The development of a race engine is most often concerned with maximising efficiency. An integral part of many engine development projects is improved ‘breathing’, and in addition to developing the inlet system, combustion chamber and exhaust, the optimisation of valve lift profiles to suit the engine is a common way to find performance from the top end of the engine.
In a number of articles in Race Engine Technology magazine, the importance of maintaining control of the valvetrain over the working range of the engine has been discussed. ‘Maintaining control’ means having an understanding of how the valvetrain is working dynamically. As simulation and testing has become more widespread, what has become clear to people is that valvetrain stiffness is important, as is component mass.
The valvetrain can be considered as a ’spring/mass’ system, albeit a complex one with many different degrees of freedom, numerous springs and numerous dampers. You may think you have only one set of springs, but every component in the system has a spring rate and a damping characteristic. The spring retainer is an important part of the valvetrain in terms of mass, because it is reciprocating, but also in terms of stiffness. If we try to pare the retainer down too much in pursuit of low mass, we might compromise stiffness (effectively making the spring less stiff) and increase stress.
Therefore, consideration of spring retainer material is an important design consideration. Passenger car retainers are generally made of steel, and this remains a popular option for race engines. High-strength steel has a high fatigue limit, and a number of surface treatments such as carburising, nitriding and nitrocarburising can improve this significantly. The downside to using steel is mass, and effort needs to be put into the design to minimise unstressed material to keep mass low.
Titanium is another popular material for spring retainers, as it has a low density compared to steel and good mechanical properties. It does suffer though from poor wear characteristics, and these parts are often coated on contact faces (collet support face and spring contact faces) with a coating such as titanium nitride or chromium nitride. Titanium metal matrix composite (MMC) materials would be a good spring retainer material, and such alloys are proven for valvetrain applications (1).
High-strength aluminium alloys are also used for spring retainers, as are aluminium-based MMC materials. Aluminium has lost some of its initial popularity as a retainer material, possibly because people failed to understand that aluminium components need to be looked after more carefully and replaced more often than steel or titanium components. There is no actual problem with using the material, so long as you understand its inherent limitations.
Typical high-strength aluminium alloys used for spring retainers are 2024 and 7075, and in addition to those bespoke designs produced for race engines, many companies still sell aluminium retainers for tuned roadcars. Aluminium MMCs have been used in race engines for at least 20 years (2) and, compared to a conventional aluminium alloy, benefit from dramatically increased stiffness with comparatively little increase in density.
1. Hunt, W., Miracle, D.B., Automotive applications of metal-matrix composites, in ASM handbook, ASM International, vol 21: Composites, 1029-1032, 2001
2. Dwivedi, R., Altland, G., Barron-Antolin, P., Leighton, J. et al, “Applications of Metal Matrix Composites in High Performance Racing Engines”, SAE Technical Paper 911770, 1991
Fig. 1 - A sectioned CAD image of a titanium spring retainer for a racing application
Written by Wayne Ward