Ideal Diesel cycle composition: How many and what types of thermodynamic processes constitute the air-standard Diesel cycle?
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Atwo constant volume and two isentropic
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Btwo constant pressure and two isentropic
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Ctwo constant volume and two isothermal
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Done constant pressure, one constant volume, and two isentropic
Answer
Correct Answer: one constant pressure, one constant volume, and two isentropic
Explanation
Introduction / Context:The Diesel cycle is the idealized model for compression-ignition engines. Knowing the nature of each process in the cycle is essential for writing energy equations, deriving efficiency formulas, and interpreting p–V and T–s diagrams.
Given Data / Assumptions:
- Air-standard analysis with constant specific heats.
- Internally reversible compression and expansion.
- Heat addition at constant pressure; heat rejection at constant volume.
Concept / Approach:The Diesel cycle consists of four processes: (1) isentropic compression, (2) constant-pressure heat addition, (3) isentropic expansion, and (4) constant-volume heat rejection. This combination distinguishes Diesel from Otto (constant-volume heat addition) and Dual (split CV then CP addition).
Step-by-Step Solution:
Identify compression: 1→2 isentropic.Heat addition: 2→3 at constant pressure.Expansion: 3→4 isentropic.Heat rejection: 4→1 at constant volume.Verification / Alternative check:Plotting on p–V shows a vertical up-right CP line for 2→3 and a horizontal CV line for 4→1; T–s exhibits an isentropic pair with s constant during 1→2 and 3→4.
Why Other Options Are Wrong:
- Two CV + two isentropic: That is the Otto cycle, not Diesel.
- Two CP + two isentropic: Rejects constant-volume rejection.
- Two CV + two isothermal: Not an air-standard engine cycle.
Common Pitfalls:Assuming both heat interactions are constant pressure; confusing Diesel with Dual, where heat addition starts at CV and then continues at CP.
Final Answer:one constant pressure, one constant volume, and two isentropic