To ignite very lean natural gas mixture it is necessary to use prechambers. The extension of lean limit of a combustion system is given by citing an example. The orifice diameter from the prechamber to the main chamber has a strong effect. The magnitude of the effect for one particular example is shown. The prechamber has another very beneficial effect, that is it increases the turbulence, which has the effect of producing more power at the same fuel to air ratio mixture.A generalized curve is given where BSNOx (grams of NOx per brake HP-HR) vs. λ (A/F/(A/F)S) is given for natural gas.A section is given to illustrate the importance of having a uniform mixture (homogeneous) to keep NOx values to a minimum.Two plots are given, one based on NOx formed in the prechamber is frozen when it emerges from the prechamber, and the other the NOx emerging is equilibrated during burning. These plots can be used to compare engine data.Several actual engine data on lean burn engines which were recently presented in literature are compared with the value given in the plots.Sample computation are given for the prechamber.