How does fuel systems work?

Hi, I know a thing or two about this.

On your Lycoming the basic description is as follows.

You have a mechanical fuel pump that will generally run around 35 psi or so to a servo (exact settings vary). This servo has a venturi (similar to a carb). The Delta-P that you get from this venturi moves a piece of rubber inside which then kicks that ~35 PSI down to a lower number. The delta P is caused by airflow going through the venturi, so this works well for measuring actual airflow.

Either before or after that metering (I forget which) is the pressure reduction caused by the mixture knob, which is essentially a variable orifice to reduce pressure.

After the servo, the fuel goes up to a fuel distributor (or spider) which then splits that fuel flow (pressure) to each of the nozzles (one per cylinder). Those nozzles have calibrated orifices to produce equal flow. This is part of why it's important to keep them clean. The nozzles have screens to allow some small amount of air in with the fuel to help atomize it. This is a continuous injection system so it's a constant dribble/spray/whatever. However the pressure at the nozzles will vary depending on the engine and the power setting. More pressure = more fuel. I forget the exact range here.

If you have a turbocharged engine the nozzles have boost pressure fed on the outside of the engine as well to prevent the boost from trying to blow the fuel out. It's important to keep these lines clear if you have them or it can cause all kinds of weird fuel flow imbalances.

Certain Lycomings have a fuel return for venting purposes to aid hot starting. I think the restart Cessnas were the main ones with that.

Continental is another ball of wax. It's a simpler concept that ends up being more complex in execution.

The function of the vehicle fuel system is to store and supply fuel to the engine. The engine intake system is where the fuel is mixed with air, atomized, and vaporized. Then it can be compressed in the engine cylinder and ignited to produce energy or power. Although fuel systems vary from engine to engine, all systems are the same in that they must supply fuel to the combustion chamber and control the amount of fuel supplied in relation to the amount of air.

The fuel is stored in the fuel tank and the fuel pump draws fuel from the tank. It then travels through the fuel lines and is delivered it through a fuel filter to the fuel injectors (carburetors and throttle body injection were used on older vehicles). As the fuel is delivered, the final conditions for providing complete combustion are atomization and the spray pattern of the fuel. Atomization is accomplished as a result of the injection pressure, due in part to the diameter of the holes in the injector. The spacing, angle and number of holes in the injector tip determine the spray pattern.

Depending on whether your vehicles fuel system is a return type or returnless type system, the fuel pressure is regulated differently. A return type system has a fuel pressure regulator that varies the fuel pressure based on the amount of vacuum from the intake system. This is so the amount of fuel pressure and flow of fuel as it reaches the injectors remains consistently the same. Whereas a returnless type system uses the powertrain control module (PCM) to regulate fuel delivery. There is a fuel pressure sensor mounted to the supply rail of the fuel injectors to allow the PCM to monitor fuel pressure. When the fuel pressure and flow starts to drop due to increase of engine speed or load the PCM compensates by increasing injector duration and/or operating speed of the fuel pump.

The basic symptoms of any type of vehicle fuel system that is showing signs of wear or deterioration are:

• Difficult Engine Starting
• Slow or Hesitation at Acceleration
• Stalling While Driving
• Intermittent Power Loss
• Check Engine Light or Service Engine Soon Light Illuminated
• Engine Idling Rough
• Excessive Engine Smoke
• Noticeable Fuel Odors
• Decreased Fuel Economy

If you notice any of these types of symptoms, we recommend having it checked out before something fails on your vehicle and leaves you stranded.

The fuel pressure, flow and operation of the fuel system components will need to be tested in order to diagnose the problem.

Maintenance on the fuel system is pretty simple. The main component is to keep clean fresh fuel in your vehicle. Contamination and debris are the number one cause of fuel system failures. If your vehicle is equipped with an inline fuel filter it is recommended that the filter is replaced on a yearly basis or approximately every 15,000 miles. Having the fuel system cleaned approximately every 20,000 miles with a professional decarboning and fuel system cleaning service to keep the buildup of fuel byproducts to a minimum.

Steve and Karen Johnston are owners of All About Automotive in Historic Downtown Gresham. If you have questions or comments, call them at 503-465-2926 or email them at [email protected].