Certified Flight Instructor - Flight Instructor Airplane Practice Exam

What is the operating principle of float-type carburetors based on?

1. Measurement of the fuel flow into the induction system

2. Difference in air pressure at the venturi throat and the throttle valve

3. Increase in air velocity in the throat of a venturi causing a decrease in air pressure

4. Temperature differential in the fuel mixture

The correct answer is: Increase in air velocity in the throat of a venturi causing a decrease in air pressure

The operating principle of float-type carburetors is primarily based on the relationship between air velocity and pressure within the venturi. As air passes through the venturi, its velocity increases as it moves through the narrowed section. According to Bernoulli's principle, as the velocity of a fluid increases, the pressure within that fluid decreases. This drop in pressure at the venturi throat creates a suction effect that draws fuel from the float chamber into the airstream, mixing with the incoming air to form an appropriate fuel-air mixture for combustion. This interplay between increased air velocity and decreased pressure is crucial for the functioning of float-type carburetors as it enables the carburetor to effectively meter fuel into the airstream in response to engine demands. The other options do not accurately capture the core operating principle of float-type carburetors. While fuel flow, air pressure differences, and temperature differentials are relevant to carburetor function, they do not govern the fundamental mechanism of how a float-type carburetor operates. The principle that the increase in air velocity in the venturi induces a decrease in air pressure is key to understanding how these carburetors create the necessary fuel-air mixture for engine operation.