Dearman's liquid air vehicle. Some common inert fuels include: Liquid nitrogen Liquid air Liquid helium Liquid neon Combustible These fuels utilize the beneficial liquid cryogenic properties along with the flammable nature of the substance as a source of power. These types of fuel are well known primarily for their use in rockets. Some common combustible fuels include: Liquid hydrogen Liquid natural gas (LNG) Liquid methane Engine combustion Combustible cryogenic fuels offer much more utility than most inert fuels can. Liquefied natural gas, as with any fuel, will only combust when properly mixed with right amounts of air. As for LNG, the bulk majority of efficiency depends on the methane number, which is the gas equivalent of the octane number. This is determined based on the methane content of the liquefied fuel and any other dissolved gas, and varies as a result of experimental efficiencies. Maximizing efficiency in combustion engines will be a result of determining the proper fuel to air ratio and utilizing the addition other hydrocarbons for added optimal combustion. Production efficiency Gas liquefying processes have been improving over the past decades with the advent of better machinery and control of system heat losses. Typical techniques take advantage of the temperature of the gas dramatically cooling as the controlled pressure of a gas is released. Enough pressurization and then subsequent depressurization can liquefy most gases, as exemplified by the Joule-Thomson effect. Liquefied natural gas While it is cost effective to liquefy natural gas for storage, transport, and use, roughly 10 to 15 percent of the gas gets consumed during the process. The optimal process contains four stages of propane refrigeration and two stages of ethylene refrigeration. There can be the addition of an additional refrigerant stage, but the additional costs of equipment are not economically justifiable. Efficiency can be tied to the pure component cascade processes which minimize the overall source to sink temperature difference associated with refrigerant condensing. The optimized process incorporates optimized heat recovery along with the use of pure refrigerants. All process designers of liquefaction plants using proven technologies face the same challenge: to efficiently cool and condense a mixture with a pure refrigerant. In the optimized Cascade process, the mixture to be cooled and condensed is the feed gas. In the propane mixed refrigerant processes, the two mixtures requiring cooling and condensing are the feed gas and the mixed refrigerant. The chief source of inefficiency