Propane, also referred to as propane autogas, has been used for decades as a clean-burning alternative fuel. An alkane gas composed of three carbon atoms and eight hydrogen atoms (C3H8), propane is a colorless and odorless liquid when stored under pressure inside a tank. When pressure is released, the liquid propane vaporizes, transforming into a gas that is used in combustion. To aid in the detection of leaks, an odorant called ethyl mercaptan is added.
Propane is an LPG (Liquefied Petroleum Gas) among a group of others, such as butane, propylene, butylene, butadiene, isobutylene, and combinations of these. Despite having a lower volumetric energy density, propane has a higher gravimetric energy density and produces less pollution than gasoline or coal when burned. Propane is a popular fuel due to its abundance in the U.S., its high-energy density, clean-burning qualities, and cost-effectiveness. It is the third most common transportation fuel in the world, behind gasoline and diesel, and is recognized as an alternative fuel under the Energy Policy Act of 1992.
It is created as a by-product of two distinct procedures: natural gas processing and petroleum refining. The raw natural gas is processed in order to remove propane, butane, and large quantities of ethane, so as to prevent the condensation of these volatiles in the natural gas pipelines. In oil refineries, propane is also produced as a result of breaking down petroleum into gasoline or heating oil.
Propane is an ideal fuel for spark-ignited internal combustion engines due to its high octane rating, as well as its safety in the event of a spill or release from a vehicle, as it does not harm soil, surface water, or groundwater. In the United States, only 2% of energy is used from propane, and of that, less than 3% is used for transportation. The majority of propane is used for home and water heating, cooking, refrigerating food, drying clothes, and powering farm and industrial equipment. Additionally, propane is utilized by the chemical industry as a foundation for creating plastics and other compounds.
HD-5 propane, which is used in vehicles, is a mixture of propane with small amounts of other gases. Specifications stipulate that HD-5 propane must be comprised of a minimum of 90% propane as specified by the GPA’s HD-5 guideline, with no more than 5% propylene and the remaining 5% being made up of other gases, mainly butane and butylene.
When propane is stored onboard a vehicle, the fuel is pressurized in a tank at 150 psi, which converts it to a liquid form with 270 times more energy density than its gaseous form. It has a higher octane rating than gasoline, meaning it can be used with higher engine compression ratios and is more resistant to engine knocking; however, more fuel by volume is needed to drive the same distance.
For comparison, propane is much more efficient to store than compressed natural gas (CNG). Propane can be liquefied and stored in a relatively small space, unlike CNG, which requires very high pressure and a large cylinder volume to store useful quantities. The hazard associated with the storage of CNG is that in the case of an accident, the cylinder may burst with great force or become a self-propelled missile.