Marine Engineering Study Materials

How to Draw Indicator Diagrams

The engine indicator or indicator is employed for taking indicator diagrams, whereby the combustion chamber pressures can be measured while the engine is running.

The draw diagram or draw card is used for measuring the compression pressure and maximum pressure, and for evaluating the ignition characteristics of the fuel oil.

The indicator diagram (pv diagram or power card or work diagram), illustrates the pressure variations in the engine cylinder as a function of the main piston position. The diagram area can be integrated by means of a planimeter, and the mean indicated pressure calculated. The power developed in the particular cylinder can then be found by multiplication by the engine speed and the cylinder constant.

See: Calculation of Shaft Power in Marine Engines on Ships

In order to ensure true indicator/draw diagrams, and correct evaluation of data, the following instructions should be followed in detail.

Maintenance of the Indicator

Friction in the indicator piston movement, as well as slackness in the stylus (writing) mechanism, will distort both the shape and the area of the diagram. Test and maintain the indicator in the following way:

Friction and tightness of piston

1. Remove the indicator spring.
2. Dismantle the upper part of the indicator, and remove the piston from the cylinder.
3. Wipe the piston and cylinder with a clean cloth.
4. Mount the upper part again.
5. Hold the indicator upright.
6. Pull the piston to the upper position.
7. Block the bottom of the cylinder with a finger.
8. Check that the piston fits so tightly that it remains in the upper position.
9. Push the piston downwards and release.
10. Check that the piston springs back to the upper position.
11. Tighten the top screw, which retains the spring, firmly against the ball-head of the spring.
12. Check that the ball is not loose on the spring (older spring types).
13. Check that the coils of the spring have not worked loose at the soldered joint in the base.
14. During mounting, check that the piston sinks slowly down the liner, by its own weight, when the cylinder is held vertically.
15. Hold the indicator upright.
16. Pull the piston to the upper position.
17. Block the bottom of the cylinder with a finger.
18. Check that the piston fits so tightly that it remains in the upper position.
19. Push the piston downwards and release.
20. Check that the piston springs back to the upper position.
21. Tighten the top screw, which retains the spring, firmly against the ball-head of the spring.
22. Check that the ball is not loose on the spring (older spring types).
23. Check that the coils of the spring have not worked loose at the soldered joint in the base.

Stylus (writing) mechanism

1. Check that the stylus is sharp.
2. Check for slackness in the writing mechanism.
3. Replace any worn parts.
4. Adjust the stylus so that, with a light writing pressure, a single passage over the paper can just be seen.
5. To obtain sufficiently distinct work diagrams, trace the diagram two or three times.
6. Lubricate the mechanism with thin oil.

Indicator Valve

During the running of the engine, soot and oil will accumulate in the indicator bore. Clean the bore by opening the indicator valve for a moment. To protect the valve against burning:

• Open the valve only partially
• Close the valve after one or two ignitions.

Fitting the Indicator

1. Dismantle the upper part.
2. Give the piston a little cylinder oil.
3. Check that the various recesses are clean.
4. Otherwise the parts could be positioned askew, and this would cause the piston to move sluggishly in the cylinder.
5. Mount the upper part.
6. Fit the indicator and the cord.
7. Engage the indicator drive.
8. Check the cord alignment.
9. Adjust the length of the indicator cord so that the diagram is traced in the centre of the paper and the cord is tight in all positions.

Taking the Diagrams

Atmospheric line

1. Keep the indicator valve closed.
2. Press the stylus against the paper.
3. Release the stylus when the indicator drive has turned the drum one or two times.

Indicator diagram or Power Card

1. Attach the cord to the indicator drive.
2. Open the indicator valve.
3. Press the stylus against the paper.
4. Release the stylus, when the drum has turned two or three times.
5. Close the indicator valve.

Draw diagram or Draw Card

1. Release the cord from the indicator drive.
2. Open the indicator valve.
3. Watch the movement of the stylus.
4. At the moment it moves upwards, simultaneously; Press it against the paper and Pull the cord just quickly enough for the stylus to trace the compression and ignition sequence.
5. This operation requires some practice to ensure that both compression and maximum pressures are clearly recorded.
6. Close the indicator valve.
7. Remove the indicator from engine and repeat the procedure for all units.
8. Check that the diagrams have been correctly taken and are distinct.
9. If the indicator quickly becomes very hot, and the piston is black after use, then this means that there is a leakage. In such a case, exchange the piston and liner.
10. Lubricate the piston with a drop of cylinder oil after about six diagrams have been taken.
11. When diagram taking is finished, unscrew the indicator head.
12. Clean and lubricate both the cylinder and the piston with cylinder oil.

Diagram Faults

The most common faults are shown below.

• Vibrations in the cord, or drive, give a wavy indicator diagram, but a smooth draw diagram.

• The drum hits the stop at one of the end points, before the diagram is completed: The cord is too long or too short.

• The indicator piston works sluggishly in the cylinder, and moves in jerks: If only the expansion curve is wrong (wavy), the cause may be gas pulsations in the combustion chamber or indicator bore.

• The indicator spring is too weak. The piston strikes against the top of the indicator cylinder. Change to a more rigid spring.
• The indicator valve leaks: Gives an untrue atmospheric line.

Calculation of Area of Indicator Diagram with Planimeter

1. Position the indicator as illustrated below.
2. Mark a starting point on the expansion line (to exactly localize the start/stop position of the diagram circumscription).
3. Begin tracing from the starting point, and circumscribe the diagram at even speed, and not too slowly.
4. Employ good illumination and, if necessary, use a magnifying glass for accurate reading of the vernier, before and after circumscription.
5. The difference in readings is proportional to the area of the indicator diagram.

NB: The calibration should be checked by measuring a known area, e.g. a precisely drawn rectangle.

 

Reference:

Instruction Book ‘Operation’ for 50-108MC/MC-C Engines