ELECTRIC WATER PUMPS by Paul Lamar

INTRODUCTION.

It never ceases to amaze me this subject keeps coming up. According to the charlatans promoting and selling these fantastically 1,875% efficient electric water pumps one should be able to feed the output flow into a water wheel and extract 3 HP out of the water wheel by putting in only 12 volts at only 10 amps or 120 watts or .16 HP. Note the 20 amp fuse in the power line to the Meziere electric pump.



A kind person loaned me a Mezier electric water pump to run some numbers on.



First thing I noticed was the 20 AMP fuse in the 12 gage motor wiring. Whoa! Wait a minute! That is a limit of 20 amps times 13 volts or 260 watts or .35 HP. As soon as you put some back pressure on this pump either the impeller will cavitat or the fuse will blow. The stock pump handles ten times that amount of HP or around 3 HP.

Then I took it apart and saw the unsophisticated toy impeller and decided testing it was a waste of time. A stock Mazda second gen impeller looks like rocket science along side this toy. I expected a much better design based on the questionable advertising claims.

There is no magic in water pumps. Impellers are impellers and scrolls are scrolls. There has been no breakthroughs in designe for the last 100 years. Therefore it takes the same amount of HP to drive an electric pump as it does a mechanically driven pump for a given GPH and head pressure. The impeller in the stock 13B water pump is head and shoulders better than the puny and crude impeller in a Meziere electric pump.





The inlet and out let holes are limited AN -12 or 3/4 inch ID. No way in hell are you going to get much flow through 3/4 inch holes both on the inlet and the outlet. That is .44 square inches and the usual water plumbing is 1.5 inch or 1.76 square inches or four times more flow area. No way to increase the size of this plumbing. In order to test this pump I would need to adapt the 3/4 inch plumbing to the 1-1/2 plumbing of my water pump test rig. Not worth it!!!

Using this pump in a high powered aircraft engine risks cracking the rotor housings due to local if not over all overheating.

A 20 AMP electric water pump can only pump enough water to cool the engine at some low duty cycle say 15% on average of max peak power.

There is no free lunch and there is no perpetual motion machines. No way is a .16 HP electric water pump going to cool your rotary engine while it is generating 75% to 85% max power or 150 to 200 HP continuously. To think otherwise is to have ones head firmly up a boldly orifice where the sun does not shine:)

People want to believe in the free lunch, 70 virgins in heaven, or 100 MPG carburettor and there is absolutely nothing one can say that will shake their belief. I think Psychologist have a term for it. They call it an emotional belief system or some such. In other words: "Don't confuse me with the facts, my mind is made up." It seems to be independent of intelligence or verbal skills. Perhaps these people are logic and reason challenged. The logic and reason part of their brain must be genetically disconnected. I see this in a lot in women :)

The rotary requires 30 gallons per minute at high engine power to force the water through the resistance of the block and the radiator to insure adequate scrubbing action and heat transfer from the metal to the coolant and vice verse when the hot coolant gets to the radiator.


Here is path of heat transfer.


Here is the temperature distribution on the surface of the rotor housing.



Here is the first indication of an overheated rotor housing. Oil coking on the wear surface. If this happens the engine won't stop right away. It may run OK for tens of hours. I don't have any hard and fast numbers on the length of time the engine will keep running However serious damage will obviously be done to the rotor housings.





The water temp alone could be misleading. What you need are cylinder head temperature gages. Just because the water temp is reasonable does not mean the metal is cooled enough.



The following pictures courtesy of Mark Steitle.





CALCULATIONS ON THE AMOUNT OF HP REQUIRED

It is ironic that James Watt invented the concept of HP while pumping water out of a mine. He found that one horse, using a large bucket, a rope and a pulley could lift 1000 pounds of water 33 feet per minute out of the mine.

I wrote a basic program to calculate the absolute minimum amount of HP required to pump X amount of water against Y pressure. "Head" and pressure are interchangeable. In other words 30 psi back pressure is like pumping water to the top of a seven story building.

Why might you ask is this the case? Well a container one foot square (one cubic foot) holds enough water to weigh 64 pounds. The bottom of the container being one foot square is 144 square inches. Therefore each square inch on the bottom supports .4444 pounds of water. Putting it another way that is.4444 psi. Since the typical back pressure we have seen on a complete cooling system can be as high as 30 psi.... 30 psi divided by .4444 is 67.5 feet.

