Hydrocarbons

Oil is largely composed of hydrocarbons which can burn if ignited. Still another important property of motor oil is its flash point, the lowest temperature at which the oil gives off vapors which can ignite. It is dangerous for the oil in a motor to ignite and burn, so a high flash point is desirable. At a petroleum refinery, fractional distillation separates a motor oil fraction from other crude oil fractions, removing the volatile components which ignite more easily, and therefore increasing the oil's flash point.Another test done on oil is to determine the Total Base Number TBN, which is a measurement of the reserve alkalinity of an oil to neutralize acids. The resulting quantity is determined as mg KOH/gram of lubricant. Analogously, Total Acid Number TAN is the measure of a lubricant's acidity. Other tests include zinc, phosphorus, or sulfur content, and testing for excessive foaming.Different motor oils are sold for Diesel fuel engines, with many claimed to contain a higher level of detergents to keep fine combustion soot in suspension. However, for some brands only the packaging varies the oil is the same, and in general a diesel engine can use any good quality oil of the correct grade.