Not all diesel fuels have the same destination. Home heating oil, for example, is designated to be pumped into above-ground storage tanks and used as a source of heat. The diesel fuel in a home heating unit is atomized and then ignited to provide warmth. Diesel fuel for automobiles contains more sulfur than home heating oil, and is designed strictly for use in diesel engines. Automotive diesel fuel is also subject to taxes not levied on home heating oil. In order to tell the two fuels apart, a special red dye is added to home heating oil to create red diesel.
Red diesel fuel is only slightly different chemically from regular automotive diesel fuel, but there can be a significant difference in cost. The cheaper red diesel fuel could conceivably work in place of the more expensive automotive diesel fuel, but that would defeat the purpose of a fuel tax. In order to ensure that home heating oil, which is minimally taxed, is not used as diesel fuel, which can be heavily taxed, revenue agents require home heating oil to receive a special red dye. This liquid red dye can be detected in even the smallest samples taken for examination.
Using red diesel for reasons other than home heating is generally considered a criminal act, since the buyer did not pay the proper tax for regular diesel fuel. If there is reasonable cause to inspect a vehicle's fuel tank or storage tank, inspectors can quickly and conclusively identify the presence of red diesel and take appropriate action against the offender. Without the dye, it would take a trained chemist to differentiate between home heating oil and automotive diesel fuel.
Different countries use different solvent dyes to create their red diesel, but the purpose is generally the same. The dye has no negative effects on the fuel itself, but it does allow revenue agents to determine at a glance if a violation has occurred.
