Natural Gas

Natural gas has been used now for more than 50 years as fuel for stationary engines for power generation, and agricultural machinery. Several million  natural gas vehicles are in operation in Argentina, Brazil, Australia, Italy, India and Pakistan,   besides other countries. In the USA, stringent particulate emission standards for the urban buses implemented from the year 1994 was the motivation for development of natural gas operated urban buses.
The principal constituent of natural gas is methane (80 to 95% by volume). The balance is composed of small amounts of other hydrocarbons  such as ethane, propane and other gases like nitrogen, carbon dioxide, water, hydrogen sulphide and some trace gases. Composition of natural gas varies from source to source.  Composition of natural gas from two different sources is given in Table 8.15.
Large variations in gas composition can have significant effects on engine performance and emissions, especially if the engine performance and emissions are optimized on a fixed gas composition. The natural gas before transportation or use is upgraded by removing water, hydrogen sulphide and condensable higher hydrocarbons.
To minimize variations in engine emissions and performance, and to ensure a minimum heating value to customers,  specifications for natural gas sold commercially as fuel have been established.

Natural gas liquefies at -161º C at atmospheric pressure. To use liquefied natural gas (LNG) as automotive fuel, cryogenic systems are required. Therefore, natural gas is stored on board in high-pressure cylinders at pressure of 200 to 300 bars as compressed natural gas (CNG).  Storage of natural gas at high pressure on board provides an acceptable range of vehicle operation.
High antiknock quality of natural gas makes it a fuel that is better suited for spark- ignition engines. The following types of natural gas engines have been developed: