Inlet and outlet valves seal off the combustion chamber and control the charge exchange process in the engine. Valves are thermally and mechanically, highly strained components which are additionally subject to corrosive infl uences. The mechanical strain is a result of the bending of the valve head under combustion pressure and impact when closing (impact stress). With corresponding construction like strength and shape of the valve headand respective choice of material, these strains are brought to a manageable level. The outlet valve is additionally heated by the passing hot exhaust gases during outlet clock opening. The valves are cooled down mainly by conducting heat via the valve seat insert to the cylinder head. The smaller proportion of the heat is conducted via the valve guide to the cylinder head.. Inlet valves reach temperatures of approx. 300 °C to 550 °C, outlet valves can get up to 1,000 °C.
- Mono-metallic valve
- Mono-metallic valve with hardened seat
- Mono-metallic valve with valve seat armouring
- Bimetallic valve
- Bimetallic valve with seat armouring
Mono-metallic valves are only made from one material. For this, one material is chosen that is suited to both requirement profiles, namely high heat resistance and good sliding properties.
Bimetallic valves enable material combination of a highly heat resistant agent (headpiece) with a skirt material which can be hardened on the one hand (skirt end) and also has good sliding properties for valve guidance. The materials are combined through friction welding.
Hollow outlet valves are mainly used for lowering the temperature in the particularly vulnerable fillet area and are additionally filled with sodium. A desired and positive side effect is a reduction in weight. Hollow, unfilled inlet valves are only used for this reason of mass reduction. In order to achieve a temperature decrease for valves, the skirt that is drilled hollow is filled with sodium by approx. 60 % of the volume and closed by a friction welding procedure. Sodium melts at 97.5 °C, has a density of 0.97 g/cm³ and is a very good heat conductor. During engine operation the sodium becomes liquid and is moved backwards and forwards in the skirt by the inertial forces. This is also called “shaker effect” in this context. In this process, the sodium transports a proportion of the heat generated during combustion from the valve head to the skirt area. Here, the heat is dissipated via the valve guide. Temperatures on the valve head can thus be decreased from 80° C to 150° C.
Handling sodium-filled hollow valves
The machining and cutting open of sodium filled hollow valves requires respective care. It must be ensured that the cavity is not opened my mistake, as sodium reacts strongly with water or drilling and grinding emulsion. When sodium reacts with water, hydrogen and caustic soda is produced.
Examination and disposal
Small quantities of hollow valves can be scrapped in the usual way. No special regulations are to be observed. If sodium-filled valves are to be examined or larger quantities are to be disposed of, the cavity is to be opened either by drilling holes in two different places without using coolant, or through splitting the valve open in the middle. The valves that are prepared in this way are individually thrown into a bucket with water to render the sodium harmless. Once the reaction has run its course, the valves can be scrapped as normal. Disposal of the remaining caustic soda occurs according to the respective local regulations.
Due to the sometimes strong reaction and release of hydrogen during reaction of sodium with water, the neutralization of the valves should only occur in well ventilated rooms or outside. Contact with skin or eyes must be avoided. For this reason, sodium must only be handled by respectively trained personnel wearing corresponding protective clothing (gloves, safety goggles etc.). The usual safety provisions when dealing with aggressive and caustic materials and explosive gases are to be observed.
Seat armouring and hardened seats
Special outlet valves are highly strained thermally as well as with regard to wear. For this reason, these valve seats are often armoured. Inlet valves for highly strained engines are mostly hardened inductively. The impact and the wear of the valve seat inserts are avoided with these measures.
The valve stem face
The valve stem face is heavily used by the valve actuation (tilting lever, finger type rocker arm, tappet). To avoid wear and tear here, valve stem faces are hardened from curable steel. Valve stem faces from noncurable steel get stellite armouring or have a hardened platelet welded on.
Dimensions and technical terms
The service life of the valves and thus effectiveness of the engine strongly depends on correct assembly. During assembly, always observe the fitting guidelines and adjustment values of the engine manufacturers.
Valves are to be handled with care. Valves must neither be treated with abrasive paper nor marked with centre punch or punch numbers on the base.
A suitable tool must be used for installing the valve into the cylinder head. When installing new valves, new cotters must also be used. The internal cone of the valve-spring retainer is to be checked for wear and tear and damage. The valve-spring tension is to be checked for the limit values of the engine manufacturer.