Fuel oil refers to the heavier liquid fractions derived from petroleum distillation, encompassing products like diesel fuel, heating oil, and heavy bunker fuel, which are burned to generate heat or power engines. While diesel dominates road transport in Europe, home heating oil has declined in favor of natural gas and heat pumps, though it remains common in regions like the Northeastern United States. The cheapest form, residual heavy fuel oil, is highly viscous and sulfur-heavy, restricting its use to large ships and power plants that can handle special heating equipment, having largely replaced steam locomotives and steamships. Environmental concerns drove notable shifts, such as New York City's PlaNYC plan, which phased out No. 4 and No. 6 fuel oils by 2015 after they contributed to 86% of the city's building soot pollution. As a result, residual fuel oil's share of U.S. electricity generation plummeted from 16.8% in 1973 to just 3% by 2005, as cleaner natural gas took precedence.