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Fuel System


  • Fuel supply and return system
  • Fuel tank
  • Activated carbon filter
  • Electric fuel pump
  • Fuel Pump Error Symptoms
  • suction jet pump
  • Float

Fuel supply and return system:
The fuel delivery system moves the fuel from the tank to the engine. The electric fuel pump pumps the fuel from the tank and moves the fuel through the supply line and through the fuel filter to the fuel gallery (also called fuel rail).
The fuel pressure then prevails at the inlet of the injectors. The moment an injector is controlled by the ECU, the fuel will be injected into the cylinder. The pressure regulator prevents excessive rail pressure. When the rail pressure rises too much, the pressure regulator ensures that the fuel flows back to the fuel tank via the return line.

When the fuel level falls, more air must therefore enter the fuel tank; otherwise there is a vacuum, or negative pressure. The same applies the other way around; when the tank level rises, such as when refueling, the air must escape from the tank. The vapors that are released during this process must not be released into the open air. That is why an activated carbon filter is used that provides aeration and venting of the fuel tank. The tank is supplied with or emptied of air via a tank venting hose.

The following sections describe the fuel system components of the gasoline engine.

Fuel tank:
The function of the fuel tank is to store fuel. The fuel tank is almost always mounted at the rear under the car, at the height of the rear wheels, under the rear seat. The tank is attached to the body with suspension brackets. The fuel tank is never within the crumple zone of the vehicle. The fuel tank is almost always made of plastic because of its low weight and the possibility of applying all kinds of strange shapes. By shaping the tank in such a way that every available space is used, the largest possible volume is created.

The fuel tank in the picture below is called a 'saddle tank' because of its shape. This tank is mounted on a rear-wheel drive car. In the elevation in the middle, space has been made for the cardan shaft. With a front-wheel drive car, the tank will be flatter at the bottom. The fuel pump in the tank provides fuel to the engine. A so-called suction jet pump is also used in a saddle tank, which transfers the fuel to the other half of the tank. The operation of these fuel pumps is described further down this page.

There are always 2 fuel lines from the tank to the engine, namely a supply line and a return line. The supply line, as the name implies, carries the fuel from the fuel pump to the engine. The return line returns the excess fuel back to the tank. The tank also always has a tank vent valve that is connected to the activated carbon filter with a vent hose.

Activated carbon filter:
The image above shows the activated carbon filter. The activated carbon filter ensures that the HC emissions (fuel vapours) do not end up in the outside air. This filter sucks the fuel vapors out of the tank and filters it through the special absorption carbon material. After the fuel vapors have been filtered, they are discharged to the outside air or to the engine's intake system. The vapors are mixed with the intake air and then burned. In this way the fuel vapors are removed as cleanly as possible.
The activated carbon filter can be mounted near the fuel tank, but sometimes it is also located under the hood. On some cars where it is mounted under the hood, an audible ticking noise can sometimes be heard that often goes away and then comes back. That is when the activated carbon filter works.

Fuel pump:
In classic cars we often find a mechanical fuel pump that is driven by the camshaft. The mechanical fuel pump has been replaced by an electronic booster pump: this provides the necessary pressure for an indirectly injected petrol engine. Today (almost) all car manufacturers use high-pressure injection; the fuel is injected directly into the combustion chamber at high pressure. This high pressure is obtained thanks to the high pressure fuel pump.

The operation and application of these fuel pumps are described on the page Petrol engine fuel pump explained.

Suction Jet Pump:
As described in the second paragraph, the saddle tank consists of two parts. The fuel levels must always be kept the same on both sides. The suction jet pump moves the fuel from one half of the tank to the other. The fuel then ends up in the tank half where the electric fuel pump is located. The electric fuel pump moves the fuel to the engine.

There is a negative pressure in the suction jet pump. Allowing fuel to flow through a restriction causes the flow of fluid to accelerate (see the figure below). As a result, a negative pressure is created after the constriction with which fuel can be sucked in. A suction jet pump works with the same venturi principle as a carburettor, only the venturi operation is not via an air flow, but through a liquid flow.

A float in the fuel tank has the job of measuring the fuel level and relaying this signal to the instrument panel. The tank contents can be read there. If this is equipped with an on-board computer, the estimated range is calculated based on the driving style. The float has a styrofoam part. This styrofoam block floats on the fuel. When the fuel level drops, so does the Styrofoam block. This mechanical movement causes a needle to move across a potentiometer (a variable resistor). A high or low resistance creates a low or high current. Based on this current, the needle on the instrument panel moves from low to high.