Thermodynamics: Air Standard Cycles and Otto Cycle
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Questions and Answers

What is one of the approximations made for the thermodynamic analysis of internal combustion engines?

  • The combustion process is replaced by an equivalent energy addition process from an external source (correct)
  • The air is assumed to behave like a real gas
  • The engine is assumed to operate on an open cycle
  • The engine is assumed to be a perfect converter of energy

    • What is the name of the inventor of the Air Standard Otto cycle?

  • James Watt
  • Nikolaus A. Diesel
  • Rudolph Diesel
  • Nikolaus A. Otto (correct)

    • What is the process during which a mixture of fuel vapour and air is drawn into the cylinder in the Otto cycle?

  • Expression
  • Constant pressure suction (correct)
  • Compression
  • Constant volume combustion

    • What happens to the temperature of the gas during the compression process in the Otto cycle?

    <p>It increases</p> Signup and view all the answers

    Why do we need to consider only the cycle 1-2-3-4 for thermodynamic analysis of the Otto cycle?

    <p>Because the work done during the process 0-1 is exactly balanced by the work done during 1-0</p> Signup and view all the answers

    What type of gas is the air assumed to behave like in the air standard cycle?

    <p>Ideal gas with constant specific heat</p> Signup and view all the answers

    What is the name of the cycle that models the operation of a spark-ignition internal combustion engine?

    <p>Air Standard Otto Cycle</p> Signup and view all the answers

    What is the purpose of the air standard cycle?

    <p>To analyze the thermodynamic performance of internal combustion engines</p> Signup and view all the answers

    What is the main assumption in the ideal Otto cycle?

    <p>Constant specific heat capacities</p> Signup and view all the answers

    What is the formula for the efficiency of the Otto cycle?

    <p>1 - (1/r)^(-1)</p> Signup and view all the answers

    What is the main difference between the Otto and Diesel cycles?

    <p>Compression ratio</p> Signup and view all the answers

    What is the formula for the heat input in the Otto cycle?

    <p>Q_in = m * cp * (T3 - T2)</p> Signup and view all the answers

    What is the advantage of the Diesel cycle over the Otto cycle?

    <p>Higher thermal efficiency</p> Signup and view all the answers

    What is the formula for the efficiency of the Diesel cycle?

    <p>1 - (1/r)^(-1) * (1 - (1/)^(-1))</p> Signup and view all the answers

    What is the term for the ratio of the specific heat capacities in the ideal cycle?

    <p>Adiabatic index</p> Signup and view all the answers

    What is the Dual Cycle also known as?

    <p>Semidiesel cycle</p> Signup and view all the answers

    What is the primary difference between the Dual Cycle and the Otto cycle?

    <p>The Dual Cycle has a higher compression ratio</p> Signup and view all the answers

    What is the assumption made about heat transfer during the compression and expansion strokes in the Dual Cycle?

    <p>There is no heat transfer during compression and expansion strokes</p> Signup and view all the answers

    What is the equation that represents the efficiency of the Dual Cycle?

    <p>= 1 - (1/r)^(-1) * (1 - (1/)^(-1)) * (T3/T2)</p> Signup and view all the answers

    What is the process that occurs between points 2 and 3 in the Brayton cycle?

    <p>Isobaric heat addition</p> Signup and view all the answers

    What is the assumption made about gas behavior in the Brayton cycle?

    <p>Ideal gas behavior</p> Signup and view all the answers

    What is the equation that represents the efficiency of the Brayton cycle?

    <p>= 1 - (1/PR)^(-1) * (TR - 1) / ( * (TR - 1))</p> Signup and view all the answers

    What is the primary application of the Brayton cycle?

    <p>Gas turbine engines</p> Signup and view all the answers

    What is the difference between the Dual Cycle and the Diesel cycle?

    <p>The Dual Cycle has a higher compression ratio and shorter combustion duration</p> Signup and view all the answers

    What is the main advantage of the isobaric heat addition process in the Brayton cycle?

    <p>It allows for more efficient energy conversion</p> Signup and view all the answers

    What is the assumption made in the Joule cycle regarding heat transfer during compression and expansion strokes?

    <p>There is no heat transfer during compression and expansion strokes</p> Signup and view all the answers

    What is the expression for the efficiency of the Joule cycle in terms of the pressure ratio (PR) and the temperature ratio (TR)?

    <p>η = 1 - (1/PR)^(-1) * (1 - 1/TR)</p> Signup and view all the answers

    Which cycle has the highest efficiency and work output for the same maximum pressure and temperature?

    <p>Diesel cycle</p> Signup and view all the answers

    What is the primary difference between the Rankine cycle and the Modified Rankine Cycle?

    <p>The inclusion of reheat, regeneration, and superheating processes</p> Signup and view all the answers

    What is the main limitation of the efficiency of the Rankine cycle?

    <p>The temperature difference between the heat source and sink</p> Signup and view all the answers

    What is the purpose of the isentropic expansion process in the Joule cycle?

    <p>To generate power</p> Signup and view all the answers

    Which cycle has a lower mean effective pressure (MEP) and work output compared to the diesel cycle?

    <p>Auto (Otto) cycle</p> Signup and view all the answers

    What is the purpose of the reheat process in the Modified Rankine Cycle?

    <p>To increase the average temperature of heat addition</p> Signup and view all the answers

    What is the expression for the work done in the Joule cycle?

    <p>W = pdV = (p3V3 - p4V4) + (p2V2 - p1V1)</p> Signup and view all the answers

    What is the characteristic that makes the Carnot cycle an ideal cycle?

    <p>All of the above</p> Signup and view all the answers

    What is the primary advantage of the Modified Rankine Cycle over the Rankine cycle?

