Marine Engine Q & A - Indicator card

With reference to engine performance monitoring:

explain how it is determined that a faulty indicator is the reason for diagram deformation rather than an engine fault.
describe the ways in which an indicator can give a false diagram.
describe how engine performance may be assessed when the use of an indicator is not suitable.

a.





Power Card From 2 Stroke Engine
A typical indicator diagram from an engine in good operating condition is shown opposite. Note that the curves are smooth, and that maximum pressure occurs just after TDC.
The length of the atmospheric line is the same as the length of the diagram which is in direct proportion to the length of the piston stroke.


Indicator diagrams are taken to compare power outputs of the engine cylinders and to help in cylinder and combustion diagnostics. They are not viewed in isolation, but as part of an overall picture which includes exhaust temperatures, fuel rack settings and scavenge pressures. Should a diagram indicate a low power for a particular cylinder, whilst the other readings are as they should be, and the draw card does not show any faults with combustion, then it would be sensible to check the indicator equipment before proceeding further.






Draw card (out of phase diagram)
If the diagram is jagged throughout the whole diagram then this would be an indication of a fault with the equipment. However, if the diagram is jagged around TDC then this is indicative of a fuel injection fault which can be confirmed and analysed by taking an out of phase card. If the diagram has a flat top, this is again is showing a fault with the equipment.

b.

Indicator cam out of phase with engine. This can be checked by taking a compression card where the compression and expansion lines should coincide.

Choked indicator cock. These should be blown through before taking cards.

Leaking indicator cock. Detected by hearing.

Sticky indicator piston. Should not happen with well maintained piece of equipment. Will give jagged diagram.

Wrong spring fitted. Diagram will have a flat top (weak spring) or be to short (strong spring: note this will only affect outcome if wrong spring constant is used)

Indicator cord wrong length. Drum will not rotate fully during engine cycle.

c.

Power cards using mechanical equipment previously discussed is only accurate on engines operating at low revs (ie slow speed engines). They are not normally attempted with medium or high speed engines because of the higher speeds causing vibrations in springs and drive mechanisms.

In these cases, because power in the engine is related to the peak or maximum cylinder pressure, this is measured using a peak pressure indicator. This can be a mechanical device, using compression of a spring to indicate pressure or a pressure transducer linked to a digital read out.

It is more accurate to use the difference between the peak and the compression pressure when assessing cylinder powers and attempting to balance the engine. The compression pressure is taken at about 80% MCR with fuel shut off the cylinder.

In addition the other cylinder parameters must be taken into account: Exhaust temperatures, fuel pump rack setting, scavenge pressure and temperature are the most usual. It should be noted that if this method is used as a basis for engine balancing, then it must be known that the fuel injection equipment is in good condition and correctly timed.

Modern computer equipment now allows power and out of phase diagrams to be taken for engines operating at any speed.