How Liquefied Petroleum Gas Works

A gas station attendant fills up a taxi with liquefied petroleum gas. See pictures of alternative fuel vehicles.­
A gas station attendant fills up a taxi with liquefied petroleum gas. See pictures of alternative fuel vehicles

­Liquefied petroleum gas, or LP gas, is one of the most common alternative fuels used in the world today. In fact, in many places, it isn't an alternative fuel at all: LP gas is a mainstay for heating and cooking in certain areas of India and some rural areas of the United States. When you cook out at a campground or in your backyard using a gas grill or camping stove, you're using some form of LP gas. Known for its versatility and efficiency, these days LP gas is looking like an attractive source of energy for people fed up with high heating bills and skyrocketing prices at the gas pump.

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In this ­article, we'll find out where LP gas comes from, what it's used for and how you can convert your car to run on this lighter and cheaper fuel. ­

What is LP Gas?

There are two LP gases that can be stored in liquid form with only moderate pressurization -- propane and butane. Isobutane, which has the same simple chemical formula as butane but has a different chemical structure, is also used. Usually, butane and isobutane are mixed with propane in various proportions, depending on the intended use of the fuel.

Propane is particularly useful as a portable fuel because its boiling point is


-44 F (-42 C). That means that even at very low temperatures, it will vaporize as soon as it is released from its pressurized container. This results in a clean-burning fuel that doesn't require a lot of equipment to vaporize it and mix it with air. A simple nozzle will suffice.

Butane's boiling point is approximately 31 F (-0.6 C), which means it will not vaporize in very cold temperatures. This is why butane has more limited uses and is mixed with propane instead of being used by itself.

A single pound of propane can generate 21,548 BTU (British Thermal Units) of energy, while butane can produce 21,221 BTU per pound [ref]. For comparison, here is how LP Gases stack up to other fuels in terms of energy:

  • Propane: 21,500 BTU per pound
  • Butane: 21,200 BTU per pound
  • Gasoline: 17,500 BTU per pound
  • Coal: 10,000 BTU per pound
  • Wood: 7,000 BTU per pound

In the next section, we'll find out where LP gas comes from.

The Source of LP Gas

Oil refineries produce LP gases in the process of producing other, more commonly used fuels.
Oil refineries produce LP gases in the process of producing other, more commonly used fuels.
Photo courtesy Phillips Petroleum Company

LP gas is a fossil fuel, like oil and natural gas. It can be refined from oil and natural gas the same way gasoline is refined from crude oil. While most energy companies are not focused on LP gases, they produce them nonetheless because they are a byproduct of the refining process for other fuels.

When an energy company draws natural gas from the ground, about 90 percent of it is methane. The rest is in the form of various LP gases, which the company separates from the methane before the methane is fed into pipelines for use in our homes. The amount of LP gas that comes from natural gas varies, but it is usually from 1 to 3 percent [ref].


LP gases are separated from crude oil, as well. The refining process produces about a 3 percent yield of LP gases, although if refineries were retooled to focus on LP-gas extraction, that number could be as high as 40 percent [ref]. See How Oil Refining Works to learn about the process.

Next, we'll find out about some of the many uses for LP Gas.

A Versatile Fuel

Propane lighter and propane stove
Propane lighter and propane stove
Photo courtesy

LP gas is easy and safe to store, which makes it a very portable fuel. It has been utilized for many different applications, some of which you've probably used yourself. Small, disposable butane lighters use the LP gas you are most likely to see on a daily basis. They contain a mixture of butane and isobutane.

If you have a portable gas grill, or if you go camping in a trailer or motor home, you use LP gas there, as well. The refillable gas tank on your grill uses propane, and the propane tanks on your motorhome can power a furnace, heat a hot-water tank or even power the refrigerator and freezer. Small tanks of camping fuel available at most sporting goods stores also use propane. The nozzles on the tanks have been standardized throughout the camping industry, so these tanks can be attached to camp stoves, lanterns, small water heaters and a variety of other devices.


Propane is useful in any situation where fuel is needed but it's not practical (or possible) to run lines for natural gas to the site. You can find LP gas on boats, at isolated cabins and hunting lodges or in rural areas that aren't served by commercial energy companies. In countries like India, where some large cities do not have reliable natural gas or other fuel service, LP gas makes up a major share of the energy market. Everyday heating and cooking needs are supplied by propane instead of oil or coal.

