What is meant by open period of a two stroke engine?

Question

I have seen in the P-V diagram of a two stroke engine that there is a blank space in the "swept volume" termed as "piston travel during open period!" so what is it about?

Answer 1

At the bottom of the stroke, both valves are open for exhaust to escape and for fuel-air to enter the cylinder.  On very small 2-stroke engines, there is actually an opening to the atmosphere for exhaust to escape (no valve).

Since the cylinder is essentially vented to the atmosphere, there is no 'P' so PV is meaningless.

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