Our example, Cummins  is a major supplier of diesel engines to the modern truck manufacturer. The diesel engine is a type of  internal-combustion engine invented by the German engineer Rudolf Diesel and patented by him in 1892.  Although his engine was designed to use coal dust as fuel, the diesel engine now burns low-cost fuel oil.

The diesel engine differs from the gasoline engine in that the ignition of fuel is caused by compression of air in its cylinders instead of by a spark: the high compression ratio allows the air in the cylinder to become hot enough to ignite the fuel. Because of the high temperatures of operation, a diesel engine must be water-cooled. The construction of the diesel engine is heavier than that of the gasoline engine; there are usually three or more cylinders (supported on a framework and bedplate) and a heavy flywheel. The cylinders are set to work alternately to give a smooth-turning effect, and the flywheel contributes further to smooth action.

There are two classes of diesel engines. In the two-stroke, or two-cycle, type there is a complete cycle of operation in every two strokes of a piston. This type of engine requires a supply of compressed air for operating and for starting. In the four-stroke, or four-cycle, type the first downstroke of the piston draws in air, which is compressed on the upstroke to about 500lb per sq. inch. At the top of the stroke a jet of oil is sprayed in through an injector. The oil is ignited and the rapid expansion of the gas created by the explosion forces the piston down in the working, or firing, stroke. The next upstroke drives the waste gases out through the exhaust valve, and the cycle is complete.

The speed and power of the diesel are controlled by varying the amount of fuel injected into the cylinder, not the amount of air admitted as in the gasoline engine. Small and medium sized ships may have several diesels producing as much as 50,000 hp. Heavy-duty land transports such as trains, trucks, buses,and tractors are often diesel powered. Even planes have been known to have diesel engines.