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Engine Assessment

The Light Spring Diagram



The light spring diagram is taken on a two stroke engine with the indicator drum in phase with the engine piston, but with the indicator spring replaced with a much lighter spring. It enables conditions to be examined around BDC when the exhaust valve opens and scavenging of the cylinder is taking place.


This means that as the piston comes up on compression, the spring will be fully compressed against the limiting stop at about 10 bar. The compression line will then stay horizontal as compression pressure increases in the cylinder.


During expansion the line remains horizontal until the exhaust valve opens (or the exhaust ports are uncovered), when the pressure will drop in the cylinder, and the spring will start to expand.


As the scavenge ports open the pressure rises slightly and the remaining exhaust gas is scavenged from the cylinder.









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