Notes from Windward: #63

Wood Gas Conversion - Step 2.2

     The truck that's being converted to run on propane is 23 years old, so there's a number of systems that need repair, replacement or modification as part of the conversion process. This step involves the truck's heater system.

     The natural question would be "What does the heater have to do with running the engine on propane?" Actually, it plays an essential role in the process of getting the propane out of the tank and into the engine.

     We think of propane as a gas, but it's actually stored in the tank as a liquid under pressure. Since propane's boiling point is below the surrounding temperature, it wants to boil and will boil until the vapor pressure above the liquid rises to the point where the vapor starts to condense again. Any gas drawn off from the top of the tank will be immediately replaced as the drop in pressure allows some of the liquid propane to boil.

     The equilibrium pressure that balances the tendency of the liquid to boil, and the vapor to condense, is a function of the temperature of the liquid propane. As a liquid turns into a vapor, the liquid becomes cool because it takes heat to convert any liquid into a vapor. This heat, called "the heat of vaporization," has to come from somewhere. Initially it can come from the room temperature heat of the liquid propane itself, but as the liquid propane boils, it gets cooler. If you draw off enough propane fast enough, it will actually freeze into a solid block of propane.

     So if you tried to run your engine by drawing vapor off the top of the tank, that would work for a while, but before very long, your fuel would freeze solid and you'd be stuck by the side of the road waiting for it to warm up again.

     The way this problem is dealt with is by drawing the propane, not from the top of the fuel tank as a gas, but rather from the bottom as a liquid. The fuel line conducts liquid propane to the engine compartment where a device called an evaporator uses the water circulating through the car's heater to provide the heat of vaporization needed to convert liquid propane into a gas.

     And so, that's why the next step of the conversion involved refurbishing the heater system. The heart of a car's heater is a miniture radiator, and after 23 years of use, it was time to replace it. I installed a new heater core installed and all new hoses, and so that part of the work is done.

Notes From Windward - Index - Vol. 63