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Old 16-09-2019, 03:34   #1
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Diesel tank over heating

Hi All.
Just purchased a second hand 41 foot sailing boat with yanmar 3jh5e engine. Has a stainless steel 70 liter diesel tank.
Just finished a 24 hour delivery in no wind so motored all the way. Checked diesel tank after several hours and found it to hot to touch. Maybe 70 deg c at a guess. Motor was running at 80 constant so no over heating. Guessing it's just the fuel return coming back hot and slowly building up heat.
Any suggestions of fuel coolers?
Was a little concerning having the fuel tank so hot.

Any suggestions appreciated.

Cheers
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Old 16-09-2019, 07:29   #2
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Re: Diesel tank over heating

A 70 liter tank in a 41 foot boat is rather small, so the amount of fuel returned by the engine is proportionally greater than it would be with a larger tank, and that makes the fuel in the tank heat up faster than it otherwise would.

That said, the fact that the fuel heats up isn't really hurting anything as long as the engine has enough reserve power to push the boat in adverse conditions. When engine manufacturers rate the power of an engine, the rating conditions usually state the temperature and density of the fuel used, and that is usually something like 845 g/ liter density and 20*C temp.

If your fuel is heated up to perhaps 70*C, it will require a greater throttle setting to push the boat at the same speed as a lesser setting when the fuel is only 20* C. As stated earlier, this may not be a problem if you have adequate power reserve.

Should you install a fuel cooler? Why not, it won't hurt anything. Lots of high performance engines are equipped with coolers so that they don't suffer power and performance loss, and modern electronically controlled engines routinely measure fuel temp so that the ECU can compensate for power loss automatically.

Get yourself a small cooler and install it in the seawater circuit between the seawater pump and the heat exchanger, and make sure the seawater passages in the cooler are large enough that there is no seawater flow restriction. Run the return fuel line from the injection pump to the cooler and then back to the tank. It's best to plumb the fuel flow so that it's flow within the cooler is in the opposite direction as the seawater is flowing. And that's about it. If nothing more it will give you piece of mind.

DougR
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Old 16-09-2019, 15:58   #3
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Re: Diesel tank over heating

My GUESS is that is way too hot for peace of mind. I'd suspect if it is actually 70oC you'd be boiling off volatiles & reducing your cetane rating over time.
A chemical engineer might chime in with more qualified analysis but if it was me I'd definitely going for a fuel cooler install.
What is the ambient air temp?
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Old 16-09-2019, 16:31   #4
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Re: Diesel tank over heating

Quote:
Originally Posted by yoting View Post
Any suggestions appreciated.

Cheers
My guess is:

If it were a problem it would long ago been apparent and you are worrying about nothing!
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Old 17-09-2019, 09:18   #5
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Re: Diesel tank over heating

Sounds like a ventilation problem.
Do not cut up your engine cooling system.
Do not chop up your fuel lines.
Turn on the engine space fan, or otherwise improve the ventilation around the tank.
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Old 17-09-2019, 09:49   #6
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Re: Diesel tank over heating

How much fuel was in the tank? You may just need to keep the fuel level up in that small a tank for heat dissipation. ie, carry jerry cans and fill underway.
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Old 17-09-2019, 10:42   #7
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Re: Diesel tank over heating

Was the tank nearly empty when it felt hot? Diesel engines pump extra fuel to cool the injection pump. If the tank was nearly empty that "coolant fuel" was probably recirculated too much. Try keeping the tank from getting too low.
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Old 17-09-2019, 12:01   #8
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Re: Diesel tank over heating

Your 20 gallon tank is too small and poorly ventilated.
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Old 17-09-2019, 20:15   #9
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Re: Diesel tank over heating

Sendure makes a dandy oil cooler for small transmissions like velvet drives, Paragons, etc. They are about a foot long, have a 5/8" hose fitting on each end and 1/4" pipe for material cooling fittings. I've seen these used on many occasions on fuel returns and on additional oil cooling. Very easy to install, just install on the down side of your present transmission cooler. I've ran across your problem mainly in yachts that put fuel return to the center day tank. Lessens the cross feed from main tanks to day tanks, but the hot diesel does reduce engine efficiency, as does a hot engine room..
Hope this gives you some insight, Capt. Vince Rakstis, Ret.
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Old 18-09-2019, 02:55   #10
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Re: Diesel tank over heating

Hi All.
Thanks for replies. Yes I think mainly for piece of mind. Boat is 25 years old and I think original setup. With what we are using it for and to keep weight down for racing the tank is more than big enough.
When we first checked and noticed the problem the tank was about 2/3 full. It's under a bunk so took the bunk cushion of and left the lid off so was all exposed but didn't make much difference.
Redpcx what's your reason for not adding a cooler?

Sorry for slow replies keep getting errors on computer trying to post. Can only reply on phone.
Thanks everyone for replies and suggestions appreciated.
Cheers
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Old 18-09-2019, 03:03   #11
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Re: Diesel tank over heating

You lose 1% of power for every 5.6oC temperature rise above 20oC according to a study I read. Also reduces storage life. You can find the studies using google. Sorry forgot to save them.
I'm still amazed it gets that hot.
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Old 18-09-2019, 07:24   #12
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Re: Diesel tank over heating

Hot fuel doesnít cause a loss of power, itís hot air that does.

Fuel is heated mostly because itís compressed which puts energy into the fuel and ends up as heat.
The higher you pressurize the fuel, the hotter it gets, normal Diesels only pressurize to about 600 PSI or so, common rails go up to 30,000 or higher.
Itís very common on common rails to have a fuel cooler, itís nothing more than a transmission cooler mounted into the return line of the fuel, on my Duramax truck it was mounted in front of the fuel tank.

Fuel is recirculated as itís used as a coolant for the HP pump and on some common rails also for the electronics

Mounting a cooler on the return fuel would be simple and easy to do and as there is no pressure on return fuel, itís not likely to be a leak issue.
Why not? Canít hurt could help.
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Old 18-09-2019, 07:48   #13
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Re: Diesel tank over heating

Sounds like a wonderful opportunity to extract hot water for showers.
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Old 18-09-2019, 15:45   #14
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Re: Diesel tank over heating

[QUOTE=a64pilot;2979198]Hot fuel doesnít cause a loss of power, itís hot air that does.

Only 1/2 right both do

https://pdfs.semanticscholar.org/73e...3cc69e6733.pdf

If the fuel is hot you need to burn more = loss of power
Cant be bothered to look for the horsepower study but the one above should give you the idea
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