Ideal Diesel cycle composition: How many and what types of thermodynamic processes constitute the air-standard Diesel cycle?

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:

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