Transcript
HEAT ENGINES
1
Course : B.Tech Mechanical
Subject : Elements of Mechanical Engineering
Unit-3
Heat Engines
EXIT
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Heat Engine
• A heat engine takes in energy by heat and partially converts it to other forms
• In general, a heat engine carries some working substance through a cyclic process
Heat Engine, cont.
• Energy is transferred from a source at a high temperature (Qh)
• Work is done by the engine (Weng)
• Energy is expelled to a source at a lower temperature (Qc)
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Heat Engine, cont.
• Since it is a cyclical process, ΔU = 0– Its initial and final internal
energies are the same
• Therefore, Qnet = Weng
• The work done by the engine equals the net energy absorbed by the engine
• The work is equal to the area enclosed by the curve of the PV diagram
Thermal Efficiency of a Heat Engine
• Thermal efficiency is defined as the ratio of the work done by the engine to the energy absorbed at the higher temperature
• e = 1 (100% efficiency) only if Qc = 0
– No energy expelled to cold reservoir
1eng h c c
h h h
W Q Q Q
Q Q Q
e
Followings are the sources of heat:
Chemical Energy: It is conversion into heat energy by burning of fuels.
Nuclear Energy: Heat energy of fission or fusion of atoms
Heat energy: Obtained by natural resources
• It is known as medium to carry heat.
• When a gas or mixture of gases or a vapour is used in engine for transferring heat, it is known as working substance.
• They are able to absorb heat, store within them and give up heat when required.
• It coat energy converts heat energy of working substance into mechanical work.
• It may be reciprocating type, rotary type, or jet type.
• Reciprocating type:– Piston reciprocates inside hollow
cylinder
• Rotary type– Blades or vanes are fitted on
wheel which is mounted on shaft that rotates action of working substance
• Jet type:– Fluid is discharged with high
velocity jet and produce thrust which causes the motion
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Second Law of Thermodynamics
• Constrains the First Law
• Establishes which processes actually occur
• Heat engines are an important application
Second Law of Thermodynamics
• No heat engine operating in a cycle can absorb energy from a reservoir and use it entirely for the performance of an equal amount of work– Kelvin – Planck statement
– Means that Qc cannot equal 0• Some Qc must be expelled to the environment
– Means that e must be less than 100%
Summary of the First and Second Laws
• First Law
– We cannot get a greater amount of energy out of a cyclic process than we put in
• Second Law
– We can’t break even
Reversible and Irreversible Processes
• A reversible process is one in which every state along some path is an equilibrium state– And one for which the system can be returned to its initial
state along the same path
• An irreversible process does not meet these requirements– Most natural processes are irreversible
• Reversible process are an idealization, but some real processes are good approximations
Carnot Engine
• A theoretical engine developed by Sadi Carnot
• A heat engine operating in an ideal, reversible cycle (now called a Carnot Cycle) between two reservoirs is the most efficient engine possible
• Carnot’s Theorem: No real engine operating between two energy reservoirs can be more efficient than a Carnot engine operating between the same two reservoirs
Carnot Cycle, A to B
• A to B is an isothermal expansion at temperature Th
• The gas is placed in contact with the high temperature reservoir
• The gas absorbs heat Qh
• The gas does work WAB in raising the piston
Carnot Cycle, B to C
• B to C is an adiabatic expansion
• The base of the cylinder is replaced by a thermally nonconducting wall
• No heat enters or leaves the system
• The temperature falls from Th to Tc
• The gas does work WBC
Carnot Cycle, C to D
• The gas is placed in contact with the cold temperature reservoir at temperature Tc
• C to D is an isothermal compression
• The gas expels energy QC
• Work WCD is done on the gas
Carnot Cycle, D to A
• D to A is an adiabatic compression
• The gas is again placed against a thermally nonconducting wall– So no heat is exchanged with
the surroundings
• The temperature of the gas increases from TC to Th
• The work done on the gas is WCD
Carnot Cycle, PV Diagram
• The work done by the engine is shown by the area enclosed by the curve
• The net work is equal to Qh - Qc
