diesel fuel - that portion of crude oil that distills out within the temperature range of approximately 200°C (392°F) to 370°C (698°F), which is higher than the boiling range of gasoline. See distillation. Diesel fuel is ignited in an internal combustion engine cylinder by the heat of air under high compression - in contrast to motor gasoline, which is ignited by an electrical spark. Because of the mode of ignition, a high cetane number is required in a good diesel fuel. Diesel fuel is close in boiling range and composition to the lighter heating oils. There are two grades of diesel fuel, established by the American Society for Testing and Materials (ASTM): Diesel 1 and Diesel 2. Diesel 1 is a kerosene-type fuel, lighter, more volatile, and cleaner burning than Diesel 2, and is used in engine applications where there are frequent changes in speed and load. Diesel 2 is used in industrial and heavy mobile service. Diesel fuel is also widely used in drilling mud formulations. See drilling oil.