Abstract: A method of operating a spark ignition internal combustion engine having a main combustion chamber into which there is introduced a fuel-lean mixture of fuel and air, an auxiliary combustion chamber into which there is introduced a fuel-rich mixture of fuel and air, and an ignition means located in said auxiliary combustion chamber. In said method the engine is operated to approach an extended minimum lean misfire limit, and thus obtain minimum emissions of objectionable pollutants, e.g., nitrogen oxides. Said method comprises providing the engine with an auxiliary combustion chamber having a predetermined length over diameter (L/D) ratio and then operating the engine at certain predetermined values for the interacting parameters of spark gap location, volume percent of auxiliary chamber filled with said fuel-rich mixture, and percent of stoichiometric fuel in said fuel-rich mixture; with said predetermined values for said parameters having been predetermined relative to said L/D ratio.