Mechanical Engineering - IC Engines and Nuclear Power Plants - Discussion

Exhaust valve opens before 50 degree of BDC and remains open even after completion of exhaust stroke after 20 degree TDC so that it ensures complete removal of flue gaseous from cylinder.

Hence it avoids deterioration of fresh incoming charge during suction stroke which may mix with these burnt gaseous. This time lag of opening and closing of valve is essential during high speed as less time is available for flue gaseous to be thrown away.

In valve timing diagram it is a 4 stroke called exhaust stroke hence exhaust valve opens before BDC at 50 and closes after TDC 15.

What @J bhargavi baaji told above is absolutely right.

Can anyone explain the total diagram?

Here the angle doesn't matters. As per the value timing diagram, value open before the the cycle starts and closes after the cycle. As per the question, the value opens before the exhaust process ie before the BDC and closes after TDC.

Valve timing diagram: opening of valves are before and after dead centers.

In valve timing diagram any valve open always before either bdc or tdc.