Marine Engineering - Motor

Indicator Diagrams

Indicator Diagrams, Power card, Draw card, Power calculation

Indicator Diagrams, Power card, Draw card, Power calculation


1)  The propeller is fully immersed.

2) The ship is fully laden at an even keel.

3) The ship is running neither with nor against a head wind or heavy sea.

4) The hull is not fouled by an under growth.

If the ship is lightly laden with the propeller partly immersed or if the ship is running with a favour of wind or sea the engine does not develop its full power than normal at same speed. Ship may be under exploited or overloaded…all these condition the diagram will be deceiving.



2) Spring rating.

3) Engine R.P.M.

4) fuel pump index and governor index.

5) Load index.

6) Exhaust temp and pressure.

7) JCW outlet temp, piston cooling oil, cooling water outlet temp.

8) Scavenge temp and pressure.

9) Turbocharger R.P.M / exhaust gas back pressure.

10) Differential pressure in air cooler.

11) Air filter temp of air before filter.

12) Fuel consumption (Flow meter reading before and after on a time bound), cylinder oil consumption.

13) Ship draught

Precaution to be taken-

1) To reduce effect of friction the indicator card piston is removed, the piston and the cylinder is cleaned and lubricated.

2) The tightness of the indicator piston in the cylinder is checked. This is done by dismantling the piston and allowing it to drop slowly through the cylinder by its own weight. The bottom of the cylinder is closed and the piston should be held in its position.

3) The drum should not hit the stop at end position. Play in the pencil mechanism will distort he diagram.

4) The cock should be free from accumulation of soot and oil.

5) The stylus should be adjusted to a light writing pressure.

6) The drum when actuated the motion of the power piston by a cam should be correctly position. It then represents the travel of the power piston. In that case the compression and expansion diagrams with the fuel cut off should coincide.

Types of Indicator cards

The taking of indicator cards, allows the engineer to receive more information about the combustion process (via the draw or out of phase card), measure the cylinder power output of the engine (via the power cards), and check the cleanliness of the scavenging process (via the light spring diagram).

There are five types of indicator cards, which can provide the following information:

  1. Power or in-phase cards

Power or in-phase cards

  • Cylinder power, calculated from the area within the P~V diagram
  • Indicates after burning present when card shape enlarged during the expansion stroke


  1. Draw or out-of-phase cards

Draw card

  • Measurement of the compression pressure
  • Measurement of the point of fuel ignition
  1. Light/weak spring card

Light-weak spring card

  • Fouling of exhaust or scavenge gas flows
  1. Compression card

Compression card

– Measurement of the compression pressure

– Timing check of the indicator cam

  1. Pressure derivative

Pressure derivative card

  • Measurement of the point of fuel ignition
  • Measurement of the maximum rate of pressure rise following initial combustion



  1. Take proper PPE protections like gloves, goggles, safety shoes and proper outfit.
  2. Use proper tools supplied with the instrument to open and close the cocks.
  3. Stretch diagram paper firmly over the drum.
  4. Before connecting the instrument to the Indicator cock, open indicator cock, allow two or three firing strokes, to blow out soot and combustion residues in the cock.
  5. fasten the instrument to the cock so that the cord is taut.
  6. Take the atmospheric line with the cock shut.
  7. Open the indicator cock and lightly press the stylus against the paper and make vertical lines as the piston moves up and down, when you are ready to pull the roller string, make sure it is in the bottom and pull the string till the cycle is described.
  8. Close the cock
  9. Disconnect the cord.
  10. make sure that the instrument is not exposed to high temperature for long time. This may affect its accuracy. Remove the hook, turn the drum by hand to a place clear form for the power diagram, take compression pressure line with the fuel cut off.

After taking indicator cards from all the cylinders, open the instrument and clean all the parts, and lubricate the same.

Analysis of the cards

Normal Diagram

Normal draw card

Maximum firing pressure is too high


High firing pressure

– Compression pressure is very normal but the maximum firing pressure is too high compared to the normal diagram.

– The causes could be due to Early Injection; which is caused by

            + Incorrect fuel timing of the cams.

            + Incorrect VIT setting

            + Leaky fuel injector

Peak pressure is too low.

max firing pressure 

Below diagram shows; the compression is same but the peak pressure is too low.

This is due to late injection caused by the following factors:-

+Bad quality of fuel.

+Fuel injector nozzle blocked.

+Fuel pumps leaking.

+Low fuel pressure.

+Injector seized.

Compression pressure is low, and the peak pressure is also too low

High compression and peak pressure

Below deficiency diagram shows diagram shows that the compression pressure is low, and the peak pressure is also too low.

This can be due to the following reasons;

+ Leaking exhaust valve.

+ Leak through piston rings i.e broken or worn out piston rings.

+ High Liner wear.

+ Burnt piston crown.

+ Low scavenge pressure.

High compression pressure

Low compression pressure

Below diagram shows high compression pressure together with high peak pressure.

This can be as a result of the following:

+ Exhaust valve opening too late i.e incorrect exhaust valve timing.

+ Overload of the engine.