Efficiency of a Carnot Engine
• Carnot showed that the efficiency of the engine depends on the temperatures of the reservoirs
• Temperatures must be in Kelvins
• All Carnot engines operating between the same two temperatures will have the same efficiency
h
Cc
T
T1e
Notes About Carnot Efficiency
• Efficiency is 0 if Th = Tc
• Efficiency is 100% only if Tc = 0 K– Such reservoirs are not available
• The efficiency increases as Tc is lowered and as Th is raised
• In most practical cases, Tc is near room temperature, 300 K– So generally Th is raised to increase efficiency
Real Engines Compared to Carnot Engines
• All real engines are less efficient than the Carnot engine
– Real engines are irreversible because of friction
– Real engines are irreversible because they complete cycles in short amounts of time
2. Rankine cycle
Generation power plants• practical Carnot Cycle• heat addition and ejection are isobaric (and not isothermal)Working fluid is alternatively vaporized and condensed
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Alternative Rankine cycles
Super Heat Reheat Regenerative6
3. Constant volume cycleOR
Otto cycle
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Process of otto cycle
• 1) Intake stroke occurs; a mixture of both air and gasoline are drawn into the engine (5–1)
2) Compression stroke occurs; pressure and temperature increases through an adiabatic process(1–2)
3) Combustion occurs via the spark plug igniting the fuel; volume remains constant in this stage (2–3)
4) Power stroke occurs; adiabatic expansion process (3–4)
5) Valve opens to exhaust waste gas (4–5)
6) Exhaust stroke; piston moves back to push any remaining waste gas out of the engine. (1–5)
4. Constant pressure cycle or
Diesel cycle
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• Process 1 to 2 is isentropic compression of the fluid • Process 2 to 3 is reversible constant pressure heating • Process 3 to 4 is isentropic expansion • Process 4 to 1 is reversible constant volume cooling • The Diesel engine is a heat engine: it
converts heat into work. The isentropic processes are impermeable to heat: heat flows into the loop through the left expanding isobaric process and some of it flows back out through the right depressurizing process, and the heat that remains does the work.
• Work in (1-2) is done by the piston compressing the working fluid
• Heat in (2-3) is done by the combustion of the fuel• Work out (3-4) is done by the working fluid expanding
on to the piston (this produces usable torque)• Heat out (4-1) is done by venting the air
Entropy
• A state variable related to the Second Law of Thermodynamics, the entropy
• Let Qr be the energy absorbed or expelled during a reversible, constant temperature process between two equilibrium states. Then the change in entropy during any constant temperature process connecting the two equilibrium states can be defined as the ratio of the energy to the temperature
Entropy, cont.
• Mathematically,
• This applies only to the reversible path, even if the system actually follows an irreversible path
– To calculate the entropy for an irreversible process, model it as a reversible process
• When energy is absorbed, Q is positive and entropy increases
• When energy is expelled, Q is negative and entropy decreases
T
QS r
More About Entropy
• Note, the equation defines the change in entropy
• The entropy of the Universe increases in all natural processes– This is another way of expressing the Second Law of Thermodynamics
• There are processes in which the entropy of a system decreases– If the entropy of one system, A, decreases it will be
accompanied by the increase of entropy of another system, B.
– The change in entropy in system B will be greater than that of system A.
Perpetual Motion Machines
• A perpetual motion machine would operate continuously without input of energy and without any net increase in entropy
• Perpetual motion machines of the first type would violate the First Law, giving out more energy than was put into the machine
• Perpetual motion machines of the second type would violate the Second Law, possibly by no exhaust
• Perpetual motion machines will never be invented
Entropy and Disorder
• Entropy can be described in terms of disorder
• A disorderly arrangement is much more probable than an orderly one if the laws of nature are allowed to act without interference
– This comes from a statistical mechanics development
Entropy and Disorder, cont.
