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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
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