FAA Powerplant Written Practice Test

Which of the following is an effect of using a too lean fuel mixture?

1. Increased fuel consumption

2. Risk of overheating engine

3. Improved engine responsiveness

4. Lower emissions

The correct answer is: Risk of overheating engine

Using a too lean fuel mixture can significantly increase the risk of overheating the engine. A lean mixture means there is less fuel relative to the amount of air in the combustion chamber, which can lead to higher combustion temperatures. When the mixture is too lean, there may not be enough fuel to absorb the heat produced during combustion, resulting in excessive heat that can damage engine components and ultimately lead to engine failure. This overheating can affect various parts including the cylinders, pistons, and valves, compromising their integrity and performance. In contrast, increased fuel consumption typically occurs with a rich mixture, where more fuel is supplied than necessary. Improved engine responsiveness is usually associated with an optimal fuel-to-air ratio, not a lean one. Similarly, lower emissions are generally a benefit of a properly tuned mixture rather than one that is excessively lean, which can produce higher levels of unwanted emissions due to incomplete combustion.