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Thermal Engineering SystemsMEE 216
Lecture 02
Senthur Prabu S.
Asst. Professor(Sr.)
SMBS
Contact information:
Room no.: GDN G 09 E
Open hrs: Mon 05.00 pm- 06.00 pm
Thu 04.00 pm- 05.00 pm
Email: [email protected]
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Review of construction and
working of engines
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Definition of Engine
It is a device which transforms one form of energy into
another form.
It can convert thermal energy into mechanical energy
Definition of Heat Engine
It is a device which transforms the chemical energy of a
fuelinto thermal energy and utilizes this thermal energy to
perform useful work.
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Heat Engines
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Internal Combustion
Engines
(Combustion of fueltakes placewithin the
engine)
External Combustion
Engines
(Combustion of fuel
takes place outside theengine)
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Classification of Heat Engines
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Advantages of I.C. Engines
Overall efficiency is high (35%)
Mechanical Simplicity
Low weight to power ratio
Absence of heat exchangers
Maximum possible expansion ratio
Greatest possible working speed
Lower initial cost
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Easy starting from cold conditions
Compact and require less space
Largest possible cylinder volume with minimum
boundary surface
Disadvantages of I.C. EnginesLow starting torque
Problem of vibration caused by reciprocation ofcomponents
Limited number of fuels
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Components of Engine
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Basic Nomenclature of Engines
Cylinder bore (d) :It is the nominal inner diameter of the working cylinder.
Piston Area (A):
It is the area of the circle of diameter equal to cylinder
bore.
Stroke (L):
It is the nominal distance through which a working piston
moves between two successive reversals of its direction ofmotion.
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Displacement or Swept Volume (Vs):
The nominal volume swept by the piston when travelling
from one dead centre to the other.
Vs = (/4) d2L = A X L
Clearance Volume (Vc):
The nominal volume of the combustion chamber above the
piston when it is at its top dead centre.
Cubic Capacity or Engine Capacity:
The displacement volume of a cylinder multiplied by the
number of cylinders in an engine.
Example: If there are n cylinders in an engine, then
Cubic Capacity = n * Vs
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Compression ratio (r):
The ratio of total cylinder volume when the piston is at the
bottom dead centre.
r = VT/VC= (VS+ VC)/VC= 1 + (VS/VC)
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WORKING OF 4 STROKE SI ENGINE
(OTTO CYCLE)
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P-V CURVE FOR OTTO CYCLE
IDEAL ACTUAL
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WORKING OF 4 STROKE CI ENGINE
(DIESEL CYCLE)
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P-V CURVE FOR DIESEL CYCLE
IDEAL ACTUAL
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QUIZ TIME
Which cycle has higher efficiency?Otto Cycle
or
Diesel Cycle
The efficiency of diesel cycle differs from efficiency of Otto
cycle by the quantity in brackets. This quantity is always
greater than 1.
Therfore , Otto cycle has a higher efficiency.
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VALVE TIMING DIAGRAMS FOR 4
STROKE ENGINE
IDEAL ACTUAL
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TWO STROKE ENGINE
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WORKING OF 2 STROKE SI ENGINE
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PORT TIMING DIAGRAM FOR
TWO STROKE SI ENGINE
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WORKING OF 2 STROKE CI ENGINE
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ACTUAL PORT TIMING DIAGRAM
FOR TWO STROKE CI ENGINE
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INDICATOR DIAGRAMS FOR TWO
STROKE ENGINES
IDEAL ACTUAL
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COMPARISON OF SI AND CI ENGINES
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COMPARISON OF 2 AND 4 STROKE ENGINES
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THANK YOU