Prerequisite Knowledge:
If you remember from the previous article, an engine requires gas, air, and a spark in order to run. This article will explore the gas aspect of that triangle is managed.
Fuel Tanks and Lines
Light aircraft generally have two fuel tanks, with one in each wing. The fuel tanks are connected to each engine via a series of fuel lines, which all converge at the fuel selector (in most planes that is). The fuel selector is a valve controlled by the pilot, which lets the pilot controls which tank(s) the engine draws fuel from.
If you remember from the previous article, an engine requires gas, air, and a spark in order to run. This article will explore the gas aspect of that triangle is managed.
Fuel Tanks and Lines
Light aircraft generally have two fuel tanks, with one in each wing. The fuel tanks are connected to each engine via a series of fuel lines, which all converge at the fuel selector (in most planes that is). The fuel selector is a valve controlled by the pilot, which lets the pilot controls which tank(s) the engine draws fuel from.
Fuel Venting
Fuel tanks need to be vented, which means that there has to be a way for air to come in and replace the space in the tank taken up by fuel. Think of a fuel tank like a water bottle; if you try and drink from the water bottle without letting air enter the bottle, a vacuum will be created inside the water bottle that prevents water from draining. This vacuum is created because there is nothing replacing the space that the water once took, and it will grow stronger and stronger as more water is drained. Eventually, a point will be reached where the vacuum is so strong that no more water can be removed from the water bottle, unless something (for example, air) is allowed to enter and fill in the vacuum. Fuel tanks are the same, if it is not vented, a point will be reached where the vacuum inside the tank prevents any more fuel from being drained.
Fuel tanks need to be vented, which means that there has to be a way for air to come in and replace the space in the tank taken up by fuel. Think of a fuel tank like a water bottle; if you try and drink from the water bottle without letting air enter the bottle, a vacuum will be created inside the water bottle that prevents water from draining. This vacuum is created because there is nothing replacing the space that the water once took, and it will grow stronger and stronger as more water is drained. Eventually, a point will be reached where the vacuum is so strong that no more water can be removed from the water bottle, unless something (for example, air) is allowed to enter and fill in the vacuum. Fuel tanks are the same, if it is not vented, a point will be reached where the vacuum inside the tank prevents any more fuel from being drained.
This is the fuel vent, under the wing of a Cessna 172. The inlet is pointed forwards, which means that as the plane flies forward, air is forced into the vent line, creating pressure in the fuel tanks that helps force fuel down the tank drains and through the fuel system. There is only a single vent inlet on the Cessna 172, and it's located under the left wing, connected to the left tank. In order to vent the right tank, a vent line connects the left and right tanks, allowed air or fuel to move between them.
Gravity Fed vs. Pump Systems
The vent lines help provide fuel presssure using air, but this alone is not enough to push the fuel through the fuel systems. This is where aircraft designers can choose between two fuel systems.
Gravity fed systems use gravity to force the fuel through the fuel lines to the engine. In a gravity fed system, the weight of the fuel in the tanks helps force it down towards the engine. Gravity fed systems are extremely simple, but they only work in aircraft where the fuel tanks are located above the engine. If the engine is located above the fuel tanks, fuel will flow in the wrong direction (from the engine to the tanks), which will starve the engine of fuel. Similarly, gravity fed systems only work in positive-g flight (e.g. level flight). In negative-g environments (acrobatic maneuvers or inverted flight), the system will be unable to provide the engine with fuel.
In a pump driven system, a fuel pump is used to maintain a constant fuel pressure, which allows fuel to move consistently to the engine. This type of system is slightly more complicated, but it eliminates the problems with the gravity fed system. A gravity fed system may also have fuel pumps incorporated, in order to increase reliability (e.g. the Boeing 737 is primarily gravity fed, but still has fuel pumps).
Gravity Fed vs. Pump Systems
The vent lines help provide fuel presssure using air, but this alone is not enough to push the fuel through the fuel systems. This is where aircraft designers can choose between two fuel systems.
