Diesel fuel pump: why does my pump "push" more diesel when the fuel is cold?

Question

Two theoreticaly IDENTICAL pumps set at the same pressure, pumping the exact same fluid, but the temperature of the fluid is the only difference. Why will the pump "pushing" the cold fluid pass more volume?

Answer 1

It's because the cold diesel is more dense than the warm diesel.

Let's say both pumps are pushing ten liters of diesel per minute. At the end of a one-minute pumping operation, both pumps would have transferred ten liters of diesel.

But if the warm diesel has a density of 0.8 kg/liter, while the cold diesel has a density of 0.9 kg/liter (both are hypothetical numbers), then you can readily see how they'd have moved different masses of diesel The warm diesel would have delivered 8 kg, while the cold diesel would have transferred 9 kg.

Now let's bring both of those samples to the same temperature, at which point their densities are the same. The diesel from the cold operation would have delivered 1 kg more diesel, and thus a larger volume.

We see the same principle in operation at gasoline filling stations. In the winter, when the fuel is colder, the density is higher, so you move a greater mass of fuel with each unit volume. Unfortunately, in the winter, your auto also burns more fuel to heat the engine and the passenger compartment, so you end up with lower fuel efficiency.