Engine speeds

Within the diesel engine industry, engines are often categorized by their rotational speeds into three unofficial groups:

High-speed engines (> 1,000 rpm),
Medium-speed engines (300–1,000 rpm), and
Slow-speed engines (< 300 rpm).
High- and medium-speed engines are predominantly four-stroke engines; except for the Detroit Diesel two-stroke range. Medium-speed engines are physically larger than high-speed engines and can burn lower-grade (slower-burning) fuel than high-speed engines. Slow-speed engines are predominantly large two-stroke crosshead engines, hence very different from high- and medium-speed engines. Due to the lower rotational speed of slow- and medium-speed engines, there is more time for combustion during the power stroke of the cycle, allowing the use of slower-burning fuels than high-speed engines.