Liquefied Natural Gas or LNG 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.
The disadvantage, however, is the high cost of cryogenic storage on vehicles and the major infrastructure requirement of LNG dispensing stations, production plants and transportation facilities. LNG has begun to find its place in heavy-duty applications in places like the US, Japan, the UK and some countries in Europe. For many developing nations, this is currently not a practical option.
Liquefied Petroleum Gas or LPG (also called Autogas) 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.