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 he 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

Power Card
Power Card

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

Compression Diagram
Compression Diagram

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

Draw Card
Draw Card

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

Light Spring Diagram
Light Spring Diagram

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Comments 5 comments

ankit 5 years ago

why draw card is 90 degree out of phase diagram n how is it indicated ??

Tmr  5 years ago

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

>> How to confirm or know it is 90 deg out of phase with piston stroke as it string is pulled by hand, Experience? or else?

TMR 5 years ago

jabelufiroz profile image

jabelufiroz 5 years ago from India Author

First of all indicator cards are not taken by pulling with hands; there is a special cam which can be engaged with the engine indicator and can be adjusted for any degree of offset.

Second, indicator cards can also be taken with great accuracy by experience.

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