Gravity fed systems use gravity to force the fuel through the fuel lines to the engine. In a gravity fed system, the weight of the fuel in the tanks helps force it down towards the engine. Gravity fed systems are extremely simple, but they only work in aircraft where the fuel tanks are located above the engine. If the engine is located above the fuel tanks, fuel will flow in the wrong direction (from the engine to the tanks), which will starve the engine of fuel. Similarly, gravity fed systems only work in positive-g flight (e.g. level flight). In negative-g environments (acrobatic maneuvers or inverted flight), the system will be unable to provide the engine with fuel.
In a pump driven system, a fuel pump is used to maintain a constant fuel pressure, which allows fuel to move consistently to the engine. This type of system is slightly more complicated, but it eliminates the problems with the gravity fed system. A gravity fed system may also have fuel pumps incorporated, in order to increase reliability (e.g. the Boeing 737 is primarily gravity fed, but still has fuel pumps).
Fuel Selector/Strainer
Most aircraft have a fuel selector, which is a valve controlled by the pilot that lets them control which tank(s) the engine draws fuel from. The fuel selector valve is where the individual lines from each tank come together. Below is the fuel selector on the Cessna 172, located on the floor of the cockpit.
Most aircraft have a fuel selector, which is a valve controlled by the pilot that lets them control which tank(s) the engine draws fuel from. The fuel selector valve is where the individual lines from each tank come together. Below is the fuel selector on the Cessna 172, located on the floor of the cockpit.
The fuel strainer (also known as a gascolator) is used to strain the fuel of any debris or contaminents. Sending contaminated fuel to the engine can cause engine damage or failure, so it is important that the fuel is clean.
Caruburetors vs. Fuel Injection
Past the fuel selector, the details of the fuel system depend on the type of engine, and whether it is carbureted or fuel injected.
Carbureted systems rely on a carburetor to control the mixing of air and fuel, before that fuel/air mixture is sent off to the engine. Most light aircraft rely on float type carburetors, as shown below:
Caruburetors vs. Fuel Injection
Past the fuel selector, the details of the fuel system depend on the type of engine, and whether it is carbureted or fuel injected.
Carbureted systems rely on a carburetor to control the mixing of air and fuel, before that fuel/air mixture is sent off to the engine. Most light aircraft rely on float type carburetors, as shown below:
A float carburetor has two main parts, the venturi and the float chamber. The venturi will be discussed more in the next article.
The float chamber is where the amount of fuel being sent into the carburetor is controlled. Inside the float chamber is a carburetor float, which floats on top of the fuel inside the chamber. When there is too much fuel in the float chamber, the float gets pushed upwards, which constricts the fuel inlet and reduces the amount of fuel that enters the carburetor. When the fuel level inside the float chamber gets too low, the float drops, and the fuel inlet widens, allowing more fuel to enter the chamber. The fuel then exits the float chamber, passing by the mixture needle. The mixture needle is moved up and down by the mixture control in the cockpit, and it's movement either lets more or less fuel exit the float chamber through the outlet. Once the fuel exits the float chamber, it enters the air stream through the discharge nozzle, and vapourized into the air. This fuel/air mixture is then sent off to the cylinders.
Fuel injection systems work very differently from carbureted systems. In a carbureted engine, the fuel and air are mixed outside of the combustion chambers of the cylinders. In a fuel injection engine. fuel injectors are used to discharge an exact amount of fuel directly into each cylinder at a certain time. The fuel and air meet and mix inside the cylinder, right before ignition.
The float chamber is where the amount of fuel being sent into the carburetor is controlled. Inside the float chamber is a carburetor float, which floats on top of the fuel inside the chamber. When there is too much fuel in the float chamber, the float gets pushed upwards, which constricts the fuel inlet and reduces the amount of fuel that enters the carburetor. When the fuel level inside the float chamber gets too low, the float drops, and the fuel inlet widens, allowing more fuel to enter the chamber. The fuel then exits the float chamber, passing by the mixture needle. The mixture needle is moved up and down by the mixture control in the cockpit, and it's movement either lets more or less fuel exit the float chamber through the outlet. Once the fuel exits the float chamber, it enters the air stream through the discharge nozzle, and vapourized into the air. This fuel/air mixture is then sent off to the cylinders.
