It is the valve's job to open one or several ports in such a way that the cylinder receives the optimal air (diesel engine) or air-
fuel mixture (petrol engine). Secondly, the used gasses are supposed to utterly leave the cylinder. During the other strokes, the valves should seal the cylinders against the inlet port.
How it works
From the cam of the camshaft, bucket tappets, push rods
or rocker arms transfer the power to the valve, and open it up, before the piston achieves the bottom
(exhaust valve) or the top dead center (intake valve).This assures that the gasses are as early as possible flowing in respectively out. One or two valve springs close the valve after the top dead center for fresh gasses
to flow in.
The cross-section of the opening of the inlet side is usually larger than on the outflow side. A valve consists of stem and head. At the stem there is a chamfer of 45° milled-in. This matches exactly its
counterpart, the valve seat, which is usually shrink-fitted as round ring into the cylinder head.
|Outlet side - Petrol engine||600 - 850°C|
|Outlet side - Diesel engine||800 - 1050°C|
|Intake side - both||approx. 500°C|
The temperatures can reach 1050°C. At maximum speed, valves are opening and closing more than 50 times per minute. Closing, the valve is pulled on its upper end of the stem and slams with its head into the valve
seat. The valve is not only exposed to mechanical problems of lengthening, but also to chemical problems, like corrosion. Intake valves therefore consist of chrome silicon steel. The valve stem might be chromium-
plated. The exhaust valve's head is manufactured from chrome manganese steel and the stem from chromium-nickel stainless steel. Wear-stressed places are hardened. Some stems of exhaust valves are hollow
and filled with sodium.
Due to its high demand, the valve seat and the stem end are armored with carbid metall.
Special valve stems prevent engine oil to penetrate into the ports.
The valve spring is a key issue. It has to completely shut off the valves. It should not be engaged too strongly, because this affects the hydraulic lifter must be serviced in regular intervals with the feeler gage.The cams
may not operate the valves. A certain valve clearance is necessary with adjustable valves, so that they close reliably. Too large play causes power losses and noises in the valve train, too few play causes a
burning of the valve head, because the valve has lost its contact with the cooling cylinder head. A petrol engine might be set on fire when the hot, explosive mixture finds a way into the inlet port.