Compressed Natural Gas
Compressed Natural Gas (CNG) is a substitute for gasoline or diesel fuel. It is considered to be an environmentally "clean" alternative to those fuels. It is made by compressing methane (CH4) extracted from natural gas. It is stored and distributed in hard containers, usually cylinders. Many bus makers use this fuel, including Orion, and New Flyer.
Buses using CNG usually have large tanks on the roof of the bus in an enclosure that takes up the majority of the roof area.