Diesel fuels are injected into the engine through precision pumps and fine injector nozzles. Dirt and water contamination must be avoided to protect these critical components. Specifications include tight limits on water and sediment, but some fuel marketers also install final filters at the nozzles of service station pumps to protect against dirt picked up in the distribution system. Fuels that leave a heavy carbon residue and contain excessive amounts of high-boiling point materials are prone to cause engine deposits. Sulfur content is the first diesel fuel property to be widely controlled by legislation aimed at limiting exhaust emissions.
Sulfur is present in all crude oils and refined products.
During combustion, sulfur compounds burn to form acidic byproducts, S02 and S03, which form sulfates in the exhaust gas stream. Depending on the crude source, diesel fuels contain various amounts of sulfur compounds, which yield corrosive sulfur oxides on combustion. These can cause high rates of engine wear and rapid depletion of engine oil additives. Engine manufacturers often relate oil change intervals to the fuel sulfur content. All diesel fuels contain small amounts of water.