Why does a fuel/air mixture become richer as altitude increases?

A fuel/air mixture becoming richer as altitude increases is primarily due to changes in the air's density. At higher altitudes, the atmospheric pressure decreases, leading to a reduction in air density. Since the amount of fuel injected into the combustion chamber remains relatively constant or is not significantly affected by altitude in many systems, the overall ratio of fuel to air becomes higher, resulting in a richer mixture.

This change in mixture can have important implications for the performance of engines, especially in lighter-than-air vehicles where achieving the correct buoyancy and power output is crucial. It's important to understand that the air's lower density means that while the amount of fuel stays the same, there is less air available for combustion, thus increasing the richness of the mixture.