THE INDICATOR DIAGRAM, WHICH DESCRIBE THE PRESSURE VARIATIONS IN THE ENGINE CYLINDER, SERVES, BY PLANIMETERING OF THE WORK DIAGRAM, TO CALCULATE THE MEAN PRESSURE, AND BY MULTIPLICATION BY ENGINE SPEED AND CYLINDER CONSTANT, TO CALCULATE THE POWER DEVELOPED IN THE PARTICULAR CYLINDER. ALSO THE COMPRESSION AND MAXIMUM PRESSURES WHICH ARE MEASURED ON THE DRAW DIAGRAM, ARE OF IMPORTANCE.
THE INDICATOR SHOULD BE FASTENED TO THE VALVE IN SUCH A POSITION THAT THE INDICATOR CORD BECOMES PARALLEL WITH THE MOVEMENT OF THE INDICATOR DRIVE TO WHICH THE HOOK OF THE CORD IS FASTENED. IF THE CORD IS NOT OBSERVED, SO THAT THE CORD IS MISALIGNED, THE MOVEMENT OF THE PAPER DRUM WILL BE UNTRUE AND THE DIAGRAM TOO SHORT. THE LENGTH OF THE INDICATOR CORD MUST BE SO ADJUSTED THAT THE DIAGRAM IS TRACED IN THE CENTRE OF THE PAPER AND SO THAT THE CORD IS TIGHT IN ALL POSITION.
OPERATION :-
- WITH THE INDICATOR VALVE CLOSED, TRACE THE ATMOSPHERIC LINE BY PRESSING THE STYLUS AGAINST THE PAPER WHILE THE DRUM TURNS 2 OR 3 TIMES.
- OPEN THE INDICATOR VALVE. PRESS THE PENCIL AGAINST THE PAPER WHILE THE DRUM TURNS 2 OR 3 TIMES. IN THIS WAY THE WORK DIAGRAM IS TRACED. CLOSE THE INDICATOR VALVE.
- RELEASE THE CORD FROM THE INDICATOR DRIVE ON THE ENGINE. OPEN THE INDICATOR VALVE. WATCH THE MOVEMENT OF THE PENCIL AND PRESS IT AGAINST THE PAPER AT THE MOMENT IT MOVES UPWARD. AT THE SAME TIME, PULL THE INDICATOR CORD SO QUICKLY THAT THE PENCIL TRACES THE COMPRESSION AND IGNITION SEQUENCE IN THE FORM OF THE SO CALLED DRAW DIAGRAM. THIS OPERATION REQUIRES SOME PRACTICE TO ENSURE THAT BOTH COMPRESSION AND MAXIMUM PRESSURES CAN BE MEASURED.