Answer for Why the exhaust valve is Opened before B.D.C. and even after T.D.C.

The opening of valve in advance does not have any ill-effect on the power of the engine because the full pressure of the gases has already pushed the piston. On the contrary, the exhaust gases get full time to come out. By keeping the valve opened even after T.D.C. we want to ensure that no burnt gases have remained inside the cylinder even if some amount of fresh mixture has to be wasted.