Abstract: Engine performance is improved and nitrogen compound emission is reduced by introducing supplemental air to the intake manifold of an internal combustion engine which is fitted with a PCV valve. The supplemental air inlet device is connected so that it introduces air into the line between the PCV valve and the intake manifold. That line connects to the manifold immediately downstream from the carburetor at a special adaptor. The supplemental air inlet valve is specially constructed to utilize frictional forces arising out of air flow past the valve head, Venturi effect as the head nears closure, and finally pressure sealing at complete closure of the valve, in an efficient but low-cost design. The supplemental air valve is operated at a temperature, and supplemental air is introduced in a way, that results in condensation on its inner walls of oil vapors drawn through the PCV valve so that the condensed oil entraps particles of foreign matter that find their way through the supplemental air valve.