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 position of the exhaust cams for astern running.
Engine builders may not time the exhaust valve symmetrically about BDC; instead, to achieve more economical and efficient operation when running ahead may retard the opening of the exhaust valve by up to 15°. For instance the exhaust valve may be timed to open at 125° after TDC and close at 95° before TDC. This of course will mean when the engine is running astern, the exhaust valve will open and close early. However, because the engine runs astern for only a very small percentage of it's operating life, the advantages gained when running ahead far outweigh the disadvantages when running astern
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