THE INDICATOR DRIVE, WHICH THROUGH THE CORD MOVES THE PAPER DRUM, REPRESENT THE STROKE OF THE POWER PISTON IN THE ENGINE CYLINDER. THE DRIVE IS USUALLY ACTUATED BY A CIRCULAR CAM FITTED ECCENTRICALLY ON THE CAMSHAFT IN WAY OF THE CORRESPONDING CYLINDER.
TO ENSURE THAT THIS CAM CAN PRODUCE A SIMULTANEOUS REPRESENTATION OF THE TRAVEL OF THE CORRESPONDING POWER PISTON, IT MUST MOVE THE PAPER DRUM IN SUCH A WAY THAT THE EXTREME POSITIONS ARE REACHED AT THE MOMENT THE POWER PISTON IS IN ITS TOP AND BOTTOM POSITION.
THIS IS CHECKED BY COMPRESSION DIAGRAM WHICH IS A WORK DIAGRAM TAKEN BY PUTTING THE FUEL PUMP OUT OF OPERATION AND THE COMPRESSION CURVE WHICH IS TRACED WHILE THE POWER PISTON MOVES UPWARD SHOULD NOW COINCIDE WITH THE EXPANSION CURVE WHICH IS TRACED WHILE THE POWER PISTON MOVES DOWNWARDS.
IF INDICATOR CAM IS INCORRECTLY POSITIONED, THESE TWO LINES WILL NOT COINCIDE BUT CIRCUMSCRIBE AN ARE AS SHOWN IN THE FIGURE. THIS ERROR IS CORRECTED BY TURNING THE INDICATOR CAM UNTIL NEW COMPRESSION DIAGRAM SHOWS THAT BOTH THE COMPRESSION AND EXPANSION CURVES ARE COINCIDING.
