As mixtures become either rich or lean, what happens to the autoignition temperature (AIT)?

The autoignition temperature (AIT) is the minimum temperature at which a material will ignite without an external ignition source. As the composition of a fuel-air mixture becomes lean (less fuel for a given amount of air) or rich (more fuel for a given amount of air), the AIT typically increases.

In a rich mixture, there is insufficient oxygen to properly facilitate combustion, which means that a higher temperature is required for autoignition to occur. In contrast, while a lean mixture might contain enough oxygen to support combustion, the overall energy and reactivity of the mixture may still require an increase in temperature to initiate the reaction.

Thus, both rich and lean mixtures generally lead to an increase in the AIT, explaining why the temperature needs to rise to achieve conditions favorable for combustion to begin without an external flame or spark. This understanding is crucial for handling and working with flammable mixtures in various industrial and laboratory settings, ensuring safety and preventing unintended ignition.