Fuel injection systems work very differently from carbureted systems. In a carbureted engine, the fuel and air are mixed outside of the combustion chambers of the cylinders. In a fuel injection engine. fuel injectors are used to discharge an exact amount of fuel directly into each cylinder at a certain time. The fuel and air meet and mix inside the cylinder, right before ignition.
The amount of fuel that is sent to each injector is controlled by a fuel metering unit (FMU). The FMU uses throttle and mixture settings to control how much fuel is sent to each cylinder, with high throttle/mixture settings resulting in more fuel being sent, and low throttle/mixture settings resulting in less fuel being sent.
Carbureted systems can either be gravity fed or driven by a pump, but fuel injection systems rely on a pump to provide constant fuel pressure. Usually this takes the form of an engine driven pump, which is a pump turned by the engine. Aircraft will generally also have a standby electric pump for use if the main engine driven pump fails.
Fuel injection systems hold many advantages over older carbureted engines. Since the exact amount of fuel being sent to each cylinder can be controlled with a much higher degree of precision, fuel injected engines are generally much more efficient. Fuel injection engines also don't suffer from carburetor icing (discussed in the next article). However, they are prone to a situation known as vapour lock, where fuel vapourizes inside of the fuel lines in hot conditions, which stops the movement of fuel to the engine.
Engine Priming
Priming the engine is the act of send small amounts of fuel into the engine before startup, in order to make starting easier. Carbureted aircraft generally have a dedicated primer, which the pilot uses to send small amounts of fuel directly into each cylinder. Fuel injection engines don't have a dedicated primer, so instead the engine is primed via other memes, such as momentarily activating the electric fuel pump. Specific priming procedures vary from airplane to airplane.
Carbureted systems can either be gravity fed or driven by a pump, but fuel injection systems rely on a pump to provide constant fuel pressure. Usually this takes the form of an engine driven pump, which is a pump turned by the engine. Aircraft will generally also have a standby electric pump for use if the main engine driven pump fails.
Fuel injection systems hold many advantages over older carbureted engines. Since the exact amount of fuel being sent to each cylinder can be controlled with a much higher degree of precision, fuel injected engines are generally much more efficient. Fuel injection engines also don't suffer from carburetor icing (discussed in the next article). However, they are prone to a situation known as vapour lock, where fuel vapourizes inside of the fuel lines in hot conditions, which stops the movement of fuel to the engine.
Engine Priming
Priming the engine is the act of send small amounts of fuel into the engine before startup, in order to make starting easier. Carbureted aircraft generally have a dedicated primer, which the pilot uses to send small amounts of fuel directly into each cylinder. Fuel injection engines don't have a dedicated primer, so instead the engine is primed via other memes, such as momentarily activating the electric fuel pump. Specific priming procedures vary from airplane to airplane.
The primer on a carbureted Cessna 172N
The amount of priming an engine needs depends on the situation, and varies from airplane to airplane. Underpriming an engine will mean it's more difficult to start. Overpriming can increase the risk of a fire on startup, and in extreme cases, it can flood the engine. Engine flooding is a situation where the fuel is not vapourized in the cylinders, but instead present as a liquid. Liquid fuel is not flammable at all, so the engine cannot run until the fuel eventually vapourizes.
Leaning
Leaning is the last thing that will be discussed here, but also one of the most important concepts to know.
Piston engines need a certain ratio of fuel and air to function correctly, with the correct ratio of air to fuel being about 14 parts air to 1 part fuel. The engine is said to be running rich when there is more fuel than what is required (e.g. a 13:1 ratio is said to be rich, the engine is getting less air so it needs less fuel to maintain the balance). The engine is running lean when this there is less fuel than what is required (e.g. a ratio of 15:1 is said to be lean, there is more air so the engine needs more fuel to stay in balance). Maintaining this ideal ratio is a challenge for aircraft, as they fly at a wide range of altitudes. At sea level, the air is relatively dense, which means that the engine gets more air, and therefore needs more fuel in order to maintain the 14:1 ratio. However, the density of the air drops with altitude, so an aircraft flying at a higher altitude needs less fuel being sent to the engine to maintain the ideal 14:1 ratio. This is where the mixture control becomes important.
