Seems to me that my 1989 Audi 200 Turbo (God, I loved that car...) had an
electrically driven water pump for the purpose of maintaining flow through
the block after shutdown.
As for power consumed, the equation is:
Horsepower = (TDH X Flow)/(3960 X Pump Efficiency)
where TDH is the total discharge head (in feet) and the flow is measured in
gpm. Required flow rate at full engine load (max heat rejection) can be
calculated from:
Heat rejected = mass flow X specific heat X temperature rise
where specific heat = 1, for water. The temperature rise is the temperature
difference between the cooled water supplied to the block and the heated
water leaving the block. Thus, FOR THE SAKE OF ILLUSTRATION ONLY(!!!),
let's assume that an engine discharges heat from the block equivalent to 33%
of it's energy input, and that the engine is 33% efficient. Thus, for a 100
BHP engine operating at full output, the heat equivalent of 100 BHP is
rejected to the cooling system. This is equivalent to approximately 254,600
BTU/Hr. If we assume a 20 degree temperature difference across the block,
then a water mass flow of about 12,700 lbs/hr, or a little over 25 gpm, is
required. If we assume that the pressure drop through the cooling system is
around 5 psig, or 11.5 ft head, and we'll assume a pump efficency of 65%,
then the power consumed by the pump is around 0.11 BHP. Not much, eh?
As a sanity check for this calculation, I looked in my TR2 and 3 factory
manual and found an interesting graph that showed that the engine consumed a
constant 0.5 (imperial) pints/BHP/hour of fuel at full load. This is
equivalent to about 1,000,000 BTUH input, assuming fuel at 20,000 BTU/lb,
which means the engine is somewhere between 25 and 30% efficient. This will
change the heat rejected to the cooling jacket, but I can't see the water
pump consuming much more than .5 BHP, in a TR3. OK, I'm ready for the
responses...
-----Original Message-----
From: Dave Massey <105671.471@compuserve.com>
To: TR List <triumphs@autox.team.net>
Date: Wednesday, March 17, 1999 10:19 AM
Subject: Electric Waterpumps
>
>All this talk about electric water pumps raises a few questions:
>1) If the pump and drive pulleys are sized for adequate coolant flow
>at idle will the pump caviate at red-line?
>2) How much power does the pump use?
>
>The Pump question is much like the fan question with the exception that
>there is no induced coolant flow form the forward movement of the vehicle
>as there is with the air. So if the pump were driven from an electric
>motor
>what speed would be used?
>
>The Ideal setup would be to use a variable speed electric drive using
>coolant temperature to determine the pump speed.
>
>On start-up a cold engine requires no pump at all. As the engine begins to
>
>warm up the pump would be required to allow the heater to work. Once
>warmed up, the engine would need a modest amount of coolant flow at idle.
>At speed, underload the coolant flow requirements would increase but
>not as a function of speed of the engine but as a function of the load and
>the heat generated.
>
>I could design such a system for, oh, about $10,000 complete with working
>prototype. ;-)
>
>Talk about gilding the lilly. Or is it Rube Goldgerg?
>
>Seriously, though, one advantage to an electric driven pump is that it can
>continue to run after the engine is shut off and cool down the motor
>between
>runs (eg:at an autocross) a la John Lye.
>
>Dave Massey
>St. Louis MO USA
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