Engine Valve

Most engines use poppet valves: a round head with a stem, closed by a spring and opened by the camshaft. Engines commonly have two or four valves per cylinder. Exhaust valves run much hotter than intake valves and are made from heat-resistant alloys; some are filled with sodium to carry heat away.

Valve stem seals at the top of each guide stop oil from running down the stem into the combustion chamber. Hardened seals are a classic cause of blue smoke at start-up after the car has stood.

The cam lobe pushes the valve open against its spring, either directly or through a follower or rocker. When the lobe passes, the spring closes the valve on its seat, sealing the combustion chamber.

Burnt or badly seated valves cause misfires and low compression. If a timing belt or chain fails on an interference engine, pistons can hit open valves and bend them. Carbon deposits on intake valves are common in direct-injection petrol engines, because fuel no longer washes over the valves. Heavy deposits can cause rough running and are removed by cleaning.