Air standard cycles_PPT KM1.pptx

1. Air Standard Cycles

2. Introduction to Thermodynamic power cycles
• Two important applications of thermodynamics:
(i) Power generation
(ii) Refrigeration.
 The required purpose is accomplished by a system which
operates in a thermodynamic cycle.
 The power or refrigeration cycles are further classified as:
(i) gas cycles (ii) vapor cycles
 In the case of gas cycles, the working substance will be in gaseous
phase throughout the cycle.
 In a vapor cycle, the working substance will be in liquid phase in
one part of the cyclic process and will be in vapor phase in some
other part of the cycle. 2

3.  Thermodynamic cycles are also classified as “closed cycles” and
“open cycles”.
 In a closed cycle, the working fluid is returned to its original state
at the end of each cycle of operation and is recirculated.
 In an open cycle, the working substance is renewed at the end of
each cycle instead of being re-circulated.
 In automobile engines, the combustion gases are exhausted and
replaced by fresh air-fuel mixture at the end of each cycle.
 Though the engine operates in a mechanical cycle, the working
substance does not go through a complete thermodynamic cycle.
 The accurate analysis of IC engine processes is very
complicated.
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4. Air standard cycles used in IC engines
• Petrol or gasoline engines, diesel engines and gas turbines operate
on gas cycles and called “Internal combustion engines”.
• Due to combustion of the fuel, the composition of the working fluid
changes from a mixture of air and fuel to products of combustion
during the course of the cycle.
• Since air is predominantly Nitrogen which hardly undergoes any
chemical reaction during combustion, the working fluid closely
resembles air at all times.
• The actual gas power cycles are complex. Hence actual gas power
cycles are approximated by air standard cycles
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5. Air standard Cycles definition and assumptions
 Air standard cycles are one of the approach to understand IC
engine cycles and to analyze the performance of an idealized
closed cycle which closely approximates the real cycle.
 The assumptions made in the analysis of air standard cycles:
(i) The working fluid is air which continuously circulates in a closed loop.
(ii) Air behaves as a perfect gas.
(iii) All the processes that make up the cycle are internally reversible.
(iv) The combustion process is replaced by a heat addition process from
an external source.
(v) The exhaust process is replaced by a heat rejection process that
restores the working substance to its original state.
(vi) Changes in kinetic and potential energies of the working substance
are very small and hence negligible.
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6. Carnot Cycle
• Carnot was the first, who proposed hypothetical cycle and studied the
performance of heat engine.
• The hypothetical cycle consists of four processes as shown in the figure:
 Two isothermal processes – one heat addition and other heat rejection
 Two reversible adiabatic processes- one compression and the other
expansion
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7. • Process 1-2: Reversible isothermal heating of the working
substance from state1to state 2.
• Process 2-3 : Isentropic expansion of the working
substance from state 2 to state 3. During this process work is done
by the working substance on the surroundings.
• Process 3-4: Reversible isothermal cooling of the working
substance from state 3 to state 4.
• Process 4-1: Isentropic compression of the working substance so
that it comes back to its initial state. During this process work is
done on the working substance by the surroundings.
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8. • Expression for Thermal Efficiency of the Cycle
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9. • The thermal efficiency of Carnot cycle depends only on two
temperatures TH and TL and is independent of working substance.
• It is the most efficient cycle that can be executed between given heat
source at temperature TH and a heat sink at temperature TL.
• However reversible isothermal heat transfer process is difficult to
achieve in practice, because,
 It would require very large heat exchangers
 It would take a very long time (a power cycle in a typical engine has
to be completed in a fraction of a second).
• Therefore it is not practical to build an engine that would operate on a
cycle that closely approximates a Carnot cycle.
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10. Air standard Otto cycle
• Otto cycle is the ideal cycle for spark ignition engines named after
Nikolaus A Otto, a German who built a four – stroke engine in 1876
• The p– V and T – s diagrams for an Otto cycle are shown below:
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11. Processes
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12. Thermodynamic Analysis of Otto cycle
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13. • Let compression ratio is rc = V1/V2
 Thus thermal efficiency of the Otto cycle depends only on the compression ratio rc.
 The efficiency increases with increase in compression ratio (rc).
 The increase is steep at low values of rc , but becomes flatter as rc exceeds 8 as
shown in figure.
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14. Diesel Cycle
• The diesel cycle is the ideal cycle for CI engines
• CI engine was first proposed by Rudolph Diesel in 1890.
• In diesel engines which works on the principle of compression
ignition, only air is compressed and at the end of the compression
process, the fuel is sprayed into the engine cylinder containing
high pressure air.
• The fuel ignites spontaneously and combustion occurs.
• Since only air is compressed during the compression stroke, the
possibility of auto ignition is completely eliminated in diesel
engines.
• Hence diesel engines can be designed to operate at much higher
compression ratios (12 -24).
• Another benefit of not having to deal with auto ignition is that
fuels used in this engine can be less refined (thus less expensive).
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15. Processes
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17. Thermodynamic analysis of Diesel Cycle
• Let’s define the volume ratios:
Let’s define the volume ratios:
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23. Dual Combustion cycle
• In the earlier versions of CI engines (Low speed) engines the
fuel was injected when the piston reached TDC and thus
combustion happens during the expansion stroke.
• In the modern high speed CI engines the simulated pressure
and volume variation do not match with the actual engine
cycle.
• In modern engines the fuel injection starts well before the
TDC (about 15o ).
• The dual combustion cycle is the closer approximation of the
modern CI engine in which some part of the heat is added to
the air at constant volume and remainder at constant
pressure.
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24. Mean effective pressure for Diesel cycle
• We have already shown for the diesel cycle
• On substituting in the equation 1 we get
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25. Processes
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34. Efficiency Comparison of Otto, Diesel cycle at different CR
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The figure shows the comparison of efficiency verses compression ratio of Otto, Diesel and
Dual cycle at various compression ratio.
It is clear that as the compression ratio increases the cycle efficiency increases.
For the given compression ratio the Otto cycle has maximum efficiency and diesel cycle has
minimum efficiency.
The efficiency of dual cycle always lies in between them