A diesel engine is a type of engine that burns fuel differently than the gasoline engines in most cars. Invented by German engineer Rudolf Diesel in the 1890s, it works by squeezing air so tightly inside a cylinder that the air becomes extremely hot, hot enough to ignite fuel without needing a spark plug. When diesel fuel sprays into this superheated air, it ignites and pushes a piston, creating power.

Diesel engines are prized for their efficiency and strength. They burn less fuel than gasoline engines while producing more pulling power, which is why you'll find them in semi trucks hauling heavy loads across the country, tractors plowing fields, locomotives pulling freight trains, and large ships crossing oceans. The characteristic rumbling sound of a diesel engine comes from the powerful bursts inside the cylinders.

While gasoline engines are lighter and accelerate faster (better for passenger cars), diesel engines excel at sustained work and long distances. A diesel locomotive can pull a hundred loaded freight cars. A diesel semi truck can haul 40,000 pounds of cargo up steep mountain passes. That combination of power and fuel economy made diesel engines essential to modern transportation and industry. When you see a big rig rolling down the highway or a combine harvester working through a wheat field, you're watching a diesel engine at work.