Leaning
Leaning is the last thing that will be discussed here, but also one of the most important concepts to know.
Piston engines need a certain ratio of fuel and air to function correctly, with the correct ratio of air to fuel being about 14 parts air to 1 part fuel. The engine is said to be running rich when there is more fuel than what is required (e.g. a 13:1 ratio is said to be rich, the engine is getting less air so it needs less fuel to maintain the balance). The engine is running lean when this there is less fuel than what is required (e.g. a ratio of 15:1 is said to be lean, there is more air so the engine needs more fuel to stay in balance). Maintaining this ideal ratio is a challenge for aircraft, as they fly at a wide range of altitudes. At sea level, the air is relatively dense, which means that the engine gets more air, and therefore needs more fuel in order to maintain the 14:1 ratio. However, the density of the air drops with altitude, so an aircraft flying at a higher altitude needs less fuel being sent to the engine to maintain the ideal 14:1 ratio. This is where the mixture control becomes important.
The mixture control is generally the red knob in the pilot. Pushing it forward, enrichens the mixture, while pulling it back leans the mixture.
Under normal operations, the mixture is kept at full rich for takeoff, landing, and flight below 3,000' MSL (3,000' above mean sea level). When operating outside of those areas, the mixture should be leaned.
Under normal operations, the mixture is kept at full rich for takeoff, landing, and flight below 3,000' MSL (3,000' above mean sea level). When operating outside of those areas, the mixture should be leaned.
Leaning is generally done using the tachometer, although the manifold pressure gauge should be used instead when flying a plane with a constant speed propeller (if it has a blue handle near the throttle and mixture, it has a constant speed propeller). The tachometer is the gauge that shows the engine RPM. If your plane has an EGT gauge (exhaust gas temperature), that is the most preferred gauge to use when leaning, even if you have a constant speed propeller.
When leaning, gradually move the mixture knob out, to lean the mixture. As you do so, the value on your chosen gauge will begin to rise, eventually reaching a peak. As you continue to lean the engine, the value on the gauge will eventually start dropping as it has passed the peak. At this point, the engine is said to be running lean of peak, where the peak represents the ideal ratio. You can leave the engine running slightly lean of peak, or start pushing the mixture knob forward again to enrichen the mixture. As you keep enrichening the mixture again, you will see the gauge rise again, peak once more, then start to fall. At this point, you are running rich of peak. Running rich of peak has a slightly higher fuel consumption, but it is generally easier to lean an engine for rich of peak rather than lean of peak (when operating lean of peak, small mixture changes have large effects). Running lean of peak has a lower fuel consumption, but is harder to obtain.
Leaning the engine helps it remain at its ideal air to fuel ratio during most of its operation, which not only lets it produce more power, but also extends its life. An engine running at its ideal air to fuel ratio is less prone to spark plug fouling, as there isn't an excess of fuel inside of the combustion chamber. This also reduces the risk of detonation and pre-ignition.
That's it for fuel systems! The next article will cover how the engine manages air.
When leaning, gradually move the mixture knob out, to lean the mixture. As you do so, the value on your chosen gauge will begin to rise, eventually reaching a peak. As you continue to lean the engine, the value on the gauge will eventually start dropping as it has passed the peak. At this point, the engine is said to be running lean of peak, where the peak represents the ideal ratio. You can leave the engine running slightly lean of peak, or start pushing the mixture knob forward again to enrichen the mixture. As you keep enrichening the mixture again, you will see the gauge rise again, peak once more, then start to fall. At this point, you are running rich of peak. Running rich of peak has a slightly higher fuel consumption, but it is generally easier to lean an engine for rich of peak rather than lean of peak (when operating lean of peak, small mixture changes have large effects). Running lean of peak has a lower fuel consumption, but is harder to obtain.
Leaning the engine helps it remain at its ideal air to fuel ratio during most of its operation, which not only lets it produce more power, but also extends its life. An engine running at its ideal air to fuel ratio is less prone to spark plug fouling, as there isn't an excess of fuel inside of the combustion chamber. This also reduces the risk of detonation and pre-ignition.
That's it for fuel systems! The next article will cover how the engine manages air.