Ricardo worked on spark-ignition engines throughout his career, including pioneering work on direct injection gasoline engines for aircraft engines in the 1930s, but his best-known work was in the development of high-speed diesel engines, in particular for cars. He was responsible for designing the combustion-chamber for the first diesel car engines produced in quantity for the Citroën Rosalie in the mid-thirties. Later, various versions of Ricardo's Comet pre-combustion chamber were deployed in almost all car diesel engines over six decades, up to the mid 1990s, and the Comet remains one of Ricardo's best-known personal achievements in automobile engineering.