Although Ricardo did not invent the sleeve valve, in 1927, he produced a seminal research paper that outlined the advantages of the sleeve valve, and suggested that poppet valve engines would not be able to offer power outputs much beyond 1500 hp (1,100 kW). A number of sleeve valve aircraft engines were developed following this paper, notably by Napier, Bristol and Rolls-Royce. Bristol produced the Perseus, Hercules, Taurus and the Centaurus, Napier produced the Napier Sabre, and Rolls-Royce produced the Eagle and Crecy, all using sleeve valves.[14]