Design Methodology of Centrifugal Automatic Decompression System in Small Gasoline Engine

This paper deals with designing methodology of centrifugal type automatic decompression system (CADS) for small gasoline engine. CADS reduce the operator’s fatigue to start the engine. When engine cranked, CADS releases combustion pressure of the engine via opening of exhaust valve momentarily during compression stroke, which drastically reduces the hand pulling force required to start the engine with recoil starter unit. A 172 cc gasoline engine, which has applications in agricultural purposes, has been used for designing and development of CADS, which has to be installed at camshaft cam gear assembly of engine. With the new developed concept operator’s hand pulling force for starting the engine has been reduced to 41 % and henceforth durability of engine starting system increased significantly. In this paper detailed design approach has been discussed of working model of CADS. Based on predicted failure modes and generated RPN values, design calculations were carried out for various geometrical parameters of sub parts of CADS like mass of centrifugal flyweight, number of coils, wire diameter, spring rate of torsion spring. PTC CREO 3.0 has been used to make rough 3 D model of assembly and further it is optimized based on calculated design values and ease of manufacturability. A testing approach also has been discussed for checking desired spring rate of torsion spring and on engine overall working mechanism of CADS.