What Engines Are Heavy Oils Used In

Heavy oil (fuel oil / heavy fuel oil) is generally not used as fuel for small automotive engines such as passenger cars, but is mainly used for large‑scale heavy machinery and equipment. 

Machines that commonly use heavy oil

Industrial boilers (steam boilers), for example in factories, palm‑oil mills, and cement plants, to produce steam. 

Power‑generation engines (coal‑fired or oil‑fired power plants) as fuel for electricity‑generation units. 

Large marine engines (ships) that use heavy fuel oil as the main propulsion fuel.

Industrial furnaces / industrial kilns in chemical plants, metallurgy plants, and similar facilities. 

Important note

Heavy oil has high viscosity and more impurities, so it usually must be preheated and is used only in engine systems specifically designed for heavy loads and high temperatures.