Electric water pumps (was) alt charging cont sw

Greg Hermann bearbvd at sni.net
Sun Mar 14 19:23:51 GMT 1999


>Greg Hermann wrote:
>>
>> >-----Original Message-----
>> >From: steve ravet <Steve.Ravet at arm.com>
>> >To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
>> >Date: Friday, March 12, 1999 2:50 PM
>> >Subject: Re: Electric water pumps (was) alt charging cont sw
>> >
>> >The reason water pumps are such a large HP lose is cause they need to be.
>> >You
>> >can slow em down some, and in some applications take em out.  But, for a
>> >street motor,
>> >they is what they is.
>> >  For you to have an electric motor pump as much as an oem waterpump you'd,
>> >have to have a
>> >big a-----  motor/pump to replace it.
>> >  For fast warm ups use a recirculating thermostat
>> >Bruce
>>
>> Bruce is right on this, completely. (Or is it one of the little guys?)
>>
>> There is even a good debate possible as to whether a high pressure fuel
>> pump should be electrically or mechanically driven, let alone a water pump,
>> or an engine fan (if you are working an engine any kind of hard at low
>> speeds for any length of time)!
>>
>> For any sizable auxiliary drive loads, a mechanical drive is way more
>> efficient than an electrical one.
>>
>> Regards, Greg
>
>But, isn't it inefficient to spin all those things at engine RPM rather
>than a constant RPM?  Seems like a pump/compressor that has to work over
>a 10:1 input speed ratio wouldn't be as efficient as one designed to
>work at a constant RPM.  Like A/C, and esp power steering.  If those
>were designed to run w/electric drive at constant RPM maybe they'd be
>more efficient.  Especially if you had a dual voltage alternator and ran
>the accessories at 100V 3phase.  maybe?

Well--for the water pump and hp fuel pump and fan, generally the demand for
coolant/fuel / air flow is pretty much proportional to engine speed, so
IMO, no. In fact, I think you are better off with those three accessories
spinning at a speed which varies with engine rpm! An excellent way to
reduce the HP used by an engine (belt) driven cooling fan is precisely the
way that heavy trucks do it: use a good tight fan shroud and
thermostatically operated shutters in front of the radiator! When the
shutters are closed, the fan does not move much air at all, and the fan HP
drops WAY off!! Take a peek at some fan and centrifugal pump curves--HP
goes up as flow volume goes up, out on the far right part of the curve,
drops off at the high head, low flow portion of the curve.

For the power steering pump, I suspect that a variable displacement, vane
type pump (or swash plate type variable displacement pump, if you wish)
would give the variability you want for less weight, more simplicity, and
greater overall efficiency than an electric drive.

There are variable displacement refrigeration compressors available
(usually screw type), but, and this one is based on having done some
serious refrigeration design work, I think the simplest solution to the AC
compressor would be to run a regulator valve on the suction line (known as
an evaporator pressure regulator in the trade). One of these valves gives
you modulating control over the compressor capacity, rather than just
bang-bang, on/off control, just by letting the low side pressure to the
compressor drop well below the pressure in the evaporator (thus reducing
the compressor's VE. under light load and/or high compressor speed
conditions)

Electric cooling fans are acceptable on cars only because cars in regular
service rarely, if ever, see high engine loads for any length of time at
low vehicle speeds--as Gary pointed out, taxis and cop cars are different,
cuzza the amount of idling they do (and the amount of auxiliary equipment
they have working when idling). But for any kind of towing or heavy
vehicle--you can get sustained heavy loads on the engine at low vehicle
speeds--where forward motion of the vehicle is insufficient to move an
adequate volume of air over the radiator. And electric fans (of the sort
commonly used in cars) are totally inadequate to move the needed amount of
air.

Consider the size of the cooling fans used on heavy equipment--where
vehicle speed is no factor at all in moving air over the radiator. Even
better--consider the size (and type of drive used) on the cooling fans used
for big stationary engines--where there is no air movement, other than what
the fan makes. And where weight and first cost are both thoroughly
subordinate factors to minimizing bsfc and maximizing reliability!
Plus--most of these engines have a 460v (or more) 3 ph. generator coupled
to their output shaft, so making the juice to drive the cooling fan
electrically would be already taken care of, if it were advantageous!

Regards, Greg

>
>--steve
>
>--
>Steve Ravet
>ARM, INC
>steve.ravet at arm.com
>www.arm.com





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