|Marine Fuel||Max. Sulfur Content|
|High sulfur fuel oil (HSFO)||3.5%|
|Low sulfur fuel oil (LSFO)||1.0%|
|Ultra low sulfur fuel oil (ULSFO)||0.1%|
Heavy fuel oils are mainly used as marine fuel, and HFO is the most widely used marine fuel at this time; virtually all medium and low-speed marine diesel engines are designed for heavy fuel oil. But older steam locomotives and oil-fired power plants also generate energy from heavy fuel oils.