Diesel Cycle p-V Diagram Characteristics

The ideal Diesel cycle consists of four processes:

• 1-2: Isentropic Compression: Volume decreases, pressure and temperature increase adiabatically.
• 2-3: Constant Pressure Heat Addition: Heat is added at constant pressure while volume increases. This represents fuel injection and combustion.
• 3-4: Isentropic Expansion: Volume increases, pressure and temperature decrease adiabatically. This is the power stroke.
• 4-1: Constant Volume Heat Rejection: Heat is rejected at constant volume, causing pressure and temperature to drop rapidly.

Identifying the Correct p-V Graph

We need to find the graph that represents these processes on a pressure-volume (p-V) scale:

• Process 1-2 should be a curve representing an adiabatic (isentropic) compression, where pressure rises sharply as volume decreases.
• Process 2-3 must be a horizontal line, indicating constant pressure as volume increases.
• Process 3-4 should be a curve representing an adiabatic (isentropic) expansion, where pressure decreases as volume increases.
• Process 4-1 must be a vertical line, indicating constant volume as pressure decreases.

Option 1 correctly depicts these features: a compression curve (1-2), followed by a constant pressure expansion (2-3), then an expansion curve (3-4), and finally a constant volume pressure drop (4-1). This matches the working of an ideal Diesel cycle on a p-V diagram.

Option 3 represents the Otto cycle, which has constant volume heat addition (vertical line 2-3) instead of constant pressure.