The aircraft's fuel system is responsible for delivering a continuous flow of fuel to the aircraft's engines throughout the duration of a flight. In terms of complexity and importance, few aircraft elements place higher than the fuel system. Although not homogenous in their design, the difference in fuel systems between two aircraft of the same class are generally composed of similar parts. In order to provide a better understanding of fuel systems, this blog will discuss their most important components and the function that each performs.
Most large aircraft, particularly passenger and cargo variants, store their fuel in large wing tanks. This placement makes the wings' structure more stable, helping to reduce vibration. It is easy to make the assumption that all the fuel stored in a tank is available. However, up to several gallons of fuel is purposely unusable, helping aid in the trapping of dense contaminants.
Since the fuel tanks are exposed to cold atmospheric temperatures, there is a chance that the contained fuel may freeze when it is under 0 degrees Celsius. Fuel heating elements help prevent freezing and optimize combustion by warming the fuel before it is sent downstream.
In order to effectively deliver fuel to the engine at an optimal volume, most aircraft implement an engine-driven fuel pump. This pump may carry fuel directly to the engines or transfer it between tanks, as is sometimes needed to help balance fuel levels or weight distribution. In addition to the engine-powered pump, most planes also contain an auxiliary electric pump to provide backup in case of failure.
The fuel reaching the engine should be free of debris and as many microscopic contaminants as possible. To aid in this, fuel systems implement a mesh filter at the lowest point in the injection system. This area in the fuel system is equipped with a drain to allow maintenance personnel to inspect and empty the contents between flights.
Fuel Selector Valve
When more than one fuel tank is installed on the aircraft, the pilot may choose which one to pull from using a selector valve. Generally, the mechanism only allows fuel to be pulled from one tank at a time in order to prevent vapor lock, which may occur when air enters one of the lines. However, some aircraft feature a safeguard to avoid vapor locks.
Fuel injection systems implement servomechanical regulators to test the pressure of fuel before it reaches the nozzle. Depending on the thrust demands of the plane, the regulator modulates the amount of fuel entering the nozzle. If fuel predominates in the mixture, it is considered "rich," while a "lean" mixture denotes a higher air content.
As there are typically more than one fuel nozzles equipped in the engine cylinder, a fuel distributor is required to divide the fuel evenly.
In the nozzle, fuel is mixed with bleed or turbocharged air to create a mixture with optimal combustion capability. The nozzle is responsible for atomizing this mixture and delivering it to the engine with uniform spray patterns.
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