LP gas isn't only for home and leisure use, however. Many industries use LP gas as a source of heat for metal working, glass working or ceramics. Many industrial forklifts are LP-gas powered because LP gas provides enough power to do heavy lifting while generating reduced fumes and pollutants in confined warehouse spaces.

One of the fastest growing uses of LP gas is in automobiles. Cars built or converted to run on LP gas are becoming more and more common -- we'll find out why in the next section.


LPG filling station
LPG filling station
Photo courtesy U.S. Department of Energy

According to the World Liquefied Petroleum Gas Association (WLPGA), more than 9 million vehicles in 38 countries currently operate on LP gas. It's not a new idea: Propane-powered vehicles have been around for decades. The benefits include reduced emissions, quoted by WLPGA as "50% less carbon monoxide, 40% less hydrocarbons, 35% less nitrogen oxides (NOx) and 50% less ozone forming potential compared to gasoline" [ref]. With government incentives and tax breaks figured in, LP gas used in cars (known as autogas) can be much cheaper than gasoline. Even without the incentives, it is usually much cheaper. Autogas is a high-octane fuel, offering performance comparable to gasoline and diesel, and many owners claim that autogas runs more smoothly, resulting in less wear and tear on engine components.

The easiest way to start using propane for transportation is to buy a vehicle that was designed and built to use autogas. A wide range of manufacturers, including major American car companies like Ford, General Motors and Daimler-Chrysler, offer propane-powered options on some models. Typically, you won't find these cars on the showroom floor -- they have to be special ordered. In some cases, there is no production model powered by LP gas, but an LPG system can be factory installed as an option. Examples of cars that are offered as LP gas models include the Ford Crown Victoria, Dodge Ram, Ford F-150 truck, Chevrolet Cavalier and Toyota Camry.


Photo courtesy The Greenfuel Company

Rarely (though more often in Europe), cars are built solely with an LP-gas fuel system. The engine is tuned and adjusted specifically to run at maximum efficiency on propane, which results in reduced engine wear and better mileage in the long run. However, many consumers are worried that a propane filling station could be hard to find. For that reason, most LP gas cars have dual fuel systems -- one for gasoline and one for propane. The systems are set to switch automatically between the two fuels as appropriate. This allows owners to use clean, cheap propane when it's available, but they still have the option of a quick fill-up at the local gas station if they need it.

How hard is it to find a propane filling station? Actually, it's not very difficult at all. Unlike some other alternative fuels (such as hydrogen), a large and stable infrastructure already exists for vehicles running on propane. You can use the Alternative Fuels Data Center to find the nearest LP-gas refueling stations. As an example, there are five LP-gas refueling points within 25 miles of the author's house in Buffalo, N.Y.

Modified Prius: LPG-electric hybrid
Modified Prius: LPG-electric hybrid
Photo courtesy Green Car Congress

In the next section, we'll discuss the basics involved in converting a gasoline-powered car to run on autogas.


Example dual-fuel conversion system
Example dual-fuel conversion system
Photo courtesy WTV-UK

The process of converting a car to run on propane is fairly demanding and requires a good knowledge of automotive systems in general to accomplish.* Several companies offer kits that include all the needed parts to perform the conversion. Those without the necessary know-how should try to find a local mechanic with experience in LP-gas conversions to get the job done. Although propane is very safe as an automotive fuel, if the system is not installed correctly, there can be safety problems.

The first step is choosing a tank. Most conversions are dual-fuel conversions, meaning you won't be replacing your old fuel system, you'll simply be adding a second. As a result, the propane tank will take up some of the storage space in your car, usually in the trunk.


Donut tank (left) and torpedo tank
Donut tank (left) and torpedo tank
Photo courtesy WTV-UK

Tanks come in "torpedo" or "donut" form. Torpedo tanks generally have more capacity, but will take up more space in your car. Donut tanks are designed to fit in the spare tire well of your car. They are smaller tanks, and you'll have to sacrifice your spare tire. In larger vehicles, you could mount multiple tanks for increased capacity.

