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Carbon composite exhausts

In the V8 era of Formula One, which ended with the introduction of new turbocharged and ERS-equipped power units in 2014, the level of component optimisation undertaken was, frankly, incredible. With many years of regulatory stability, teams looked to every quarter in order to find performance gains, with the key areas of investigation being aerodynamics and weight reduction. This led to the development of some unique components that used lightweight materials in areas where they had not previously been seen. One particularly interesting avenue of investigation was undertaken by the Honda team (and possibly others) into the use of composite-based exhaust systems. 

Traditionally, the usual material for exhaust construction in Formula One is either Inconnel or titanium, both of which provide relatively low weight, mechanical strength and reliability. However, with the weight of the exhausts sitting relatively high in the car, the potential gains from further weight saving are significant. 

Honda therefore developed a carbon-carbon (CC) exhaust system, which saw extensive dyno testing and some limited track use. Regular carbon composites, as used in panel work and chassis structures, use carbon fibres in a resin matrix. It is this matrix that is the limiting factor when it comes to temperature resistance, since although the carbon fibres themselves can withstand exceptionally high temperatures, most resins lose their integrity above 200 C. There have been recent developments in higher temperature resins, but there are none as yet that would survive reliably at the temperatures seen in an exhaust system. 

Honda’s answer was to use a CC material similar to that used for making brake discs and pads. It is a carbon fibre composite that uses carbon as a matrix, in essence a synthetic form of charcoal. While by no means weak, CC structures have nowhere near the mechanical properties of regular composites or metallic materials, so Honda’s development started with a target durability of 25 laps, aiming to use the parts for qualifying only. It began by developing collector and tail parts, but the ultimate goal was a complete set of CC exhaust pipes including the primaries, which if realised could net a weight reduction of 39% over the metal equivalents. 

Despite the challenges of using CC, there were other potential benefits in addition to the weight savings in terms of geometric freedom and packaging. For example, the use of Inconel entails the use of welded fabrication methods, which in turn requires that a certain material thickness be used to ensure weld integrity; it also limits the geometrical complexity of the parts. The CC parts are created in a mould, so such constraints were not a problem, and the result – Honda hoped – would be a lighter and more compact system.

Unsurprisingly, development was not trouble-free, and issues were encountered along the way, for example the integrity of the interface between the CC collector and the still Inconel primaries. CC has a very low coefficient of thermal expansion, and at operating temperatures gaps opened up between the two materials, making it hard for the engine control electronics to maintain a steady air-fuel ratio (which is measured using lambda probes in the primaries). To combat this, Honda developed a gasket material to fill the gaps. 

Additionally, the low resistance to oxidation of the CC material at high temperatures meant that a surface coating had to be developed to prevent this. Ultimately, the team was able to prevent surface oxidisation and maintain structural integrity for up to three hours at 900 C, which was found to equate to 30 laps at racing speed. 

Unfortunately for Honda though, regulation changes removed the possibility of running qualifying-only parts and ultimately the exhausts were never raced. However, the development process proved that the concept was feasible and it would be reasonable to expect that, given the renewed emphasis on weight saving in the new powertrains, such avenues of investigation may well come to the fore again.

Written by Lawrence Butcher

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