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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 77°ATDC.
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.
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
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