Valve train components from MAHLE: more than just valves
In cooperation with international engine manufacturers, MAHLE develops the tribological system of valve train components: valves, valve guides, and valve seat inserts. The greatest challenge with valves is defining the design. In order to withstand the extreme mechanical, chemical, and thermal loads, and to guarantee optimal heat dissipation, our valves are designed for a wide variety of operating conditions.
The demands on our valves are pretty tough
Exhaust valves can reach 800–1,000°C, while intake valves can still reach 300–550°C. Enormous ignition pressures and hard valve springs hammer the red glowing valves into the valve seat up to 70 times per second. Aggressive chemicals in the exhaust gases can cause corrosion or carry deposits. In order to withstand these conditions for years or even decades, the valve, valve guides, and valve seat inserts must be made from carefully formulated, high-quality materials.
Materials: for the safety of people, engines, and the environment—only the best
Valves are generally divided into monometal and bimetal valves, typically made of solid material. Depending on the loads, various materials are selected: from high-alloy CrSi steel for intake valves to CrMoV and austenitic CrMnNiNb or CrMnNi steels with added nitrogen, to nickel superalloys for extremely highly stressed exhaust valves. Special cases call for so-called hollow valves, which are filled with sodium.
Sodium-filled hollow valves have a melting point of about 97.5°C that is distributed within the cavity. The so-called shaker effect transports the heat from the valve disc through the entire stem to the valve guide. This reduces the temperature at the valve disc by 80–150°C—which significantly improves the knocking properties of the engine. This can increase in the engine output and reduce consumption.
In order to meet these high requirements, valves can also be reinforced around the seat, hardened, nitrided, or chrome-plated.
A CuZnAl alloy is used for valve guides with medium loads. When the load increases, a grey cast iron alloy with pearlite microstructure is used. Strength can be even increased by means of an additional phosphorous component. A CuSn or CuNi alloy is suitable for high loads, and sintered metal withstands even the highest temperatures.
Valve seat inserts
For valve seat inserts (and their counterparts on the valve), MAHLE uses materials such as grey cast iron with a high carbon content, or cobalt with chromium and tungsten. Steels with chromium and molybdenum are also used, as well as steels with chromium and vanadium or even extremely high-quality sintered materials in order to meet the requirements of high-performance engines.
MAHLE produces valve seat inserts using centrifugal casting or sintering. For the first method, the liquid steel alloy is poured into a rotating casting mould. The melt adapts to the shape of the casting mould as it hardens, thus producing a hollow cast component. For sintering, metal powder is pressed together. The pre-pressed component, known as a “green body”, is heated (though not melted), further compacted, and hardened. The type of powder mixture is based, among other factors, on the temperature that the rings will be exposed to during operation and the required wear resistance. This means that valve seat inserts are precisely tailored to their subsequent use in the engine.
Research and development for the repair shop: so that you can continue to install safe products in the future
Innovative development: the TopTherm valve
MAHLE developed the TopTherm® valve specifically for downsizing engines. In contrast to the classical hollow valve and EvoTherm® valve, this is an assembled valve. The large cavity, partially filled with sodium, reduces weight and allows temperature reduction at the highly thermally stressed component surfaces. The disc temperature can potentially be reduced by significantly more than 100 Kelvin. At the same time, consumption at full load and high speeds is reduced as it is no longer necessary to enrich the mixture to protect the valves. Greater baseline compression can save some fuel in transient operation as well. With all of these advantages, the TopTherm® valve demonstrates that MAHLE is a global leading manufacturer of hollow valves for good reason.