Lecture - 01 - Gas Power Cycles
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Transcript of Lecture - 01 - Gas Power Cycles
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EE 3278Engineering Thermodynamics
Chapter 1: Gas Power Cycles
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Introduction and basic considerations
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Introduction and basic considerations
• Devices or systems used to produce a net power output engines
• Actual cycle vs. Ideal cycle– Actual cycles are difficult to
analyze– Friction, insufficient time for
equilibrium conditions, i.e. irreversibilities
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The Carnot Cycle
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The Carnot Cycle
• The most efficient cycle that can be executed between a heat source at temperature TH and a sink temperature TL
• Can be executed in closed systems (a piston-cylinder device) or steady-flow systems(utilizing turbines and compressors)
• Working fluid can be gas or vapour
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The Carnot Cycle
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The Carnot Cycle
• 12: isothermal heat addition• 23: isentropic expansion• 34: isothermal heat rejection• 41: isentropic compression• Thermal efficiency for Carnot cycle
ηth = 1 −
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The Carnot Cycle
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Example
Show that the thermal efficiency of a Carnot cycle operating between the temperature limits of TH and TL is solely a function of these two temperatures.
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Reciprocating Engines
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Reciprocating Engines
Nomenclature for reciprocating enginesDisplacement and clearance volumes of a reciprocating engine.
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Reciprocating Engines
Compression ratio, r
푟 =푉푉 =
푉푉
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Reciprocating Engines
Mean effective pressure, MEP
MEP used as a parameter to compare the performances of reciprocating engines of equal size; larger value of MEP delivers more net work per cycle and thus performs better
MEP =푊
푉 − 푉=
푤푣 − 푣
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The Otto Cycle
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Reciprocating Engines: Otto Cycle• The ideal cycle for spark-ignition reciprocating
engines• Named after Nikolaus A. Otto who built a
successful four-stroke engine in 1876
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Reciprocating Engines: Otto Cycle
1-2: Isentropic compression2-3: Constant-volume heat addition3-4: Isentropic expansion4-1: Constant-volume heat rejection
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First law analysis on Otto Cycle (closed cycle):- Energy balance:
also,
- Thermal efficiency
Processes 1-2 and 3-4 are isentropic, and 푣 = 푣 and 푣 = 푣 . Thus,푇푇
=푣푣
=푣푣
=푇푇
푞 − 푞 + 푤 − 푤 = ∆푢
푞 = 푢 − 푢 = 푐 푇 − 푇 and 푞 = 푢 − 푢 = 푐 푇 − 푇
휂 , =푤푞
= 1 −푞푞
= 1 −푇 − 푇푇 − 푇
= 1 −푇 푇
푇 − 1
푇 푇푇 − 1
∴ 휂 , = 1 − where 푟 = = = and 푘 =
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Reciprocating Engines: Otto Cycle
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Reciprocating Engines: Otto CycleExample
An ideal Otto cycle has a compression ratio of 8. At the beginning of the compression process, air is at 100 kPaand 17 °C, and 800 kJ/kg of heat is transferred to air during the constant-volume heat-addition process. Utilizing the cold-air standard assumptions, 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. Take 푐 = 0.718kJ/kgK.
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The Diesel Cycle
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Reciprocating Engines: Diesel Cycle• The ideal cycle for CI reciprocating engines• CI engine: first proposed by Rudolph Diesel in
the 1890s
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Reciprocating Engines: Diesel Cycle
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Reciprocating Engines: Diesel CycleFirst law analysis of Diesel cycle (closed system)
and
Therefore, the thermal efficiency;
Defining cutoff ratio, 푟 = = ;
푞 − 푤 , = 푢 − 푢 →푞 = 푃 푣 − 푣 + 푢 − 푢= ℎ − ℎ = 푐 푇 − 푇
−푞 = 푢 − 푢 → 푞 = 푢 − 푢 = 푐 푇 − 푇
휂 , =푤푞
= 1 −푞푞
= 1 −푇 − 푇
푘 푇 − 푇= 1 −
푇 푇푇 − 1
푘푇 푇푇 − 1
휂 , = 1 −1
푟푟 − 1푘 푟 − 1
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Reciprocating Engines: Diesel CycleExample
An air-standard Diesel cycle has a compression ratio of 16 and a cutoff ratio of 2. At the beginning of the compression process, air is at 95 kPa and 27 °C. Utilizing the cold-air-standard assumptions, determine (a) the temperature after the heat-addition process, (b) the thermal efficiency, and (c) the mean effective pressure
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ReferencesCengel, Y. A. & Boles, M. A. (2011). Thermodynamics: An Engineering Approach, 7th edition, New York: McGraw-Hill
Eastop, T. D. & McConkey, A. (1993). Applied Thermodynamics for Engineering Technologists, 5th edition, Harlow: Pearson Education Limited.