What is the fuel tank function

Fuel tank 1


Fuel for internal combustion engines has a particularly high energy density. That is why the most important task of the fuel tank is to protect it from escaping fuel, even in the event of minor and possibly moderate accidents. Since modern vehicles are also not allowed to emit fuel gases, these must be temporarily stored and fed to the engine at the appropriate time. Motorists expect problem-free refueling and the designer wants to use the tank volume to fill empty spaces that are otherwise unusable.


The classic fuel tank made of two pressed sheet metal parts, which are connected to one another by rolling welding tongs and coated with an anti-corrosion layer, is becoming increasingly rare in cars. The plastic tank allows a much better use of the existing cavities and is lighter. Since the fuel tank is increasingly mounted under the rear seat bench in rear-wheel drive vehicles too because of the cardan shaft, it has a noticeable depression in the floor. There are now two recesses, each of which should have its own fuel pump. One of them can be replaced by a suction jet pump (picture on the left). The fuel delivered by the electric pump on the other side comes from above. In the return, when pushed through a nozzle, a negative pressure is created. The fuel on this side is added through the line below. Both leave the suction jet pump to the right.

Since the filler neck can easily tear off and / or leak in the event of an accident, the plastic tank has an advantage. If, in the case of metal tanks, twisting walls have to be used to stiffen the container and to prevent extreme fuel displacement, e.g. when cornering, this can be taken into account in the shape of plastic tanks or cast in. Recently, however, American legislation in particular has demanded such a tight seal from the tank that the sheet metal tank now has a slight advantage again. The plastic tank has no disadvantages in the event of an accident. Even after its introduction in 1973, no specialist in vehicle fires has ever experienced such an explosion.

PE is used as the plastic tank material, which is more economical than sheet steel in the event of a fire. It does not glow and thus transfers heat, but even covers fuel residues and possibly heat-insulating.
In the picture above, you can see a fuel tank with an inner tank pump, which is mounted under the large screw-on cover together with the flow, return, surge pot and level sensor. The surge pot is always filled with fuel and prevents the dreaded drop in fuel supply in classic cars in long curves.
Several lines are connected to the actual fuel tank:
- flow to the injection system (small cross-section),
- return from the injection system (small cross-section),
- ventilation as pressure compensation during suction (small cross-section),
- Venting when refueling (large cross-section).

None of these lines may lead into the atmosphere in modern vehicles. While the tank ventilation ends at the top of the filler neck and is meanwhile being sucked off through the tap, the ventilation ends in the activated carbon filter. Here the gasoline components are held up until they are fed to the engine via a timing valve at a point in time determined by the engine control unit and burned there.

If you look at the last picture, you will also discover the sender for the fuel gauge, which in this case consists of a rotary potentiometer with float, the voltage of which shows the fuel level in the front display. From the thickness of the float you should recognize that this arrangement cannot give any precise information when the flat tank is completely full and completely empty.


Whether a vehicle is shut down with a full or empty tank depends on its material. Plastic tanks do not have to be full, while metal tanks are better if they do not have any special protection on the inside, for example. This applies, for example, to motorbikes, where the examination of the inside of the tank is an important measure when determining the used value.