Electric vs. mechanical water pump 1

[globi5]

Actually it is more about the power requirements of a mechanical waterpump. Most electric water pumps draw between 100W and 200W. However some claim that mechanically driven water pumps draw several HPs.
Does anyone have any data about how much power a mechanically driven waterpump requires? If it indeed draws several HP, where does this come from? Is the belt drive that inefficient or are mechanical pumps simply oversized to flow enough at idle?

Several HP just appears to be rather high. After all a water pump doesn't even need to work against a pressure head. On the contrary it is even supported by convection through the radiator and engine.

 

[Fabrico]

Belt effeciency is hardly relavent because a belt will be driving the pump one way or another. On top of that, the water pump usually has a bigger pulley than the alternator making direct drive of the pump more efficient.

In almost every case, an electric pump will pump too much at low engine speeds and not enough at times of sustained high engine output.

There are variable speed systems but they too will be lacking at full engine load. The speed of a mechanical pump is always fairly well matched to engine load. If it's drawing HP it most likely needs to be, and should be.

There are reliability issues. Unlike it's counterpart, the electric fan, there is no relief. The electric pump will work hard all the time regardless of vehicle or engine speed. Of course this all adds to demands on the alternator.

 

[GregLocock]

"Does anyone have any data about how much power a mechanically driven waterpump requires?"

Yes I do.

" If it indeed draws several HP, where does this come from?"

The belt


"Is the belt drive that inefficient or are mechanical pumps simply oversized to flow enough at idle?"

The latter.

Also fuel consumption at full power is not an issue for production car engines. Fitting a larger more efficient pump would give little benefit to most buyers, and would penalise them all.

Nonetheless I agree that an electric pump would be a good idea if you can handle the reliability issues.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[globi5]

Greg, would you mind disclosing that data?

I guess, with an electric pump, the buyer would also benefit from a slight power gain at full throttle. Not just because of the higher efficiency but because some newer alternators are turned off at full throttle. In this case the pump would run of the battery and consequently reduce engine load.

 

[GregLocock]

I wouldn't mind, but it isn't worth losing my job over.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Fabrico

Mechanical driven pumps are tied to engine rpm, not load. They do not change output with throttle position nor with manifold pressure, simply with rpm.

An electric pump can easily be controlled to react to water temperature.

Durable electric water pumps can be made. They are used in household water systems in rural areas that rely on rainwater tanks for water supply. Also, I am sure water pumping stations in town water supply systems are reliable.

I expect the problem is making reliable pumps at a cost, size and weight acceptable to automotive engine manufacturers.

Regards

eng-tips, by professional engineers for professional engineers
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[globi5]

Ok, let's ask this way:
Is it true that a mechanical waterpump of a 2 l to 3 l engine draws more than 1kW or 2kW at max rpm?

 

[GregLocock]

2 kW for a 3 litre engine would be pretty good.

You are doing the right thing, implementation is the hard bit, the idea is sound.

Would it be so hard to build a rig test, or instrument the engine of interest? All you need to know is the resistance characteristic of the coolant passages in the engine and rad, and the efficiency of the pump.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

In the shop you can rig an electric motor powered by a Variable Frequency Drive and generate your own data.
Change speed and monitor the power consumption.
If the budget is tight, just use an electric motor and change speed by changing pulleys. At 80% to 100% load, the current is a good indication of output power. Less than 75% or 80% load and the current is less accurate at indicating the actual power. If the motor is in the car, you can put a board across the fenders and mount the motor on that. Belt drive the water pump in place.
Let us know what you find out.
yours

 

[globi5]

Actually, I just asked for curiosity. I've kept on reading claims of mechanically driven water pumps needing several HP or even up to 20 HP and I found these claims ridiculous. After all some people go fishing with a 6 HP outboard engine.
I just wanted to hear a ballpark figure from someone with experience.

Here's an example of an electric water pump that draws 7.5A (approx. 0.1kW):

http://www.hrpworld.com/index.cfm?form_prod_id=747,52,363_3529&action=product

Though, I'm still surprised that a mechanical water pump draws more than 10 times as much power.

 

[MGaberik]

I have seen drag racers or even some high hp street cars address water pump losses by switching to electric.
The hot rod suppliers have these for typical v8 applications for $300-400 as I recall. I believe the logic is that a mechanical waterpump set up to cope with hot day idle with the air conditioning operating... is drawing quite a lot of power at 7000 rpm.
The electric pump is operating mostly off the battery under these conditions and therfore not as large a parasite to accelerating the car. The high RPM and power events are transients so the reduced total cooling capacity needed is acceptable.
I cannot imagine these would work well for continous high demand or regular street use.

 

[globi5]

Apparently BMW is the first car manufacturer that sells cars with electric water pumps:

http://www.mwerks.com/artman/publish/features/printer_960.shtml

They actually mention a 3 HP drag as well.

 

[Fabrico]

Patprimmer
“Mechanical driven pumps are tied to engine rpm, not load.”

I never said anything to the contrary.

“An electric pump can easily be controlled to react to water temperature…Durable electric water pumps can be made. They are used in household water systems in rural areas that rely on rainwater tanks for water supply. Also, I am sure water pumping stations in town water supply systems are reliable… I expect the problem is making reliable pumps at a cost, size and weight acceptable to automotive engine manufacturers. ”

I hope you don’t consider those to be helpful, real world, practical, or relevant, automotive related statements.
Cheers

 

[globi5]

Found some data:

http://superchevy.com/technical/engines_drivetrain/accessories_electronics/0408sc_pump/index1.html