4 stroke engine diagram

If a timing diagram for a two stroke engine is
examined, it can be seen that the exhaust valve starts to open at
about 110º after TDC (position 4 on the diagram). After the
initial blowdown of the exhaust gas from the cylinder, the
scavenge ports are opened at about 140º after TDC (position 5),
as the piston moves down the cylinder.

The position of the scavenge ports is fixed in the cylinder
liner, and so it should be obvious that their opening and closing
must be symmetrical about BDC, and therefore they close at 140º
before TDC as the piston moves up the cylinder on the compression
stroke. When the engine is operating in the reverse direction, the
timing of the opening and closing of the scavenge ports remains
the same.

The exhaust valve can be timed to open and close symmetrically
about BDC, and so again it means that when the engine is reversed, the exhaust
valve will open and close at the same time as when the engine is
running ahead. This means that there is no need to alter the