CHAPTER 0 LPG AND LPG FUEL TANKS LPG Fuel Supply Liquefied petroleum gas (LPG) consists mainly of propane, propylene, butane, and butylenes in various mixtures. LPG is produced as a by-product of natural gas processing or it can be obtained from crude oil as part of the oil refining process. LPG, like gasoline, is a compound of hydrogen and carbon, commonly called hydrocarbons. In its natural state, propane is colorless and odorless; an odorant (ethyl mercaptan) is added to the fuel so its presence can be detected. There are currently three grades of propane available in the United States.
A propane grade designation of HD5 (not exceeding 5% propylene), is used for internal combustion engines while much higher levels of propylene (HD10) are used as commercial grade propane along with a commercial propane /butane mixture. APPROXIMATE COMPOSITION OF HD5 PROPANE BY VOLUME The same principle is applied to LPG in a container, commonly referred to as an LPG tank or cylinder. Typically an LPG tank is not filled over 80% capacity allowing for a 20% vapor expansion space. Outside air temperature effect’s an LPG tank and must be considered when using an LPG system. (Figure 2) shows the relationship between pressure and temperature in a LPG tank at a steady state condition.
With 128 PSIG vapor pressure acting against the liquid propane the boiling point has been raised to slightly more than 80 deg. F / 27 deg. C. NOTE Vapor pressure inside an LPG tank depends on the ambient air temperature outside the tank, not the amount of liquid inside the tank. A tank that is ѕ full of liquid propane at 80 deg. F will contain the same vapor pressure as a tank that is only ј full of liquid propane. LPG’s relative ease of vaporization makes it an excellent fuel for low-rpm engines on start-and-stop operations. The more readily a fuel vaporizes the more complete combustion will be.
Because propane has a low boiling point (-44F), and is a low carbon fuel, engine life can be extended due to less cylinder wall wash down and little, if any, carbon build up. LPG Tank Pressure VS Temperature 300 250 Pressure, psig 150 200 100 50 Figure 2 0 -20 0 20 40 60 80 100 120 140 Temperature, deg F Compressed Vapor 128 PSIG Liquid Propane An advantage of LPG is the ability to safely store and transport the product in the liquid state. In the liquid state propane is approximately 270 times as dense as it is in a gaseous form. By pressurizing a container of LPG we can effectively raise the boiling point above –44 deg. C / -42 deg. C, keeping the propane in liquid form.
The point at which the liquid becomes a gas (boiling point) depends on the amount of pressure applied to the container. This process operates similarly to an engine coolant system where water is kept from boiling by pressurizing the system and adding a mixture of glycol. For example water at normal atmospheric pressure will boil at 212 deg. F / 100 deg. C. If an engines operating temperature is approximately 230 deg. F / 110 deg. C, then the water in an open un- pressurized cooling system would simply boil off into steam, eventually leaving the cooling system empty and over heating the engine.
If we install a 10 PSIG cap on the radiator, pressurizing the cooling system to 10 PSIG, the boiling point of the water increases to 242 deg. F / 117 deg. C, which will cause the water to remain in liquid state at the engines operating temperature. Figure 3 LPG Tank G420E/G424E Tier LP Engine LPG and LPG Fuel Tanks 8 Propane (C3H8) Propy lene Butane (C4H10) Iso- Butane Methane (CH4) TOTAL 90.0% min. 5% max. 2.0% 1.5% 1.5% 100%