FAA Powerplant Written Practice Test

When comparing fuel mixtures in air cooled reciprocating engines, which statement is true?

1. The mixture at idle is poorer than at rated power

2. The mixture at rated power is thinner than at cruise

3. The mixture at idle is richer than at rated power

4. All statements are incorrect

The correct answer is: The mixture at idle is richer than at rated power

In air-cooled reciprocating engines, the fuel-air mixture is richened at idle to ensure proper engine operation with minimal risk of stalling or misfiring. This is particularly important because, at idle, the engine runs at a lower RPM and is more susceptible to rough running if the mixture is too lean. A richer mixture increases fuel availability, allowing for smoother combustion in these conditions. As the engine accelerates to higher power settings, such as rated power, the fuel-air mixture becomes leaner. This is not only necessary to optimize power output but also to improve fuel efficiency and reduce emissions during full power operation. Therefore, at idle, the mixture is intentionally set richer than it would be at rated power, making the statement that the mixture at idle is richer than at rated power accurate. The focus on engine performance considerations at different operating conditions and the design of the fuel delivery system explains why the mixture at idle needs to be richer for optimal functioning.