As the name implies, natural gas is a naturally occurring fuel source and also a byproduct of petroleum extraction. In its raw state, natural gas can be extracted either pure or as a heady cocktail of methane, ethane, propane, butane, and pentane, as well as nitrogen, carbon dioxide, water vapor, and other compounds. Refineries strip away virtually everything in the mix, leaving methane as the primary component, with a few other compounds added. Methane is the simplest of carbon-based gases: just one carbon atom and four hydrogen atoms. As a result, it's also clean burning, with combustion resulting in one carbon-dioxide molecule and four water molecules, lower greenhouse emissions than any other fuel except hydrogen.
Under the Hood: CNG Conversion
All CNG tanks must withstand the same standards for impact and puncture resistance as an under-car gasoline fuel tank while also vastly exceeding the strength requirements. There are four types of tanks, each lighter and more expensive than the next. Type I is an all-metal tank, generally steel or aluminum. Type II is a thinner metal tank wrapped around the middle with a fiberglass or carbon composite. Type III is a thinner metal tank fully wrapped in composite. Type IV is a plastic tank fully wrapped in carbon-fiber composite--very lightweight, but it comes at a hefty price.
What You'll Need for a Natural Gas Conversion
It doesn't take much besides a new fuel tank to convert a gasoline-burning engine to one that also runs on natural gas. Attached to the fuel tank  is the regulator , which reduces tank pressure from 3600 psi to 125 psi. Fuel is then fed to a parallel fuel rail  and to new, secondary injectors plugged into an adapter . A wiring harness  plugs into the factory engine-control unit and intercepts throttle information, sending it to a new fueling computer , which slightly alters the data and passes it to the CNG injectors  through a parallel wiring harness .