Diesel fuel oil ranks next to gasoline in quantity used. It can be produced as cheaply as gasoline, but its use is limited to Diesel type engines. The use of kerosene as a fuel is usually limited to farm tractors, marine and stationary engines, all which operate at a fairly constant speed. Its traits are such that it cannot be properly mixed with air and controlled in variable speed engines. Gasoline from asphaltic base crude oil produces less knock than one from paraffinic base crude.
Cracked gas has less tendency to knock than straight run gas.
All marketed gasolines are a blend of straight run and cracked gasolines, so unless their blending is controlled, the anti-knock qualities will vary. These ratings range from 50 in third grade gasolines to 110 in aviational fuels. If the octane rating of a gasoline is naturally low, the fuel will detonate as it burns and power will be applied to the pistons in hammer-like blows. The ideal power is that which pushes steadily on the pistons, rather than hammer against them. The octane rating of a gasoline can be raised by treating it with a chemical which is not a fuel.