Q: How Does the Fuel System Work?

asked by on November 24, 2015

How does the fuel system work?

The fuel system in our vehicles is made up of many parts. The fuel tank stores the fuel and is generally made of galvanized metal or poly composite plastics. Some use an internal bladder to hold the fuel as well and all are sealed with a cap to reduce emission. In racing they refer to the tank as a fuel cell.

Next we have the pump that will deliver the fuel to the engine for combustion. In the past, this was usually a very low pressure mechanical pump which was driven by the engine. This 3 to 5 psi pump only needed enough delivery pressure to fill the bowl of the carburetor. With the advent of fuel injection systems that required higher delivery pressures fuel pumps, output was increased to 14+ psi for TBI (throttle body injection) and in some instances 30 to 80 psi or greater.

These injection systems came in both mechanical and electrical versions requiring higher delivery pressure which meant we needed a dependable pump that could produce these increased pressures so the in tank electric fuel pumps were brought into service. The modern in tank fuel pump module contains the actual pump, the fuel level sensor or sender, and a filter when used with a non-return style fuel system. The fuel delivery systems were either return style systems, which sent fuel to the carburetor or injectors that returned the unused portion of fuel to the tank, or return-less type, which only sent the actual fuel needed in a single feed system.

As with the early carburetor systems, the first fuel injection systems were developed as a return style system. This means that they pumped fuel to the carburetor or injectors and allowed the excess to return to the tank. An external filter was included in line with these systems as the fuel often made several trips from the tank and back as the demand at the injector or carburetor changed and required more frequent service intervals. The fuel pump module now is treated as a one piece unit and used in systems that only send the required amount of fuel to be used allowing for one pass of the filter, which is generally now in the tank within the unit.

Moving on, we have fuel lines which are a combination of flexible rubber lines used where the body of the vehicle will flex and solid steel lines which run along the chassis or undercarriage of the vehicle to the engine bay. Flexible lines again are used to route the fuel to the engine intake manifold area and connect to the fuel rail. Next, we have the fuel injectors themselves which are commonly small electromechanical solenoids that operate when activated and allow fuel to be sprayed out much like water comes out of a standard garden hose nozzle. This is the final stop for the fuel which can be delivered to a throttle body and injected into the incoming engine air stream or found with multipoint injection systems that are mounted to deliver the fuel at the intake manifold near the entry of the intake valve.

Finally we also have direct injection systems that spray the fuel as implied directly into the engine cylinder. So to recap, we have fuel storage, a pump for fuel transfer, filters to clean the fuel, lines to allow the fuel to travel safely to the point of use, and some sort of injection system to deliver atomized fuel to the engine. Once the fuel mixes in the cylinder it will vaporize and that’s when the power is made as the atomized fuel air mixture is ignited by a spark plug and the resulting power created pushes the pistons to provide the power for our vehicle. One last system included in the fuel system is an evaporative emission system that keeps the fuel vapors trapped in the system especially when refueling reducing hydrocarbon emissions.

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