1893

Two-Stroke Diesel Engine

Rudolf Diesel (1858–1913)

Four-stroke diesel engines work well and they are extremely common in road applications that require hundreds of horsepower. Most tractor-trailer rigs, school buses, and passenger buses use a four-stroke diesel engine. But in some applications, diesel engines need to scale up to thousands of horsepower, as in a diesel locomotive, or tens of thousands of horsepower, as in a container ship or supertanker. In these cases, engineers use two-stroke diesel engines, patented by German engineer Rudolf Diesel in 1893, instead.

Why? One big advantage of a two-stroke diesel is that it produces twice as much power from an engine of any given size. In an eight-cylinder four-stroke diesel engine running at 1,000 rpm, there are 4,000 power strokes per minute. If the same engine is two-stroke instead, there are 8,000 power strokes.

In a two-stroke diesel engine, as the piston moves downward in the power stroke, it bottoms out and uncovers intake ports cut into the cylinder wall. Right before that happens, exhaust valves open at the top of the cylinder to release exhaust gases. The air coming in through the intake ports is pressurized by a supercharger, so it forces its way into the cylinder at 2x atmospheric pressure or more. As the piston finishes bottoming out, it starts moving back upward and compresses the air in the cylinder. Right before top dead center the diesel fuel is injected. It ignites spontaneously in the hot, compressed air, providing power to push the cylinder back downward in the cylinder.

The two-stroke diesel engine in a typical diesel locomotive is already massive, weighing 30,000 pounds (13,600 kg) or more. If it were a four-stroke design instead, with the same power, it would need to be twice as big. This is why two-stroke diesel engines are so common in large applications like locomotives and cargo ships.

In smaller applications, the two-stroke’s requirement for a supercharger often offsets the power-to-weight advantage of the two-stroke approach. Engineers have to balance the parameters to achieve optimal results in any given application—the weight, power draw, cost, and complexity of a supercharger may outweigh the other advantages of the two-stroke approach when only 100 hp is needed.

SEE ALSO Supercharger and Turbocharger (1887), Diesel Locomotive (1897), Seawise Giant Supertanker (1979), Container Shipping (1984).

Pictured: The Wärtsilä X72 and the Wärtsilä X62 two-stroke engines in Doosan Engine Co., Ltd, in Chagwon, South Korea.