Indicator diagrams indicates, simultaneously , the pressures and the relative position of the piston for a particular engine cylinder. They are taken for every voyage, both in loaded and ballast condition. It is a practice to take indicator cards after every major overhauls.

Purpose

• To calculate indicated power of the engine
• To determine peak pressures and compression pressures
• To evaluate the process of combustion inside the engine
• To evaluate scavenging and exhausting conditions

Procedure

• Check whether the spring fitted on the indicated instrument will meet the peak pressure (maximum combustion pressure) to be expected
• Stretch diagram paper firmly over the drum.
• Before taking diagram, open indicator cock, allow two or three firing strokes, to blow out soot and combustion residues in the cock.
• After drawing atmospheric pressure line, hook the cord to indicator drive, open indicator cock, take power card and shut off the cock.
• 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 from 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.

Types of Indicator Diagrams

See: Types of Indicator Diagrams

Calculate Indicated Power

See: How to Calculate Indicated Power

Evaluation of Indicator Diagrams

Power Diagram

• To calculate indicated power, also gives peak pressure.

Out of Phase Diagram

• Required for accurate determination of the compression pressure.
• To evaluate peak pressure (more accurately), compression pressure and ignition delay period.
• It shows combustion process (early or late combustion).
• To evaluate injection, fuel quality, ignition delay, combustion, loss of compression, expansion, fuel pump timing and after burning.

Troubleshooting with Indicator Diagrams

See: How to Troubleshoot with Indicator Cards

Precautions While Taking Indicator Cards

• Always lubricate indicator instrument parts to prevent seizure at high temperatures.
• Tightness of the indicator piston inside the cylinder to be checked. It should be a free sliding fit.
• The cock to be free from accumulated carbon particles.
• Do not apply much pressure on the stylus while taking the diagram.
• Allow sufficient cooling time for the instrument after taking diagrams from each units.

 

Reference

“Reference of Marine Engineering Knowledge for MOT 2nd Class Part B (Oral)” by Min Zar Tar