Exhaust valve materials
The poppet valves of a four-stroke engine have an arduous life, and the current design trend of engine downsizing in production vehicles and several race series is likely to make their life more difficult still. With downsizing comes boosting, mainly in the form of turbocharging; any form of boosting leads to increased temperatures and pressures in the combustion chamber, and much higher exhaust gas temperatures. The exhaust valve, which already has much higher temperatures to deal with than the inlet valve in a naturally aspirated engine, will see its temperature increase still further.
Many naturally aspirated race engines (and some production engines) use titanium as an exhaust valve material. Titanium alloys benefit from low density and are hence attractive to a valvetrain designer. The material may be the same as the inlet valve material in such instances, although there are some titanium alloys available which are chosen specifically for exhaust valve use. These come at a premium, but will run reliably to higher temperatures compared to 'old favourites' such as Ti-6242. Polmear (1) suggests that exhaust valve materials such as Ti-834 have found use in production engines. As ever, motorsport benefits from aerospace development of materials: most high-strength titanium alloys used for valves were originally developed for gas turbine engine compressor blades.
While titanium has found use in turbocharged engines as a racing exhaust valve material, this was in the days of the comparatively low-boost and alcohol-fuelled CART engines.
Steel materials have proved a very popular choice of exhaust valve material for naturally aspirated engines. Austenitic steels such as 21-4N remain popular today for racing applications, and more than 40 years ago it was cited in technical papers and books on the subject of valves (2, 3). These days there are materials with slightly better mechanical properties in this category, such as DIN 1.4882
For the more extreme applications, we have to look beyond steel materials and towards superalloys. Typically specified for turbocharged race engines, these alloys are nickel-based and offer much improved strength and stiffness at temperature. Nimonic 80A is a typical offering from valve manufacturers. Owing to the consistently high market price of the elements from which such materials are made - mainly nickel and chromium in the case of Nimonic 80A - and the difficulties in machining them, such valves don't come cheap, but they do make the difference between an engine working and not working.
(Sources: G&S Valves Technical Information, Issue 2, 2003, for 21-4N, DIN 1.4882 and Nimonic 80A, and Timet datasheet for Ti-834)
Not only must we consider the strength of the exhaust valve material at working temperature, we must also consider other properties such as creep and corrosion resistance. Creep is a measure of the 'relaxation' of material, and is measured by observing time-dependent strain under a fixed load, or by observing time-dependent stress under fixed strain. Books containing relevant creep data include that by Conway (4).
1. Polmear, I.J., "Light Alloys", 4th Edition, Butterworth-Heinemann, 2005
2. Cowley, W.E., Robinson, P.J., and Flack, J., "Internal Combustion Engine Poppet Valves: A Study of Mechanical and Metallurgical Requirements", Proc. IMechE Auto. Div. 1964
3. Smith, P.H., "Valve Mechanisms for High-Speed Engines", Foulis, 1967
4. Conway, J.B., "Stress Rupture Parameters: Origin, Calculation and Use", Gordon and Breach, 1969
Fig. 1 - Material selection is key to successful valve operation; strength is only part of the equation
Written by Wayne Ward