A Natural Gas Vehicle (NGV) is an alternative fuel vehicle that uses compressed natural gas (CNG) or liquefied natural gas (LNG). Natural gas vehicles should not be confused with autogas vehicles powered by liquefied petroleum gas (LPG), mainly propane, a fuel with a fundamentally different composition. In a natural-gas-powered vehicle, energy is released by the combustion of methane gas () fuel with oxygen () from the air to form carbon dioxide () and water vapor () in an internal combustion engine. Methane is the cleanest burning hydrocarbon, and many contaminants present in natural gas are removed at the source. Existing gasoline-powered vehicles may be converted to CNG or LNG and can be dedicated (running only on natural gas) or bi-fuel (running on gasoline or natural gas). Diesel engines for heavy trucks and buses can also be converted and can be dedicated with the addition of new heads containing spark ignition systems or can be run on a blend of diesel and natural gas, with the primary fuel being natural gas and a small amount of diesel fuel being used as an ignition source. It is also possible to generate energy in a small gas turbine and couple the gas engine or turbine with a small electric battery to create a hybrid electric motor-driven vehicle. Convenient fuel storage and refueling is a key challenge compared to petrol and diesel vehicles because natural gas is pressurized and liquefied (in the case of LNG). The lower energy density of gases compared to liquid fuels is mitigated greatly by high compression or gas liquefaction but requires a trade-off in terms of size, complexity, or weight of the storage container and range of the vehicle. Although similar storage technologies may be used and similar compromises would apply to a hydrogen vehicle as part of a proposed new hydrogen economy, methane as a gaseous fuel is safer than hydrogen due to its lower flammability, low corrosivity, and better leak tightness due to its larger molecular weight, resulting in lower price hardware solutions. Other factors holding back NGV popularization for individual mobility applications, i.e. private vehicles, include the additional weight, unfamiliarity with the technology and lack of refueling infrastructure in some countries due to relatively small demand compared with other petroleum-based fuels such as gasoline and diesel. Worldwide, there were almost 28 million NGVs by 2019. In 2016, the market was led by China (5.4 million), Iran