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The effect of thermal barrier coatings on engine efficiencies

Thermal barrier coatings have been big news at various times in the past, but they have never been adopted as widely as might have been expected given their supposed advantages. As the name suggests, they aim to prevent the transfer of heat. They are often used in motorsport on exhausts, and heat shields which are used to prevent damage due to the harmful effects of radiated exhaust heat. However, they were expected to become common in engines, and indeed they continue to be marketed with a view to coating everything from piston crowns, combustion chambers and ports to poppet valves.

The idea behind them is a very attractive one. For every unit of fuel burnt in the engine, less heat escapes directly to the components next to the combustion chamber, so there is a reduced cooling requirement. This reduction is very attractive to engineers in charge of cooling systems and aerodynamics, as lower heat rejection is associated with smaller coolers, lower cooling air flows and reduced aerodynamic drag. It also means more energy is retained in the combustion chamber, where it can be used for producing extra torque. Good news all round, or so you may think.

There are two problems though that need to be overcome before the coatings become common in race engines. One is the issue of longevity. The aim will be to take maximum advantage of such coatings, and the reduced heat transfer will naturally lead design engineers to redesign components to work at lower temperatures, but if the coating fails and heat transfer is locally increased, it will cause component failure. People who stand to gain the most from the technology also need to be convinced the most of the reliability of the coatings. The coating process also needs to be carried out at a temperature that does not harm the component – especially important for pistons which are very highly stressed but which can stand only relatively modest temperatures before their mechanical properties are degraded.

The other problem is the management of heat and temperature. If volumetric efficiency is to be maintained, so that the amount of air drawn into the engine is not adversely affected by increased component wall temperatures, the coatings’ surfaces need to be able to cool quickly. If the effect of the coating is to increase component surface temperatures such that volumetric efficiency is impaired, the result could be a reduction in performance. To present an overall advantage in terms of performance, any increased fuel conversion efficiency – that is, the ratio of mechanical energy extracted divided by the chemical energy in the fuel – needs to outweigh any loss in volumetric efficiency.

Having worked with a group of engineers who had used low thermal conductivity pistons for extensive trials and found large gains in fuel conversion efficiency, combined with large losses in volumetric efficiency, this can be a real concern. The conductivity of the coating and its thickness need to be managed to give an overall advantage in terms of performance for most racing applications. However, with racing increasingly rewarding fuel efficiency, it might be that thermal barriers which lead to an overall reduction in performance but do this while achieving increased fuel economy may prove useful in certain types of racing, such as categories where a loss of 5% of performance might be tolerated if the car can go one more lap before entering the pits for refuelling.

Written by Wayne Ward