What happens when an engine running on a leaned mixture is returned to full rich?

When an engine running on a leaned mixture is returned to full rich, the expected behavior is for the power to initially increase followed by a potential decrease. Initially, there is a richening of the fuel-air mixture, which allows for more fuel to enter the combustion chamber. This can lead to an increase in power output, as the engine can burn the richer mixture more effectively under certain conditions.

However, as the mixture goes from lean to rich, it can also lead to some operational changes within the engine. If the mixture becomes excessively rich after being leaned, it may result in inefficient combustion, leading the engine to either experience a temporary loss of power after the initial increase or exhibit rough running.

This dynamic behavior is largely influenced by the specific engine design, conditions at the time of the transition, and how closely the mixture is adjusted towards the ideal stoichiometric ratio for combustion. Thus, while there is an immediate increase in power with the richer mixture, the subsequent decrease can happen due to various factors, such as engine overload or inefficient combustion if the mixture becomes overly rich.