Thermodynamic Analysis of Internal Combustion Engines

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Questions and Answers

What is the main purpose of the approximations made in the thermodynamic analysis of internal combustion engines?

To simplify the analysis by neglecting certain complexities (correct)

To reduce the fuel consumption of the engine

To increase the power output of the engine

To make the engine operate more efficiently

What is the assumption made about the combustion process in an air standard cycle?

It is replaced by an equivalent energy addition process from an external source (correct)

It is an adiabatic process

It is a reversible process

It is an isothermal process

What is the characteristic of the air in an air standard cycle?

It behaves like an ideal gas with constant specific heat (correct)

It behaves like a perfect gas with zero specific heat

It behaves like a mixture of ideal and real gases

It behaves like a real gas with variable specific heat

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 effect of isentropic compression on the temperature of the gas in the Otto cycle?

It increases the temperature Signup and view all the answers

Why is the cycle 0-1-0 excluded from the thermodynamic analysis of the Otto cycle?

Because the work done during suction is exactly balanced by the work done during exhaust 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

How many strokes are there in the complete Otto cycle?

4 Signup and view all the answers

What is the compression ratio of a Dual Cycle engine compared to a Diesel engine?

Lower Signup and view all the answers

What is the combustion duration of a Dual Cycle engine compared to a Diesel engine?

Shorter Signup and view all the answers

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

Higher Signup and view all the answers

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

Isentropic compression Signup and view all the answers

What is the process that occurs during the expansion stroke of the Dual Cycle engine?

Isentropic expansion Signup and view all the answers

What is the equation for the efficiency of the Dual Cycle engine?

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

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

Isobaric heat addition Signup and view all the answers

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

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

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

No heat transfer occurs during compression and expansion strokes Signup and view all the answers

What is the formula for the efficiency of the Otto cycle in terms of the compression ratio (r = V1/V2)?

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

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

Higher compression ratio Signup and view all the answers

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

Q_in = m * cv * (T3 - T2) Signup and view all the answers

What is the characteristic of the Diesel cycle that reduces heat loss during combustion?

Fuel injection at the top of the compression stroke Signup and view all the answers

What is the formula for the efficiency of the Diesel cycle in terms of the compression ratio (r = V1/V2) and the cutoff ratio ( = V3/V2)?

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

What is the advantage of the Diesel cycle compared to the Otto cycle?

Higher thermal efficiency Signup and view all the answers

What is the characteristic of the Dual Cycle?

Combines elements of the Otto and Diesel cycles Signup and view all the answers

What is the primary advantage of the Joule cycle over the Otto cycle?

Isobaric heat addition process Signup and view all the answers

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

No heat transfer during compression and expansion strokes Signup and view all the answers

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

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

Which cycle has the highest efficiency and work output among the Otto, diesel, and dual cycles?

Diesel cycle Signup and view all the answers

What is the characteristic of the compression stroke in the diesel cycle compared to the Otto cycle?

Longer compression stroke Signup and view all the answers

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

Isobaric heat addition process Signup and view all the answers

What is the effect of the pressure ratio on the efficiency of the Joule cycle?

Increases the efficiency Signup and view all the answers

What is the condition for which the diesel cycle has a higher efficiency than the Otto cycle?

Same compression ratio and heat rejection Signup and view all the answers

What is the main characteristic of the dual cycle?

It has a compression stroke longer than the Otto cycle but shorter than the diesel cycle. Signup and view all the answers

What is the primary function of the boiler in the Rankine cycle?

To add heat to the working fluid and generate steam. 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 Modified Rankine Cycle over the Rankine cycle?

It has a higher efficiency due to reheat, regeneration, and superheating. Signup and view all the answers

What is the main difference between the Rankine cycle and the Carnot cycle?

The working fluid used in the cycles. Signup and view all the answers

What is the maximum possible efficiency of a heat engine?

1 - (Tc/Th). Signup and view all the answers

What is the main limitation of the Rankine cycle?

The efficiency of the turbine and condenser. Signup and view all the answers

What is the primary application of the Carnot cycle?

As a benchmark for comparing the efficiency of different heat engines. 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

This quiz covers the introduction to air standard cycles, focusing on the approximations made for thermodynamic analysis of internal combustion engines, including assumptions about the engine's operation and combustion process.