propane from on-site storage tanks, these generators can be arranged for automatic starting upon power failure. Transport Liquefied natural gas (LNG) engines are expanding into the marine market, as the lean-burn gas engine can meet the new emission requirements without any extra fuel treatment or exhaust cleaning systems. Use of engines running on compressed natural gas (CNG) is also growing in the bus sector. Users in the United Kingdom include Reading Buses. Use of gas buses is supported by the Gas Bus Alliance and manufacturers include Scania AB. Use of gaseous methane or propane Since natural gas, chiefly methane, has long been an economical and readily available fuel, many industrial engines are either designed or modified to use gas, as distinguished from gasoline. Their operation produces less complex-hydrocarbon pollution, and the engines have fewer internal problems. One example is the liquefied petroleum gas, chiefly propane. engine used in vast numbers of forklift trucks. Common United States usage of "gas" to mean "gasoline" requires the explicit identification of a natural gas engine. There is also such a thing as "natural gasoline", but this term, which refers to a subset of natural gas liquids, is very rarely observed outside the refining industry. Technical details Fuel-air mixing A gas engine differs from a petrol engine in the way the fuel and air are mixed. A petrol engine uses a carburetor or fuel injection. but a gas engine often uses a simple venturi system to introduce gas into the air flow. Early gas engines used a three-valve system, with separate inlet valves for air and gas. Exhaust valves The weak point of a gas engine compared to a Diesel engine is the exhaust valves, since the gas engine exhaust gases are much hotter for a given output, and this limits the power output. Thus, a Diesel engine from a given manufacturer will usually have a higher maximum output than the same engine block size in the gas engine version. The Diesel engine will generally have three different ratings — standby, prime, and continuous, a.k.a. 1-hour rating, 12-hour rating and continuous rating in the United Kingdom, whereas the gas engine will generally only have a continuous rating, which will be less than the Diesel continuous rating. Ignition Various ignition systems have been used, including hot-tube ignitors and spark ignition. Most modern gas engines are essentially dual-fuel engines. The main source of energy is the gas-air