Could steel rods be the optimal solution?
There have been a number of RET-Monitor articles on con rod materials over the past few years. The last article to cover steel as a con rod material, in late 2009, discussed the general benefits compared to other materials. In general terms, people have tended to consider steel as the safe option and titanium as the material for the optimum rod if the budget allows.
If we decided to look beyond traditional steels for rod manufacture and towards some of the more exotic steels available, could we make something to rival titanium? Without going into a major design study, we can’t know for certain, but we should not write steel off.
One advantage of using steel rather than titanium is that we can make both ends of the rod physically smaller while maintaining the required stiffness. At the small end of the rod, this may mean that we can push the piston pin axis closer to the piston crown. There are two possible advantages here. We may either run a longer rod with a less extreme ratio of crankshaft throw to con rod length, which should reduce piston thrust load through lower maximum rod articulation angle. The second is that we can maintain the same rod length and reduce the height of the block. Of course, this is often impractical unless we are at the early design stage of the engine project.
At the small end of the rod, it is common to run a steel rod without an interference-fit little-end bush, which allows the small end to be made even smaller. Titanium requires a bush to be used, and the section around the small end has to cope with the interference stresses and an allowance for possible damage due to galling as the bush is fitted.
The stiffer material means the load coefficient of the big-end fasteners is lower, so the fasteners are less stressed. We could therefore use smaller fasteners, positioning their axes closer to the big-end axis and reducing bending loads. When designing a vee engine from scratch, for example, we might be able to lower crankshaft height.
Given the advances in CNC machining in recent years and the opportunities to fully machine all surfaces of the rod, we should be able to produce the main beam of the rod with enough axial and torsional stiffness to rival a titanium rod without committing excess mass to the design. Steels in general have an elastic modulus of about 210 GPa and a density of around 7.85g/cc, giving a ‘specific modulus’ of 26.75; by contrast, titanium alloy Ti-6Al-4V has a specific modulus of 25.73. So, in order to produce a beam of a certain length and with a desired axial stiffness, the steel part would be lighter.
Remember though that these figures are for a general engineering steel. We don’t have to look too hard to find a steel with a significantly higher specific modulus which, for all the design areas discussed above, will offer an even greater advantage compared to titanium.
An optimised steel rod will require expensive material, more machining time and quite possibly various heat treatments and surface treatments, so it is not likely to be cheap – in fact it may be far more expensive than a titanium rod.
Written by Wayne Ward