In other words to pump water against a back pressure of 30 psi is the same as pumping it straight up 67 feet. Now I ask you, do you really think this puny electric pump can pump 30 gallons of water per minute to the top of a seven story building from ground level?

OK 30 gallons of water weighs 240 pounds. We are lifting 240 pound 67 feet straight up in one minute. Putting it another way that is 16,080 pound feet per minute. Since James Watt proved that one horse could lift 33,000 pound feet per minute it takes 16,080/33,000 or .487 HP to do the job. That is the absolute minimum amount of HP to do the job assuming a 100% efficient pump. No if ands or buts. No getting around it. It is a FACT. No lie. It is not just a good idea it is the law!

Now, as I pointed out above, an electric motor is about 80% efficient and a centrifugal pump at best is 70% efficient. The over all efficiency of the electric water pump is therefore .7 times .8 or .56 efficient. You can quibble with these numbers but they are not far off with the possible exception the mechanical efficiency of the centrifugal water pump may be much lower than 70%. From the test we have run so far on the Mazda pump I suspect that is indeed the case.

This meanS you are going to have to double the HP number calculated above.

Here is a table of the amount of power it takes to pump 30 GPM of water against pressure. The last number indicates the current required by an electric water pump to achieve these numbers. The HP number is the minimum HP mentioned above and the watts and amps are corrected for the electric pump efficiency.



WATER PUMP DYNO.

My late rocket scientist friend Vance Yaqua built a water pump dyno to verify the calculations.







Vances motor died before we got any good data.

On Vances recommendation I stopped in at Harbour Freight and bought a CC 5 HP 220 volt motor. $80 on sale. Spent the day adapting it to the existing dyno mount and static balancing it. Now works great. Pump runs at 4500 RPM at 40 gal per minute and generates about 5 psi across the pump. I am not happy with the valve that Vance used as there is always a press. drop across it even when it is wide open. With the original ball valve there was no drop when it was wide open and one could see almost 70 GPM at 4500 pump RPM.

At 40 GPM and 4500 pump RPM the digital fish scale was reading about 1 pound one ounce or 1.06 pounds. Lever arm is 13 inches or 1.08 feet. Torque was therefore 1.15 foot pound. At 3650 motor RPM that is about .8 HP.

Again BTW that would be 597 watts or about 50 amps if it were a 100% efficient electric motor. Typical 80% electric motor efficiency would be about 62 amps.

4500 pump RPM is of course only about 3400 engine RPM so we are still not at take off or climb RPM. Worst case for cooling would be at a PSRU ratio of 2.17 would be WOT around 6000 engine RPM and about 8000 pump RPM.

Next step was fitting the 8 inch pulley on the 5 HP electric motor and running the pump at 7200 RPM to check the flow and HP.

I ran the pump at 7000 RPM using the eight inch pulley. Max flow was about 55 GPM at a 5 psi pressure differential across the pump. The fish scale read 2.9 pounds. This worked out to over 2.1HP !!!

If it were electric driven that would be 1567 watts or 131 amps at 12 volts 100% eff. At 80% eff. it is 163 amps!!!!!!!!!

My guess was the pump was starting to cavitat but I was not sure. Vance witnessed the test and we both felt if you are running a 2.85:1 PSRU ratio you should use the Racing Beat smaller E-shaft pulley to slow the pump. Other wise the pump will be pushing 8000 RPM on take off.

Next step was to mount the flow meter on the dyno engine along with all the pressure gages to see how flow is related to RPM in an actual engine. In other words we were measuring the pressure drop through the block and through the rad.

BTW by shutting off the valve I was able to force the 16 psi pressure cap open. It is a good idea to mount the pressure cap on the inlet side of the pump. If you are using a remote pump do not mount the pressure cap between the outlet of the pump and the engine block. This is not possible on the stock set up of course as the pump outlet is bolted to the block. Most cars have it mounted between the block and the rad. My suggestion is mount it after the rad and before the pump inlet. That will directly affect the boiling temperature and up the cooling ability of the system greatly.

Paul Lamar