A two-stroke diesel engine is a diesel engine that uses compression ignition in a two-stroke combustion cycle. It was invented by Hugo Güldner in 1899.
In compression ignition, air is first compressed and heated; fuel is then injected into the cylinder, causing it to self-ignite. This delivers a power stroke each time the piston rises and falls, without any need for the additional exhaust and induction strokes of the four-stroke cycle.
According to the engineer who drew up Rudolf Diesel‘s design for of the first operational diesel engine, Motor 250/400, Imanuel Lauster, Diesel did not originally intend using the two-stroke principle for the diesel engine. Hugo Güldner designed what is believed to be the first operational two-stroke diesel engine in 1899, and he convinced MAN, Krupp and Diesel to fund building this engine with M 10,000 each. Güldner's engine had a 175 mm work cylinder, and a 185 mm scavenging cylinder; both had a stroke of 210 mm. The indicated power output was . In February 1900, this engine ran under its own power for the first time. However, with its actual power output of only and high fuel consumption of 380 g·PS−1·h−1 (517 g·kW−1·h−1), it did not prove to be successful; Güldner's two-stroke diesel engine project was abandoned in 1901.
In 1908, MAN Nürnberg offered single-acting piston two-stroke diesel engines for marine use, the first engine from MAN Nürnberg was made in 1912 for an electric power plant. In collaboration with Blohm + Voss in Hamburg, MAN Nürnberg built the first double-acting piston two-stroke engine for marine use in 1913/1914. Paul Henry Schweitzer argues that the opposed piston two-stroke diesel engine was originally invented by Hugo Junkers. During World War I, MAN Nürnberg built a six-cylinder, double-acting piston, two-stroke diesel engine with a rated power of . MAN moved their two-stroke diesel engine department from Nürnberg to Augsburg in 1919.
By 1939, several two-stroke diesel types were in widespread use, and others were being developed for high-power applications,.