The role of FEATags : con-rods
The modern race engine is something of a technical marvel, not simply in terms of power output but also reliability. In terms of the basic concepts and the components involved, however, there is very little that the race engine designers of the 1920s and ‘30s would not be familiar with. While theirs was very much an era of trial and error in terms of the design of complex components, the modern engineer has at his fingertips an array of tools that can help optimise any number of race engine components. From the ubiquitous calculator to the spreadsheet, even the most basic of our modern electronic aids gives us a huge advantage over our forebears in terms of speed and accuracy of calculation.
However, no amount of hard work with a calculator or spreadsheet can hope to achieve what finite element analysis (FEA) can do for us in the optimisation of a stressed component. Even where we are able to extract an answer from a basic calculation, the ability of FEA to present the results in a graphical form and to generate graphs from the data makes its use very compelling. FEA is especially useful in the case of con rods, and a number of rod manufacturers use the software to good effect. The following is a simple overview of how such software is applied.
In terms of the simple stresses on the component, FEA can easily calculate the stress at any point and is capable of calculating fatigue data given the correct operating conditions over the whole cycle. The cyclic loads can be superposed over other loads such as the stresses due to small-end bush insertion, hydrodynamic loads and so on.
The stress and deformation around the small-end bush and big-end bearing can be monitored to prevent the designer going too far in his exertions to reduce component mass. If either bearing suffers from insufficient support, it will deflect easily under pressure and lead to excessive pressure on better-supported areas, causing failure. Conversely, FEA can prove to be a useful tool in determining where mass can be removed without affecting bearing support.
The deformation of the rod can be calculated so that one includes the correct figure in the calculations for valve-to-piston clearance. We can make a stab at this with hand calculations but this is either laborious if we try to achieve some semblance of accuracy, or we make assumptions based on an average beam section – which will definitely be incorrect.
If the mass and the moment of inertia of the piston assembly components are known then FEA can calculate the natural frequencies of the rod in bending and torsion. Some rod makers who use FEA routinely will ask for this information (and should do if you have asked them to calculate the natural frequencies).
FEA can give us a lot of information about fastener loads and stresses, including predictions of joint separation. A promising new high-speed engine concept that I was involved with came to grief when we realised through the use of FEA that we wouldn’t be able to keep the rod joint together at the proposed engine speeds.
Written by Wayne Ward