What are Fuel Metering Systems in Aircraft?

The fuel metering system is an often overlooked piece of equipment in an aircraft's engine. Fuel economy and thrust rely on a deliberate fuel-to-air ratio entering the engine, that of which is dependent on speed and altitude. This task could not be completed without a fuel metering system. In this blog, we discuss the purpose and function of the fuel metering system. 

It is crucial to understand the two main types of engine systems in aircraft, turbine and reciprocating engines, before understanding the role of fuel metering systems in the fuel delivery process. Turbine engines have a fan in the front that sucks cold air from the atmosphere and pulls it into an inlet. From there, the air enters a component called a compressor, which works by increasing the air's pressure and, thereby, its temperature. Next, the air is mixed with fuel in the combustion chamber and is ignited, producing a very hot stream of gas. Then, the exhaust travels past a turbine which cools it, and finally, it exits through an exhaust nozzle in the back of the engine. This expulsion of accelerated gas going backward propels the plane forward because of Newton's third law of motion. 

Reciprocating engines are very similar to turbine engines in principle and steps. They work by converting pressure into rotational motion. The power in a reciprocating engine comes from the burning and discharge of fuel in a cylinder. Like turbine engines, a fuel and air mixture enters the cylinder where it is compressed, increasing its temperature and thermal efficiency. Then, the mixture is ignited by a spark plug which causes an explosion that powers the rotational motion. Finally, the exhaust from the explosion is expelled, and the cycle repeats. 

In both of these engines, an appropriate ratio of fuel to air is required before combustion. If too much fuel were to enter the engine, there would be a lower than desired temperature in the combustion chamber. The consequences of a fuel-rich combustion chamber are a loss of power, excessive fuel consumption, and rough engine operation. On the contrary, if there were not enough fuel injected into the engine, there would be an overheating effect with a resultant loss of power. 

The two fuel metering systems most used in aircraft are carburetors and fuel injection systems. In carbureted aircraft, fuel and air meet and are vaporized before entering the combustion cylinder. Then, as the pilot throttles forward, more fuel and air are let into the carburetor as there is a greater demand for power. In fuel injection systems, the air and fuel do not combine until they reach the cylinder. With this system, a regulator measures the airflow entering the engine, and a pump delivers a concordant amount of fuel to the cylinder. Fuel injection systems are more precise with fuel delivery; thus, they are more fuel-efficient but harder to start. 

As altitude increases, the role of fuel metering systems becomes even more critical and complex. At a higher altitude, the density of air decreases, which means less air is entering the engine to be combusted. This means that, unless the fuel being delivered is altered, the ratio will be too rich, causing the engine to lose efficiency and power. Fuel injection systems adapt to the altitude automatically with a regulator, whereas the carburetor's fuel mixture will be adjusted manually to maintain a proper ratio.

To maintain your aircraft engines' safe and efficient operation, it is essential to have properly working fuel metering systems. Veritable Aviation is an online distributor of many aircraft parts, including carburetor and fuel injection system components. We invite you to search our catalog of parts and components sourced from trusted manufacturers, where you can find over 2 billion parts available for purchase. Please fill out an instant RFQ or contact our team today for a quick and competitive quote.


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