Understanding Indicator Diagram and Different Types of Indicator Diagram Deficiencies

Indicator diagrams are used to assess the performance of each unit of the main engine on a ship. It is based on the indicator diagram that the overall performance of the main engine is found out. Indicator diagrams are taken at regular intervals of time and matched with that of the ship’s sea trial diagrams to check if there is any major difference in performance. If there is any difference, it is important that the problem is rectified before starting the engine.

 Understanding Indication Diagram

Generally there are two types of indicator diagrams – one is power card and other is draw card. With the help of these two diagrams we can determine the compression pressure, peak pressure and the engine power.

High loading is to be prevented on main engine’s units or else it can lead to several problems such as bearing damage, cracking etc.It is therefore for this reason very important to read these diagrams properly as they provide several details about the cylinder working pressures and load.

In earlier days, the indicator diagram was taken with the help of mechanical indicator which was to be fitted on top of the indicator cocks.

Bur nowadays a pressure transducer is fitted on the indicator cocks with the help of which the indicator diagram can be taken at any moment and displayed in the computer.

How Can You Assess or Interpret Just by Looking at the Card Diagram.

The indicator diagram shown below is a normal diagram (Diagrams taken before the use of the engine) and the diagrams that are taken from the engine are taken and compared for deficiency.

Types of Deficiencies

We will take a look at some of the common deficiencies found in indicator diagrams.

Deficiency type 1

When the above diagram is compared with the normal diagram it can be seen that the compression pressure is normal and the maximum firing pressure is too high.

This can be due to early injection, a result of incorrect fuel timing of the cams, incorrect VIT setting, or leaking fuel injector.

Deficiency Type 2

In this diagram, it can be seen that the compression is same but the peak pressure is too low.

This effect can be a result of following factors:-

  • Bad quality of fuel.
  • Fuel injector nozzle blocked.
  • Fuel pumps leaking.
  • Low fuel pressure.
  • Injector seized.

Deficiency Type 3

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

This can be due to the

  • Leaking exhaust valve.
  • Leak through piston rings i.e broken or worn out piston rings.
  • High Liner wear.
  • Burnt piston crown.
  • Low scavenge pressure.

Deficiency Type 4

This 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.


Operation and maintenance of machinery In motorship By N.E Chell

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