easily combusts, making any leakage potentially extremely dangerous. Gasoline sold in most countries carries a published octane rating. The octane number is an empirical measure of the resistance of gasoline to combusting prematurely, known as knocking. The higher the octane rating, the more resistant the fuel is to autoignition under high pressures, which allows for a higher compression ratio. Engines with a higher compression ratio, commonly used in race cars and high-performance regular-production automobiles, can produce more power; however, such engines require a higher octane fuel. Increasing the octane rating has, in the past, been achieved by adding 'anti-knock' additives such as lead-tetra-ethyl. Because of the environmental impact of lead additives, the octane rating is increased today by refining out the impurities that cause knocking. Diesel Conventional diesel is similar to gasoline in that it is a mixture of aliphatic hydrocarbons extracted from petroleum. Diesel may cost more or less than gasoline, but generally costs less to produce because the extraction processes used are simpler. Some countries (particularly Canada, India and Italy) also have lower tax rates on diesel fuels. After distillation, the diesel fraction is normally processed to reduce the amount of sulfur in the fuel. Sulfur causes corrosion in vehicles, acid rain and higher emissions of soot from the tail pipe (exhaust pipe). Historically, in Europe lower sulfur levels than in the United States were legally required. However, recent US legislation reduced the maximum sulfur content of diesel from 3,000 ppm to 500 ppm in 2007, and 15 ppm by 2010. Similar changes are also underway in Canada, Australia, New Zealand and several Asian countries. See also Ultra-low-sulfur diesel. A diesel engine is a type of internal combustion engine which ignites fuel by injecting it into a combustion chamber previously compressed with air (which in turn raises the temperature) as opposed to using an outside ignition source, such as a spark plug. Kerosene Kerosene is used in kerosene lamps and as a fuel for cooking, heating, and small engines. It displaced whale oil for lighting use. Jet fuel for jet engines is made in several grades (Avtur, Jet A, Jet A-1, Jet B, JP-4, JP-5, JP-7 or JP-8) that are kerosene-type mixtures. One form of the fuel known as RP-1 is burned with liquid oxygen as rocket fuel. These fuel grade kerosenes meet specifications for smoke points and freeze points. In the mid-20th century, kerosene or "TVO" (Tractor