Effect of H-DLC coatings on direct acting cam-tappet friction forces

In internal combustion engines, the valvetrain is responsible for up to 25% of the friction losses, at rotational frequencies below 2000 rpm. Important friction losses in valvetrain come from the contact between the cam and the tappet. Despite to manufacturer specifications for the engine lubricant, the lubricant viscosity can be changed under some conditions, affecting the cam-tappet contact forces. Also, the surface conditions of cam and tappet can affect these friction forces. This work presents a bench mono-valve experimental apparatus and results for frictional force investigations, using aged engine lubricant SN5W30, with viscosity modifications. The influence of a tappet crown coated with H-DLC is investigated, comparing to standard tappets. These experiments showed that, under specific experimental conditions, the friction forces between the cam and the tappet are the lowest with H-DLC application, for all lubricant viscosity applied.