• Isolated systems tend toward greater disorder, and entropy is a measure of that disorder
– S = kB ln W• kB is Boltzmann’s constant
• W is a number proportional to the probability that the system has a particular configuration
• This gives the Second Law as a statement of what is most probable rather than what must be
• The Second Law also defines the direction of time of all events as the direction in which the entropy of the universe increases
I.C. ENGINES
I.C. Engines• Introduction• Classification • Engine details• I.C. Engine Terminology • Otto four-stroke cycle• Diesel-four-stroke cycle • Difference between otto cycle and Diesel cycle• Two-stroke cycle• Difference between two-stroke and four-stroke cycle• Governing of IC engine • Indicated power (I.P.)• Brake Power (B.P.)• Efficiencies
Topics covered
Heat Engines - A machine or device which derives heat from thecombustion of fuel and converts part of this energy intomechanical work is called a heat engine. Heat engines may beclassified into two main classes as follows:
1. External combustion engines
2. Internal combustion engines.
Introduction of I.C. Engine
Internal Combustion Engines –
In this case, combustion of fuel with oxygen of the air occurswithin the cylinder of the engine. The internal combustionengines group includes engines employing mixtures ofcombustible gases and air, known as gas engines, those usinglighter liquid fuel or spirit known as petrol engines and thoseusing heavier liquid fuels, known as oil, compression ignition ordiesel engines.
The important applications of I.C. engines are: (i) Road vehicles,locomotives, ships and aircraft, (ii) Portable standby units forpower generation in case of scarcity of electric power, (iii)Extensively used in farm tractors, lawn movers, concrete mixingdevices and motor boats.
Introduction of I.C. Engine
Cylinder Arrangement
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Cylinder Arrangement
Inline-4
V-6
Flat-4
Overhead Cam-410
Internal Combustion Engines
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The internal combustion engines may be classified in the following ways:1. According to the type of fuel used
a) Petrol engines, b) Diesel engines, and c) Gas engines.2. According to the method of igniting the fuel
a) Spark ignition engines, and b) Compression ignition engines.
3. According to the number of strokes per cyclea) Four stroke cycle engines, and b) Two stroke cycle engines.
4. According to the cycle of operationa) Otto cycle engines, b) Diesel cycle engines, and c) Dual cycle engines.
Classification of I.C. Engines
5. According to the speed of the engine
a) Slow speed engines, b) Medium speed engines, and
c) High speed engines.
6. According to the cooling system
a) Air-cooled engines, and b) Water-cooled engines.
7. According to the method of fuel injection
a) Carburettor engines, and b) Air injection engines.
8. According to the number of cylinders
a) Single cylinder engines, and b) Multi-cylinder engines.
Classification of I.C. Engines
9. According to the arrangement of cylinders
a) Vertical engines, b) Horizontal engines, c) Radial engines,
d) In-line multi-cylinder engines, e) V-type multi-cylinder
engines,
f) Opposite-cylinder engines, and g) Opposite-piston engines.
10. According to the valve mechanism
a) Overhead valve engines, and b) Side valve engines.
11. According to the method of governing
a) Hit and miss governed engines, b) Quantitatively
governed engines, and Qualitatively governed engines.
Classification of I.C. Engines
The basic idea of internal combustion engine is shown in Fig.(Basic idea of I.C. engine). The cylinder which is closed at oneend is filled with a mixture of fuel and air. As the crankshaft turnsit pushes cylinder. The piston is forced up and compresses themixture in the top of the cylinder. The mixture is set alight and,as it burns, it creates a gas pressure on the piston, forcing itdown the cylinder.
This motion is shown by arrow ‘1’. The piston pushes on the rodwhich pushes on the crank. The crank is given rotary (turning)motion as shown by the arrow ‘2’. The flywheel fitted on the endof the crankshaft stroes energy and keeps the crank turningsteadily.
Basic Idea of I.C. Engines
A cross-section of an air-cooled I.C. engine with principal parts is shown in Fig. (Air-cooled I.C. engine).
A. Parts common to both Petrol and Diesel engine:
1. Cylinder, 2. Cylinder head, 3. Piston,
4. Piston rings, 5. Gudgeon pin, 6. Connecting rod,
7. Crankshaft, 8. Crank, 9. Engine bearing,
10. Crank case. 11. Flywheel, 12. Governor,
13. Valves and valve operating mechanism.
B. Parts for Petrol engines only:
1. Spark plug, 2. Carburettor, 3. Fuel pump.
C. Parts for Diesel engine only :
1. Fuel pump, 2. Injector.
Constructional details of I.C. Engines
Fig. Air-cooled I.C. engine 12
The details of the I.C. Engine parts are:
1. Cylinder - It is one of the most important part of the engine, inwhich the piston moves to and fro in order to develop power.The engine cylinder has to withstand a high pressure (more than50 bar) and temperature (more than 2000 deg C). Thus thematerial for the engine cylinder should be such that it can retainsufficient strength at such a high pressure and temperature. Forordinary engines, the cylinder is made of ordinary cast iron. Butfor heavy duty engines, it is made of steel alloys or aluminumalloys.
