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Introduction :
Power cycles
An important application of thermodynamics is the analysis of power cycles through which the energy absorbed as heat can be continuously converted into mechanical work . A thermodynamics analysis of the heat engine cycles provides valuable information regarding the design of new cycles or for improving the existing cycles.
These four processes make a cycle.. 1.Compression 2. Heat Addition 3.Expansion 4.Heat rejection 1.Compression and expansion are opposite to each other, Whereas heat addition and heat removal are opposite to each other. 1.Compression process in all cycles is isentropic Compression, because it is ideal process for the compression. An isentropic compression process can be shown on P-V and T-S diagram as below.
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WWW.MECHDIPLOMA.COM 3. Expansion process in all cycles is also isentopic expansion..
3. The difference among the various cycles is the process by which heat is added or rejected from the system. Heat addition may be isothermal, constant volume,constant pressure or combinations. This makes the different cycles. 5 cycles Cycle... Process . 1-2 {comp} 2-3 {Heat ad} 3-4 {expans.}
Carnot Cycle
Otto cycle
Brayton cycle
Diesel cycle
Dual cycle
Isentropic Isentropic Isentropic Isentropic Isentropic compr. compr. compr. compr. compr. Isothermal Const.Vol
Const Pr
Const Pr
Const.vol + Const. Pr
Isentropic Isentropic Isentropic Isentropic Isentropic exp. exp. exp. exp. exp.
4-1 Isothermal Const vol {Heat rej.)
Const pr
Const vol
Const vol
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WWW.MECHDIPLOMA.COM OTTO cycle The air-standard-Otto cycles is the idealized cycle for the idealized cycle for the spark-ignition internal combustion engines .This cycles is shown above on p-v and T-s diagrams .The otto cycle 1-2-3-4 consist of following four process : Process 1-2 ; Reversible adiabatic compression of air Process 2-3: Heat addition at constant volume Process 3-4 : Reversible adiabatic expansion of air Process 4-1 : Heat rejection at constant volume
Where r= compression ratio =V1/V2 γ = ratio of specific heats
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From the above equation, it can be observed that the efficiency of the Otto cycle is mainly the function of compression ration for the given ratio of Cp and Cv. Diesel cycle Air standard diesel cycle is a idealized cycle for diesel engines. It is as shown on P-V diagram and T-s Diagram, the processes in the cycle are as follows :
Process 1-2 : Reversible adiabatic compression Process 2-3 ; Constant Pressure heat addition Process 3-5 : Reversible adiabatic compression Process 4-1; Constant volume heat rejection Air Standard efficiency of diesel cycle
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Limited pressure cycle or Dual cycle This cycle has heat addition split into two processes, part is at constant volume and remaining part is at constant pressure. Thus this cycle combines the advantages of both Diesel cycle and Otto cycle.Hence the process of heat addition 2-3 is split into two processes, as 2-3’ & 3’-3. This cyles is very close to the actual cycle because the heat addition in actual engines does not occur exactly at constant volume or at constant pressure but it is a combination of both.
Process 1-2: Reversible adiabatic(Isentropic) compression Process 2-3’: Constant Volume heat addition Process 3’-3: Constant Pressure heat addition Process 3-4: Reversible adiabatic(Isentropic) expansion Process 4-1: Constant volume heat rejection
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Carnot cycle
Carnot cycle is ideal cycle in which heat addition and heat rejection is carried out at constant temperature(isothermal process). Actually it is not practically possible to carry out the heat addition and rejection at constant temperature..Carnot cycle is used for comparing the actual cycles. Process 1-2 Isentropic compression Process 2-3 Isothermal heat addition Process 3-4 isentropic expansion Process 4-1 isothermal heat rejection
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Brayton Cycle(Constant pressure closed cycle):
Process 1-2 Isentropic compression Process 2-3 Constant pressure heat addition Process 3-4 isentropic expansion Process 4-1 Constant pressure heat rejection The Brayton cycle is a theoretical cycle for simple gas turbine. This cycle consists of two isentropic and two constant pressure processes. The cycle is similar to the diesel cycle in compression and heat addition. The isentropic expansion of the diesel cycle is further extended followed by constant pressure heat rejection. The efficiency of the ideal Brayton cycle is given by
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