A diesel engine is a type of internal combustion engine that uses diesel fuel to operate. It works by compressing air in the cylinder, which raises the temperature, and then injecting diesel fuel into the hot air. This causes the fuel to ignite, producing power to move the engine's pistons. Diesel engines are commonly found in vehicles like trucks, buses, and ships due to their fuel efficiency and durability. They are known for providing more torque at lower speeds compared to gasoline engines, making them suitable for heavy-duty applications.