluting. The primary pollutants
from internal-combustion engines are unburned
hydrocarbons, carbon monoxide, and oxides of
nitrogen.
In external-combustion engines, the CH
x
and CO can
be drastically reduced by carrying out the combustion
with excess air and the NO
x
production can be
markedly reduced by lowering the combustion
temperature. By burning the fuel with excess air, more
energy is released per pound of fuel.
There are three general ideal external-combustion
engine cycles, the Stirling and Brayton are ideal gas-
power, and vapor power cycles.
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The
Brayton
cycle was first proposed by George Brayton
for use in the reciprocating oil-burning engine that he
developed around 1870.
Brayton Cycle:
The Ideal Cycle for Gas-Turbine Engines
Fresh air at ambient conditions is drawn into the compressor,
where its temperature and pressure are raised. The high-
pressure air proceeds into the
combustion chamber, where
the fuel is burned at constant
pressure. The resulting high-
temperature gases then enter
the turbine, where they
expand to the atmospheric
pressure, thus producing
power. (An open cycle.)
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Brayton Cycle continued)
The open gas-turbine cycle can be modeledas a closed
cycle, as shown in the figure below, by utilizing the air-
standard assumptions.
The ideal cycle that the working
fluid undergoes in this closed
loop is the Brayton cycle, which
is made up of four internally
reversible processes:
1
2 Isentropic compression (in a
compressor)
2
3 Constant pressure heat addition
3
4 Isentropic expansion (in a
turbine)
4
1 Constant pressure heat rejection
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T sDiagram of Ideal Brayton Cycle
Notice that all four processes
of the Brayton cycle are
executed in steady-flow
devices (as shown in the
figure on the previous slide,
T-sdiagram at the right), and
the energy balance for the
ideal Brayton cycle can be
expressed, on a unit-mass
basis, as
1414
2323
TTChhq
TTChhqhhwwqq
pout
pin
inletexitoutinoutin
and
w here
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P Diagram and
th
of Ideal Brayton Cycle
Then the thermal efficiency of
the ideal Brayton cycle under
the cold-air-standard
assumptions becomes
k/kp
p
p
in
out
in
netBrayton,th
r
T/TT
T/TT
TTC
TTC
q
q
q
w
1
232
141
23
14
11
1
111
1
ratio.pressuretheisand,w here1
2
4
3
1
4
3
1
1
2
1
2
P
Pr
T
T
P
P
P
P
T
Tp
k/kk/k
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Thermal Efficiency of the Ideal Brayton Cycle
Under the cold-air-standard
assumptions, the thermal
efficiency of an ideal Brayton
cycle increases with both the
specific heat ratio of the
working fluid (if different from
air) and its
pressure ratio
(as
shown in the figure at right) of
the isentropic compression
process.
The highest temperature in the cycle occurs at the end
of the combustion process, and it is limited by the
maximum temperaturethat the turbine blades can
withstand. This also
limits the pressure ratios
that can
be used in the cycle.
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With the demise of the steam powered tractor in thelate 1800s, most modern tractors are equipped withinternal combustion engines.
Internal combustion engines are identified by thenumber of strokes in the cycle and by the fuel that is used to run them.
Common Tractor Classifications:4 stroke cycle
- gasoline- diesel
- LP
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IntakeExhaust
Lubricating
Electrical
Cooling
Fuel
Hydraulic
Drive Train
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Parts:1. Crankcase Oil Reservoir (Oil
Pan)
2. Oil Pump
3. Oil Filter
4. Oil Passages
5. Pressure Regulating Valve
Oil goes to:1. Camshaft Bearings
2. Crankshaft Main Bearings
3. Piston Pin Bearing
4. Valve Tappet Shaft 47
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Parts:
1. Battery
2. Ground Cable
3. Key Switch
4. Ammeter5. Voltage Regulator
6. Starter Solenoid
7. Starter
8. Distributor * Gasoline
Only
9. Coil
10. Alternator
11. Spark Plug
12. Power Cable48
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Cooling System
Liquid & Air
Parts:
1. Radiator
2. Pressure Cap
3. Fan4. Fan Belt
5. Water Pump
6. Engine Water Jacket
7. Thermostat8. Connecting Hoses
9. Liquid or Coolant
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P T i i
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Power Transmission
Mechanical & Hydraulic
Parts:
1. Clutch Pedal
2. Clutch
3. Shift Controls
4. Transmission
5. Differential
6. Differential Lock Pedal
7. Final Drives
8. Power Take Off (PTO)
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TWO STROKE ENGINES
Two-stroke engines do not have valves,
which simplifies their construction and
lowers their weight.
Two-stroke engines fire once every
revolution, while four-stroke engines fire
once every other revolution. This givestwo-stroke engines a significant power
boost.
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TWO STROKE ENGINES
Two-stroke engines don't last nearly as
long as four-stroke engines. The lack ofa dedicated lubrication system means
that the parts of a two-stroke engine
wear a lot faster.
