Materials for Extreme Applications (3)
In the previous article on this subject we looked at the use of Inconel as a valve material. This month I'll look at an application of another high-temperature valve material, Nimonic alloys.
These are nickel-based alloys which, similar to Inconel materials, maintain a high proportion of their room-temperature strengths at elevated temperatures. As with Inconel alloys they are commonly used in turbocharged or supercharged applications, where the high operating
temperatures in these engines call for high-temperature materials for the exhaust valves.
The main constituents of typical Nimonic valve alloys are nickel and chromium. A typical exhaust valve material will contain 65-70% nickel with about 20% chromium, the other main metallic ingredients being aluminium, titanium and iron. The ultimate tensile strength and proof strength properties of a Nimonic valve alloy are almost undiminished at 600ÂÂºC compared with ambient temperature.
Turbocharged and supercharged engines have a higher power density than their naturally aspirated counterparts, with the same power being available from a lighter base engine. Supercharged engines are not common in many circuit racing series, owing to poor fuel economy, although their performance and response make them ideal for other forms of competition, especially drag racing.
Turbocharged engines are found in WRC rallying, where some very clever anti-lag strategies have been used to counter their perceived disadvantage of 'turbo-lag'. Equally clever are strategies and systems applied by some engine manufacturers to circuit racing applications. Turbocharged engines commonly race in endurance racing; the heavily funded factory Audi R8 cars with their turbocharged gasoline engines, for example, had great success at Le Mans and on the circuits of Europe and the US. Audi has since switched to diesel-powered cars and found continuing success.
One leading supplier of turbocharged racing engines to Le Mans and the US and European Le Mans Series continues to compete successfully using its turbocharged gasoline engines in the LMP1 and LMP2 categories. Advanced Engine Research (AER) of Basildon, UK, has supplied teams competing in these events with turbocharged engines for almost a decade, and it uses Nimonic exhaust valves in all its turbocharged engines, including the current works-backed Mazda LMP2 engines.
The photo here (figure 1) shows the company's turbocharged P32 V8 LMP1 engine, which is currently competing in the US Le Mans series.
This engine, and the latest P70 Mazda LMP2 engine, both use a similar design of Nimonic exhaust valve, as shown in figure 2. AER's Mark Ellis said of its valves, which are made to their own design but from an undisclosed supplier, "The Nimonic exhaust valves have proved reliable at Le Mans race distance and beyond". This was proven in 2009 when a car using its LMP2 engine claimed third place after a gruelling 24 hours at Le Mans.
As the company develops both turbocharged and normally aspirated engines we have no reason to doubt Ellis's word. Given that the 2 litre LMP2 engines all produce about the same power, and that the normally aspirated V8s enjoy a 70% larger capacity, we can expect significantly higher combustion temperatures and pressures.
Fig. 1 - AER's turbocharged V8 endurance engine uses Nimonic exhaust valves (Courtesy of Advanced Engine Research)
Fig. 2 - These Nimonic exhaust valves are typical of those used in turbocharged endurance engines (Courtesy of Advanced Engine Research)
Written by Wayne Ward