Sometimes, a liner or sleeve is inserted into the cylinder, whichcan be replaced when worn out. As the material required forliner is comparatively small, it cab be made of alloy cast ironhaving long life and sufficient resistance to rapid wear and tearto the fast moving reciprocating parts.
Constructional details of I.C. Engines
2. Cylinder head - It is fitted on one end of the cylinder, and actas a cover to close the cylinder bore. Generally, the cylinderhead contains inlet and exit valves for admitting fresh chargeand exhausting the burnt gases. In petrol engines, the cylinderhead also contains a spark plug for igniting the fuel-air mixture,towards the end of compression stroke. But in diesel engines,the cylinder head contain nozzles, (i.e. fuel valve) for injectingthe fuel into the cylinder.
The cylinder head is cast as one piece and bolted to one end of the cylinder. The cylinder block and cylinder head are made from the same material. A copper or asbestos gasket is provided between the engine cylinder and cylinder head to make an air-tight joint.
Constructional details of I.C. Engines
3. Piston – It is considered as the heart of an I.C. engine, whosemain function is to transmit the force exerted by the burning ofcharge to the connecting rod. The piston are generally made ofaluminium alloys which are light in weight. They have good heatconducting property and also greater strength at highertemperature.
4. Piston rings – These are circular rings and made of specialsteel alloys which retain elastic properties even at hightemperatures. The piston rings are housed in the circumferentialgrooves provided on the outer surface of the piston. Generally,there are two sets of rings mounted for the piston. The functionof the upper rings is to provide air tight seal to prevent leakageof the burnt gases into the lower portion. Similarly, the functionof the lower rings is to provide effective seal to prevent leakageof the oil into the engine cylinder.
Constructional details of I.C. Engines (contd..)
5. Connecting rod – It is a link between the piston andcrankshaft, whose main function is to transmit force from thepiston to the crankshaft. Moreover, it converts reciprocatingmotion of the piston into circular motion of the crankshaft, in theworking stroke. The upper (i.e. smaller) end of the connectingrod is fitted to the piston and the lower (i.e. bigger) end of thecrank.
The special steel alloys or aluminium alloys are used for themanufacture of connecting rods. A special care is required forthe design and manufacture of connecting rod, as it is subjectedto alternatively compressive and tensile stresses as well asbending stresses.
Constructional details of I.C. Engines
6. Crankshaft – It is considered as the backbone of an I.C.engine whose function is to covert the reciprocating motion ofthe piston into the rotary motion with the help of connecting rod.This shaft contains one or more eccentric portions called cranks.This part of the crank, to which bigger end of the connecting rodis fitted, is called crank pin. Special steel alloys are used for themanufacture of crankshaft. A special care is required for thedesign and manufacture of crankshaft7. Crank case – It is a cast iron case, which holds the cylinder
and crankshaft of an I.C. engine. It also serves as a sump for thelubricating oil. The lower portion of the crank case is known asbed plate, which is fixed with the help of bolts.8. Flywheel – It is a big wheel, mounted on the crankshaft,
whose function is to maintain its speed constant. It is done bystoring excess energy during power stroke, which, is returnedduring other stroke.
Constructional details of I.C. Engines
The various terms relating to I.C. engines are elaborated in Fig.
1. Bore – The inside diameter of the cylinder is called bore.
2. Stroke – As the piston reciprocates inside the engine cylinder,it has got limiting upper and lower positions beyond which itcannot move and reversal of motion takes place at these limitingpositions. The linear distance along the cylinder axis betweentwo limiting positions, is called stroke.
3. Top Dead Centre (T.D.C.) – The top most position towardscover end side of the cylinder is called “top dead centre”. Incase of horizontal engines, this is known as inner dead centre.
4. Bottom Dead Centre – The lowest position of the pistontowards the crank end side of the cylinder is called “bottomdead centre”. In case of horizontal engines it is called outerdead centre.
