An internal combustion piston engine has a prechamber or auxiliary chamber associated with each main combustion chamber and connected thereto by a plurality of torch openings. Ignition of a relatively rich mixture in the auxiliary chamber causes flames to be projected through the torch openings to ignite the relatively lean mixture in the main combustion chamber. At least three torch openings are provided in the bottom wall of the auxiliary chamber; one is directed toward the main intake valve in the main combustion chamber, another is directed toward the exhaust valve in the main combustion chamber, and a third directs a flame toward a region of the main combustion chamber remote from both valves. Additional torch openings are employed for larger engines. The relative positions of the torch openings are defined as well as their total cross sectional area compared to the volume of the auxiliary combustion chamber.