What does fuel cleanliness level mean?

ISO 14/13/11 is a target rating specified for diesel fuel. Fuels typically leave refineries clean, but contamination levels increase and occur throughout the shipment and distribution process. Fuel-hauling tanker trucks are not used exclusively for one particular fuel. Today, they could be hauling ethanol-blended gasoline and tomorrow they could be hauling diesel fuel in the same tanker. This meaning that ethanol-blended gasoline remaining in the bottom of the tanker would end up mixing with the diesel on a later delivery. Common contaminates include rust, scale, airborne dust, various types of oxidation, sludge, bacteria, and water. Gulf Coast Fuel Polishing meets the ISO standard through various types of cleaning procedures. Fuels that meet the ISO standards are at the optimal levels designed for diesel engine performance.