Wish I knew how to engineer.

"HammerheadFistpunch" (hammerheadfistpunch) 12/10/2014 at 22:32 • Filed to: None

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I have an idea but I have no way to validate if its even possible because I'm not an engineer and know only enough about heat science to get me into trouble.

My cruiser as a separate liquid powered rear heat unit that branches off the main coolant line. I love this heater but its generally regarded with disdain the community because it can cause problem in unmaintained cooling systems which mine isnt. Let me tell you what my idea is and maybe there are some smart engineering types that could help. It pisses me off that my dad is gone, because:

A. This was his living and his passion (engineer specializing in heating and cooling)

B. This is exactly the kind of project he would have loved. And I could have even got him to talk...maybe.

Anyway. So I've got this small heater that will easily heat the entire car by itself, I'm guessing its in the 3000-5000 BTU class. The only problem with it is that because its engine coolant fed, when the engine is off, the party is off.

The whole reason this came into my head was thinking about ways to stay warm when I go camping with my little girls. My thought was that if i could find an alternative heat source for the water, I could bypass the coolant and run a closed circuit system that would heat the car without having to run to engine.

I'm guessing that the supply temp is ~210 F and the return is likely in the 200's for a delta T of 10, and the flow rate is ~8 gpm from the water pump currently

My goal would be to design an 12v healing element and aux pump to circulate hot water (coolant, really) through this forced air heating unit powered off the battery. The things I would need then would be

1. the smallest heating element I can get away with

2. A voltage regulator to make sure to not flatten the house battery

3. a suitable low power waterpump in the 5-10 GPM range.

The trouble is, I don't know what it would take to heat 5 gallons a minute 10 degrees so Im not sure what heating element would work.

In the end, its looking like in order to heat a car interior from 32 outside to 50 inside I would need something like 1000-2000 btu/h, which works out to 300-600 watts/h, not including high flow high temp pump and when it all comes down to it...I'm probably looking at 50 Ah draw. Can't sustain that on a battery.

Damn. It was a neat idea.

thedevilinside > HammerheadFistpunch 12/10/2014 at 23:22

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I took my Heat Transfer Monday. I'd try to help, but it didn't go well.

On the electricity/power side my mind says this will draw a ton of power.

ly2v8-Brian > HammerheadFistpunch 12/10/2014 at 23:25

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cut a hole in your roof and install a wood burning stove.

Seriously though most things I think of need enough power that the vehicle needs to be on anyway.

BJ > HammerheadFistpunch 12/10/2014 at 23:27

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Nope, too complicated.

You can buy 12V heaters for in-car use that plug into a lighter socket. Maybe it's possible to tear one apart and fit it in with the blower assembly. Wire the two together and magic!

HammerheadFistpunch > ly2v8-Brian 12/10/2014 at 23:29

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yeah, Batteries bite the big one in terms of energy storage. I would need like 400 amp hours, or the total capacity of 4 good group 31M deep cycle AGM marine batteries.

HammerheadFistpunch > thedevilinside 12/10/2014 at 23:30

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Yeah, best case its a 300 watt heater, at 13.8 volts thats 21 amps alone. even a really good huge deep cycle battery would be done after just 5 hours.

HammerheadFistpunch > BJ 12/10/2014 at 23:31

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yeah, I looked at those. most people had the same review:

uh, this does nothing.

ly2v8-Brian > HammerheadFistpunch 12/10/2014 at 23:31

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then you lose space for gear because of the batteries, not to mention the cost.

HammerheadFistpunch > ly2v8-Brian 12/10/2014 at 23:32

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I certainly wouldn't bother at that point.

Short-throw Granny Shifter is 2 #blessed 2b stressed > HammerheadFistpunch 12/10/2014 at 23:33

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You would be better off with an electric space heater rather than the inefficiency of pumping your working fluid around and using a heating element. The advantage of the stock setup was its ability to utilize excess heat from the engine coolent. You have a cool idea and I'm sure you could make it work, but whats the point?

orcim > HammerheadFistpunch 12/11/2014 at 04:01

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Engine running to make heat only? Bad efficiency, as is resistive heating. Though I have to admit, getting a resistive heater and fan to run on electricity is a no brainer - no need for pumps or heat transfer. My old F150 (covered back, foam bottom, carpeted sides - obviously not a working man) had 3 (2 under the hood and one in back) batteries all charged by this wild looking alternator that apparently could handle the load (I've seen others with 2 batteries with the std. HD alternator.)

