Chilling a Gasoline Return Line.




Here is an interesting scenario. On rainy days or mornings, my fuel gauge reads significantly higher than warmer days. Where I live a cold day is actually 27 Celsius.

Could it be that the lower temperature effectively lowers the vapor pressure of the gasoline in the tank? Therefore there is a better ratio of liquid to vapor during those cool times?

My car tank is 45 litres however when I full up from empty, the most I can put is 33 litres. I apparently have 12 litres in reserve and at 12km/l fuel economy, I can drive significant distance in my island home using that reserve volume. I want to be able to register more volume on my fuel gauge without having to modify the fuel sending unit.

Also, the vapor vent line has been clogged for the past 10 years. I have tried to fix it but it is permanently clogged due to a defective roll over valve in the fuel tank.

I want to get more fuel consumed between fill ups. Here is my experiment to lower the vapor pressure in the fuel tank without introducing liquid or vapor leaks.

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Step 1: Finding the Return Line.

This line is connected between the fuel pressure regulator and the return line to the tank.

Step 2: Deciding on the Means to Chill the Return Fuel.

The suction line on my car's air conditioning is very cold and gets as low as 8 Celsius within the engine bay.

Step 3: Getting a Heat Exchanger.

For the purpose of this experiment, I used a length of aluminium tubing to allow return fuel to flow.

Step 4: Mounting the Heat Exchanger.

I used cable ties to secure the tubing to the air conditioning suction line. This way there will be some heat exchange when the air conditioning is running.

Step 5: Testing.

I turned on the engine to verify all connections are liquid tight. I used a piece of paper towel to check for leak at each new connection I made.

Using my seek thermal infrared camera, I started the engine and measured the return line temperature after 10 minutes. It read approximately 33 Celsius.

Putting the air conditioning on, I tried measuring the same line after 10 minutes. Due to thermal bloom from a nearby power steering engine hose, it was difficult to get a good reading. The tubing however got cold to touch but since it is shiny, my thermal camera is useless to measure it. The air conditioning return line got predictably cold.

Step 6: Adding Insulation and Closing Up.

I installed some thermal insulation to help keep the chilled return line cold. I used plumber's mate and duct insulation to help reduce engine heat leeching into the mini heat exchanger.

I will update this instructables in the coming week to post my real world results.

Step 7: Test Results.

The first pic is after my second fill up after implementation of fuel chilling. I am pleasantly surprised by what my range is at slightly below half tank. I have not gotten such figures before in the 12 years I have owned the car.

Update 26 Nov 2015:

For my third fill up after implementing fuel chilling I got 530km on a tank! On fill up it took 32 litres. My fuel economy jumped to 16.21 km/l. I'm quite happy. I'm hoping this new trend becomes the standard for my car.

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    6 Discussions


    3 years ago

    Gasoline expands and contracts a lot with temperature. It has nothing to do with the vapor pressure. The vent cap keeps excessive pressure from building up in the tank. It allows air in as you use gas as well. As far as chilling the gas so the tank holds more, that will work,. Just a note of caution, do not chill it, add gas to absolutely full, then Park the car. Expansion of the gasoline may cause the tank to leak. The people that test pumps deliver a gallon into a special container. They have to calculate based on temperature so they know if the pump is within tolerance.

    1 reply

    Reply 3 years ago

    actually gasoline liquid wont change volume in significant values in the temperature range cars are operated. Its the gasoline vapor in the tank I want to return to liquid phase since my fuel tank does not vent due to that stuck rollover valve. I will keep you guys updated as I make a couple fill ups. The main thing for me is zero gasoline leaks so safety wise I am happy.


    Reply 3 years ago

    I'm very curious too. I just want the most out of my fuel tank, more liquid fuel than useless vapor.


    3 years ago

    cool project and interesting idea!
    keep us updated. :)

    1 reply

    Reply 3 years ago

    Most definitely! I just need to wait until it's time to fill the tank.