Terms relating to I.C. Engines
5. Clearance volume – The volume contained in the cylinder
above the top of the piston, when the piston is at top dead
centre, is called the clearance volume.
6. Swept volume – The volume swept through by the piston in
moving between top dead centre and bottom dead centre, is
called swept volume or piston displacement. Thus, when piston
is at bottom dead centre,
Total volume = swept volume + clearance volume.
Terms relating to I.C. Engines
In a four-stroke engine, the working cycle is completed in four strokesof the piston or two revolutions of the crankshaft. This is achieved bycarrying out suction, compression, expansion and exhaust processes ineach stroke.
Four-stroke Cycle
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The sequence of operation in a cycle are as follows:
1. Suction stroke – In this stroke, the fuel vapour in correct proportion, is applied to the engine cylinder.
2. Compression stroke –. In this stroke, the fuel vapour iscompressed in the engine cylinder.
3. Expansion stroke – In this stroke, the fuel vapour is fired justbefore the compression is complete. It results in the sudden riseof pressure, due to expansion of the combustion products in theengine cylinder. This sudden rise of pressure pushes the pistonwith a great force, and rotates the crankshaft. The crankshaft, inturn, drives the machine connected to it.
4. Exhaust stroke – In this stroke, the burnt gases (orcombustion products) are exhausted from the engine cylinder,so as to make space available for the fresh fuel vapour.
Sequence of Operation
In a four-stroke engine, the working cycle is completed in two strokes ofthe piston or one revolutions of the crankshaft. It's called a two-stokeengine because there is a compression stroke and then a combustionstroke
Two-stroke Cycle
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The sequence of operation in a cycle are as follows:
1. First stroke –. In this stroke, the fuel is inlet which removes the burnt gases from combustion chamber and fill new fuel ti ignite.
2. Second stroke – In this stroke, the fuel vapour is fired Itresults in the sudden rise of pressure, due to expansion of thecombustion products in the engine cylinder. This sudden rise ofpressure pushes the piston with a great force, and rotates thecrankshaft. The crankshaft, in turn, drives the machineconnected to it.
Sequence of Operation
Following are the advantages and disadvantages of two-strokecycle engines over four-stroke cycle engines:
Advantages
1. A two stroke cycle engine gives twice the number of powerstrokes than the four stroke cycle engine at the same enginespeed.
2. For the same power developed, a two-stroke cycle engine islighter, less bulky and occupies less floor area.
3. As the number of working strokes in a two-stroke cycleengine are twice than the four-stroke cycle engine, so theturning moment of a two-stroke cycle engine is more uniform.Thus it makes a two-stroke cycle engine to have a lighterflywheel and foundations. This also leads to a highermechanical efficiency of a two-stroke cycle engine.
Comparison of Two-stroke and Four-stroke Cycle
Engine
4. The initial cost of a two-stroke cycle engine is considerably
less than a four-stroke cycle engine.
5. The mechanism of a two-stroke cycle engine is much simpler
than a four-stroke cycle engine.
6. The two-stroke cycle engines are much easier to start.
Disadvantages
1. Thermal efficiency of a two-stroke cycle engine is less than
that a four-stroke cycle engine.
Comparison of Two-stroke and Four-stroke Cycle
Engine (contd..)
2. Overall efficiency of a two-stroke cycle engine is also lessthan that of a four-stroke cycle engine because in a two-strokecycle, inlet and exhaust ports remain open simultaneously forsometime. A small quantity of charge is lost from the enginecylinder.
3. In case of a two-stroke cycle engine, the number of powerstrokes are twice as those of a four-stroke cycle engine. Thusthe capacity of the cooling system must be higher. There is agreater wear and tear in a two-stroke cycle engine.
4. The consumption of lubricating oil is large in a two-strokecycle engine because of high operating temperature.
5. The exhaust gases in a two-stroke cycle engine creates noise,because of short time available for their exhaust.
Comparison of Two-stroke and Four-stroke Cycle
Engine (contd..)
It is also known as compression ignition engine because theignition takes place due to the heat produced in the enginecylinder at the end of compression stroke. The four strokes of adiesel engine sucking pure air are described below:
1. Suction stroke – In this stroke, the inlet valve opens and pureair is sucked into the cylinder as the piston moves downwardsfrom the TDC. It continues till the piston reaches its BDC asshown in Fig.
