The first diesel engine was successfully operated in 1897. Rudolf Diesel’s early prototypes in Augsburg proved the compression-ignition concept, but the machine still required substantial redesign and testing before it was accepted as a successful working engine.
The decisive test took place on 17 February 1897, when engineer Moritz Schröter tested Diesel’s third prototype. It produced about 13.1 kilowatts and achieved an effective efficiency of 26.2 percent—an impressive result for the period. The engine’s success helped establish Diesel’s design as a practical alternative to steam engines and spark-ignition engines.
A common mix-up is 1893: that was when Diesel’s first prototype was built and first ignition occurred. Another milestone came in 1898, when the first commercial diesel engine was installed. Thus, 1897 is the standard answer for the successful operation and validation of the first practical diesel engine.