In previous articles we have looked at the various materials used for valves, and examined some of the relative merits of these materials. The 'super-alloy' materials are particularly suited to high-temperature applications, having been developed initially with a view to use in gas-turbine engines. But they are relatively heavy.
Titanium is much more favourable in terms of component mass, and is widely used in motor racing. The material comes with problems though; titanium requires surface treatments to prevent oxidation. Also, titanium has low stiffness, and its specific stiffness is equal to that of most other alloys, meaning that to arrive at the same stiffness in compression, we would need about the same mass of titanium as we would steel, for example.
Titanium aluminide, as we have seen, has better high-temperature capabilities than titanium and is both less dense than titanium and has a higher modulus. It is, however, very expensive and is banned in some racing series, notably Formula One.
If we were to ask a valvetrain designer for a list of properties he would find look for in an ideal valve material, he would certainly mention low density, high elastic modulus, good high-temperature capability and good wear behaviour. All these attributes are found in the aforementioned materials but, perhaps with the exception of titanium aluminide, they are not all to be found in one material.
There is a class of materials though that might offer the valvetrain engineer exactly what he wants - ceramics. These offer an excellent combination of properties.
A typical silicon nitride material used for valvetrain component evaluation has an elastic modulus of more than 300GPa (about 50% higher than steel) and a density which is 25% less than that of a typical titanium alloy - less even than titanium aluminide. Silicon nitride has been used successfully for poppet valves in heavy-duty diesel engines, and has passed qualification tests for military use, indicating the durability of the material.
A stated advantage of using ceramic valves with regard to the diesel engine tests is reduced heat rejection, owing to the material's low thermal conductivity. For a race vehicle, lower heat rejection means less vehicle drag and a smaller radiator area.
Silicon nitride has found use in engine valvetrains for components other than valves, in applications for rollers on pushrod valvetrain rockers; ceramic rollers on pushrod lifters would be another worthy application. Moreover, the car companies have taken a keen interest in these materials for valvetrains, with successful dyno and vehicle studies being undertaken more than a decade ago by European and Japanese manufacturers.
With high stiffness and low density, valve control is made much easier using a material such as silicon nitride. Race engine valvetrain designers would naturally put this to good use with more aggressive valve lift profiles, although an equally valid strategy would be to run with lower spring pressures and so reduce valvetrain friction. In the current era of increasing fuel costs and emissions legislation, any gains due to reduction of friction are not to be lightly dismissed.
For this very reason the Japanese have for a long time shown particular interest in the use of ceramics in valvetrains. The 1998 SAE paper, "Advantage of Lightweight Valve Train Component on Engines" (980573) highlights some of the benefits to be had here in terms of fuel economy.
Fig. 1 - Silicon nitride has been widely used in rolling-element bearings for many years. Could it be the ideal valve material?
Written by Wayne Ward