HISTORICALLY, FUELS HAVE BEEN MADE TO MEET THE REQUIREMENTS of specific engine types. This tailoring of the fuels has been accomplished in an iterative process, in which problems are uncovered in the field and efforts are made to resolve the problems through a process of specification and qualification. An example of this proces is the development of the currently used “cetane number” (CN). Cetane number is the measured parameter defined to provide a rating of the ignition quality of fuels for diesel engines. As refinery processes became more complex and focused on the production of gasoline in the 1920s, and as the diesel engine gained acceptance, the quantities of high quality, straight run diesel fuel became limited. The fuels that were available in that time period demonstrated problems with cold starting and low temperature white smoke.