Internal combustion reciprocating-piston engine precombustion chamber in which a mass of air or of air mixed with selective amounts of fuel and a spark-ignitable air-fuel mass are relatively segregatively compressed with the latter mass enveloping spark electrodes preparatory to ignition. The chamber is provided by recessing the air intake valve of an engine cylinder into the air intake passage for that cylinder so the chamber forms a section of that passage between the valve and the variable volume space above the piston in the cylinder. Contaminant products of combustion residual in the precombustion chamber upon completion of each combustion stroke are, during the next air intake stroke, vented into the cylinder variable volume space where they become mixed with the fresh intake air and any fuel entrained therein and then forced with this air back into the chamber pursuant to the ensuing compression stroke to be recycled during the next combustion stroke.