Engineering Question:

CNG (Compressed Natural Gas) is stored on the vehicle in high-pressure tanks - 20 to 25 MPa (200 to 250 bar, or 3,000 to 3,600 psi). Natural gas consists mostly of methane and is drawn from gas wells or in conjunction with crude oil production. As delivered through the pipeline system, it also contains hydrocarbons such as ethane and propane as well as other gases such as nitrogen, helium, carbon dioxide, sulphur compounds, and water vapour. A sulphur-based odourant is normally added to CNG to facilitate leak detection. Natural gas is lighter than air and thus will normally dissipate in the case of a leak, giving it a significant safety advantage over gasoline or LPG.

LPG (Liquefied Petroleum Gas) consists mainly of propane, propylene, butane, and butylene in various mixtures. It is produced as a by-product of natural gas processing and petroleum refining. The components of LPG are gases at normal temperatures and pressures. One challenge with LPG is that it can vary widely in composition, leading to variable engine performance and cold starting performance. At normal temperatures and pressures, LPG will evaporate. Because of this, LPG is stored in pressurised steel bottles. Unlike natural gas, LPG is heavier than air, and thus will flow along floors and tend to settle in low spots, such as basements. Such accumulations can cause explosion hazards, and are the reason that LPG fuelled vehicles are prohibited from indoor parkades in many jurisdictions.

LNG (Liquefied Natural Gas) is natural gas stored as a super-cooled (cryogenic) liquid. The temperature required to condense natural gas depends on its precise composition, but it is typically between -120 and -170°C (-184 and -274°F). The advantage of LNG is that it offers an energy density comparable to petrol and diesel fuels, extending range and reducing refuelling frequency.