Barry,
Your question requires a multipart answer. First, a little something about electric motors: while horsepower is a function of torque and rpm, an electric motor produces the most torque (and has the highest current draw) just before it's stalled. At operating RPM, current draw is proportional to the load, which on a water circulation system would be mainly a function of the head pressure.
The EWP doesn't need restrictors to limit the volume of
coolant flow, it does it by varying the pump speed - therefore head pressure will be comparitively low - therefore the current draw will be low.
The mechanical water pump has to be geared to produce sufficient flow on a hot engine at idle RPM, and low driving speeds, AND it's doing it against artificially high head pressure - therefore it has *way* excess capacity at 5000 RPM, which translates into drag/wasted_energy_consumption.
Once the EWP has spun up, it's running on nearly its lowest possible energy draw, against very little head pressure, and it slows down every time the heat exchanger gets ahead of it.
I suspect that one of the reasons the "studies" gave such
off-base data is that no one tried to actually determine how *little* power was required just to move the water.
my $.02,
Dale
>
> By Ed's law--I mean Ohm's law--below, the EWP current drain is .134 HP. So why the difference?
>
> 1) The pumps could have differing efficiencies. That's possible but it doesn't explain the magnitude nor why auto manufacturers would persist with an inferior pump design if reverse engineering a Davies Craig pump could yield such superior results.
> 2) Electric motors are more efficient than internal combustion engines. While this is true, the 20 HP savings is measured at the crank of the gas engine so the efficiency loss of the internal combustion engine (i.e., the heat balance and all that stuff) has already been taken into account.
> 3) ???
>
> One interesting datum from the Davies Craig website is that the EWP pumps less coolant. Maybe you guys saw these lines in there:
> a.. --"Maximum flow rate of [the EWP is] only 88L/min compared with a maximum of 240L/min for the same engine using an older mechanical water pump design.
> a.. --The extra flow rate is not required and is therefore a waste of engine power."
>
> I'm all for matching my pumping needs to my cooling needs but it seems to me that all the cooling problems people have had would not argue for pumping less coolant, off-hand. I realize that a lot of the cooling problems have been caused by difficulties getting good airflow through the heat exchangers but still, reducing the coolant rate would not be first thing I'd propose.
>
> So help me out: how can both these claims be true simultaneously? How can you save 20 HP at the crank but spend only .134 HP on an electric water pump?
>
> I'm pleased that Todd has had this good luck but puzzled why it worked.
>
> Please educate me. My physics education must have been lacking. Thanks.
>
> Barry Gardner
> Wheaton, IL
>
>
>
> Well at 7.5 amps and 13.8 volts the power consumed is approx 100 watts which equal approx 0.134 HP. A couple orders of magnitude less power than some "experts" had predicted would be required. Your "experiment" has debunked another long-held myth about the amount of power required to keep sufficient coolant flowing. Good work, Todd - a great data point!
>
>
> Ed Anderson
> RV-6A N494BW Rotary Powered
> Matthews, NC
> eanderson@carolina.rr.com
>
>
>
> From: Haywire
> To: Rotary motors in aircraft
> Sent: Wednesday, October 08, 2003 4:31 AM
> Subject: [FlyRotary] Re: EWP - Success at last
>
>
> Hi Marko;
> Full specs can be seen at http://www.daviescraig.com.au/ Last year I did full tests on the current draw, but I can't find that info handy right now and it's to late to look for it, but IIRC it was approx 4amps with the inrush current only ~6amps. The web site shows max draw 7.5amps. If I didn't have so many other things to do in the next few weeks I'd install a volt/amp meter on the pump/controller to take readings at various coolant flow rates and plot them on a chart with temps; engines speed, MAP, etc... but I just don't have the time right now. maybe in the spring.
>
> S. Todd Bartrim
> Turbo 13B RV-9Endurance
> C-FSTB
> http://www3.telus.net/haywire/RV-9/C-FSTB.htm
>
>
>
>
>
Guys, let me play the role of skeptic here. I'm not
Paul Lamar's lockstep disciple but I thought his argument sounded good. Can
you guys help me understand why the EWP works with such less power?
I looked at the Davies Craig website, which claimed
that substitution of the EWP for the mechanical variety could save "up to 20
HP." A tech article inside the site looked at the improvement on a drag car
which estimated the HP addition of lessening the load on the crank at 30 HP. So,
the claims are that the EWP saves 20 to 30 HP load on the engine.
By Ed's law--I mean Ohm's law--below, the EWP
current drain is .134 HP. So why the difference?
1) The pumps could have differing efficiencies.
That's possible but it doesn't explain the magnitude nor why auto manufacturers
would persist with an inferior pump design if reverse engineering a Davies Craig
pump could yield such superior results.
2) Electric motors are more efficient than internal
combustion engines. While this is true, the 20 HP savings is measured at the
crank of the gas engine so the efficiency loss of the internal combustion engine
(i.e., the heat balance and all that stuff) has already been taken into
account.
3) ???
One interesting datum from the Davies Craig website
is that the EWP pumps less coolant. Maybe you guys saw these lines in
there:
--"Maximum flow rate of [the EWP is]
only 88L/min compared with a maximum of 240L/min for the same engine using an
older mechanical water pump design.
--The extra flow rate is not required
and is therefore a waste of engine power."
I'm all for matching my pumping needs to my cooling needs but it seems to me that
all the cooling problems people have had would not argue for pumping less
coolant, off-hand. I realize that a lot of the cooling problems have been caused
by difficulties getting good airflow through the heat exchangers but still,
reducing the coolant rate would not be first thing I'd
propose.
So help me out: how can both these
claims be true simultaneously? How can you save 20 HP at the crank but spend
only .134 HP on an electric water pump?
I'm pleased that Todd has had this good
luck but puzzled why it worked.
Please educate me. My physics education
must have been lacking. Thanks.
Barry Gardner
Wheaton, IL
Well at 7.5 amps and 13.8 volts the power consumed
is approx 100 watts which equal approx 0.134 HP. A couple orders of
magnitude less power than some "experts" had predicted would be
required. Your "experiment" has debunked another long-held myth about
the amount of power required to keep sufficient coolant
flowing. Good work, Todd - a great data point!
Sent: Wednesday, October 08, 2003 4:31
AM
Subject: [FlyRotary] Re: EWP - Success
at last
Hi
Marko;
Full specs can be seen at http://www.daviescraig.com.au/
Last year I did full tests on the current draw, but I can't find that info
handy right now and it's to late to look for it, but IIRC it was approx
4amps with the inrush current only ~6amps. The web site shows max draw
7.5amps. If I didn't have so many other things to do in the next few weeks
I'd install a volt/amp meter on the pump/controller to take readings at
various coolant flow rates and plot them on a chart with temps; engines
speed, MAP, etc... but I just don't have the time right now. maybe in the
spring.
S. Todd Bartrim
Turbo 13B RV-9Endurance
C-FSTB
http://www3.telus.net/haywire/RV-9/C-FSTB.htm
|