<div class="htmlview paragraph">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.</div> <div class="htmlview paragraph">A generalized curve is given where BSNO_x (grams of NOx per brake HP-HR) VS. λ(^A/F/(_A/F)S) is given for natural gas.</div> <div class="htmlview paragraph">A section is given to illustrate, the importance of having a uniform mixture (homogeneous) to keep NO_x values to a minimum.</div> <div class="htmlview paragraph">Two plots are given, one based on NO_x formed in the prechamber is frozen when it emerges from the prechamber, and the other the NO_x emerging is equilibrated during burning. These plots can be used to compare engine data.</div> <div class="htmlview paragraph">Several actual engine data on lean burn engines which were recently presented in literature are compared with the value given in the plots.</div> <div class="htmlview paragraph">Sample computation are given for the prechamber.</div>