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The authors have reported on a study on extended expansion linkage engine to enhance thermal efficiency since 2006. This report discusses the use of a test engine applied to a Micro Combined Heat and Power Generation Unit for household use, in order to reduce engine noise at a rated operation. Test engine noise is mainly caused by gear meshing for the multiple linkage system, so helical gear with higher contact ratio than that of spur gear was used. Measurement of engine noise revealed that test engine noise increased by 3.2 dB(A) over compared conventional engine. From results of behavior analysis by mechanical simulation, when transmission direction of the relative torque between the crankshaft and the eccentric shaft is reversed, the direction of the thrust force acting on the gear is reversed. For this reason, the test engine noise increases because each shaft vibrates, and rattle noise occurs.