Issues with aux tank for portable generator

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Old 05-22-18, 09:31 AM
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Issues with aux tank for portable generator

I recently purchased an A-iPower SUA4500 portable generator. It is rated for 3500W continuous and 4500W starting. I intend to use it to power a camper when at unimproved camp sites. It comes with a 4 gallon metal gas tank that sits on top of the engine/generator assembly. In theory it should provide fuel for 4-8 hours depending on demand.

I wanted to add an aux fuel tank to increase the run time, so I searched the Internet for ideas. As I already had an unused 6 gallon tank for an old boat, I decided to use the technique of running a fuel line (with priming bulb) from the aux tank to the main tank. According to one DIYer, you could drill a hole in gas cap for the main tank and install a fitting for the hose from the aux tank. In theory as the fuel level in the main tank dropped during use, a vaccum in the main tank would draw fuel from the aux tank as needed. It seemed to make sense to me so I looked at my generator.

The cap on the main fuel tank is completely sealed with no vents. That left me wondering how the tank vented as the fuel level dropped. I quickly found that there was a vent hose plumbed from the top of the tank to the air cleaner assembly. It allowed fumes from the tank to be consumed by the engine and allowed air to travel to the tank as the fuel level dropped. As the tank was still empty I tested my thesis by removing the vent hose from the air cleaner and blowing into it. No air escaped the tank and as soon as I stopped blowing the air in the tank rushed back out. It was a closed system except for that vent hose.

So I set up my aux system as follows. First I blocked off the opening in the air cleaner assembly where the vent hose had gone. I then connected the hose from my aux tank to the open end of the vent hose using a fuel filter as the connection. I put the aux tank on a platform at about the same height as the main tank. With fuel in the aux tank and the cap off the main tank, I pumped the bulb and heard gasoline splashing into the main tank. I then filled the main tank about 3/4 full, put the cap on, opened the vent on the aux tank and fired up the generator for its breakin period. I ran it for about three hours with a light load and the fuel level in the main tank dropped to less than 1/2. I saw no indications it was drawing fuel from the aux tank. As I was going to have to leave for a while, I shut off the generator and then something interesting happened.

As the hot generator sat, it heated the fuel in the main tank and I soon heard the sound of bubbles in the aux tank. I conclude that the heat vaporized gasoline in the main tank causing a build up of pressure. That pushed vaporized gasoline out throught the vent hose into the aux tank hose, past the valves in the bulb and into the aux tank. Everything was outside in a well vented area so I left it for a while. When I returned the main tank was full of fuel and the aux tank had much less fuel. It appears that when the generator cooled completely, the heat process that pushed gasoline vapors out of the main tank reversed and pulled fuel from the aux tank. That process appeared to continue until the main tank was completely full and there was no longer any vacuum in it.

So finally I am to the point where I ask for thoughts and advice. If my understanding of what happened is correct, then heat from the running generator is causing enough vapor pressure in the main tank to prevent the aux tank from refilling it while the generator is running. It's slightly convenient to refill the main tank this way if I'm not in a hurry, but that's not why I wanted an aux tank. So does anyone have a different explanation for what is happening, or a simple, elegant solution that will allow an aux tank to work?
 
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Old 05-22-18, 10:24 AM
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I don't know if this arrangement will ever work like you hope. For one, the little carb on that engine needs free-flowing gas to keep the tiny bowl full and sucking it from a 2nd tank is too much work. Plus--if the checkball in your primer bulb were doing its job your heat/cool pumping wouldn't happen, AND the check is another restriction to free flow.

And I wouldn't be comfortable with 6 gallons of fuel ready to drench the hot motor if the hose gives outor the primary tank overflows.
 
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Old 05-22-18, 10:28 AM
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I would disconnect/remove the tank included with your generator. Get a separate, larger fuel tank and use a pulse type fuel pump to move the fuel from the tank to the generator's carburetor.
 
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Old 05-22-18, 11:00 AM
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Pilot Dane,
I'm liking the impulse fuel pump idea. I'm reading up on them now.

Others: I'm still open to thoughts and ideas.
 
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Old 05-22-18, 12:39 PM
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Piloit Dane's suggestion is the only method I can get behind with regard to safety.
 
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Old 05-22-18, 01:16 PM
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While I was thinking through the pulse pump idea I was considering how to connect the fuel line from the pump to the carburetor. That's when a simple, elegant solution came to me. I realized that because the aux tank is on a platform at the same level as the main tank, I can simply connect the hose from the aux tank to the hose that feeds the carburetor with a T-ftting upstream from the shutoff valve. That will allow both tanks to feed the carburetor by gravity. As they are about the same heights, that should not increase the pressure going to the carburetor. And if there is any difference in the height of the fuel in each tank, it should equalize automatically and feed evenly from both tanks. .... Please let me know if you see any flaws in this approach or have a way to do it better.
 
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Old 05-22-18, 05:01 PM
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The big problem I see is the shape of your generators fuel tank. It's likely rather low profile (short height). That means that your aux tank also must be low profile. Any part of your aux tank higher than the main tank can't be used as the fuel will cross feed and leak out the main's fuel cap or vent. So, you will be limited in the aux tank you can use. Using a fuel pump will allow you a lot more flexibility for a tank.
 
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Old 05-22-18, 06:40 PM
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Correct, and another danger in that is that if one tank gets a clogged vent or warms up more than the other or whatever the situation may be, it could force fuel out of it and into the other tank, overfilling it. If you want to connect them both, then you're going to need a fuel switch to shut off one tank and open the other.
 
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Old 05-22-18, 07:33 PM
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Good thoughts. Thank you.
 
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