SEMT PIELSTICK has developed since 1971 a range of medium-speed dual-fuel engines with relatively high air-fuel ratios, which enable ratings similar to diesel engines. The PC 2.3 DF.C of 1971 was developed up to 535 hp/cyl. and was followed by the PC 2.5 DF.C of 600 hp/cyl. This later engine was applied in the West German cogeneration plan of a textile factory, giving more than 82 percent use of primary energy. This engine may also be used as a pollution-abating machine, because it traps toxic solvent vapors, which are burned in the engine, reducing furthermore the apparent (paid) energy consumption. Thanks to the lean air-gas mixture, the very severe West German limits on pollution could be fulfilled without any extra depolluting device. The newest development is the PA 5 DF engine of the same philosophy, which will cover the 1000 to 3600 kW range.