Assumeing a constant pump speed, when the thermostat closes and head pressure goes up,  power required to drive the waterpump does NOT go up.  It actually goes DOWN.  Reason:  There is less mass being accelerated (energy) at lower flow rates.  In the extream example  (zero flow) the same water in the pump housing is being spun around at a constant velocity which requires no energy.  Of course there are losses in the pump so the energy consumed is not zero.

 

This argument applies ONLY to centrifugal pumps (of which automotive waterpumps are an example) and not positive displacement types (like oil pumps).

 

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Tracy, I agree with you with respect to the amount of energy being used to pump water, however the frictional losses in the pump increase with rpm and decreasing flow.  The point I was trying to make (apparently poorly) is that the power dissipated in the pump can rise rapidly with RPM. The energy into the pump is turned into heat energy, and although this effect is small at lower RPM, it rises rather quickly with RPM.

 

A water brake dynomometer is just a pump that is dumping its energy into raising the temperature of the water.

 

Bill Schertz

 

All true.  But my point was that there is still *less* shaft HP required to turn the pump when the thermostat restricts the flow through the pump and we are not wasting engine HP by having a restriction in the circuit as is occasionally claimed.  True, the pump is operating in a less efficient part of its operating curve, but do we really care?

 

Tracy