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Engine Assessment

Indicator Diagrams: Out of Phase or Draw Cards



The conventional power card as used for measuring indicated power in the cylinder has its limitations. Because it is a pressure/volume diagram, the piston piston movement and therefore volume change is very small either side of Bottom Dead Centre and Top Dead Centre (i.e. each end of the diagram), and so what is happening at the end of compression and start of combustion is squeezed into a very small area.


In the example shown, which is a powercard for an engine of 2.5m stroke with a conrod length of 2.88m, the piston displacement per degree of crank angle is greatest at about 70 ATDC where the displacement is almost 24mm/degree. Compare this with the movement at 10 ATDC where the piston displacement is only 5mm/degree.


This means that because the indicator drum movement is in direct relation to the piston position, between about 50 and 90 of crank angle represents the largest change in volume on the diagram, and it is here that the drum on the engine indicator equipment is moving at its fastest. Note also that the mid point on the diagram is not at 90 because on an engine of this stroke and conrod length, the mid stroke is reached at about 77ATDC.



If the drive to the indicator drum is moved 90 out of phase with the piston position, then the drum will be moving at its fastest as the piston comes over TDC and BDC. This means that the pressure rise in the cylinder about TDC is shown over a wider horizontal axis and because of this, faults with timing and injection can be identified.


Instead of advancing the cam by 90, conditions in the cylinder around TDC can be assessed by rotating the drum by pulling on the cord at the the critical moment as the piston comes over TDC. This is known as a draw card.



As can be seen from the above diagrams, a draw card can show up faults such as early or late combustion of the fuel. This could be due to incorrect timing or because of varying ignition delays.


Other faults which can be seen are a leaking or blocked fuel injector and afterburning.



Not all engines are fitted with the equipment to take power cards. However, a draw card can be taken on any engine fitted with an indicator cock. A competent engineer, with practice, can take a draw card on an engine running at up to 450rpm



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