The introduction of the PA5 series of high-speed engines in 1982, brought to the SEMT-Pielstick family of engines a machine able to operate on modern heavy fuel oils which producing up to 220 kw (300 bhp)/cylinder at speeds in the region of 1000 rev/min. In addition to these factors the PA5 engine is able to stop and start on heavy oil and maneuver for reasonable periods without distress. The PA5 engine and its components are described with special emphasis on the effect of heavy fuels on the engine components. Some of the tests are described.