Fuel Pump efficiency and life span

Does the weight of petrol in your tank help Pump to work better by pushing against/through pump so the Pump not having to pump so hard the Petrol?

• The pump wont notice the difference, its negligible.
– Moab
Commented Apr 20, 2020 at 14:51
• The best way to achieve pump lifespan is to keep the tank at least half full. It's an electric pump that's cooled by the fuel. When you run low on fuel, you overheat the pump. The weight of the fuel on the pump is negligible. Commented Apr 20, 2020 at 15:25
• @Moab some pumps will fail to "pull" fuel if the pipes run dry... Commented Apr 20, 2020 at 15:25
• Only the cheap inefficient ones, haven't seen that issue in many years.
– Moab
Commented Apr 20, 2020 at 16:23
• In the old days of mechanical pumps putting out a few psi to the carburetor , the fuel level made a difference. Today with high pressure injectors it makes no significant difference. Commented Apr 20, 2020 at 16:38

Only possible if the tank is above the pump, if not, then the pump has to work to get the fuel - terms like positive head and suction head are used.

Does the weight of petrol in your tank help Pump to work better by pushing against/through pump so the Pump not having to pump so hard the Petrol?

In order to understand this, you need to understand how hydrostatic pressure operates.

Any liquid has a hydrostatic pressure calculated from its surface. The pressure is its density multiplied by gravity multiplied by the height difference from its surface.

Example:

You have gasoline fuel (750 kg/m3), on earth (9.81 m/s2), and your tank is 0.3 meters deep and full of gasoline. The hydrostatic pressure at its bottom is `750*9.81*0.3 = 2207 Pa`. As 1 bar is 100 000 Pa, in other words it is 0.02 bar. Quite negligible, and in fact nothing compared to something like 10 bar pressure in low-pressure fuel injection systems.

If your fuel injectors are above the surface of the gasoline in the tank (very likely), for example 0.7 meters above it, then you have pressure `750*9.81*(-0.7) = -5150 Pa = -0.05 bar`. As the pressure is negative, the fuel doesn't naturally flow and a pump has to provide the pressure.

The fuel pump has to provide this pressure and also the far greater pressure that is present in the injection system.

However, if your tank is nearly empty having only 10 cm of gasoline in it, then the height difference between the injectors and surface of gasoline is 0.9 meters and the pressure is then -6620 Pa = about -0.066 bar.

So, yes, the more fuel you have in the tank, the easier time the fuel pump has to raise the fuel to the level of the injectors. However, all of this is practically nothing compared to the pressures the fuel pump has to anyway provide, so in practice we can safely say: the effect is negligible.