Abstract: A technique is disclosed for operating an internal combustion engine near the lean burn limit. This technique includes sensing a first characteristic corresponding to a mean effective pressure of the engine and a second characteristic corresponding to fluid pressure in the intake manifold. An engine performance criterion is established as a function of the first and second characteristics. This performance criterion corresponds to an observed emission rate of a pollutant from the engine. A target for the performance criterion is selected which corresponds to lean burn operation of the engine with a limited emission rate of a pollutant. A mixture of air and fuel provided for combustion by the engine is controlled in accordance with the performance criterion and the target. The performance criterion may correspond to: ((BMEP.times.IMT)/IMP). Furthermore, this technique may be applied to lean burning, gaseous-fueled engines configured to drive an electrical power generator.