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Comment by ErroneousBosh | Hacker News Reader

Apparently some engines now have a solenoid-operated shroud that pops up to surround the water pump impeller, so if the coolant is still warming up it doesn't circulate. This is supposed to reduce the parasitic load on the engine from the ancillaries.

I can't help but think that the water pump must require about 3 brake gerbil power to turn, and the weight of the solenoid, plunger, spring, shroud, and extra cabling - not to mention more seals to go hard and leak - probably takes more power to haul around.

I don't really care about a car's 0-60 time or fractions of a mile per gallon. If you want to save fuel, lighten your right foot.

I want the car to be simple enough to be reliable and repairable when it eventually does go wrong.

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I agree with everything you said, but I believe the pump shroud is for faster engine warmup, not saving a fraction of a horsepower. Cold engines run rich, producing more hydrocarbon emissions, and the cold startup phase emissions are heavily penalized. There’s also additional wear on the engine due to cold oil and looser tolerances, which affects nearly every aspect of the engine.

It might be, yeah. But why wouldn't the thermostat solve that problem, since it won't let water through the rad? And wouldn't the faster warmup come at the expense of the heater taking longer?

If you really wanted to not run the coolant pump until you'd got the engine a little warmer I would have thought a magnetic clutch like an aircon compressor would have been better. Although these days, maybe even an electric pump could be more efficient.

The thermostat bypasses the radiator when cold, but not the engine. The coolant has to be allowed to flow in order for the hot coolant to fully open the thermostat. Being electronically controlled means there just needs to be a sensor near a known hot spot to trigger flow from the pump.

I’m not familiar with the impeller shroud you mentioned, but I looked it up and the description seems to agree: “This pump includes the shroud and control valve to restrict flow while the engine heats up.”

Whether or not it affects the time required for the heater core to be operational would depend on how they decided to route it, and if the solenoid offers variable positioning. I imagine it is variable, otherwise they’d create thermal shock every time the engine heated up and the pump suddenly started flowing colder coolant through the block, so technically it should be possible to fully replicate the general functioning of the thermostat and heater core. Now that I think about it, it’s most certainly variable and it’s why they didn’t go with a clutch system.

It's not variable, it just pulls in and out with a solenoid, either fully surrounding the pump or fully retracted.

I hadn't thought about the thermal shock thing but I did wonder how it could possibly help the coolant warm up if it's not circulating at least through the block. The engine doesn't warm up evenly.

Oh wow, it’s upsetting that it’s not variable. The total system might hold 2x (or more) of the amount of coolant in the engine water jacket. When the coolant around the engine gets up to ~200 degrees and the pump suddenly snaps to 100% it’s going to flood the engine with coolant at ambient temperature. Imagine getting the engine up to operating temperature then dropping it into a swimming pool; even in the kitchen you find out that’s what causes pans to warp and glassware to shatter, and the engine is just a funny shaped pan with bolts.

My only other guess is that it’s not 100% on/off, like maybe a bit is still allowed to flow when “off”, but then it would still need to bring the entire coolant mass up to temperature so I’m not sure how that would be a benefit for faster warmup. Either there’s some clever engineering I’m not seeing, or you’re buying a few points of regulatory compliance for them by needing to replace head bolts and gaskets sooner.

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