What does a admission valve do in a engine?

Answer 1

There is no such valve. You must be referring to emission. There are numerous parts that make up the emissions system. It is designed to burn off unburnt fuel by recirculating it.

This answer is very wrong. There is a such thing as an admission valve. It is used in lean burn prechambered engines to deliver the rich fuel to the prechamber. The air/fuel ratio drawn in by way of the carburetor is too lean to be ignited by a spark alone. Upon the intake stroke the admission valve is open and the prechamber is filled with rich fuel. When the compression stroke is performed, the admission valve is forced closed by compression, and some of the lean mixture that is in the cylinder is forced into the bottom of the prechamber through torch-like holes. The lean mixture mixes with the rich and the spark plug, which is mounted so that it is inside the prechamber, ignites this, almost stoich mixture. The flame then propagates out of the torch-like holes which in turns ignites the lean fuel/air mixture in the cylinder. Example engine: Waukesha 7042 GL ( an industrial natural gas burning engine)