Valves have to perform in harsh conditions: under heat, high pressure, friction – and sometimes without sufficient lubrication. Of course, they are expected to work as long and reliably as possible. At the same time, they are supposed to get lighter with every new engine generation. In short: valves have a tough job. The challenges for the designers are accordingly huge. High-tech materials offer convincing solutions … and design ideas. The individual components of the valve train system are subject to different influences. The valves have to endure the highest mechanical, thermal and resulting tribological stresses in this system. And as moving masses they affect directly the mechanical loads in the valve train and the friction losses. The basis of valve production: metal alloys
Depending where valves are used (at the inlet or exhaust side, in turbocharged or naturally aspirated engines) they are made from different materials. The standard material is a chromium silicon alloy (X 45 Cr Si 9-3). For the highly stressed exhaust valves, the MAHLE developed nickel super alloy Nimonic 80A© is used, which also sets new standards in terms of malleability, weldability and temperature resistance.  | | The micrograph of the MAHLE developed super alloy Nimonic 80A© for highly stressed exhaust valves. |
 | |
MAHLE valve seat with armouring. |
Mono, bi or hollow?
Different loads require different valve characteristics – and therefore different designs. The mono-metal valves consist of only one material or alloy, mainly the above mentioned chromium silicon alloy or the nickel super alloy. Bimetal valves are made from two different steel types as the name implies. The shaft of the valve is connected to the valve head by friction welding. The head is made from highly alloyed and high-temperature resistant steel, while the shaft is made from a different alloy steel that can be hardened.  | | The friction welding joint connects the valve shaft with the valve head. |
An innovative design is the hollow valve with optimum cooling characteristics: the valve cavity is partly filled with sodium, which becomes liquid at 97.5 °C, and then dissipates the heat better from the hot head to the shaft.
 | |
The hollow valve: temperature control thanks to the sodium filling. |
Well protected with stellite
In order to cope with extreme stresses such as high temperatures and chemical corrosion, highly stressed valves are “armoured” with an additional stellite layer. This layer based on cobalt and chromium is first welded on and later machined.  | | The valve head – prepared for stellite facing ... and with stellite layer welded on. |
 | |
The different production stages of a bimetal valve. |
The future of valve design: lighter and lighter?
The future of engine components production is characterized by further weight reductions. Valves offer here a great potential: if the mass of valves is reduced, the components driving them such as camshaft & co. can also be designed significantly lighter. With conventional valve steels the intended weight optimisation is limited by the demands for component strength. New materials with very favourable ratios between strength and density are still in the trial stage. As trendsetter in engine component development, MAHLE follows an alternative and innovative design approach: the MAHLE lightweight valves. To make these valves, different moulded and formed metal parts and a solid valve shaft end are joined together using precision laser and friction welding technology. A hot topic: A look at valve production |