Two-stroke oil is expensive, and you
need about 4 ounces of it per gallon ofgas. You would burn about a gallon of
oil every 1,000 miles if you used a two-
stroke engine in a car.
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COMPRESSION
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FOUR CYCLE ENGINES
conventional Otto engines
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Unusual Four stroke engines
applications
ROTARY CYLINDER VALVE ENGINE RCV ENGINE
ROTARY ENGINES WANKEL ENGINE
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The heart of a rotary engine is the rotor.This is roughly the
equivalent of the pistons in a piston engine. The rotor is
mounted on a large circular lobe on the output shaft. This
lobe is offset from the centerline of the shaft and acts like thecrank handle on a winch, giving the rotor the leverage it
needs to turn the output shaft. As the rotor orbits inside the
housing, it pushes the lobe around in tight circles, turning
three timesfor every one revolution of the rotor. 69
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How Rotary Engines Work
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How Rotary Engines Work
If you watch carefully, you'll see the offsetlobe on the output shaft spinning three times
for every complete revolution of the rotor.
As the rotor moves through the
housing, the three chambers
created by the rotor change size.
This size change produces apumping action. Let's go through
each of the four stokes of the
engine looking at one face of therotor.
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Thi t t t th
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This starts at thehighest pointknown as topdead center andends at bottomdead center
The intake strokeallows the pistonto suck fuel andair into thecombustionchamber through
the intake valve 74
Compression stroke
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Second stroke
The piston moves from BDC to TDC
Intake and exhaust valves stay closed
Air and fuel mixture is compressed8:1 to 12:1
The pressure in the cylinder is raised
75
Compression startsat bottom dead
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at bottom deadcenter and ends attop dead center.
The second motion ofthe stroke takes allthe fuel and air that
was stored andcompresses it intoone tenth its originalsizes. Making theair/fuel mixtureincrease intemperaturepreparing it for thenext stage in its
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The power stroke starts as
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The power stroke starts as
soon as the piston reaches
top dead center allowing the
spark plug to ignite.
This electric current created
by the spark plug ignites the
fuel and air mixture sending
the piston back down the
cylinder with a pressure
reaching high as 600 PSI.
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The final stage of the
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The final stage of the
stroke releases all the
burned fuel through the
exhaust valve.
As the piston moves
from bottom dead center
to top dead center it
takes all the burned fuel
and pushes it out of thecylinder, preparing it for
the next cycle of strokes.
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Inlet Valve : Valve Timing Diagram
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PcylPatm
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When expressed as a percentage, the thermal efficiency must bebetween 0% and 100%. Due to inefficiencies such as friction,
heat loss, and other factors, thermal engines' efficiencies aretypically much less than 100%. For example, a typical gasolineautomobile engine operates at around 25% efficiency. The largestdiesel engine in the world peaks at 51.7%.
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The term indicated work is used to define the net work doneon the piston per cycle
the indicated mean effective pressure (imep),can be defined
by;
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The imep is a hypothetical pressure that would produce the
same indicated work if it were to act on the piston throughoutthe expansion stroke. The concept of imep is useful because itdescribes the thermodynamic performance of an engine, in away that is independent of engine size and speed and frictionallosses.
Unfortunately, not all the work done by the gas on the piston isavailable as shaft work because there are frictional losses inthe engine. These losses can be quantified by the brake meaneffective pressure (bmep,), a hypothetical pressure that acts onthe piston during the expansion stroke and would lead to thesame brake work output in a frictionless engine.
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Mechanical Efficiency contd
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Mechanical efficiency depends on pumping losses
(throttle position) andfrictional losses (engine design and engine speed).
Typical values for automobile engines at WOT are:90% @2000 RPM and 75% @ max
speed.
Throttling increases pumping power and thus themechanical efficiency
decreases, at idle the mechanical efficiencyapproaches zero.
93
Brake Specific Fuel Consumption (BSFC) is a measure of fuel
efficiency within a shaft reciprocating engine It is the rate
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efficiency within a shaft reciprocating engine. It is the rate
of fuel consumption divided by the power produced. Specific
fuel consumption is based on the torque delivered by the
engine in respect to the fuel mass flow delivered to the engine.
Measured after all parasitic engine losses is brake specific fuel
consumption [BSFC] and measuring specific fuel consumption
based on the in-cylinder pressures (ability of the pressure to do
work) is indicated specific fuel consumption [ISFC].
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Volumetric Efficiency
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Volumetric efficiency a measure of overall effectiveness ofengine and its intake and exhaust system as a natural
breathing system. It is defined as:
If the air density ra,0is evaluated at inlet manifold conditions, thevolumetric efficiency is a measure of breathing performance of the
cylinder, inlet port and valve.
If the air density ra,0is evaluated at ambient conditions, the volumetricefficiency is a measure of overall intake and exhaust system and other
engine features. The full load value of volumetric efficiency is a design feature of entire
engine system.
