Small Single-Engine Aircraft Fuel Systems
Single-engine aircraft may utilize any of several types of fuel systems, depending upon the fuel metering unit (carburetor or fuel injector) used and whether the aircraft is a high-wing or low-wing design.
a. Gravity-feed Systems
The most simple aircraft fuel system is that found on the small high-wing single-engine training-type airplanes. This type of system is illustrated in figure 1.1. These systems normally use two fuel tanks, one in either wing. The two tank outlets are connected to the selector valve that can draw from either tank individually, or both tanks can feed the engine at the same time. A fourth position on the selector valve turns off all fuel to the engine. Since both tanks can feed the engine at the same time. the space above the fuel in both tanks must be interconnected, and this space vented outside of the airplane. The vent line normally terminates on the underside of the wing where the possibility of fuel siphoning is minimized.
After the fuel leaves the selector valve, it passes through the main strainer and on to the carburetor inlet. Fuel for the primer is taken from the main strainer.
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Figure 1.1: Typical gravity-feed fuel system for a small single-engine, high-wing airplane.
b. Pump-feed systems
Low wing airplanes cannot use gravity to feed the fuel to the carburetor, and these airplanes use a fuel system similar to that in figure 1.2. The selector valve used in these systems can normally select either tank individually, or shut off all flow to the engine. But they do NOT have a both position, because the pump would pull air from an empty tank rather than fuel from a full tank. After leaving the fuel selector valve, the fuel flows through the main strainer and into the electric fuel pump. You will notice that the engine-driven pump is in parallel with the electric pump. so the fuel can be moved by either pump, and there is no need for a bypass feature to allow one pump to force fuel through the other. In order to assure that both pumps are functioning. note the fuel pressure produced by the electric pump before starting the engine, and then, with the engine running. turn the electric pump off and note the pressure that is produced by the engine driven pump.
The electric pump is used to supply fuel pressure for starting the engine and as a backup in case the engine-driven pump should fail and to assure fuel flow when switching from one tank to the other.
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Figure 1.2: Typical pump-feed fuel system for a small single-engine, low-wing airplane.
c. High-wing Airplane Using A Fuel Injection System
The fuel injection system requires an engine driven fuel pump, and the system in uses a Teledyne-Continental system that returns part of the fuel from the pump back to the
fuel tank. This fuel contains any vapours that could block the system, and by purging all of these vapours from the pump and returning them to the tank they cannot cause any problems in the engine.
Fuel flows by gravity from the wing tanks through two feed lines, one at the front and me at the rear of the inboard end of each tank, into two small accumulator (reservoir) tanks, and from the bottom of these tanks to the selector valve.
The selector valve directs fuel from the desired reservoir tank to the engine, and at the same time directs the fuel vapour from the engine-driven pump back to the selected reservoir tank. This vapour then returns to the wing tank that supplies the reservoir tank.
Tine electric auxiliary fuel pump picks up the fuel at the discharge of the selector valve and forces it through the strainer and on to the inlet of the engine-driven fuel pump. From the engine driven fuel pump, the fuel flaws to the heel-air control unit where the fuel that is needed for engine operation goes to the cylinders. and all of the excess fuel returns to the inlet side of the pump. Some of the fuel that. is taken into the engine driven pump has vapour in it and this fuel is returned to the selector valve through the fuel return check valve.
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