FAA Powerplant Written Practice Test

What role does manifold pressure play in a fuel injection system?

1. It indicates the fuel viscosity

2. It determines the air-fuel mixture quality

3. It affects the propeller efficiency

4. It controls the ignition timing

The correct answer is: It determines the air-fuel mixture quality

In a fuel injection system, manifold pressure is a crucial parameter that directly influences the air-fuel mixture quality. Manifold pressure is a measure of the absolute pressure of the air-fuel mixture present in the intake manifold and is typically expressed in inches of mercury (inHg) or kilopascals (kPa). When the manifold pressure varies—due to changes in engine load, throttle position, or altitude—the fuel delivery system adjusts the amount of fuel injected into the engine to maintain the optimal mixture ratio. An appropriate air-fuel mixture is essential for efficient combustion, engine performance, and emissions control. If the manifold pressure is low, the fuel injection system compensates by increasing the fuel flow, while a high manifold pressure may lead to a reduction in fuel flow. This relationship helps ensure that the combustion process is optimized, allowing the engine to perform efficiently across a range of conditions. Thus, understanding and managing manifold pressure is fundamental in achieving the proper air-fuel mixture necessary for peak engine operation.