The heart valves shut. There are two valves the mitral and the tricuspid. The mitral valve opens allowing blood into the ventricles (from the atrium) then the tricuspid valve opens allowing the blood to be pushed out into the left ventricle and out of to the body. It shuts allowing blood to fill the atrium again.

This engine (allowing it is an automotive engine) is a four valve per cylinder engine.

It increases the coverage of valves over the combustion chamber allowing faster breathing leading to better power at high RPM's. It allows the spark plug to be in the center of the chamber allowing a more complete burn.?æ

The valves will open earlier and close later, allowing more air and or fuel into the cylinder. If the clearance is too small, the valves will not close entirely and might burn.

The two semilunar valves are the aortic valve and the pulmonic valve. The aortic valve opens when the left ventricle contracts, allowing blood to pass into the aorta. The pulmonic valve opens when the right ventricle contracts, allowing blood to pass into the pulmonary artery.