It is said that humble pie is never a pleasant dish to eat. However, on this occasion, eat it I must. Only last year, in an article for RET-Monitor on the subject of crankshafts, I asked the question "Why Not Titanium?". The reasons given in the article as to why titanium crankshafts are not used are certainly valid. Titanium's low elastic modulus compared to steel means that, in order to be sufficiently stiff, a titanium crankshaft must be larger in section, and this means that bearing diameters are consequently larger. As discussed in the crankshaft article in RET 65 (September/October 2012), the frictional losses due to oil shearing in a journal bearing are increased quite markedly for even a small increase in bearing diameter.
However, it appears that people are still looking at the possibilities for titanium crankshafts, despite the relative drawbacks of using the material. Moreover, components have been made in a titanium metal matrix composite (MMC).
Both titanium and any form of MMC are banned under the FIA's Formula One regulations for crankshaft manufacture, and crankshafts made from them are certainly not destined for Formula One or any other kind of top-level motorsport. These parts have run with limited success in a real engine, and this has to be considered a step forward. The project may be a dead end, with no real push for further development, but it may also be a significant first step towards bringing the technology into everyday use.
The research has been run by a large multinational roadcar manufacturer, so we have to at least consider the fact that such materials are being developed for use in passenger cars. The advantage of using a titanium MMC material is the increase in modulus owing to the reinforcement, meaning that bearing diameters are much closer to those of a steel crankshaft than would be possible using a conventional, non-MMC titanium material. The frictional losses are therefore lower with the stiffer material, owing to the smaller bearings.
The material manufacturer has told me that the crankshafts ran for some hours in a test engine, and that it eventually failed owing to a surface-related mechanism. This should come as no surprise to people who have worked with titanium, a material whose propensity to fail due to surface damage in even lightly loaded sliding contacts is well known.
To reduce the likelihood of failure, the crankshafts were DLC-coated. However, such coatings, while hard and having low friction coefficients, are only very thin, and to be successful they need to be properly supported. Having a relatively hard and brittle thin coating placed on top of a comparatively low modulus material that is prone to surface damage often leads to premature coating failure. Besides DLC, there are a number of coatings that can be applied to titanium, although none are generally applied in layers thick enough to withstand highly loaded sliding wear, especially when lubrication is marginal, as is the case at engine start-up.
Written by Wayne Ward