Spilling Gas and $ [Fuel Cell with Gas Saver Vent]

As anyone who has ever worked with fuel tanks knows, proper venting is critical to the operation of the tank. Modern vehicles have sophisticated venting systems with fuel vapor recovery canisters, external vent lines, an external liquid/vapor separator, multiple moving parts, springs, and floats. These systems are frequently tied into the computer of the vehicle and can impact operation and/or performance if they don't function properly.

With secondary (i.e. auxiliary) fuel tanks the venting systems are very rudimentary. We typically use non-vented caps so there is no spillage in the event of a tip-over. To allow for air to offset the fuel as it is drained, we use a simple vent line located at or near the highest point on the tank. In addition to allowing air to enter the tank, this vent line also functions as a fuel overflow should the fuel expand beyond the capacity of the tank. Since we typically want a secondary tank to be as compact as we can make it there is generally little or no room for expansion. Therefore, we commonly find that some portion of our gasoline ends up on the ground. Not only is this environmentally unsound but, at the price of gasoline, it can add up to a significant expense.

I have tried various methods to prevent this gasoline from being vented overboard but in every case all I managed to do was complicate the system and cause venting problems. All that is about to change.

I just finished up a cell for a client that had a unique twist. He requested an overflow tank mounted internal to the tank. It's his design and a really neat idea, I wish I would have thought of it.

Following is a description of how it functions.

This is the CAD view of the catch tank.

Here's how it works:

• Overflow Operation:
• The overflow is a small tube that exits the top of the overflow tank 
• It extends into the filler neck and goes to the bottom of the overflow tank 
• As the fuel expands in the main tank it flows into the overflow tank. This overflow tank is engineered to hold most of the expansion expected from the 5 gallon tank.
  
  
  
  
  
• Vent Operation:

• The vent tube starts at the top of the catch tank and exits the base of the catch tank and extends through the base of the main tank
• When the auxiliary tank drains a vacuum is created at the overflow tube causing a low pressure in the catch tank, the low pressure is offset by outside air entering the tank via the vent line
• Fuel in the catch tank will be siphoned back into the auxiliary tank 

This entire assembly is mounted inside the secondary fuel tank providing a nice clean exterior. Here is a picture of the finished catch tank mounted in the secondary tank. 

In this particular case, the client asked for a straight filler neck so it was easy to make the overflow tube extend into the filler neck. 

Since most of my tank builds use the side mounted filler with a 90 degree bend I doubt that I could mount a system like this on the inside. I will experiment with making this external on tanks with a bent filler neck.

I will be offering this as an option for current and future clients.

Thanks for looking.

Mike