Insulating the thermos?

I would guess that the temp drops due to the way the EBH work, as well as the thermos. I believe that the EBH is just a passive heater element (I don't use one) and as such just transfers heat from itself to the block, and ergo into the coolant. It has no capacity to circulate the coolant except for some small passive thermosiphon. Therefore the coolant nearest the heater would be hotter than the bulk of the coolant, ie in the heater core, hoses etc. It would logically follow then, irrespective of how the thermos was injecting coolant, that as the engine starts to circulate coolant, the colder coolant would at some point come in contact with the Scan sensor and show a LOWER temp. That coupled with a general mixing of the warm and the cold as the water pump starts to circulate would also lead to a lower reading.

The bottom line, is that there is a finite number of BTU's stored in the coolant. If you have one portion hot (More BTU's) and one portion cold, (Fewer BTU's) the aggregate total is still the same, regardless of any change in the gauge.

Doesn't answer the original question however,

Icarus

 

Excellent point, Icarus.

If the Thermos container (a brand name by the way) is actually a vacuum bottle, then no extra insulation could help short of insulating the lines to and from. If the container is just a tank with foam or fiberglass insulating it, then additional insulation should help but perhaps not significantly depending on the R value of existing insulation.

Now to brush up on Japanese to understand the above links!

Lee

 

This is correct. It is the circulation within the cooling system that causes the temperature to drop after startup, even without the thermos.

Tom

 

Thank you Ken. Very interesting and helpful.

 

I wonder if it would be possible to have an alternative to the OEM passive EBH? Maybe you could splice a pump & heater combo into the coolant lines that would actively circulate the coolant while heating it? Heating via a fluid would probably more efficient as well, from my study of water cooling setups for computers. And I haven't checked, but isn't the coolant line more easily accessible than the block heater receptacle?

After searching a bit I found some manufacturers who create this type of heating circulation pump:
Engine Heaters at JC Whitney
Kim Hotstart Tank Style Engine Pre-Heaters
Diesel Engine Heaters - Watlow

I guess the question is: Can the coolant flow freely through the lines while the engine is off?

Edit:
Not sure what kind of car this is being installed into, but here's a guide to installing a circulation heater.
http://pics.tdiclub.com/data/517/ZEROSTART_SERIES_8000_HEATER_INSTALLATION.pdf

 

Look into a Webasto. Made in Germany, uses the fuel tank from the car, burns a small amount of fuel, circulates the coolant throughout. Most common on Transport trucks, but they make them for cars as well. I have a hunch you might find it a bit of a problem to find a spot for on on a Prius however.

Icarus

 

perhaps, but not by as much as the coolant would have cooled the ICE if there were no thermos. The engine has coolant in it at startup that is not circulating. Upon reaching target temps, the valve opens and cooler stuff from the radiator comes in and must be warmed. In the Prius it is a 3-way valve. The stuff brought in from the thermos should never be cooler than it would have been if allowed to sit in the radiator.

Bottom line: she needs x amount of coolant to protect the ICE in the AZ summers (e.g.), and all of that coolant must circulate and hence be heated. It would be great if the coolant could be dynamically restricted from that circulation, but the Prius thermos is the only attempt at any such management that I know of (not that I would).

 

I've been wondering the same thing. On my VW TDI diesel, I have a plug-in circulating coolant heater which is the cat's meow (or "the bomb" as my son would say). 1000W, heats up all the coolant and the block without a pump. Here is a link to info: FrostHeater.com - Welcome

Anyone make something like this for the Prius? I haven't seen such a thing. I think I'll ask Terry at Frost Heater if he will put one together for us.

Jerry

 

I think I have to agree about the earlier response suggesting that the EBH creates a sort of hot-spot which is responsible for the drop I'm seeing. It makes the most sense.

 

Do remember that while the Prius is "advanced", most of its' engineering is still based on what has already been done. To that topic, the cooling system has a "bypass" to the rad. The coolant circuit that is used to heat the interior exhausts to the rad, forcing rad coolant into the engine. This is a small amount of coolant but enough to notice a temp drop on the Scangauge. The bypass circuit is there in case the thermostat valve sticks.

BTW, I too see the temp drop, no matter how cold the engine is. I don't use my block heater. The thermos has virtually no effect here in ambients just below freezing after 24 hrs. It works well in summer though, even after two days.

The engine does warm up fast though. It starts out at -1 to -5C, and within a minute is up to 20C while idling. In just 3 min of idling it normally reaches 30C. Four blocks of driving gets it above 50C (heat in cabin - ahhhh ).