Thermal cracking was first developed by William Merriam Burton in 1913 which operated under the temperature 700 F -750 F and pressure at 90 psi. It is probably the first commercialized cracking process. By this time, it is also the history of cracking of heavy crude oil fraction to light fraction started. However, as we learned from lesson 5, thermal cracking produces short straight chain alkanes from longer straight chains found in gas oils and the reactions were governed by the free radicals. The process actually produces gasoline that contain lower octane number than that of gasoline which produced by catalytic cracking. It is due to isomerization of free radicals is not favored. During 1920, the petroleum refining industry was facing a challenge called engine knock. In order to solve the problem, it needed more powerful and stranger engines, but that means it also require gasoline which contain higher octane number. By 1930, gasoline had achieved the octane ring around 60 and 70. The newer and more powerful engine required higher octane gasoline which up to about 100, but thermal cracking could not raise its octane number any higher. It is also the reason that thermal cracking is not common process in U.S refineries. Thermal cracking once was the primary process for distillate fuel production. Even it is not anymore, it still remain as an important process in refineries to produce diesel fuel and ethylene.