What requirement must reciprocating fuel metering systems meet?

• A. Operate at constant pressure regardless of air intake
• B. Meter fuel proportionately to air to maintain proper ratio at all speeds and altitudes
• C. Always supply maximum fuel
• D. Adjust flow to match only RPM changes

Answer: (B)

In a reciprocating fuel metering system, the goal is to deliver fuel in direct proportion to the air entering the engine so the air–fuel ratio stays correct across all operating conditions. As air density changes with altitude and air flow changes with engine speed, the system must adjust fuel flow to match the amount of air the engine is ingesting. If fuel were kept at a fixed level or only tied to RPM without considering how much air is actually entering the cylinders, the mixture would become too rich or too lean at different speeds and altitudes, causing rough running, overheating, or fouling. Therefore, the best approach is to meter fuel proportional to air to maintain the proper ratio at all speeds and altitudes.