This is the same air-standard Diesel cycle as the 3D piston-and-crank simulator (γ = 1.4), redrawn as pure state-space geometry instead of a rendered engine: the large panel is a full-size P–V diagram, not a small overlay, and the strip below it is a 2D schematic cross-section (rectangles and lines, not a 3D camera view) whose piston position is driven by the identical slider-crank kinematics.
1→2 Isentropic compression (r = V1/V2), P·V^γ = const
2→3 Constant-pressure heat addition (rc = V3/V2)
3→4 Isentropic expansion (power stroke, V4 = V1)
4→1 Constant-volume heat rejection (exhaust)
T2 = T1·r^(γ-1) P2 = P1·r^γ
T3 = T2·rc P3 = P2 (constant P)
T4 = T3·(rc/r)^(γ-1)
η = 1 − (1/γ)·(rc^γ − 1) / [r^(γ-1)·(rc − 1)]
The enclosed loop area on the P–V diagram is the net work done per cycle by definition of thermodynamic work (W = ∮P dV); this engine numerically integrates that same loop with the trapezoid rule alongside the closed-form efficiency formula above, so "Efficiency (∮P dV / Qin)" is an independent numerical cross-check of "Thermal efficiency (formula)" rather than a re-display of it — the two should track each other to within integration error.
- Compression ratio r — stretches the diagram's isentropic left leg steeper and pushes P2/T2 up, raising the loop's height and its enclosed area (efficiency).
- Cutoff ratio rc — widens the flat constant-pressure top segment (more injected fuel, more peak temperature/torque) but that segment does less work per unit heat than an equivalent constant-volume rise would, so a wider top trims efficiency even as peak pressure stays fixed at P2.
- MEP (mean effective pressure) is net work divided by displaced volume (V1−V2) — the constant pressure that, acting over one stroke, would deliver the same work as the whole cycle; it is the diagram's average loop height.