2. Compression stroke – In this stroke, both the valves areclosed and the air is compressed as the piston move upwardsfrom BDC to TDC. As a result of compression, pressure andtemperature of the air increases considerably. This completesone revolution of the crank shaft. The compression stroke isshown in Fig.
Four-stroke Cycle Diesel Engine
Efficiency Of Otto Cycle)
PV DiagramTS Diagram
P
V
T
Entropy
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Total Cylinder Volume:-It is the total volume (maximum volume) of the cylinder in which Otto cycle takes place. In Otto cycle,Total cylinder volume = V1 = V4 = Vc + Vs (Refer p-V diagram above)where,Vc → Clearance VolumeVs → Stroke Volume
Clearance Volume (Vc):-At the end of the compression stroke, the piston approaches the Top Dead Center (TDC) position. The minimum volume of the space inside the cylinder, at the end of the compression stroke, is called clearance volume (Vc). In Otto cycle,Clearance Volume, Vc = V2 (See p-V diagram above)
Stroke Volume Or Swept Volume (Vs):-In Otto cycle, stroke volume is the difference between total cylinder volume and clearance volume.Stroke Volume, Vs = Total Cylinder Volume – Clearance Volume = V1 – V2 = V4 – V3
Basic terms used in derivation of efficiency of Otto cycle:-
Compression Ratio:-Compression ratio (r) is the ratio of total cylinder volume to the clearance volume.
Now that we know the basic terms, let us derive expressions for T2 and T3. These expressions will be useful for us to derive the expression for air-standard efficiency of otto cycle. For finding T2, we take process 1-2 and for finding T3, we take process 3-4.
Process 1-2:This process is an isentropic (reversible adiabatic) process. For this process, the relation between T and V is as follows:
Process 3-4:-This is also an isentropic process. The relation between T and V in this process is similar to the relation between T and V in process 1-2:
Here,
Efficiency of Otto cycle:-It is defined as the ratio between work done during Otto cycle to the heat supplied during Otto cycle.Air-Standard Efficiency (thermal efficiency) of Otto cycle,
th
th
A Diesel engines operation sequence is as follows:
Stroke 1 (intake) – only air enters cylinder.
Stroke 2 (compression) – air is compressed to high
extent, raising temperature.
Stroke 3 (power) – diesel is injected, high air
temperature ignites diesel.
Stroke 4 (exhaust) – burnt gases are expelled from the
engine.
Diesel Engine Operation
Direct Injection
Differences in Operation
Indirect Injection
Injector Injector
Pre CombustionChamber
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Feature Direct Injection Indirect Injection
Efficiency / Economy More Less
Power More Less
Emissions Less More
Direct V/S Indirect Injection
Comparison of Petrol and Diesel Engines
Petrol Engines
1. A petrol engine draws a mixture of petrol and air during suction stroke.
2. The carburettor is employed to mix air and petrol in the required proportion and to supply it to the engine during suction stroke.
3. Pressure at the end of compression is about 10 bar.
4. The charge (i.e. petrol and air mixture) is ignited with the help of spark plug.
Diesel EnginesA diesel engine draws only air during suction stroke.
The injector or atomiser is employed to inject the fuel at the end of combustion stroke.
Pressure at the end of compression is about 35 bar.
The fuel is injected in the form of fine spray. The temperature of the compressed air is sufficiently high to ignite the fuel.
Comparison of Petrol and Diesel Engines
5. The combustion of the fuel takes place at constant volume. It works on Otto cycle.
6. A petrol engine has compression ratio from 6 to 10.
7. The starting is easy due to low compression ratio.
8. As the compression ratio is low, the petrol engines are lighter and cheaper.
9. The running cost of a petrol engine is high because of the higher cost of petrol.
The combustion of the fuel takes place at constant pressure. It works on Diesel cycle.
A diesel engine has compression ratio from 15 to 25.
The starting is difficult due to high compression ratio.
As the compression ratio is high, the diesel engines are heavier and costlier.
The running cost of diesel engine is low because of the lower cost of diesel.
