Valves

Function

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.

Materials

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 stem sealings at the valve stems prevent engine oil to penetrate into the ports.

Valve springs

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 cam course and/or the appropriate roll and/or sliding friction partner. However, if it is engaged too weakly, the valve takes off, and causes valve fluttering with higher engine speeds. Even the piston on its way to TDC might catch a not yet fully closed valve, causing extensive damage.
The larger the masses in the valve train, the harder the valve spring's task. It is relatively easier with overhead camshafts. Yet, displayed in the picture above is a particularly difficult case. It is a V8-engine with down-lying cam shaft, suited for racing purposes with numbers of revolutions beyond 7500 1/min. The complicated valve spring's task is accomplished by combining two or three valve springs at the same time. Thus, there is some additional protection against a valve/piston crash and the gradual closing is less violent.

Important

Valves without hydraulic lifter must beserviced 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.