For a number of reasons metals are excellent materials from which to con rods. They are relatively cheap, and generally of excellent quality. You can make a mess of the rod’s manufacture but the material has every chance of being very good. Powder-reinforced metal matrix composites also come into this category – we take a good quality billet or forging and machine it.
Fibre-reinforced composites are another matter entirely though. The raw material can be excellent, but the people laying it up can ruin it in a number of ways, from accidentally introducing contamination (eyelashes, arm hairs, crumbs from sandwiches) to basic mistakes in lay-up (wrong material, wrong fibre orientation and so on).
For many reasons, even if well manufactured, composites tend to have a wider variation in strength than a metal, so are less predictable. Composite properties have larger standard deviations as a percentage of the property in question. For example (not based on actual figures), if you take 100 identical samples and want the weakest to break at 800 MPa tensile stress, you might need to select a material with an average tensile strength of 900 MPa. To achieve the same result for a composite though, the required average strength might be 1100 MPa.
For these reasons, as well as those of increased manufacturing complexity, we should applaud those people who seek to make complex components such as con rods from composites. From the 1980s Polimotor project to abortive Formula One composite rod projects which actually ran in engines, the potential gains of composites mean that people will keep trying. The technology with the highest potential is continuous reinforcement, whether using a polymer or metal matrix. Both techniques have been studied for con rods.
The study by Sala (1) went as far as manufacturing trials and mechanical testing, but did not extend to putting the components into a running engine. The manufacturing technique used was squeeze-casting of aluminium around a pre-form made of graphite fibres coated with silicon-carbide. The rods were of the non-split type and so would be suitable to use with an assembled crankshaft, and were of an unusual design owing to the requirement to have the fibres running in straight lines. Sala pointed out that the manufacturing technique was very expensive owing to its complexity and manual input, although the costs of the manufacturing machinery compared very well with those for more conventional techniques such as forging or casting.
The paper by Gunyaev et al (2) took a different route, using a polymer matrix and manufacturing but with split and non-split variants. They credit previous work on the Polimotor engine and some pioneering work by Mercedes-Benz on split con rods with continuous fibre reinforcement. The split type made by Gunyaev, as might be deployed in a multi-cylinder engine with a single-piece crankshaft, was made from multiple layers of conventional multi-directional pre-preg carbon fibre reinforced polymer (CFRP). The non-split type used pre-stressing to ensure that the fibres were lightly stressed in tension at all times, and the resulting stress of the polymer matrix in the con rod in its free state was one of compression. Both types of rod were tested in engines, with varying degrees of success.
So, might we see composite con rods in engines in future? It seems so, although given the strict materials regulations in force in motorsport, it is likely to come from either a car manufacturer or a university research department.
- Sala, G., “Technology-driven Design of MMC Squeeze Cast Connecting Rods”, Journal of Science and Technology of Advanced Materials, Elsevier/IOP, 2003
- Gunyaev, G.M., Borovskaya, S.M., and Panin, V.I., “Using CFRP for the design of a connecting rod for automobile applications”, Proc. IMechE, Journal of Automobile Engineering 1994
Written by Wayne Ward