Tag Archives: Indicator Diagrams

Indicator Diagrams

  • The indicator diagram is very important to know the combustion in the cylinder and also to adjust the engine.
  • The diagram is taken periodically from the indicator valve equipped on he cylinder head and combustion condition is to be confirmed.
  • The compression pressure and maximum pressure in the cylinder can be presumed from the indicator diagram.
  • Engine indicator is the device used to take the indicator diagram, which can be considered as a ‘stethoscope’ for diesel engines.
  • Indicator diagrams give efficiency of combustion in the cylinder, condition of the running gear, irregularities in fuel pumping and injection and a lot of things.


Pcom – Compression Pressure

Pmax – Maximum Pressure

There are 4 types of indicator diagrams that can be taken from the engine cylinder to know the condition and performance of the engine.

  1. Power card / Power indicator diagram
  2. Compression diagram
  3. Draw card / Out of phase diagram
  4. Light spring diagram

The area of indicator diagram is calculated by

  • Planimeter (Usually used on board)
  • Mid Ordinate Method

Power Card

  • Power card is taken with the indicator drum rotating in phase with the piston movement
  • The area within this diagram represents the work done during one complete cycle to scale
  • Mean Indicated Pressure (MIP) is obtained from this diagram to calculate power produced in the cylinder


Compression Diagram

  • Compression diagram is taken in similar manner to the power card but the fuel shut off in the cylinder
  • The height of this curve shows maximum compression pressure
  • If the compression and expansion line coincide, it indicates that indicator is correctly synchronized with the engine
  • Reduction in height of this diagram shows low compression which may be due to worn cylinder liner, faulty piston rings, insufficient scavenge air or leaky exhaust valve


Draw Card / Out of Phase Diagram

  • Draw card is taken in a similar manner to power card with fuel pump engaged but with the indicator drum 90 degree out of phase with the piston stroke
  • This diagram illustrates more clearly the pressure changes during fuel combustion. Fuel timings or injector faults may be detected from its shape


Light Spring Diagram

  • Light spring diagram is taken similar to the power card and in phase with the engine and with a light compression spring fitted to the indicator
  • This diagram shows pressure changes during exhaust and scavenge to an enlarged scale
  • It can be used to detect faults in these operations