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The key is to control the temperature of the coolant after it has passed through the radiator.
Thermostats control the flow based on the temperature of the coolant exiting the engine, but they do not control the temperature of the coolant exiting the radiator.
Radiator capacity is based on the "worst case scenario" of operation. In every other case it has too much capacity. As you reduce heat losses through hypermiling techniques, you will encounter lower coolant temperatures of coolant exiting the radiator.
When this happens the thermostat will restrict the coolant flow, but that will exacerbate the situation.
Test the temperature of the exit coolant hose in the summer and winter and you will see a huge difference in temperatures, as much as 100 degrees in coolant temperature.
One way to compensate for this is to reduce the effective surface area of the radiator, a radiator block, which has the effect of increasing the exit coolant temperature to bring it up to the temperature of the same coolant in the summer when its hot.
Two ways to fix this would be to have a thermostatically adjustable radiator block, or a thermostatically adjustable radiator bypass. Either solution would make the temperature of the coolant entering the engine constant regardless of ambient temperature or loads applied to the engine.
Basjoos does this by monitoring his coolant fan activity and adjusting his fresh air opening to allow more air flow when the coolant fan operation becomes more frequent.
Making this operation automatic by either adjusting the grille opening or bypassing the radiator would accomplish the same objective of controlling the coolant temperature as it enters the engine.
In his case since air flow through his engine compartment is much less than normal, he also benefits from warm air intake from the higher temperature of the under hood air available to the engine. This is recycling the engines radiated heat for higher efficiency.
I have personally used a radiator block on my Insight and seen a significant mileage improvement in the winter. I Have also completely blocked the lower grille openings on both my Insight and Echo, and it seems to have made a fairly significant difference in maintaining higher mileage as the weather cools down here in the fall.
I know this might seem off topic in relation to electric water pumps, but to me the real problem is not the water pump, but the inconsistency of temperatures of coolant entering the engine and its effect on mileage and thermal stresses.
regards
Mech
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