Thinking about making a vacuum thermos system.

[DaveBonds]

I've been playing around with some rudamentary ideas to bump the warm up cycle in the Scamp for some time.

Along with the upper half of the engine being rebuilt, I'm swapping over to a clutch fan and setting up a heat stove, but one of my friends who works at a Toyota dealership got me thinking about building a vacuum insulated thermos that holds a reasonable amount of antifreeze.

He was telling me that the Prius uses a vacuum thermos reservoir somewhere in the front fender to help with engine temperature regulation on the hybrid combustion engine. I guess those engines have trouble staying warm, because they are not always in use and wear out prematurely.

The idea is that a vacuum insulated reservoir holds it's temperature after driving, for hours after the engine has lost all of it's running temp, so it helps the engine raise back into the running temp range, much quicker than just a thermostat.

I'm thinking that this system's insulating barrier could be self charged with the aid of a one way vacuum valve, fed from the manifold and given how much room is under the hood on the passenger side of the engine, could hold quite a bit of coolant and keep it at engine temp for a long time. I'm thinking overnight, even. I know that small vacuum thermos cups can keep coffee warm all day in the cold.

If this works for a hybrid engine, I don't see why it couldn't work on any car. It would do the same thing as an oil heater, only it would use the wasted energy from the engine temp and isolate it for later use.

I know that modern cars have a lot of one way vacuum valves in them, but I'd like to run a test on some components before building a system like this. It wouldn't need constant vacuum. It would only use vacuum from the engine to recharge the insulation barrier, similar to how an oil accumulator works.

Hell, for that matter, I could even utilize a 2nd reservoir for oil accumulation to keep the oil warm if it works well.

Does anybody know of an aftermarket outfit that makes anything like this?

 

[67Dart273]

Couple of things-----

I have no idea what the coolant capacity of the Pree *** is, but I bet it's not much, these old Mopars hold a lot more. What this means is, that any stored liquid has to have a lot more mass to affect the overall heat of the engine.

Also, be careful of "thermal shock." You don't want to dump a bunch of either cold or hot coolant into an engine that's in the opposite condition. So you'd need a means of metering the flow

 

[jos51700]

Not worth it.

Assuming that your typical small block might have 2 gallons of coolant, let's say you park at night, and it cools off to 75 degrees (warm august night).
Think, 1 gallon in a Thermos at 200 degrees, mixed with your 2 gallons of coolant at 75 degrees, nets you a final temp of 120 degrees. Not bad.

But...factor in your 50lbs per head, times two, a 250lb cast iron block, crank, rods, waterpump, exhaust manifolds, intake, etc., all with the horrible specific heat of cast iron and all at 75 degrees, and compare that to the 24 lbs of coolant in your cooling system....No way jose'.

At 32 degrees mixing your hot n' cold coolant...your final temp not figuring all that engine mass? A measly 75 degrees on a cold day.

ALL this assumes that you lose NO heat from the Thermos.

You'd have to store tons of coolant. Hardly worth it.

The Pious pulls it off because it has a near-zero mass engine (compared to a small block Mope, anyways), that's aluminum to boot, and it has a minimal radiator/cooling system due to it's minimal heat-dispersal requirements.

Have you bothered to ask yourself why they're doing this on the Prius?


Rereading your post makes me wonder if you have considered, what reasons do people have for using oil heaters? Do you really want to add another 50+ lbs of weight to the nose of your car? What do you think that would do for handling and (gasp) fuel economy?

 

[KitCarlson]

Ignition timing is one way, but you will need something to control that.

 

[DaveBonds]

Have you bothered to ask yourself why they're doing this on the Prius?


Rereading your post makes me wonder if you have considered, what reasons do people have for using oil heaters? Do you really want to add another 50+ lbs of weight to the nose of your car? What do you think that would do for handling and (gasp) fuel economy?

Um, ok? I guess you can approach an idea with insult. That's one way to look at it.

