Several factors affect the use and selection of any fuel.
Some important properties are engine design, net energy per pound, net energy per gallon and the sulphur content of alternative fuel. Measuring a fuel's relative potential energy can easily be done by defining that fuel's Btu content. For a gasoline engine designed to run on CNG, significant modifications are needed. See Figure 1. These dual fueled vehicles are known as hybrid-demonstration vehicles to let the public know that natural gas powered vehicles perform well.