Diesel cycle idealization — mode of heat addition In the idealized Diesel (compression-ignition) engine cycle, the primary heat addition occurs at:

Introduction / Context:
Ideal cycles help engineers approximate real engine behavior. For Diesel engines, ignition occurs after high compression, and fuel is injected over a finite crank angle. Recognizing the textbook heat-addition model distinguishes Diesel from Otto cycle analysis.

Given Data / Assumptions:

• Air-standard analysis (working fluid is air with constant properties).
• Compression-ignition with high compression ratio.
• Heat addition idealized either at constant volume (Otto) or constant pressure (Diesel).

Concept / Approach:
In the Diesel cycle, heat addition is modeled at constant pressure while the piston continues to move, reflecting continued fuel injection and combustion as the volume expands. In contrast, the Otto cycle assumes instantaneous heat addition at top dead center (constant volume). While real engines exhibit mixed behavior, the Diesel ideal uses the constant-pressure assumption to derive efficiency and performance relations.

Step-by-Step Solution:
Recall ideal cycle definitions: Otto (constant volume), Diesel (constant pressure).Match Diesel to constant-pressure heat addition segment.Select the correct mode: constant pressure.

Verification / Alternative check:
Pressure–volume diagrams show the Diesel heat-addition line nearly horizontal compared to the vertical Otto line at TDC, confirming the modeling assumption.

Why Other Options Are Wrong:
Constant volume corresponds to Otto. Constant temperature resembles idealized external combustion (not applicable here). Adiabatic describes compression and expansion strokes, not heat addition.

Common Pitfalls:
Equating real DI combustion phasing directly to the ideal model; forgetting that actual pressure can rise—‘‘constant’’ is an approximation for analysis.

Final Answer:
constant pressure