FAA Powerplant Written Practice Test

What is the function of the automatic mixture control on a pressure injection carburetor?

1. To maintain a constant air-fuel ratio

2. To enrich the mixture at high altitudes

3. To compensate for changes in air density due to temperature and altitude

4. To prevent fuel vaporization

The correct answer is: To compensate for changes in air density due to temperature and altitude

The function of the automatic mixture control on a pressure injection carburetor is designed to adapt the air-fuel mixture based on variations in air density that occur due to changes in temperature and altitude. This system helps ensure optimal engine performance by automatically adjusting the fuel flow to maintain the appropriate mixture regardless of environmental conditions. When an aircraft ascends to higher altitudes, the air density decreases, which can lead to a lean mixture if not compensated for. Similarly, drastic changes in temperature can also affect the density of the air entering the carburetor. The automatic mixture control effectively manages these variables, allowing the engine to operate smoothly and efficiently across a range of conditions. It assists in maintaining engine performance by ensuring that the correct air-fuel ratio is achieved, thus optimizing combustion and preventing issues such as engine roughness or stalling.