Why does an engine have valves?

To let the fuel and air into the cylinder and to let the exhaust out. Inside a motor there is a space (cylinder) where fuel and air is compressed by a piston then ignighted by a sparkplug. A valve is a little round disk with a stem coming out of its middle. The valve moves into that space creating an outlet around it past the stem. Every cylinder will have at least two valves.. 1 for fuel & air and 1 for exhaust. Hope that was helpful I tried to keep it simple as possible. To let the fuel and air into the cylinder and to let the exhaust out. Inside a motor there is a space (cylinder) where fuel and air is compressed by a piston then ignighted by a sparkplug. A valve is a little round disk with a stem coming out of its middle. The valve moves into that space creating an outlet around it past the stem. Every cylinder will have at least two valves.. 1 for fuel & air and 1 for exhaust. Hope that was helpful I tried to keep it simple as possible.