Materials for extreme applications
In a previous RET Monitor articles, we have looked at a couple of valve materials, namely titanium and titanium aluminide. The chief advantage of these materials is their low density which has obvious appeal when trying to keep control of valves at increasing engine speeds. Titanium aluminide, besides being possessed of even lower density than titanium has the additional advantage of increased stiffness, making it an excellent choice for poppet valves.
In this article we shall begin to look at other valve materials, beginning with those materials used for supercharged or turbocharged applications. Highly charged engines extract impressive bmep and high 'power density' by forcing much greater amounts of air into the combustion chamber than is possible with even the most highly optimised naturally aspirated engines. To this greater quantity of air is added fuel in the correct ratio and much greater energy is released in a combustion chamber which is not significantly larger than found in a naturally aspirated engine. Consequently combustion pressures and temperatures are very high and the exhaust valve in particular is required to run at extremely high temperatures, having high-speed burned gases flowing over it at high velocity, with the consequent high levels of heat transfer to the valve head and stem.
Owing to the temperatures involved in such applications, those materials normally used for the manufacture of valves, such as steels or titanium are often not sufficiently strong, or may offer insufficient durability. For the majority of those racing every weekend, the option of rebuilding engines and replacing valves after each race is not an appealing one. Some very highly optimised turbocharged racing engines have successfully been able to exploit titanium valves, but these items have a short service life and are extremely carefully designed and managed.
The materials that valve manufacturers and engine designers turn to for these applications are those developed specifically for high temperature use and which find wide application in the gas turbine engine sector. They have been developed for their ability to retain high levels of strength at high service temperatures and also to resist creep. Creep is the phenomenon whereby materials will continue to stretch under constant load over time, and this is particularly apparent with increasing temperature. Interested readers will find that the particularly dry book Stress-Rupture Parameters by Conway, contains much data on the subject.
The materials which maintain high strength at high service temperatures and which are creep resistant are often expensive, as they contain large amounts of expensive elements such as nickel and cobalt. They are often described as 'superalloys'.
One of the most popular classes of materials for poppet valves on turbocharged or supercharged engines are the 'Nimonic' alloys, which are based on Nickel, but which also contain significant percentages of other expensive alloying elements. A typical material often used for valves is described as having a service temperature of around 800 degrees C, or 1500 degrees F, and which maintains around 90% of its room temperature strength at 600 degrees C (1100 degrees F). The valves shown in the accompanying picture are Nimonic parts for a motorcycle application.
Another suitable class of alloy are known as Inconel alloys. These alloys are again based on Nickel, but are not as widely used as the Nimonic alloys.
Fig. 1 - Nimonic exhaust valves.
Written by Wayne Ward