What about a more efficient heating element (kerosene heater outside, or a camp stove type thing... gas is a much better heat source) and then make a little heat transfer dealie (small pump, fan) that gathers the heaters output and pumps it into the cab? Or...a gas heater thing that you vent outside so no CO coming in, but you get the heat (a window exhaust thing? Maybe CO alarm inside in case a bear sticks its nose into the vent?)

BJ > HammerheadFistpunch 12/11/2014 at 08:31

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I took a quick look and couldn't find many for sale - I have the feeling that they're no longer as common as they used to be, probably 'cause they don't work well.

I knew someone who had one, many many moons ago. In an Acura Integra it was sufficiently powerful to make a difference during the cold Alberta winters, waiting for the little 4-cyl to warm up.

153624 - Straight Six > HammerheadFistpunch 12/11/2014 at 08:38

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According to my calculations, you'd need a ~13.3 kilowwatt heater.

Either that, or I'm going to fail my upcoming chemistry exam.

Calculations:

q=mct

=(19000g)(4.184J/g C)(10C)

=794960 J (per minute)

(794960 J/min)(1min/60s)=13250J/s or 13.3kw

I'd say this wont work without an external generator. In that case, you may as well just use the generator to power a heater rather than integrating it into the car.

Rock Bottom > HammerheadFistpunch 12/11/2014 at 09:37

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I'm currently employed as a thermal systems engineer in the auto industry... maybe I can help.

Many major manufacturers are now looking to gasoline fueled auxiliary heaters in some cars (like hybrids or bus applications). I believe "parking heaters" is the most common name for these. As you've learned, the amount of electrical power necessary to keep a car warm is pretty significant. All that glass isn't very good at keeping heat in. That's not to say it isn't possible to do build a battery pack for a hitch-mounted rack or something... And keep in mind that no matter how you generate the heat, you need to run that fan all night too. That alone can kill the on-board battery.

Lots of old hot-rodders around here (Michigan) have Southwind gas fueled heaters in their cars, but I've never payed too much attention to them. That might be a place to start. Also, Espar in Canada makes cab heaters for sleeper-cab trucks, but I think all their stuff might run off diesel. Might still be worth looking into. And Webasto is a major supplier of gas-fueled heaters to the auto industry, so they might be worth studying too. I hope this helps!

http://southwindheater.com/Products.html

http://www.espar.com/business-units…

http://www.webasto.com/int/markets-pr…

Keep us posted and let us know if you have any other questions. Also, I get the distinct feeling that even though you may not have an engineering degree, you're already light-years ahead of many people with expensive degrees. You understand the basics of heat transfer and energy management, which is startlingly rare in automotive engineers!

EDIT: There is one equation you may already know, but I'll throw it out there anyway: Q=(Mdot)(Cp)(T) where Q is watts, Mdot is the mass flow rate of either your coolant or heated air, Cp is the specific heat of either the coolant or the heated air, and T is your change in temperature of either the coolant or air. Rearrange it to solve for your unknowns, pay special attention to your units (especially the Cp term), and make sure you don't mix the values for the coolant side and air side (using Cp for air with Mdot for the coolant, as an example).

HammerheadFistpunch > 153624 - Straight Six 12/11/2014 at 10:34

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13 kw sounds excessive, since that equates to nearly 40,000 btu. about half of what i heat my house with

HammerheadFistpunch > 153624 - Straight Six 12/11/2014 at 10:37

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wrong person

HammerheadFistpunch > Rock Bottom 12/11/2014 at 10:39

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yes, thank you. i found that equation useful. i looked at forced air propane haters too. at the end of the day, its pious my plan is flawed to the point of abandoning it on the table.

i would have liked to have been an engineer, my dad, uncle and most of my friends are/were...but it math and i don't jibe. turns out it may be discalcula...which had i known early on could have been treated but now I'm in a different career.

Thanks again for the help

153624 - Straight Six > HammerheadFistpunch 12/11/2014 at 12:13

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I fucked up my units: I took your delta T to be 10 deg C rather than 10 deg F.

In reality, delta T is around 5.6.

So you'd need a 7.4 KW heater. This seems much more reasonable given this 3.5 KW heater can heat 2L/min of water to 100degF/38degC.

