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Engine Assessment

Indicator Diagrams: Computer Generated Diagrams



Indicator diagrams taken using mechanical equipment are limited by the speed of the engine and the skill of the operator. Normally it is not possible to obtain an accurate powercard on engines running above 120rpm because of vibrations in the equipment. Draw cards are reliant on the operators skill. It is possible to take drawcards on engines running up to about 450 rpm.


Modern methods allow the computer to generate indicator diagrams and calculate the power developed in the cylinder for engines running at any speed.


The equipment consists of a pressure transducer which screws onto the indicator cock which is connected to the processing unit. A crankshaft position detector is also fitted to the propshaft or engine flywheel which measures the crank angle to 0.25. The crankshaft position is also fed back to the processing unit.


The indicator cock is opened and when ready the information is stored to the processing unit. When all the cylinder readings have been completed, the information is downloaded to a computer and the results reviewed. The computer will draw a powercard and a crank angle diagram. It is possible to view the cylinder diagrams individually or overlay them on top of each other (each cylinder being represented by a different colour).





From the crank angle diagram, faults in the injection process can be determined:



It should be noted that some slow speed two strokes (MAN B&W MC engines) combustion is delayed until 2-3 after TDC to limit the pressure rise to 35 bar, to prevent overloading of the crosshead bearing. This will give a slight dip in the pressure after TDC, which should not be construed as a fault.





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