A diesel engine is an internal combustion engine that operates using diesel fuel and relies on compression ignition, where air is compressed to high pressure and temperature, causing injected fuel to ignite spontaneously. Known for their superior fuel efficiency and high torque at low RPMs, diesel engines are commonly used in heavy-duty applications like trucks, buses, and industrial machinery. They are built for durability, able to withstand high pressures and temperatures, but they also require advanced emissions control systems to manage nitrogen oxides and particulate emissions. Overall, diesel engines are favored for their reliability and power, making them essential in various sectors.