- assuming the electric pump energy requirement is comparable to the stock belt driven pump...
- our lightly driven, small displacement engines may only require, say, 1/3 of the pump's max output (that's a generous assumption, considering the EWP's max rating is likely for a 5L engine at max load/ cooling demands)
- = 2.5 A
- I think I figured my normal alternator load to be around 7-10 A
- so you could expect to improve FE roughly 3% by driving the water pump electrically (based on my 10% savings found from removing the 7-10A load from the engine by unbelting the alternator)...
BUT ... you get these savings only if you offload the power requirement to a battery which isn't being charged by the alternator. If you're driving the EWP off the alternator, the FE difference is probably negligible, and possibly worse, since you're going from direct mechanical energy transfer (belt driven pump) to mechanical/electrical through the alternator, with associated conversion losses.
Really messy assumptions & calcs, but better than nothing.
EDIT: can we estimate this? How could we figure out what the energy requirement is for running a pump at various engine speeds? We could start by looking at the energy requirement spec'd for the electric racing pump, but it likely applies to a V8 application (higher displacement pump than most of us are running).
Any (water) pumped system's power can be calculated by:
Flow (gpm) x head (ft of water column) / 3960 / efficiency.
Pump power is proportional to the cube of the engine rpm. (This is a pump "affinity law".):
HP used = K x rpm x rpm x rpm.
The cubic factor is why racers with high RPM race engines use an electric pump instead of an engine-driven pump. Drag racers don't use ANY cooling pump for the same reason.
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I think it's safe to say then that for a fuel conscious driver (like most of us are) removing the water pump and replacing it with an electric one will have no real marginal benefit. The benefits seem to be for the high RPM people.
The only opportunity for savings that I can see from an electric water pump (powered by the charging system) would be through a microprocessor controlled unit that carefully regulates the pump motor to move only the minimum amount of coolant necessary to keep the engine at the desired temp (and not have hot spots).
For example, the pump would run little, or not at all, when the engine is started cold, until it starts to warm up.
So the role of controlling coolant temp shifts from the OEM mechanical thermostat to the EWP electronic motor controller. (That kind of controller is available from Davies Craig, and their site states you need to remove the OEM thermostat for that application.)
But the savings would be so small as to be not worth the cost of the system from a financial perspective.
I wonder though if this is worth persuing considering "pumping losses below 3000rpm are minimal".
The answer to that question is in the table at the end: average gains (HP) were less than 1% going from OD to UD and from UD to Electric.
My only other comment on their testing is they don't show the OEM figures. They jump right in with the over driven setup. Considering the average HP gain between OD to UD was under 1%, the gain would have presumably been even less from OEM to UD.