Typical Fuel Systems - An Overview Richard Skiba Skiba, R. (1999). Typical Fuel Systems An Overview, Pacific Flyer, March. Skiba, R. (2001). 'Typical Fuel Systems - An Overview'. Air Sport: The Home of Australian Experimental Aircraft, February/March. Skiba, R. (2001). Typical Fuel Systems - An overview', Gyro News, Vol.15, No. 4, Summer (Dec/Jan/Feb). The Australian Ultralight Federation Inc (AUF) Technical Manual (Issue 2, Section 4.1) points out that all holders of an AUF pilot certificate are "accredited with the minimum qualifications necessary to maintain their own aircraft". AUF pilots are automatically granted this level of accreditation which is referred to as a Level One Maintenance Authority. This means that an owner-operator of an ultralight aircraft is entitled to perform the following maintenance on their own aircraft: Post assembly, daily and scheduled inspection, repairs, modifications, airworthy notice, heavy landing inspection, component overhaul and replacement, engine installation and welded repairs (Issue 2, Section 4.0 Annex A) so long as the aircraft is not used for hire and reward. This article aims to provide the basics of a typical fuel system ultralight pilots may encounter in their aircraft giving those who aren't familiar with the way the system works a fundamental introduction. The purpose of the fuel system in an engine is to provide the air and fuel to the cylinder in the correct ratio to allow for combustion. Generally, there are two common ways in which fuel is delivered to the cylinder: carburetion, where a carburetor is used to mix the fuel and air before it enters into the engine, and fuel injection. Fuel injection may be either into the cylinder(s) directly, referred to as direct injection, or right above the intake valve, called port injection. In order for the process of mixing to occur, there are typically a number of components which must be present and which constitute the fuel system. These include the fuel tank and cap, fuel line, fuel pump, fuel filter, carburetor or injectors and intake manifold. Each of these components will now be described and considered in terms of the problems they may cause to an aircraft s performance and ways in which these problems can be overcome. First of all, we will discuss the possibilities which may result in no fuel reaching the carburetor/injectors. Our investigation will begin with the fuel tank. The fuel tank s purpose is to act as a reservoir for the fuel which will eventually be used by the engine. Fuel tanks are susceptible to some obvious and some not so obvious problems. In terms of the obvious there lies rust and leaks, which may result from puncture, and in both cases the tank should be replaced. It does not need to be pointed out that an aircraft with no fuel resulting from a Page 1 of 5
leaking tank whilst in flight presents a need for concern. With regard to the not so obvious is the problem which may be generated by a blocked fuel tank vent. If the tank is not vented, the vent may be on the fuel tank cap or using some alternate means, the fuel will not be able to be sucked through the system by the pump. Those of you who have bottle fed a baby would understand this concept. The baby sucks from the bottle making an air tight seal with teet. They do however break the seal occasionally and let air back into the bottle (amazingly enough they do this instinctively, but that s another story). If the fuel pump is strong enough, it will collapse the tank trying to get the fuel out as a vacuum is created in the tank. Baffles are fitted the tanks to prevent fuel surging of fuel resulting from changes in attitude. These are basically compartment walls. These may also be a problem as loose baffles may cause blockages or obstructions which restrict fuel flow. Moving along the fuel system we come to the fuel lines. These lines connect all the components in the system thus transferring fuel from one place to another. Problems may occur with fuel lines where there are breakages, resulting in insufficient amounts of fuel delivered for combustion, or blockages, with the same effect. Breakages may occur from contact with sharp objects on the aircraft s body or from excessive engine vibration and movement and blockages generally occur as a result of dirty fuel. These sorts of problems may be avoided by checking the lines for cracks and occassionaly cleaning the fuel tank in accordance with the manufacturer s instructions. A further problem which may occur in the fuel lines is that of vapor lock when the lines are to close to the hot components of the exhaust system. Fuel filters are usually installed along the fuel line. Their purpose is to keep the fuel getting to the carburetor/injectors clean. They are usually found between the fuel tank and the fuel pump, however they may be found anywhere in the fuel system. The main contaminants removed from fuel by these filters are water, arising from condensation in the fuel tank, and rust from fuel tanks and fuel storage containers. The fuel filters need to be checked regularly and replaced/cleaned when they are dirty. A blockage in a fuel filter prevents fuel moving from the tank to the cylinder thus stopping combustion and any flying which may be occurring. In order to draw the fuel from the tank to where it is mixed with air a fuel pump may be used. A mechanical fuel pump is made up of a chamber which is divided by a diaphragm. The diaphragm is moved up and down by an arm driven by the engines camshaft. When the diaphragm moves down, it draws fuel in past the inlet valve which allows flow in one direction. A vacuum is created with the downward movement of the diaphragm thus sucking the fuel into the pump. A spring then returns the diaphragm and as it moves up, the fuel is forced out the other end. An electric fuel pump operates in a similar manner as a mechanical one except that a solenoid is used to operate the diaphragm. If the fuel pump is incorrectly Page 2 of 5