Comparison of Petrol and Diesel Engines
10. The maintenance cost is less.
11. The thermal efficiency is about 26%.
12. Overheating trouble is more due to low thermal efficiency.
13. These are high speed engines.
14. The petrol engines are generally employed in light duty vehicle such as scooters, motorcycles and cars. These are also used in aeroplanes.
The maintenance cost is more.
The thermal efficiency is about 40%.
Overheating trouble is less due to high thermal efficiency.
These are relatively low speed engines.
The diesel engines are generally employed in heavy duty vehicles like buses, trucks, and earth moving machines.
The function of governor to keep the speed of engine constantirrespective of the changes of load on engine. The governor isusually of centrifugal type.
Methods of governing:
1. Hit and Miss method.
2. Quality governing
3. Quantity governing
Governing of I.C. Engine
1. Hit and Miss method:Once the supply is cut off, engine performs idle cycles which will reduce the engine speed. Yes, this method of controlling the speed of engine is known as Hit and Miss Governing. Due to large fluctuations of speed it is recommended to use a heavy flywheel with such small gas or oil engines
2. Quality governingIn high speed diesel engines Governing can be done by regulating air-fuel ratio. Air sucked into the cylinder during the suction stroke always remain constant. But, the amount of fuel supply varies according to the speed of vehicle.
3. Quantity governingIn spark ignition engines governing is done by varying the quantity of mixture supplied, while air fuel ratio remain constant. Throttle valves are usually used to control the amount of supply.Disadvantage: less thermal efficiency
Governing of I.C. Engine
Engine Performance ≡ Indication of Degree of Success for the work assigned.
(i.e. Conversion of Chemical Energy to useful Mechanical Work)
Basic Performance Parameters :
1. Power & Mechanical Efficiency
3. Specific Output
5. Air : Fuel Ratio
7. Thermal Efficiency and Heat Balance
9. Specific Weight
2. Mean Effective Pressure & Torque
4. Volumetric Efficiency
6. Specific Fuel Consumption
8. Exhaust Emissions
Performance of I.C. Engine
A. Power and Mechanical Efficiency :
Indicated Power ≡ Total Power developed in the Combustion Chamber,
due to the combustion of fuel.
)(6010
)10(..
3
5
kWNkALpn
PI i
n = No. of Cylinders
Pmi = Indicated Mean Effective Pressure(bar)
L = Length of Stroke (m)
A = Area of Piston (m2)
k = ½ for 4 – Stroke Engine,
= 1 for 2 – Stroke Engine
N = Speed of Engine (RPM)
Performance of I.C. Engine
A. Power and Mechanical Efficiency :
Brake Power ≡ Power developed by an engine at the output shaft.
)(1060
2..
3kW
X
TNPB
N = Speed of Engine (RPM)
T = Torque (N – m)
Frictional Power (F. P.) = I. P. – B. P.
..
..
PI
PBmech
Performance of I.C. Engine
B. Mean Effective Pressure :
Mean Effective Pressure ≡ Hypothetical Pressure which is thought to be
acting on the Piston throughout Power Stroke.
Fmep = Imep – Bmep
Imep ≡ MEP based on I.P.
Bmep ≡ MEP based on B.P.
Fmep ≡ MEP based on F.P.
Power and Torque are dependent on Engine Size.
Thermodynamically incorrect way to judge the performance w.r.t. Power / Torque.
MEP is the correct way to compare the performance of various engines.
Performance of I.C. Engine
C. Specific Output :
Specific Output ≡ Brake Output per unit Piston Displacement.
LXA
PBOutputSp
...
)(.. RPMinSpeedNXBXConstOutputSp mep
D. Volumetric Efficiency :
Volumetric Efficiency ≡ Ratio of Actual Vol. (reduced to N.T.P.) of the Charge
drawn in during the suction stroke, to the Swept Vol. of
the Piston.
Avg. Vol. Efficiency = 70 – 80 %
Supercharged Engine ≈ 100 %
Performance of I.C. Engine
G. Thermal Efficiency :
Thermal Efficiency ≡ Ratio of Indicated Work done, to the Energy Supplied by the fuel.
..
.., .).(
VCXm
PIEfficiencyThermalIndicated
f
PIth
)/(..
sec)/(
kgMJfuelofValueCalorificVC
kgusedfuelofmassm f
..