HammerheadFistpunch > 153624 - Straight Six 12/11/2014 at 12:16

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7 KW sounds better, but still like a lot. I mean a 1.5 KW resistive heater would heat my interior. Granted I know we are talking about heating water and I don't doubt your math at all. and here I was thinking a 300 watt heater plug would do the trick. HA!

RamblinRover Luxury-Yacht > HammerheadFistpunch 12/12/2014 at 12:01

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Since I missed this post the first time around:

Rock Bottom is right. Fuel-powered parking heaters are the standard way to do this. Hot coolant in the engine and in the heater system when it's cold out is absolutely an effective thing, and it's better than just a gas-powered heater because the fuel doesn't have to be combusting *in* the cabin and you have the ability to pre-heat your engine as well. Huge advantage for a diesel, but less so with gas. That, and the energy density of fuel means it's really economical. Battery... not so much.

Oh, and your rear heat unit? Very similar to a Land Rover Smiths heater, which normally only exists in the main cab, but my grand-dad was known to rig a second one in the bed/rear seat area opposite the fuel fill - there's a nice little cubby that works well.

HammerheadFistpunch > RamblinRover Luxury-Yacht 12/12/2014 at 12:03

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Yeah, Its not looking cheap or easy as I had hoped....or even sane. Oh well, its fun to learn.

David Tracy > HammerheadFistpunch 12/13/2014 at 10:05

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First things first: 8GPM is a TON of flow through that small heater core. Likely far beyond the point of diminishing returns.

Basically, your heat exchanger effectiveness goes up with coolant flow up to a certain point. Any additional flow thereafter is wasted energy. If you were engineering this from scratch, you'd have a heater core out target (it might be something like: "after 8 minutes, the heater core outlet air temperature should be 85F, or something like that). You'd then take your engine and figure out what the warmup profile looks like (coolant temp vs time). You then figure out what your coolant temp is at 8minutes. You'd be given the heater core blower performance, as well as the heater core performance, and you'd be able to solve for the required coolant flow given the heater core outlet air temperature requirement, the heater core air inlet temperature (outside ambient air+temp gain through ducting), the coolant temp at 8min (from your warmup profile), and the blower airflow.

But enough about that. Ideally, you want to make sure that you don't waste power by overflowing the heater core. If you plot the heater core outlet air temperature as a function of coolant flow given your blower airflow (you can do this if you have the heater core and blower performance data), you'll see a curve that eventually flatlines. Higher coolant flows yield zero benefit and may result in an oversized pump.

Anyway, most front heater cores on cars see about 2-4 gallons per minute of coolant flow. Most rear heater cores in the industry get about 1GPM of coolant flow, so 8 is probably overkill for that little heater core.

As for how you can design this: you COULD plumb an auxiliary pump into the cooling system that just recirculates the engine coolant when you shut the engine off, since it has considerable thermal mass and will remain warm for a long time (but definitely not a whole night of camping) and it might have to be a big pump to overcome all the pressure drop in that system (engine block, radiator (while t-stat is open), oil cooler, etc.).

What would I do if I had absolutely no heat exchanger performance data and had to go off he seat of my pants? I'd probably buy 20W electric pump, plumb it in series with a 5kW heating element and make sure that when I fill the system, I'm able to get rid of all air bubbles so as to minimize the chance of pump cavitation.

HammerheadFistpunch > David Tracy 12/13/2014 at 10:31

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Thanks for that data. I obviously dont have any performance data on this heater core but im aware of the curves, i helped my dad design our hydronic heating system at a cabin, but since i didnt have performance data i guessed. The flow rate for the water pump on my engine is 500 gph so i used that. Your right though, delta t and ecxhange rate curves are key. A smaller pump sure saves me the trouble of getting a way more powerful and expensive pump, but i still need a hugh temp pump and even 5kw of electric heat is way too power hungry an soI've abandoned this idea all together. Thanks for the reply, it gave me some good data

David Tracy > HammerheadFistpunch 12/15/2014 at 12:24

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I have to amend my post. 5kW is actually too much for a tiny heater core like that. Many EVs use ~4kW for their main heaters. For a little heater like that, you probably wouldn't need more than a couple of kW to match the performance of a typical engine-coolant fed heater core.

Just came back from a BMW i3 teardown and was reminded of this post.

HammerheadFistpunch > David Tracy 12/15/2014 at 12:57

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even 1kw is too much draw, that's over 70 amps. even a big deep cycle would be done in Less than 2 hours. like i said, cool potential idea...but a non starter