Power calculations:


Measuring indicated power of diesel engine

– The area can be measured by an instrument known as ‘Planimeter’ or by the use of the mid ordinates rule. [On modern engines this diagram can be continuously taken by employing two transducers, one pressure transducer in the combustion space and other transducer on the shaft. Through the computer we can thus get on line indicated diagram and power of all cylinders.]

– The area is then divided by the length of the diagram in order to obtain mean height. This mean height, when multiplied by the spring scale of the indicator mechanism, gives the indicated mean effective pressure for the cylinder. The mean effective or average pressure [Pm] can now be used to determine the work done in the cylinder.


Power card1

Area of the indicator diagram = a [mm2]

Length of the diagram = l mm

Average height of the diagram = a [mm2] / l [mm]

Average mean indicator pressure = a [mm2] / l [mm] X k [bar / mm]; Where k = spring scale in bar per mm


Pm = ( a / l) x k [bar]

Work done in one cycle = Mean Indicated Pressure x Area of the Piston x Length of stroke

= [Pm] x [a] x [l]

To obtain the power of this unit, it is necessary to determine the rate at which work is done.

Now, Indicated Power of Unit [ip] =Mean Indicated Pressure [Pm] x Area of Piston [A] x Length of Stroke [L] x Number of Power Strokes per Second [N]


Indicated Power of Unit = Pm L A N

Unit of Final Result

Indicated Power = Pm L A N

= (A / L) X k [bar] x L [m] x A [m2] x N [1/s]

= [bar] x [m] x [m2] x [1/s]

= 105 N/m2 x m x m2 x 1/s

= 105 Nm/s

= 105 Joules/s

= 105 Watts

Hence, multiply the result obtained from calculating indicated power with 105 and the final unit will be in Watts.


About Ram Govindasamy

Ram Govindasamy is a sailing marine engineer working for a leading cruise company. Ram founded Dieselship to create an online platform for Marine engineers worldwide to bond, share knowledge & Resources. He is a computer enthusiast who loves creating web based activities, web pages & programs. He is also authoring technical articles, videos for Dieselship as well as for various other maritime websites. Ram is interested in Maritime Law and Technical Operations and looks forward to meeting new people especially those who are interested in creating web based platforms, Assets maintenance & inventory programs and Planned Maintenance software etc

12 thoughts on “Indicator Diagrams

  1. surender singh says:

    Respected sir,
    Good day.
    Sir , while explaining high compression pressure ,high max. Pressure and low compression pressure ,low max pressures, you have put diagrams wrongly.
    Which should be for low compression has been put for high and same for high compression pressure explanation.
    Kindly correct it . except this it all is explained pretty clear .
    Thank you

  2. Ram Govindasamy says:

    Thank you Surender, we have amended the change. DTP error, corrected now. Thank you for your valuable feedback.

  3. Pyle Limos says:

    Sir can you give me the formula for finding the area of the indicator diagram using only the mean height and length .

  4. Ashwini kumar says:

    I would like to know about the indicator cord , where it is connected,no idea at all.

  5. harsh bhardwaj says:

    how to derive Indicated Power [P] = Mean Indicated Pressure [Pm] x Area of Piston [A] x Length of Stroke [L] x Number of Power Strokes per Second [N] OR Indicated Power = Pm L A N

    1. Ram Govindasamy says:

      Very simple,

      It can be explained with basic physics..
      Work done= Force X Displacement

      Pressure = Force / Area
      If you apply this you will be able to explain Pm L A N

  6. mohit says:

    can you tell me the name of the instrument by which we are taking performance.

  7. Karthikeyan says:

    Hello Mr.Ram Govindasamy,
    i knew, what is BMEP & IMEP, but i can’t get clear difference both of them in practical manner( if BMEP mean avg pressure in top to bottom moving piston in power stroke , it is use in BP ). Can you explain both of them in practically. Also if you know about the any relation between them , kindly share me…
    thank you….

  8. kunal says:

    hello sir can u tell me how to check the spring incorporated in instrument is correct or not?

    1. Ram Govindasamy says:

      Dear kunal,

      The spring free length is checked and matched with recommended length.
      Other than that usual visual inspection, sound testing is done to ascertain the condition of the spring. I request the reader to comment if there is any other specific method. Thanks


    Respected Sir,
    More questions I am facing which are related with all functions MEK M, MEK G, MEP, MEP, SSEP, N/A, & MET, comprise written and oral of my present and upcoming, II Class Part B , MMD KOLKATA, Examination.
    In this regards getting maximum help/ feed back from your DIESEL SHIP study materials,that’s why I am highly grateful to you. But for my present and upcoming confused questions, need your more help feed back, thruogh e-mail correspondences.
    Thanks with best regards to you Sir,


    in your all card diagrams , you are not illustrating clearly by the arrow marks, i.e. actual direction of compression and expanssion cycles.
    I think, the Compression card is not applicable for large 2 stroke engine, it is only applicable for 4 stroke auxiliary engine.

    Same, the Pressure derivative card is not applicable for large 2 stroke engine, it is only applicable for 4 stroke auxiliary engine.
    Hope you will clear up my confussions in this regards.

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