Many of the applications discussed in the RET-Monitor articles where thermal barrier coatings are concerned have been on the subject of exhausts. Thermal barrier coatings here have the aim of reducing radiated heat, although in some cases a benefit of such coatings can be to improve the efficiency of the turbocharger turbine by retaining heat within the exhaust flow rather than rejecting it to atmosphere prior to the turbine.
There are other applications of thermal barrier coatings which have the same basic aim but which are applied to the cylinder head. Of course, all of the exhaust flow must travel through the exhaust port before it reaches the exhaust system. In the same way that insulating the exhaust system with a thermal barrier coating can reduce heat rejection to the atmosphere, a thermal barrier coating applied to the exhaust port of an engine reduces the heat rejection from the exhaust flow to the coolant water. This has the additional benefit of reducing the amount of heat to be rejected at the radiator, which has an additional aerodynamic benefit.
This is not novel thinking. Porsche were thermally insulating exhaust ports in turbocharged production engines 25 years ago. However, they did it by casting a ceramic insert into the head rather than using a coating, the aim being to improve the transient response of the engine by improving the spool-up speed of the turbocharger. Twenty-five years on, and coating technology might have allowed them to do this in a simpler manner. There are a number of companies offering thermal barrier coatings, some of which have been applied to the exhaust ports of race engines.
If you have seen a very high performance engine (especially a turbocharged engine) running at full load on the dyno, you cannot fail to have noticed the dramatic and impressive spectacle of the exhaust system glowing red hot. Irrespective of the amount of heat radiated to the atmosphere, there must be a significant amount of conduction across the joint between the exhaust system and the head. It is likely that there would be some small reduction in heat conducted to the cylinder head by coating either the face of the exhaust flanges or the cylinder head. Even in the case of having an insulating gasket between the two, there would be some conduction through the studs used to mount the exhaust systems to the cylinder head. If the exhaust flange has a thermal barrier coating where it contacts the exhaust stud and nut, there would be some reduction in thermal conduction across the head joint face.
In both of these cases, there is a clear benefit for naturally aspirated engines due to a reduction, however small, of their cooling requirements. Given the number of times we have seen under-cooled Formula One cars rolling out of the garages in pre-season tests with large holes carved into the sidepods, every little must help in this regard.
In the case of turbocharged engines, thermal barrier coatings could also be used to protect the turbine wheel from excess heat. This technology has been used to good effect on high-pressure turbine blades of modern aero engines, helping materials survive for long periods of time where they could not otherwise work satisfactorily.
Fig. 1 - Could turbine blade coating technology used in modern aircraft find application in a racecar turbocharger?
Written by Wayne Ward