An internal combustion piston engine has a main combustion chamber and an auxiliary combustion chamber connected by a torch passage, which is tapered end to end, the large end communicating with the main combustion chamber. Lean mixture is supplied to the main combustion chamber and rich mixture is supplied to the auxiliary combustion chamber. A spark plug ignites the mixture in the auxiliary combustion chamber, causing a torch flame to be projected through the tapered torch passage into the main combustion chamber near its center. A suction conduit, smaller in diameter and total area than any portion of the tapered torch passage, extends from a peripheral portion of the main chamber and intersects the tapered torch passage between its ends. Movement of the burning gases through the torch passage aspirates a flow of mixture from the main combustion chamber through the suction conduit into the torch passage.