Fuel System

One of the most widely known aircrafts in general aviation is the Cessna 172. Depending on the year and model, the aircraft may be equipped with one of the two ways that fuels may be delivered to the engine, gravity feed or fuel pump. The following sections will explore both methods.

Gravity Feed

Fuel Delivery Using Gravity

In the gravity feed method, fuel flows from the left or right fuel tank controlled by the fuel selector within the cockpit with the assistance of gravity. The fuel makes its way through the strainer to filter out unwanted contaminants and is delivered directly into the fuel air control unit.


Carburetors are used as the fuel air control unit in some older planes. The subcomponents that make up a carburetor are the fuel inlet, float chamber, air inlet, venturi, discharge nozzle, mixture needle, and throttle valve.

During fuel delivery, the fuel is received by the carburetor through the fuel inlet and fills the chamber where the amount of fuel is regulated by a float-type device. The second chamber of the carburetor resemble a throat. Its purpose is to promote air flow and withdraw fuel through the discharge nozzle from the float chamber in vapor form. This is achieved with a venturi, a restricted region near the middle of the throat that creates a low pressure area compared the incoming air and the float chamber. The higher pressure air at the air inlet allows air to flow through the venturi. Similarly, the high pressure air in the float chamber forces fuel through the mixture needle and discharge nozzle into the venturi where it mixes with the high pressure air from the air inlet. The fuel air mixture continues their way through the remainder of the carburetor and is delivered to the cylinders regulated by the throttle valve.

Carburetors are subjected to carburetor ice. When this happens, the engine may run rough or stops completely in worse case scenario. Carburetor icing is highly likely to occur when the temperature falls below 21°C (70°F) and relative humidity is greater than 80%, but it is also possible for temperature as high as 38°C (100°F) when relative humidity is greater than 50%. Use carburetor heat when carburetor ice is suspected.

Fuel Injection

Fuel Delivery Using Fuel Pump

The two additional components added to the fuel system that make up the fuel pump system are the engine-driven fuel pump and a backup electric fuel pump. Similar to the gravity feed method, fuel is drawn from the selected tank(s) indicated on the fuel selector by the engine-driven fuel pump. The fuel will flow from the selected tank(s), through the fuel selector, into the strainer to filter out unwanted contaminants, and delivered directly into the fuel air control unit. When the engine driven pump fails, the backup electric fuel pump can be used to maintain engine operation.

Fuel Injection

In a fuel injection system, fuel is delivered by fuel pump and received by the fuel air control unit or also known as servo regulator. This fuel air control unit gauges mixture to the fuel distribution valve based on the throttle position. The fuel distribution valve then routes mixture evenly to all cylinders.

Ignition System

The ignition system provides a way to ignite the fuel and air mixture in the cylinders. It is made up of magnetos, spark plugs, and the ignition switch. The magnetos are self-contained engine-driven component that delivers electrical power to the spark plugs in the cylinders. Each cylinder contains two spark plugs powered by two separate magnetos. This redundancy is employed such that when one magneto fails, all cylinders will continue to operate with the only impact being slight loss of power output. The ignition system will start by turning the ignition switch and continue to fire when the crankshaft is turning.

Four Stroke Operating Cycle

The four stroke operating cycle describes the process where the fuel and ignition systems work in unison to turn the crankshaft. The four strokes refer to the four different phases that occur in the cylinders: intake, compression, power, and exhaust.

During the intake phase, the piston moves away from the cylinder head drawing fuel and air mixture into the combustion chamber via intake valve. The piston then moves toward the cylinder head and compresses the mixture in the process during the compression phase. In the power stroke, the spark plugs fire and ignites the mixture resulting in a rapid expanding gases that pushes the piston away from the cylinder head and rotate the crankshaft. Lastly, the exhaust phase expels the burned gases from the combustion chamber through the open exhaust valve.

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