Yes, I know exactly why they do it. I explained it above. The engine shutting off constantly needs a ballast to retain heat. It's that simple.

Most people use oil heaters to keep viscosity regulated, especially in engines with 15w40 oil that can gel in cold weather.

I know that the thermal capacity of cast iron holds a lot longer than aluminum and will take longer to warm up than aluminum does and I am aware of water capacity changing the need for more storage and how it will affect total temperature, combined with the fluid in the rest of the system. This is why I don't think temp shock will be an issue, especially with a thermostat controlling flow.

I'm not looking for instant temp, but rather helping it gain temp.

Fifty pounds isn't going to touch fuel economy and two gallons of antifreeze doesn't come near fifty pounds.

 

[DaveBonds]

I'm thinking that the best way to do this within reason is to run about a 3gal reservoir that is shaped to the apron, against the firewall and incorporate it into the heater hoses for feeding the engine.

This would do two things; limit the amount of antifreeze feeding into the engine, to the capacity of a 5/8" hose and cycle within the bypass circuit, without the need of the thermostat being open to operate and if nothing else, it could also help heat the cabin of the car in the cold.

I don't think this is going to take much to build and even if it isn't very effective, would be completely reversible.

I have some idea of how effective it will be adding about 1/2 temp (possibly less) to the engine fed through the heater, but I don't know how well it will work without trying.

If I do this, I'll likely build it at the end of the summer and see how well it works with an infared thermometer at startup and elapsed time, compared to using it without.

If I use polished aluminum to tig the reservoir out of, I could suspend it with insulated rubber, inside of a fiberglass case with a hose fitting and one way valve. For testing, I may want to use two vacuum gauges, one before and after the one way valve to give me engine and insulator vacuum readings.

If I can find a 3gal internally reflective container to start with, I could heat and pressure check it and vacuum check the insulator that I use, even if I don't make it myself, before adapting it and give it a try.

I may also ask my friend at his dealership if he can give me some capacity numbers from the reservoir and total capacity dry fill on the Prius engines, so I could get some more comparitive information from a system that is currently being used. I know that changes from aluminum to cast iron will change radiation factor, but it could give me something to start with, to see if I'm within any range on a 3gal capacity increase.

I also have a friend who taught metallurgy at the School of Mines in Golden, CO who may be able to give me some info on the radiation index of aluminum vs iron by weight or cubic volume.

At that point, I may be able to get a better calculation, before practicing this.

 

[67Dart273]

A 3 gal container that's well enough insulated to maintain any meaningful heat is going to be VERY bulky. You cannot make a vacuum container that's very insulated out of the aluminum because of the thermal conductivity of aluminum. Stainless would be some better, but there's a good reason why vacuum cans are made of glass.

Otherwise, you'll have to use some sort of foam, just like most any picnic cooler.

Actually, cast iron BY WEIGHT holds LESS heat than aluminum. The specific heat of water is "1" aluminum is about .9 and cast iron is less than .5. Copper is only .39. So don't confuse thermal conductivity with specific heat.

 

[DaveBonds]

Right. I understand that it holds less per weight, because of volume decrease per equal weight of iron, compared to aluminum, which is why, as per your numbers, radiators are made from metal that transfer temp quicker.

I'll see what I can come up with for containment. I was thinking about making the inner structure from aluminum with a fiberglass outer, but I'd even be open to the idea of other materials for the inside, backed with a reflective coating. Even non vacuum thermos containers are glass, because they work well. Glass could work, but I'd be concerned about pressure and heat.

There is a really good plastic supplier near my work that deals in all kinds of plastics and mold making, including foams and different material weave.

I can make the vacuum insulator from anything, layered with foam on the inside before putting in a suspended reservoir made from something else, I may even consider a ceramic coating on the inside. There is an outfit that can reflective ceramic coat the inside of header tubes that I live nearby and I can likely have them do the same to the reservoir.