Indicator Cards, Engine Performance & Power Calculation

What is Indicator Card and what is the purpose of these cards?

Indicator Card: It is a curve taken from each unit of the engine to check the performance of unit by matching it with that of the ship’s sea trial performance curve. It shows pressure inside the cylinder with respect to the position of the piston and for each unit, there is one indicator card. On X-axis position of the piston and on Y-axis pressure inside the cylinder is shown.

Indicator card
Indicator Diagram

In above image, both compression card/draw card are shown in detail.

Following explains the purpose of Indicator Card:

  • To enable the evaluation of the power developed in each engine cylinder.
  • To highlight conditions during fuel injection, combustion and after burning.
  • To highlight conditions of scavenging/exhaust gas exchange process.
  • To show the pressure variations in the cylinder with respect to piston displacement.

Describe things or points you check before taking Indicator card?

These are points to check before taking Indicator cards:

  • The propeller of the ship should be fully immersed in water.
  • The ship should be on the even keel and preferably fully laden.
  • There are no strong winds present either against or in favor of the direction of motion of the ship. If winds are present the engine will either produce more power or will produce lesser power for the given speed because of the wind resistance or aid, as the case may be.
  • The hull is not fouled by an undergrowth.
  • Ensure the ship, and its engine is running at a constant speed in open sea.
  • Ensure the weather is calm.

What are the preparations you do for taking Indicator cards?

  • Take proper PPE, especially high-temperature gloves and eye protection.
  • Check the battery of Data Acquisition unit and change/ charge if needed.
  • Check and prepare the Digital Pressure indicator instrument and check all the wires/sensors are visually ok.
  • Use proper tools supplied with the instrument to open and close the cocks.

Describe the procedure of taking indicator cards using the electronic/digital tool?

  • Connect the probe from the incremental encoder to data acquisition unit.
  • Connect the pressure transducer probe to handheld data acquisition unit.
  • Carefully open the indicator cock of the cylinder for few seconds and blow out the cylinder. It is done to remove any stuck impurity (soot and other combustion particles) inside the cock.
  • Fix the pressure transducer unit on indicator cock and open the cock  to register the cylinder data
  • Repeat the procedure for all the cylinders.
  • After finishing the process, disconnect the pressure transducer probe and keep it aside for cooling it down.
  • Disconnect the incremental encoder probe from handheld data acquisition unit.
  • Fill the required data in the Digital Pressure Indicator software and wait for the result to be generated.

Describe the precaution you take before you taking Indicator card diagram using mechanical indicator card instruments?

  • To reduce the effect of friction the indicator card piston is removed, the piston and the cylinder are cleaned and lubricated.
  • 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.
  • The drum should not hit the stop at the end position. Play in the pencil mechanism will distort he diagram.
  • The cock should be clean so that all the pressure is made available to the piston of the engine indicator
  • The stylus should be adjusted to a light writing pressure.
  • 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.

Describe the procedure of taking Indicator card diagram using mechanical indicator card instruments?

  • Take proper PPE, especially high-temperature gloves and eye protection
  • Take the reading of all the relevant engine parameters
  • Ensure the ship, and its engine is running at a constant speed in open sea
  • ensure the weather is calm
  • Use correct tool to open the indicator cock valve
  • Take the paper provided with the instrument and fix it firmly over the drum
  • Carefully open the indicator cock of the cylinder for few seconds and blow out the cylinder. It is done to remove any stuck impurity (soot and other combustion particles) inside the cock.
  • Fix the instrument on the indicator cock so that the cord is firm.
  • Draw the atmospheric line with the cock shut
  • Slowly open the indicator cock and press the stylus against the paper lightly. Make straight vertical lines as the piston moves up and down and then pull the roller string, till the cycle is drawn on the paper
  • Close the indicator cock and remove the instrument
  • Ensure the tool does not get exposed to high temperature for an extended period as its mechanical parts like springs, the stylus will respond differently and may affect the accuracy

Describe the different types of indicator cards?

POWER CARD (In-Phase):-

Indicator Diagram
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.
  • It’s also highlight after burning.