NV
m
da
a
v
0,
2
r
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Ignition and Combustion in Spark Ignition
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and Diesel Engines
Spark ignition (SI) engines usually have pre-mixed combustion, in which aflame front initiated by a spark propagates across the combustion chamberthrough the unburned mixture. Compression ignition (CI) engines normallyinject their fuel toward the end of the compression stroke, and thecombustion is controlled primarily by diffusion.
Whether combustion is pre-mixed (as in SI engines) or diffusion controlled
(as in CI engines) has a major influence on the range of air-fuel ratios(AFRs) that will burn.
In pre-mixed combustion, the AFR must be close to stoichiometric-the AFRvalue that is chemically correct for complete combustion. In practice,dissociation and the limited time available for combustion will mean thateven with the stoichiometric AFR, complete combustion will not occur.
In diffusion combustion, much weaker AFRs can be used (i.e., an excess ofair) because around each fuel droplet will be a range of flammable AFRs.
Typical ranges for the (gravimetric) air-fuel ratio are as follows:
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Diesel engines have a higher maximum
efficiency than spark ignition engines for threereasons:
The compression ratio is higher.
During the initial part of compression, only airis present.
The air-fuel mixture is always weak of
stoichiometric.
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2
Simple Combustion Equilibrium
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p q
For a given combustion device, say a piston engine, howmuch fuel and air should be injected in order to completely
burn both? This question can be answered by balancing the
combustion reaction equation for a particular fuel. A
stoichiometric mixture contains the exact amount of fueland oxidizer such that after combustion is completed, all the
fuel and oxidizer are consumed to form products.
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Combustion stoichiometry for a general hydrocarbon fuel, withair can be expressed as;
The amount of air required for combusting a stoichiometricmixture is called stoichiometric or theoretical air.
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4
Methods of Quantifying Fuel and Air Content
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of Combustible Mixtures
In practice, fuels are often combusted with an amount of air
different from the stoichiometric ratio. If less air than the
stoichiometric amount is used, the mixture is described as fuel
rich. If excess air is used, the mixture is described as fuel lean.
For this reason, it is convenient to quantify the combustible
mixture using one of the following commonly used methods:
Fuel-Air Ratio (FAR): The fuel-air ratio, f, is given by
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Fuel Requirements
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q
Gasoline is a mixture of hydrocarbons (with 4 toapproximately 12 carbon atoms) and a boiling point range of
approximately 30-200C. Diesel fuel is a mixture of higher
molarmass hydrocarbons (typically 12 to 22 carbon atoms),
with a boiling point range of approximately180-380C. Fuelsfor spark ignition engines should vaporize readily and be
resistant to self-ignition, as indicated by a high octane rating.
In contrast, fuels for compression ignition engines should self-ignite readily, as indicated by a high cetane number.
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7
Octane number is a standard measure of the anti-knock
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properties (i.e. the performance) of a motor or aviation fuel.
The higher the octane number, the more compression the fuel
can withstand before detonating. In broad terms, fuels with ahigher octane rating are used in high-compression engines that
generally have higher performance.
Knocking (also called knock, detonation, spark knock, pinging
or pinking) in spark-ignition internal combustion enginesoccurs when combustion of the air/fuel mixture in the cylinder
starts off correctly in response to ignition by the spark plug,
Effects of engine knocking range from inconsequential to
completely destructive.
.
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The octane or cetane rating of a fuel is established by
i i i i i li i h f f l i
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comparing its ignition quality with respect to reference fuels in
CFR (Co-operative Fuel Research) engines, according to
internationally agreed standards. The most common type ofoctane rating worldwide is the Research Octane Number
(RON). RON is determined by running the fuel in a test engine
with a variable compression ratio under controlled conditions,
and comparing the results with those for mixtures of iso-octane and n-heptane.
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0
Engine Knockand thermal Efficiency of an Engine
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The thermal efficiency of the ideal Otto cycle increases
with both the compression ratio and the specific heat
ratio.
When highcompression ratios
are used, the temperature of the
air-fuel mixture rises above the
autoignition temperature
produces an audible noise,
which is called engine knock.
(antiknock, tetraethyl lead?
unleaded gas)
For a given compression ratio, an ideal Otto cycle using
a monatomic gas (such as argon or helium,
k
= 1.667) as
the working fluid will have the highest thermal efficiency.
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Combustion Chamber Designs
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Combustion Chamber Design
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Combustion Chamber Design
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A turbocharger or turbo is a centrifugal compressor
Turbocharging
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A turbocharger, or turbo, is a centrifugal compressorpowered by a turbine that is driven by an engine's exhaustgases. Its benefit lies with the compressor increasing the
mass of air entering the engine (forced induction), therebyresulting in greater performance (for either, or both, powerand efficiency). They are popularly used with internalcombustion engines (e.g., four-stroke engines like Ottocycles and Diesel cycles).
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Engine Artificial Respiratory System: An Inclusion ofCV
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CV
Turbo-Charged Engine 120
Turbo -Charger
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g