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 done with an injection system.
The propane converter used in fumigation systems accomplishes the task of changing the liquid propane to a gas by vaporizing 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 or just regulator.
With fumigation systems, propane is metered into the engine with a propane mixer or a venturi. An air-gas valve mixer supplies a fixed ratio of fuel to air and the ratio is controlled by the shape of the gas valve's cone. A venturi mixer bleeds the fuel into the air stream and this mixers require converters that include power and idle mixture control. 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. Feedback-controlled systems generally use either a fast-reacting fuel control valve between the converter and mixer or a means of controlling the reference pressure acting on the converter. Both methods are electronically controlled with an O2 sensor in the exhaust.
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 but ram air will require that the converter be pressure-referenced to the mixer's air horn.
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:
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.