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That is a good question that has people often mistakenly thinking that running lean is a bad thing for emissions.
The graph you put up is a classic used in many tomes on combustion. It does not take into account modern engines and controls. The graph does show clearly the problem. It's not the production of NOx at these lean ( 18+) AFRs but the increase of HC and CO. In the Honda lean burn engines that ran at 22:1 air/fuel mass ratios, the combustion became erratic and HC and CO would skyrocket. But, the engine actually runs COOLER past 18:1 AFR resulting in very poor catalytic converter efficiency. This is why lean burn is limited in the North American market in gasoline spark ignition vehicles. Natural gas engines on the other hand, have inherently better CO and HC profiles for combustion and it would be interesting to see more development of lean burning gaseous fueled engines.
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