Once the tank is bolted in, a fill point must be drilled into the car's body, usually near the gasoline fill point or at the back of the trunk. The ideal location is one that requires minimal piping to connect to the tank. The fuel lines themselves are copper tubes, which offer a certain amount of flexibility when the lines are routed. The tank must be connected to the fill point, and lines also have to run along the underside of the car up to the engine.

A solenoid valve (LPG valve in the above diagram) must be installed on the fuel line in between the tank and the engine. This valve cuts the flow of LP gas when the car is running on gasoline and when the engine is shut off. It also has a filter built in that removes any dirt that may be in the fuel.

The next major component is called a regulator, also referred to as a vaporizer. This device performs one of the functions that a carburetor handles in a gasoline engine -- it uses heat from the car's cooling fluids to vaporize the propane into gas form. Another safety check occurs at the regulator, as well. It includes an electronic circuit that cuts the flow of gas if the engine stops or stalls. The regulator is usually smaller than a regular carburetor, so finding space for it in the engine compartment shouldn't be a problem.

The other part of a carburetor's function is handled by a mixer mounted in the intake manifold. The mixer takes information from the car's sensors or ECU, and then it controls the amount of gas that flows into the cylinders.

Dashboard fuel switch/gauge
Dashboard fuel switch/gauge

The system must then be wired into the car's electrical system, allowing for a functioning fuel gauge, as well as proper automatic switching between propane and gasoline (along with a dashboard-mounted manual switch). There must be connections to the car's ECU so that the engine controller can adjust for different fuel settings.

Cars with an electronic injection system will probably need an electronic emulator. When the car is operating on LP gas, the fuel injectors will not be sending any information to the other sensors in the car -- this will light up the "check engine" light and give incorrect diagnostic readings. The emulator fakes the proper signals so the ECU can operate properly.

Conversion kits come with more detailed instructions, but this is a basic overview of what needs to happen in a dual-fuel conversion.

Next, we'll find out how safe LP gas is.

*Disclaimer: This should not be used as a guide for an actual LPG conversion. It is intended as an overview of the process and not a set of instructions. Unless you are very experienced in automotive fuel systems and electrical systems, you are strongly urged to have the conversion performed by a professional. All conversions should be tested and approved by a professional with experience testing LPG systems before they are used.

Safety and Storage

LP gases are actually quite safe in comparison with other fuels. Propane has a high ignition temperature, about 850-950 F (450-510 C), compared to about 495 F (257 C) for gasoline. This makes it less likely to ignite spontaneously.

Also, the tanks used to store propane are stronger than gasoline tanks because of the pressure needed to keep the propane in liquid form. This makes on-board propane storage safer than a typical gas tank -- it is more resistant to rupturing in the event of a collision. Special safety valves and cut-offs increase the safety factor.


To store LP gas safely, there are some guidelines. It's important to remember that a propane tank is never really empty. When the tank is filled, most of the propane is under enough pressure that it is in liquid form. But at safe pressures, not all of the propane is liquefied -- a small amount is in gas form, filling up the rest of the space in the tank. As more propane is used, the pressure decreases, leaving less propane in liquefied form and more propane gas filling up the remaining space.

Tanks should only be filled to roughly 80 percent capacity. Changes in temperature can change the pressure inside the tank. If you were to fill your tank to 100 percent on a cool, cloudy day, and then leave your car out in the sun the next day, the increase in temperature would cause in increase in pressure within the tank that could cause it to fail without that 20 percent headroom.

For more information on LP Gas and related topics, check out the links on the following page.

Lots More Information

Related HowStuffWorks Articles

More Great Links


  • Ford Scorpio: LPG Conv (2.0 16V)
  • Global Autogas Industry Network
  • World LP Gas Association: About LP Gas
  • Snelling Discovery
  • BTU Content Comparisons of Common Fuels
  • Propane Gas Association of Canada
  • Leicester Auto & Marine Gas Services
  • Do-It-Yourself Automotive LPG Conversion
  • Calor: LPG Conversion
  • Turner, CC. The LP Gas Man's Encyclopedia of Methods and Equipment. Moore Gas, 1955.
  • The Compressed Gas Association. Handbook of Compressed Gases. Van Nostrand Reinhold, 1990. 0-442-21881-8.