Indicator Diagrams
- The indicator diagram is very important to know the combustion in the cylinder and also to adjust the engine.
- The diagram is taken periodically from the indicator valve equipped on he cylinder head and combustion condition is to be confirmed.
- The compression pressure and maximum pressure in the cylinder can be presumed from the indicator diagram.
- Engine indicator is the device used to take the indicator diagram, which can be considered as a ‘stethoscope’ for diesel engines.
- Indicator diagrams give efficiency of combustion in the cylinder, condition of the running gear, irregularities in fuel pumping and injection and a lot of things.
Pcom – Compression Pressure
Pmax – Maximum Pressure
There are 4 types of indicator diagrams that can be taken from the engine cylinder to know the condition and performance of the engine.
- Power card / Power indicator diagram
- Compression diagram
- Draw card / Out of phase diagram
- Light spring diagram
The area of indicator diagram is calculated by
- Planimeter (Usually used on board)
- Mid Ordinate Method
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
Compression Diagram
- Compression diagram is taken in 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
- Reduction 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 valve
Draw Card / Out of Phase Diagram
- Draw card is taken in a similar manner to power card with fuel pump engaged but with the indicator drum 90 degree 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
Light Spring Diagram
- 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
