Carbon monoxide (CO) is most concentrated with fuel-rich mixtures, as there will be incomplete combustion. With lean mixtures, CO is always present owing to dissociation, but the concentration reduces with reducing combustion temperatures. Hydrocarbon (HC) emissions are reduced by excess air (fuel-lean mixtures) until the reduced flammability of the mixtures causes a net increase in HC emissions. These emissions originate from the flame quench layer - where the flame is extinguished by cold boundaries; regions like piston ring grooves can be particularly important. The outer edge of the quench can also contribute to the CO and aldehydes emissions. The formation of Nox is more complex since it is dependent on a series of reactions such as the Zeldovich mechanism:
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