Material Choices

Sunday, September 13, 2009

Tags :  con-rods

con-rodsLast month, we started to look at materials which are being used for con rods. The article looked at the use of aluminium, which finds widespread use in drag-racing. This month we shall turn our attention to engines with less than 5000 hp and con rods in another low-density material: titanium.

Titanium has long been used as a con rod material for racing, and its benefits have been slowly taken up by road car and motorcycle manufacturers in the last couple of decades, although this has only been on very high-specification machines, many of which will have found their way onto race circuits. That great racing company, Honda, pioneered the use of titanium con rods for road vehicles in the late 1980s, first with the fantastic RC30 motorcycle, and then with the NSX road car.

Titanium is favoured by many owing to its low density. A typical titanium con rod material will have a density of approximately 4.4 g/cc, compared to 7.8 g/cc for steel. If we were simply to copy a steel con rod and produce it in titanium (many people do just that), we would realise a 44% mass reduction. This alone is an extremely worthwhile saving, but the benefits go much further than this.

Any mass saving on the con rod should, if we have the resources to properly take advantage of it, be accompanied by a consequent mass saving on the crankshaft, where we add counterweighting to balance the rotating and reciprocating masses. These counterweights can thus be made lighter and so we find that any mass saving made on the con rod is effectively doubled if we reduce the crankshaft weighting accordingly. An extra benefit is reduced inertia which allows the engine to accelerate faster, giving a performance benefit to the car/motorcycle, in spite of the fact that the engine may not produce any more power. However, we should note that this may make the engine feel more ‘peaky’ or ‘highly-strung’, and this is not always what is required in some forms of motorsport, especially if traction is limited, or ways of controlling it are banned.

Image 2 - Joey Dunlop TT92

 

In common with aluminium as a con rod material, some engineers state that the elastic modulus of titanium compared to steel makes it a more forgiving material and it can therefore have a valuable role to play in protecting adjacent parts of the engine from shock loads due to, for example, irregular combustion (detonation). The elastic modulus of Ti-6Al-4V, a popular titanium alloy for con rods, is 114GPa, compared to 207GPa for a typical steel (the specific modulii of titanium and steel are very similar).

We should say that, in order to optimise the material for use in con rods, we should not simply copy a steel rod, because in doing so we miss out on some of the benefit of using titanium. If we design the small end to have acceptable stresses, we will find that it is lighter than the equivalent steel item, and therefore each part of the rod between here and the small end is consequently less stressed under maximum inertia loading than its steel counterpart. We may also find that we are able to use a smaller con rod bolt, although the ‘load coefficient’ which dictates the share of cyclic loads between the bolt and joint is changed compared to a joint of the same geometry in a steel rod.

So, what are the disadvantages of titanium as a rod material? Cost is obvious; it is still much more expensive than steel. Quality might be a concern, especially if we are tempted to buy the very cheapest material available, and I’ve seen some appalling material quality. Tribological behaviour is poor, with galling a serious concern with fasteners and interference-fit bushes, although these problems can be overcome with experience.


Written by Wayne Ward.

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