Unit 6: Air Standard Cycles

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Questions and Answers

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

It increases the temperature ratio

It decreases the compressor design constraints

It allows for more efficient energy conversion (correct)

It reduces the pressure ratio

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

There is no heat transfer during compression and expansion strokes (correct)

Heat transfer only occurs during expansion strokes

Heat transfer only occurs during compression strokes

There is some heat transfer during compression and expansion strokes

What is the assumption made about the engine in the air standard cycle?

The engine operates on an open cycle with a variable mass of air.

The engine operates on a semi-closed cycle with a variable mass of air.

The engine operates on a closed cycle with a fixed mass of air. (correct)

The engine operates on a semi-open cycle with a fixed mass of air.

What is replaced by an equivalent energy addition process from an external source?

The combustion process Signup and view all the answers

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

η = (W / Q_in) = (1 - (Q_out / Q_in)) Signup and view all the answers

What is the assumption made about the air in the air standard cycle?

The air behaves like an ideal gas with constant specific heat. Signup and view all the answers

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

Diesel cycle Signup and view all the answers

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

Nikolaus A. Otto Signup and view all the answers

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

Compression stroke length Signup and view all the answers

What type of combustion process is used in the Dual Cycle?

Short and rapid combustion Signup and view all the answers

What is the process that occurs during the stroke 0-1 in the Otto cycle?

Constant pressure suction Signup and view all the answers

What is the result of a longer compression stroke in the Diesel cycle?

Higher mean effective pressure Signup and view all the answers

What is the characteristic of the compression ratio in the Dual Cycle compared to Diesel engines?

Lower compression ratio Signup and view all the answers

What is the comparison between the Dual cycle and the Otto cycle in terms of efficiency?

The Dual cycle has efficiency between Otto and Diesel cycles Signup and view all the answers

What is the effect of the isentropic compression on the temperature of the gas?

The temperature of the gas increases Signup and view all the answers

What is the limitation of the efficiency of the Joule cycle?

Pressure ratio and temperature ratio Signup and view all the answers

What is the efficiency of the Dual Cycle compared to the Otto cycle?

Higher efficiency Signup and view all the answers

What is the purpose of considering only the cycle 1-2-3-4 in the Otto cycle?

To simplify the thermodynamic analysis Signup and view all the answers

What is the Otto cycle?

An idealized thermodynamic cycle that models the operation of a spark-ignition internal combustion engine Signup and view all the answers

What is the process that occurs during the compression stroke in the Dual Cycle?

Isentropic compression Signup and view all the answers

What is the characteristic of the Brayton cycle?

Models the operation of a gas turbine engine Signup and view all the answers

What is the process that occurs during the heat addition stage in the Brayton cycle?

Isobaric heat addition Signup and view all the answers

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

1 - (1/PR)^(-1) * (1 - 1/TR) Signup and view all the answers

What is the purpose of the exhaust stroke in the Dual Cycle?

To release the exhaust gas to the atmosphere Signup and view all the answers

What is the assumption made in the processes of the Otto cycle and Diesel cycle?

Constant specific heat capacities Signup and view all the answers

In the Otto cycle, what is the process between points 2-3?

Heat addition Signup and view all the answers

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

= 1 - (1/r)^(-1) Signup and view all the answers

Why is the Diesel cycle more efficient than the Otto cycle?

Higher compression ratio Signup and view all the answers

What is the process between points 2-3 in the Diesel cycle?

Fuel injection and combustion Signup and view all the answers

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

= 1 - (1/r)^(-1) * (1 - (1/)) Signup and view all the answers

What is the characteristic of the Diesel cycle that leads to higher thermal efficiency?

No heat loss during combustion Signup and view all the answers

What is the name of the thermodynamic cycle that combines elements of both the Otto and Diesel cycles?

Dual Cycle Signup and view all the answers

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

The addition of reheat, regeneration, and superheating processes Signup and view all the answers

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

To increase the average temperature of heat addition Signup and view all the answers

What is the primary advantage of the Carnot cycle over the Rankine cycle?

Higher efficiency due to the use of an ideal gas Signup and view all the answers

What is the typical efficiency of the Rankine cycle?

33-40% Signup and view all the answers

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

To reduce the temperature difference between heat addition and rejection Signup and view all the answers

What is the working fluid used in the Rankine cycle?

Water Signup and view all the answers

What is the primary application of the Rankine cycle?

Steam power plants Signup and view all the answers

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

η = 1 - (Tc/Th) 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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Learn about the approximations made in thermodynamic analysis of internal combustion engines, including closed cycle operation and energy addition/rejection processes.