    <p>It has higher efficiency and power output</p> Signup and view all the answers

    What is the main difference between the Joule cycle and the Otto cycle?

    <p>The Joule cycle has isobaric heat addition</p> Signup and view all the answers

    What is the formula for the maximum possible efficiency of a heat engine?

    <p>η = 1 - (Tc/Th)</p> Signup and view all the answers

    What is the purpose of the regeneration process in the Modified Rankine Cycle?

    <p>To transfer heat from the exhaust steam to the feedwater</p> Signup and view all the answers

    What is the characteristic that makes the Rankine cycle more practical than the Carnot cycle?

    <p>It is more practical and widely used in power plants</p> Signup and view all the answers

    Study Notes

    Air Standard Cycles

    • Air standard cycles are used to model the operation of internal combustion engines.
    • Assumptions:
      • The engine operates on a closed cycle with a fixed mass of air.
      • The combustion process is replaced by an equivalent energy addition process.
      • The exhaust process is replaced by an equivalent energy rejection process.
      • The air behaves like an ideal gas with constant specific heat.

    Otto Cycle

    • Named after Nikolaus A. Otto.
    • Consists of four processes: suction, compression, expression, and exhaust.
    • Assumptions:
      • Ideal gas behavior.
      • Constant specific heat capacities.
      • No heat transfer during compression and expansion strokes.
      • No friction or other losses.
    • Efficiency calculation:
      • Work done: W = pdV = (p3V3 - p4V4) + (p2V2 - p1V1).
      • Heat input: Q_in = m * cp * (T3 - T2).
      • Heat output: Q_out = m * cp * (T4 - T1).
      • Efficiency: η = W / Q_in = (1 - (Q_out / Q_in)).
    • Theoretical efficiency: η = 1 - (1/r)^(-1), where r is the compression ratio.

    Diesel Cycle

    • Models the operation of a diesel engine.
    • Consists of four processes: compression, fuel injection and combustion, expansion, and exhaust.
    • Assumptions:
      • Ideal gas behavior.
      • Constant specific heat capacities.
      • No heat transfer during compression and expansion strokes.
      • No friction or other losses.
    • Efficiency calculation:
      • Work done: W = pdV = (p3V3 - p4V4) + (p2V2 - p1V1).
      • Heat input: Q_in = m * cv * (T3 - T2).
      • Heat output: Q_out = m * cv * (T4 - T1).
      • Efficiency: η = W / Q_in = (1 - (Q_out / Q_in)).
    • Theoretical efficiency: η = 1 - (1/r)^(-1) * (1 - (1/)^(-1)), where r is the compression ratio and is the cutoff ratio.

    Dual Cycle

    • Combines elements of the Otto and Diesel cycles.
    • Consists of four processes: compression, fuel injection and combustion, expansion, and exhaust.
    • Assumptions:
      • Ideal gas behavior.
      • Constant specific heat capacities.
      • No heat transfer during compression and expansion strokes.
      • No friction or other losses.
    • Efficiency calculation:
      • Work done: W = pdV = (p3V3 - p4V4) + (p2V2 - p1V1).
      • Heat input: Q_in = m * cv * (T3 - T2).
      • Heat output: Q_out = m * cv * (T4 - T1).
      • Efficiency: η = W / Q_in = (1 - (Q_out / Q_in)).
    • Theoretical efficiency: η = 1 - (1/r)^(-1) * (1 - (1/)^(-1)) * (T2/T3).

    Brayton Cycle

    • Models the operation of a gas turbine engine.
    • Consists of four processes: compression, isobaric heat addition, expansion, and isobaric heat rejection.
    • Assumptions:
      • Ideal gas behavior.
      • Constant specific heat capacities.
      • No heat transfer during compression and expansion strokes.
      • No friction or other losses.
    • Efficiency calculation:
      • Work done: W = pdV = (p3V3 - p4V4) + (p2V2 - p1V1).
      • Heat input: Q_in = m * cp * (T3 - T2).
      • Heat output: Q_out = m * cp * (T4 - T1).
      • Efficiency: η = W / Q_in = (1 - (Q_out / Q_in)).
    • Theoretical efficiency: η = 1 - (1/PR)^(-1) * (TR - 1) / ( (TR - 1)), where PR is the pressure ratio and TR is the temperature ratio.

    Comparison of Cycles

    • Auto (Otto) Cycle: lower efficiency (~25-30%), lower mean effective pressure (MEP), and lower work output.
    • Diesel Cycle: higher efficiency (~40-45%), higher mean effective pressure (MEP), and higher work output.
    • Dual Cycle: efficiency between Otto and Diesel cycles (~35-40%), mean effective pressure (MEP) between Otto and Diesel cycles, and work output between Otto and Diesel cycles.

    Rankine Cycle

    • Models the operation of a steam turbine power plant.
    • Consists of four processes: isentropic compression, isobaric heat addition, isentropic expansion, and isobaric heat rejection.
    • Modified Rankine Cycle:
      • Reheat: increases the average temperature of heat addition.
      • Regeneration: reduces the temperature difference between heat addition and rejection.
      • Superheating: increases the steam temperature and reduces condensation losses.

    Comparison of Rankine and Carnot Cycles

    • Rankine Cycle: used in steam power plants, involves water as the working fluid, and has a typical efficiency of 33-40%.
    • Carnot Cycle: ideal cycle, not practical for implementation, involves an ideal gas as the working fluid, and has a maximum possible efficiency of η = 1 - (Tc/Th).
    • Comparison: Rankine cycle is more practical and widely used, while Carnot cycle is ideal and theoretical.

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    Description

    Understand the principles of air standard cycles and the Otto cycle, used to model internal combustion engines, including assumptions and processes.

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