The technique disclosed increases the energy conversion efficiency by means of substantial reduction of friction between the cylinder walls and the associated piston rings. The result is achieved by eliminating the unwanted carbonaceous deposits on cylinder walls and associated piston rings by means of vapor nitric acid (HNO3) delivered to the combustion chambers of an engine. Nitric acid is produced by means of chemical reaction between oxygen (O2) and water vapor (H2O) in air and nitric dioxide (NO2) generated throughout an electrical discharge in the air intake path.