Piston skirts benefit from an anti-friction coating to reduce wear and improve performance. Thermal barrier coatings can be used on the crown to protect the piston, and to extract more power from combustion.

Heat

The bulk of useful engine coatings that are not friction reducers are thermal control coatings.  These coating generally come in two flavors, the first being thermal barriers.  Reflective barriers often increase combustion efficiency when applied to valves, combustion chambers and piston tops.  The same type of coating also sees frequent use externally on intake manifolds, helping to keep the incoming air cool, and has varied uses throughout the exhaust system.  It’s not hard to imagine a multitude of other applications where keeping heat out could solve a lot of problems.

The other common class of temperature control coating is the thermal dispersant, designed to help coated areas shed heat more efficiently.  Thermal dispersants are especially useful when applied to valve covers or on the oil pan itself, helping to draw unwanted heat from the oil.  Outside of the oil system, dispersants have the flexibility to perform a range of functions, from increasing the efficiency of the intake manifold to dramatically extending the lifespan of the valve springs. 

Many coating blends designed for these purposes also have good frictional properties.  The previously mentioned frictional coating used on turbochargers, for example, is also designed to trap heat inside the unit, increasing thermal expansion and, therefore, power output.  In some cases, alcohol engines have made use of a special Teflon blend coating on the injector hat to prevent the butterflies from icing over, making use of a frictional blend to address a thermal issue.