Engine valves function and working

The valve function is crucial to the entire sequence: they have to open and close at exactly the right times, allowing an uncontrolled flow of air or exhaust gas when they are open, while creating a perfectly gas tight seal when they are shut down. 

Each valve is roughly mushroom Shaped, with a long straight stem and a flat circular head with a beveled edge and precise ground to match the slope of the hardened valve seat surrounding the tunnel mouth in the cylinder head .

Through valve stays shut for most of through cycle, held tightly by one or two very powerful valve springs against its seat . If required, it is opened by a part called a rocker, like a miniature seesaw that pivots across the cylinder head on another shaft. 

⠀In the meantime, the crankshaft drives a component called the camshaft, but at half the speed of the crankshaft.There are carefully machined bulges, called cams, which are shaped and placed so that each in turn pushes up against a rocker in each cycle at the right moment. As one end of a rocker is pushed up, the other end moves down to open the valve. 

 Although the principle is standard, the theme varies widely. For example, the camshaft can be powered by gears, a chain and sprocket system, or a toothed rubber belt, and mounted high on the engine with the cams pressing directly on the rockers; or lower down and relying on push rods to transmit the cams ‘ motion to the rockers.  

In this case, the ends of the push rods do not rest on the cams directly, but sit in small bucket-shaped components called tappets or cam followers. In some engines, cam followers are equipped with rollers to reduce wear: in others, they are designed to rotate in order to spread wear more evenly, while some engines are equipped with hydraulic tappets that adjust for wear as it happens.