A Corliss steam engine (or Corliss engine) is a steam engine, fitte with rotary valves, invented by and named after the American engineer George Henry Corliss. There are four valves for each cylinder, with steam and exhaust valves located at each end. With variable valve timing, engines fitted with Corliss valve gear offered the best thermal efficiency of any type of stationary steam engine until the refinement of the uniflow steam engine and steam turbine in the 20th century.