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| Propane is stored in the fuel tank as a pressurized liquid but is generally metered into the engine as a gas. When the gas is metered in at the central point, such as a carburetor, this is known as a fumigation system or as a pneumatic system. When the gas is metered in on the intake manifold at the entrance to each cylinder, this is know a sequential injection system. The propane converter accomplishes the task of changing the liquid propane to a gas by evaporating the fuel with heat supplied by its integral heat exchanger and then regulating the pressure in a two-step pressure regulator to a pressure just below atmospheric. The device that evaporates the fuel and regulates its pressure is also known as a converter-regulator. With fumigation systems, propane is metered into the engine with a propane mixer or a venturi. A mixer supplies a fixed ratio of fuel to air and the ratio is controlled by the shape of the air valve. A venturi bleeds the fuel into the air stream with the aid to a fast-reacting fuel control valve, which is usually electronically controlled with O2 sensor in the exhaust. A mixer-based system can also be electronically controlled but the fuel pressure at the converter is what the controller adjusts to maintain the correct fuel mixture. The main reason that propane engines have less power than gasoline engines is that the fuel is metered into the engine as a dry gas rather than as an atomized liquid. The gaseous fuel displaces some of the combustion air and the key to maximizing power is to maximize the amount of air drawn into the cylinders. An important means of overcoming this reality, which works for both gasoline and propane engines, is to use cool (preferably ram) air. A good way of doing this is with something like a Ram Air Box. For those people considering a conversion of their personal vehicle, you need to decide what kind of driving you are realistically going to do with your vehicle. At what RPM will your vehicle be spending most of its time? Use this calculator to calculate find out: http://tech.oldsgmail.com/axle_RPM.php You should also know your vehicle's transmission shift points. With this knowledge, you need to select a propane carburetor. An Impco CA425 carburetor would most commonly be used up to 4250 RPM with a Chrysler big block 440 CID engine, for example. If you plan on running an engine like this much much faster, you should consider 2 of these carbs. With 2 carbs, you would need a dual carb intake, which would obviously add significantly to the cost and make a cold air system much more difficult to install. Alternatively, you could run a single Technocarb 4bbl propane carb which would flow much more air and come already equipped for closed-loop (O2 sensor) operation. However, I wouldn't expect a daily driver to really need more fuel than what an Impco CA425 could supply.
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