Answer for Why the inlet valve is opened before T.D.C. and after B.D.C.

The inlet valve is opened even before T.D.C. and after B.D.C. so that maximum gases may come into the cylinder. Valve is opened before time so that the time spent in opening the valve is over before T.D.C. By opening the valve after B.D.C. the mixture of air and petrol is stopped from going out through the carburetor by back pressure.