Indicator Card Draw
Draw Card


  • Draw card is taken in a similar manner to power card with fuel pump engaged but with the indicator drum 90 degrees 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





Indicator Card Compression
Compression Card
  • Compression diagram is taken in a 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.
  • Reductions 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



Light Spring Card


  • 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


Describe the method of power calculation from indicator card?

For measuring the indicated power of diesel engine, we need to calculate Indicated Power of each unit. The area of power card is measured by one of these methods, whichever is available:

  • The area can be measured by an instrument known as ‘Planimeter’ or by the use of the mid ordinates rule.
  • Or with the help of superimposing graph(old method).
  • On modern engines, this diagram can be continuously taken by employing two transducers, one pressure transducer in the combustion space and other transducers on the shaft. Through the computer, we can thus get online 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.

What are the readings you will take for power calculation?

For calculating the power of the engine, we will increase the speed of the engine to the rated RPM. After some time(generally 15 to 20 minutes) take fuel oil, lube oil flow counters reading. Start taking Power Cards of all unit one by one and other reading mentioned below. After one hour again take counter readings of the flow meter, with this flow meter reading we can calculate fuel oil consumption for one hour and further SFOC. Other readings are:

  • Engine R.P.M.
  • Fuel pump index for each unit.
  • Governor index for each unit.
  • Load index.
  • Exhaust temp and pressure for each unit.
  • JCW outlet temp, piston cooling oil, cooling water outlet temp for each unit..
  • Scavenge temp and pressure.
  • Turbocharger R.P.M / exhaust gas back pressure.
  • Differential pressure in the air cooler.
  • Air filter temp of air before filter.
  • Ship draught
  • Spring rating.

Calculation of power of Main Engine:

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 × k bar/mm;

Where k = spring scale in bar per mm

Therefore Mean Indicator Pressure Pm = k×A/L bar

Please note that till now unit of length we were taking as millimeter and pressure as bar, But for further calculation, the unit of length should be taken as meter and unit of pressure will be taken as Pascal. If you don’t take unit as pascal you have to multiple correction factors explained below.

And the relation is 1 bar = 10Pascal (SI unit of pressure)

Effective force during one cycle = Mean Indicated Pressure x Area of the Piston

    = PmA  bar/mm3          {But pressure*area( bar/m×m2 )= Force (N)}

            = PmA Newton

But for N strokes/second & having Length of stroke L, Distance covered = NL m

Therefore Power(work is done in unit time) for one unit will be, Power P= Force×Distance

  = PmA×NL  or PmLAN   Watts

To obtain the power of all units, for one unit, Indicated Power P =PmLAN

Please remember this is the Indicated Power of one unit of Main Engine.

Therefore Total Indicated Power of the main engine

=Sum of Indicated Power of all units

=(PmLAN)1 +(PmLAN)2 +…………+(PmLAN)n  

for the engine having n cylinder units And Indicated Power of Unit = Pm L A N

        Please do not misunderstand Indicated power of one unit and Indicated Power of all units. Mean effective pressure for each unit is different and L, A, & N are same for each unit.

Conversion to KiloWatts(KW)

Indicated Power = Pm L A N

= Pm (bar) x L (m) x A (m2) x N (1/s)

= (bar) x (m) x (m2) x (1/s)

= 105 N/mx m x m2 x 1/s   (1 bar = 105 Pascal = 105 N/m)

= 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. Because this calculation we have done with unit taken as bar as unit of pressure (generally pressure gauges are in bar). Further indicated power is described in KW(Kilowatts). Therefore convert it into KW  with conversion factor,  1 KW = 1000 W.

 Feature Image by :

Xtrememachineuk |Public Domain | Source: Wikimedia

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Abhay Kumar

He is working as engineer in Shipping Corporation Of India, member of Nielsons Educational Forum. He is alumni of Marine Engineering & Research Institute(MERI) Mumbai. He loves driving and writing blog in his free time. Specialty : Tanker ship

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Mayank singh
Mayank singh

Why there is notch in light spring diagram?

Pratham Shukla

When the piston is coming down, it uncovers the scavenge ports. There is a pressure difference between scavenge space and volume above the piston, hence this sudden opening causes pressure fluctuation. This notch is showing the same.