Indicator diagram (PV)

An indicator diagram is the closed pressure-volume loop traced over one cylinder cycle, with cylinder pressure on the vertical axis and swept volume on the horizontal. The area enclosed by the loop equals the work done per cycle, so the diagram is the direct route to indicated power. Traditionally drawn by a mechanical indicator on the indicator cock, it is now captured electronically as a pressure-versus-crank-angle trace and replotted against volume. Engineers read peak firing pressure, compression pressure, and the timing of combustion from it to diagnose injection timing, fouled exhaust valves, leaking rings, or a worn liner. The PV diagram is the basis for indicated mean effective pressure and balance between cylinders.