Thermo-Bob Coolant Thermostat

by Dennis Urban in General Tech Discussion 10 years ago

To all the Thermo-Bob users: How is your system working? Have you had any problems?

This is how Thermo-Bob works:

[url=http://]http://www.watt-man.com/uploads/TB_FAQ.pdf[/url]

This is the Thermo-Bob that fits a Rotax 912:

[url=http://]http://shop.watt-man.com/Thermo-Bob-1-Universal-Kit-with-One-Inch-Fittings-TB1.htm[/url]

Thanks,
Dennis

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Re: Thermo-Bob Coolant Thermostat

by Kevin Machniak » 10 years ago

Kevin Machniak

6 167 40

Hi,
I have been using the thermobob for two years now . It has worked great. 912 uls in a kitfox5.

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Re: Thermo-Bob Coolant Thermostat

by Mike Goodrich » 10 years ago

Mike Goodrich

8 123 0

I haven't used thermo bob but made my own system up using a bypass thermostat from a Land Rover (lightweight ABS plastic housing, 25$ on ebay). It ran very well and I coupled it up to a Mocal oil/water heat exchanger so that when cold the water heated the oil and when hot it cooled the oil. On a 0°C (32°F) day I could get the oil temperature up to 50°C (122°F) in 3 to 4 minutes and in flight the water temperature was a constant 90°C (194°F) and the oil temperature was a constant 100°C (212°F) for all flight conditions.

I have since sold that machine (autogyro) and was in the process of making up the same system for my new autogyro when it occurred to me that my design had a serious flaw that is probably also in the thermo bob design.

If/when the thermostat fails it fails in the cold position which shuts off the radiator and leads to a very rapid overheating of the engine. This isn't a big problem for a motor bike (for which thermo bob was designed) you simply stop by the side of the road but could be a very big issue for an aircraft.

Do you really want the security of your flight to hang on a 12$ (thermo bob price) thermostat????

Mike G

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Re: Thermo-Bob Coolant Thermostat

by Dennis Urban » 10 years ago

Dennis Urban

17 30 48

Mike, I like your oil to water heat exchanger idea as well as the full bypass thermostat. I have a couple questions.

Did you find a Land Rover full bypass thermostat that fits our 1” hoses (if so, do you recall the part number), or did you make adapters? Do you recall which Mocal you used, and how much it weighed? I’ve looked at them now that you told me they exist, and I think the smallest would work for us, but could not find how much it weighs. Where in the system did you mount the Mocal, was it in series with an air-based oil cooler? Or was the Mocal the only oil cooler you used? Any info you can provide will be appreciated.

You are right that if any thermostat fails in the closed position, it will rapidly overheat the engine. On the other hand, how likely is this, and does this overweigh the consequences of running an engine too cold over and over again. I don’t hear of many automobiles failing this way. Perhaps the answer might be to go back to taping over the radiator or putting in adjustable cowl flaps to control air over the radiator - if this concern overweighs the anticipated benefits.

I think, for me, I will use some type of bypass thermostat (and take my chances). Right now I use Thermo-Bob but am worried about hot summer days and the permanent 3/8” bypass line. If I have problems, I will consider switching to a full bypass like the Land Rover or BMW (but I have not found any for 1” hoses and I don’t like lots of adapters due to lower reliability).

I do like your idea of how to heat the oil more quickly and want to pursue this further.

Dennis

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Re: Thermo-Bob Coolant Thermostat

by Mike Goodrich » 10 years ago

Mike Goodrich

8 123 0

Denis
The land rover thermostat I used was like this one,
http://www.ebay.com.au/itm/NEW-LAND-ROVER-DISCOVERY-THERMOSTAT-PEM-100990-/320863542343
it might be the same but I can't remember the number. Mine was an 85°C thermostat and I found it too cold so still had to blank off the rad a bit. I might try a 92°C thermostat next time. It had 1.25" (32mm) connections unfortunately and I used silicon hoses from a race car supplier, they include things like reducers but I've just realised that if you slip a 11/4" silicon hose over a 1" hose over a 1" piece of aluminium tube a single hose clip would seal the three pieces together. It would be shorter in length (if you have a space problem) and save having to buy reducers (if, like me, you're cheap). I'm going to try it on my next installation.
I used a Laminova heat exchanger, they're made in Norway and I think Mocal use the Laminova cooler matrix in the heat exchanger they supply. I used a Laminova C43 182 (182 is the length in mm) that I bought direct from them. I am in discussions with Laminova at the moment for my new installation and they recommend a C45 182 which is a lightweight version for the 912/914 Rotax.
You can get these with 1"or 11/4" water connections, my first installation used 11/4" because I was concerned about the extra pressure drop I was adding to the water cooling circuit but Laminova tell me that most 912/914 installations are using 1" connections so I'll go that way for my new installation.
You must specify that you want a "plug" (part number TB00176) in the heat exchanger.
You can order with just about any oil connection imaginable and it weighs 722 grams.
You don't need an air based oil cooler the heat exchanger does the job but if your water radiator is at its limit this might be a problem because the water rad now has to dissipate the heat from the oil as well as the cylinder heads.
I mounted the "Mocal" in the suction line to the water pump.

Here is an alternative made by a French company that includes the heat exchanger, thermostat and 1" connections.
http://loravia.com/shop/fr/-nouveautes-/echangeur-eau-huile-thermostat-3-voies

It weighs 1.1 kg, it's 576 euros which isn't bad but I prefer to engineer my own solutions (and I'm cheap).

I agree that thermostats don't fail very often but then neither do ignition systems and we want a double ignition system in our aero engines. It's a personal decision, each owner has to decide for himself.

I'm looking into Motorad "fail safe" thermostats, they might be a good solution but it means I have to make a by pass thermostat housing.

Good luck

Mike G

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Re: Thermo-Bob Coolant Thermostat

by Rob Seaton » 10 years ago

Rob Seaton

3 1.32k 240

Mike,
I just have to speak up before others get the wrong idea idea.
you are using a coolant thermostat so have increased your failure points, then you are using a heat exchanger that cools the oil via the coolant.
- so now if you have a coolant system leak your oil will have no cooling.
The Rotax was certified with fins on the cylinders as it is a "get-home" redundant feature; if the coolant system fails you can operate at a lower power setting and the engine will run for hours. If you have a coolant failure with your system you will very quickly lose the engine due to oil over-temp.
I do not recommend installing a coolant thermostat. It is not necessary.
For sure do not use the Laminova heat exchanger.
Rob

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