mounted such that it is unable to generate sufficient vacuum from to draw fuel through the line or to close to a heat source (causing vapor lock) a fuel problem will arise. Likewise, it is possible that the diaphragm can crack or tear. Again, vacuum will not be sufficient to suck through the fuel. The next place where faults can occur is the carburetor. The carburetor mixes the right amount of air and fuel and delivers the mixture to the combustion chamber depending on the engine's requirements at the time. Air which has passed through an air filter is sucked into the air passage within the carburetor. This passage is narrower at one point which makes the air flow faster once it gets passed the restriction (much like a bottle-neck on a four line highway narrowed to two then expanded back to four). This acceleration in the speed of the air causes a drop in pressure and a partial vacuum in the passage. The passage is normally referred to as the venturi, and this effect the venturi effect. The partial vacuum sucks fuel from the float chamber, where it is pumped to by the fuel pump. The incoming fuel mixes with the air and is passed to the engine by means of the manifold. The throttle is responsible for controlling the fuel/air mixture entering the engine. When the throttle is used, it operates a valve. If the valve is opened more it allows greater air flow through the venturi and therefore more fuel as a greater partial vacuum is created. In order for the fuel to move into the partial vacuum and mix with the air the correct amount of fuel must be stored in the float bowl. Too much fuel in the float bowl and the engine will taking in too greater an amount of fuel for the engines requirements. As a result the engine will run rich (too much fuel by weight for the amount of air). In a running engine, this will be characterised by smoke (or excessive smoke in a 2-stroke engine), a lag in acceleration, low exhaust gas temperature and excessive fuel consumption. If on the other hand, the float level is too low, an insufficient amount of fuel will enter the venturi. This means that the engine will run lean (a disproportionate amount of fuel air with an insufficient amount of fuel to air by weight). In a running engine it may backfire during deceleration, lag during acceleration, inconsistent idling (or no idle at low rpm) and a high exhaust gas temperature. The float needle ensures that the correct amount of fuel remains in the float bowl such that gravity and partial vacuum can work together to mix the right amount of fuel with the air passing through the venturi. Any problems including blockage and wear in the needle will cause the engine to function in an undesirable manner. The needle is opened by a float which senses the amount of fuel in the float bowl (much like a float in an ordinary toilet cistern) and any damage to this float would also cause fuel system problems. The above is a simplified description of the functioning of the carburetor and it is not the purpose of this article to go into depth on its function. It should be noted that there are many ways a carburetor can fail, especially given that it is a mechanical device and all mechanical devices are prone to wear. Carburetor problems can often be solved with a clean, or more Page 3 of 5
extremely with a rebuild. Using good quality fuel can prevent many problems associated with carburetor blockages, as can using a carburetor cleaner occasionally. In summary, a fuel system which is not kept clean and well maintained may cause the engine to not start, stall at idle speed, accelerate poorly, misfire or cut-out at high speeds and/or excessively burn fuel. Specific components of fuel systems will vary from one aircraft to another and any maintenance should, of course, be carried out in accordance with manufacturer's instructions, in line with proposed maintenance schedules and with regard to licencing authorities requirements. # Page 4 of 5
Fuel Tank and cap Rust Leaks Blocked fuel tank vent Blockages from dirt or loose Direction of fuel Fuel Filter Problems may be associated with: Blockages from contaminants Fault in the filter Fuel Pump Wear of components Damaged diaphragm Incorrect mounting Carburetor/Inject ors Worn or incorrectly adjusted needle and seat Incorrectly set float Incorrectly set control screws such as idle etc Fuel Lines Breakages Vapor lock due to closeness to heat sources Fuel to Inlet Manifold Diagram of a typical fuel system and some of the problems which may occur in each component Page 5 of 5