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.

The area of indicator diagram is calculated by

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 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 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 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