The main theme of this paper concerns loop scavenge two-cycle engines as a type, but most of the examples refer to trunk-piston engines of powers up to 2,000 b.h.p. The characteristics of such engines are enumerated and a comparison is made with other established types of engines—both two-cycle and four-cycle—in marine, stationary and locomotive applications. Details of performance are given. Exhausting, scavenging and charging processes of loop scavenge two-cycle engines are dealt with and in particular a new method of filling the engine cylinder (“exhaust pulse pressure charging”) is described. There then follow data regarding scavenge efficiency and design of ports.