.., .).(
VCXm
PBEfficiencyThermalBrake
f
PBth
Performance of I.C. Engine
H. Heat Balance :
Heat Balance ≡ Indicator for Performance of the Engine.
Procedure :
1. Engine run at Const. Load condition.
2. Indicator Diagram obtained with help of the Indicator.
3. Quantity of Fuel used in given time and its Calorific Value are measured.
4. Inlet and Outlet Temperatures for Cooling Water are measured.
5. Inlet and Outlet Temperatures for Exhaust Gases are measured.
Performance of I.C. Engine
H. Heat Balance :
)(.. kJVCXm f
Heat Supplied by Fuel =
)(60.. kJXPIHeat equivalent of I.P. =
)(12 kJTTXCXm ww Heat taken away by Cooling Water =
mw = Mass of Cooling Water used (kg/min)
Cw = Sp. Heat of Water (kJ/kg.°C)
T1 = Initial Temp. of Cooling Water (°C)
T2 = Final Temp. of Cooling Water (°C)
)(kJTTXCXm rePge Heat taken away by Exhaust Gases =
me = Mass of Exhaust Gases (kg/min)
CPg = Sp. Heat of Exhaust Gases @ Const. Pr. (kJ/kg.°C)
Te = Temp. of Exhaust Gases (°C)
Tr = Room Temperature (°C)
Performance of I.C. Engine
Sr. No.
InputAmount
(kJ)Per cent
(%)Output
Amount (kJ)
Per cent (%)
1.Heat Supplied
by FuelA 100
Heat equivalent to I.P.
B α
2.Heat taken by Cooling Water
C β
3.Heat taken by Exhaust Gases
Dγ
4.Heat UnaccountedE = A – (B+C+D)
Eδ
Total A 100 Total A 100
H. Heat Balance :
Performance of I.C. Engine
o Image References :-1. http://www.netanimations.net/Wankel_Rotary_engine.gif
2. http://www.antonine-
education.co.uk/Image_library/Physics_5_Options/Applied_Physics/App_06/heat_engine.gif
3. http://upload.wikimedia.org/wikipedia/commons/thumb/1/1b/Mercedes_V6_DTM_Rennmotor_199
6.jpg/300px-Mercedes_V6_DTM_Rennmotor_1996.jpg
4. http://img.bhs4.com/AB/6/AB63A4423864E7CBE847B30F58A06EF6DADD3605_large.jpg
5. http://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Rankine_cycle_layout.png/330px-
Rankine_cycle_layout.png
6. http://upload.wikimedia.org/wikipedia/commons/thumb/b/be/Rankine_cycle_Ts.png/400px-
Rankine_cycle_Ts.png
7. http://wpcontent.answers.com/wikipedia/commons/thumb/0/0b/Rankine_cycle_with_superheat.jpg/
250px-Rankine_cycle_with_superheat.jpg
8. http://upload.wikimedia.org/wikipedia/commons/thumb/6/65/DieselCycle_PV.svg/300px-
DieselCycle_PV.svg.png
9. http://www.pilotfriend.com/training/flight_training/fxd_wing/images4/flat4.gif
10. http://www.pilotfriend.com/training/flight_training/fxd_wing/images4/flat4.gif
11. 4StrokeEngine_Ortho_3D_Small.gif
Reference-Sources
Reference-Sources
12. http://content.answers.com/main/content/img/BritannicaConcise/images/72180.jpg
13. http://media.web.britannica.com/eb-media/72/93572-034-26C16785.jpg
14 http://prharikrishnan.files.wordpress.com/2014/05/2-stroke.gif
15 http://www.angelfire.com/ultra/omshome/thermalscience/powerc26.jpg
16 http://www.car-engineer.com/wp-content/uploads/2013/05/direct-indirect-injection-
system.png?46ac1a
• Content References
• – Elements of Mechanical Engineering by H.G. Katariya, J.P Hadiya, S.M.Bhatt , Books India Publication.
• -Elements of Mechanical Engineering by V.K.Manglik, PHI
• -Elements of Mechanical Engineering by R.K Rajput.
• -Elements of Mechanical Engineering by P.S.Desai & S.B.Soni
Any Question ?
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