Efficiency Mechanical Fuel Pump versus Electric

[christianroth]

I am questioning the effieceny of a mechanical fuel pump versus an electric fuel pump. In theory it looks like the mechanical fuel pump is more efficient, but I just can't get my head around it.

For example the mechanical fuel pump in question uses 1/5 HP at the selected load/RPM. At the same point the electric fuel pump used 9 Amps at the same load/RPM. In conversion 9 Amps equates to about 1/6 HP.

At this point it looks like the mechancial pump will load the engine less (not taking in the consideration of losses in coverting mechanical energy to electric and back again).

But my question is, will the electric motor truly load the engine, or will the battery supply most of the power? I guess that power has to come from somewhere and thats most likely from the alternator which would then rob the engine of HP.

 

[Fabrico]

Generating is more efficient than pumping. I would estimate around 50% more for generating the power to run the electric pump. Other figures being correct, that means 1/4 HP total for the electric pump.

 

[WGJ]

Don't forget that, unless you're running a 'total loss' electrical system (i.e. everything powered by a battery, like in a racing bike) you had to generate the power to run anything electrical in the first place.

I would estimate the alternator as 70-80% efficient, so the power consumed by the electric pump of 108 watts (9 amps x 12-ish volts) needed an input from the engine of at least 108/0.8 or about 140 watts. There's also the efficiency of the alternator drive system (the belts and pulleys) to take into account.

So, at a guess, the final result of fuel flow from an electric pump of about 108 watt power consumption is an engine power requirement of, say 150 watts, or about 1/5 hp.

Once the engine is started, the battery acts more like a giant capacitor with 13.5 volts or thereabouts at its terminals and the alternator is keeping it topped up.
Any drain is then supplied by the alernator output.

By the way, another 'loss' you don't see but wastes power is in the wiring from the battery terminals to the fuel pump.

There are resistive losses in this. Try measuring the voltage on the terminals of your pump and then at the terminals of the battery.

If you get 13.5 volts at the battery and only 12.5 volts at the pump, then the resistance of the pump supply wiring is causing a volt drop of 1 v, and at 9 amps thats worth another 9 watts wasted.

A friend of mine was running a sprint bike with a total loss electrical system and was having power loss problems under hard accelerations.
I had given him a pump to suit his fuel demands, but he hadn't told me the system was total loss. When I did realise this, I suggested either a much bigger battery or going over to a mechanical pump.
Everytime a high fuel demand was called for, the marginal electrical system voltage went low and the ECU was getting down to its lower operational limit of about 7 volts.

 

[patsvw16v]

You might want to consider that an electric pump keeps a constant flow even at low engine rpm, and that a mechanical pump has to rely on engine speed

 

[christianroth]

Thanks for the help, seems like mechanical is the way to go.

 

[christianroth]

Basically then the battery really only provids the extra power when needed. So then the alternator would provide the majority of the power to the auto's electrical system and to charge the battery.

 

[patprimmer]

Any power drawn from the battery must be replaced by the alternator or by an external charging source. It still consumes power, the source just varies.

If you have no alternator and depend on an outside source, you probably also need a larger, more expensive and heavier battery, which offsets the gains of not running an alternator.



Regards

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