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Alternate Fuels Technologies, Inc

What Is LPG?

LPG is liquefied petroleum gas commonly known as propane (C3H8), a combustible hydrocarbon based fuel. It comes from the refining of crude oil and natural gas. At normal pressure and temperatures above -44F Propane remains in it's gaseous form. At lower temperatures and/or higher pressures propane will become a liquid. Propane is colorless and odorless. For safety reasons propane is required to be odorized as to indicate positively, by distinct odor, the presence of gas in air down to a concentration of not over 1/5th the lower level of flammability 0.4% in air. This is achieved by adding 1.0 lbs of ethyl mercaptan, or 1.0 lbs of thiophane, or 1.4 lbs of amyl mercaptan per 10,000 of liquefied petroleum gas. There are currently three grades of propane available, HD5 for internal combustion engines, commercial propane and commercial propane butane mix for other uses. The exact composition of propane varies slightly between different parts of the country and different refineries. Compared to gasoline the energy content of LPG is 74%

Comparison of Fuel Properties

  LPG Pressure Versus Temperature

It can be difficult to picture in our minds the effects of temperature and pressure on propane because the propane we deal with on a daily basis is always sealed inside a storage container, out of sight. To help us to understand propane better we will compare it to a substance that we are all familiar with, water. We will use an automotive cooling system to illustrate water under pressure at different temperatures. Liquid propane and water act very similar to temperature changes, the difference being the temperature at which events take place.

Table #1 lists the vapor pressure inside a propane container, at a particular temperature, containing some liquid but not more than 80% total capacity. This allows for a 20% vapor space

Table #2 lists the boiling point of water at a particular pressure

Table #3 lists the similarities of water and propane

  How can propane be stored as a Liquid above its boiling point of -44 degrees

We can increase the boiling point of liquid propane by applying pressure against it similar to the way a vehicle cooling system raises the boiling point of water by holding pressure in the system through the use of a radiator pressure cap. For example if we look at table #2 we see that at normal atmospheric pressure water boils at 212 Degrees Fahrenheit. If we use a 10 psig radiator cap to hold 10 psig against the water it's boiling point increases to 242 Degrees Fahrenheit. Therefore the water remains liquid at 242 Degrees Fahrenheit. Liquid propane reacts in much the same way to temperature and pressure as water, its just that the boiling point of propane is much lower on the thermometer. For example if we look at table #1 we can see that if we had a bucket full of liquid propane and the temperature was below -44 degrees Fahrenheit the propane would remain a liquid at normal atmospheric pressure. The propane would look and act just like water does in its liquid form. If we raise the temperature of the propane in the bucket above -44 degrees it would look and act just like water in a pot on the stove, it would boil and vaporize. The propane would continue to boil and vaporize until the bucket was empty. If we take that bucket of propane and pour it into a propane container and seal it and the temperature is below -44 degrees not much happens. The propane remains in its liquid state. However if we raise the temperature of the container to 80 degrees the propane will boil and vaporize. Since the propane is now in a sealed container with a fixed volume and we now know from table #3 that propane expands 270 times in volume when it changes from a liquid to a vapor, the propane vapor begins to compress. As the propane vapor compresses the pressure inside the container will begin to increase. Table #1 shows that at 80 degrees the vapor pressure inside the container should be 128 psig. Therefore the propane will continue to boil and vaporize, the vapor will continue to compress, the pressure will continue to increase until it reaches 128 psig. With 128 psig vapor pressure acting against the liquid propane the boiling point acting against the liquid propane the boiling point has been raised to slightly more than 80 degrees. Therefore the propane will stop boiling. It is the vapor pressure that governs the boiling point of the liquid propane inside the container. In turn the amount of vapor pressure generated inside the container is governed by the ambient temperature outside the container. To review, liquid propane will boil above -44 degrees unless the pressure is held against it. The amount of vapor pressure required to stop the liquid propane from boiling depends on the ambient temperature outside the container.

Does the amount of Liquid effect the pressure inside a propane storage container?

No. And important fact to remember is that since the vapor pressure inside the propane container is governed by ambient temperature outside the container not the amount of liquid inside. A container that is 1/4 full at 80 degrees will contain the same vapor pressure as a container that is 3/4 full at 80 degrees. The vapor pressure is not generated by the amount of liquid in the tank. As long as there is some liquid and not more than 80% liquid inside the container, ambient temperature outside the container will govern the vapor pressure inside the container.

Comparison of Energy Content

Energy content per unit of fuel (energy density) is an important factor affecting range and power output of internal combustion engines. The following chart compares the energy content of alternate fuels to gasoline.

Comparison of Auto Ignition Temperature

The auto ignition temperature is the temperature at which a fuel will ignite without the need for a spark or flame. In respect to auto ignition temperature LPG, CNG, and LNG are much safer than gasoline or diesel because the auto ignition temperature is much higher. The following chart compares the auto ignition temperature of various fuels.

Comparison of Flammability Range

The flammability range is the distance from the leanest (LEL - Lower Explosion Limit) to the richest (UEL - Upper Explosion Limit) mixture of fuel and air that will burn. Fuels with narrower ranges are safer to work with but are less versatile because they offer less choice of air to fuel ratios. The following table compares the flammability range of various fuels.

Comparison of Peak Flame Temperature

The following chart compares the peak flame temperature of various fuels. You can see that CNG (Compressed Natural Gas) has a peak flame temperature of 1790 C & 3254 F whish is 187 C & 337 F or 9.5% cooler than the peak flame temperature of gasoline at 1977 C & 1591 F. The peak flame temperature of propane at 1991 C & 3614 F is only
13 C & 23 F or less than 1% higher than gasoline.

Comparison of Volumetric Efficiency

The amount of air entering an engine at a particular throttle angle and load is fixed. Any fuel added to the air before it enters the cylinder will displace an equal volume of air and will reduce the volumetric efficiency and power output of the engine. The table below illustrates the reduction of volumetric efficiency of various fuels.

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