Abstract: In a spark-ignition internal combustion engine of the type having a combustion chamber which presents at least one surface area on which there is applied at least one layer which presents to the combustion chamber a layer defining surface having very low reflectivity to electromagnetic waves in the wavelength range of about 0.2-7.0 microns. The layer reflects non-absorbed incident waves in a diffuse fashion, thereby to reduce the octane number requirement of the engine. The total surface area thus treated for each combustion chamber of the engine is equal to at least 10% of the top surface area of the piston in said chamber. The overall efficiency of the engine can be further improved when at least one of the layers has an underlying defining surface distal from the combustion chamber which reflects substantially all incoming electromagnetic waves in the wavelength range above 2.5 microns in a diffuse fashion.