Piston Ring Movement during Blow-by in High-Speed Petrol Engines

Abstract

This paper deals with electrical recording of the axial and radial movement of a piston ring, relative to its groove, in a petrol engine running under conditions of heavy blow-by. The engine was delivering power and records were made at speeds up to 5,500 r.p.m. The records show that the excessive blow-by often experienced at high engine speeds is not due, as generally supposed, to a vibratory piston ring radial “flutter” of frequency associated with the natural frequency of the piston ring. The records show instead that, at high engine speeds, the piston rings collapse radially inwards some 25 deg. before the end of the compression stroke and remain steadily collapsed until some 60 deg. after the start of the firing stroke. It is this steady collapse which causes the heavy blow-by. An explanation, supported by experimental results, is given of this radial collapse of piston rings. A description is given of two designs of piston ring and groove which were evolved as a result of this investigation. These two designs have not yet been thoroughly tested, but preliminary trials are very promising and suggest that with such designs blow-by should not become excessive at any practicable engine speed.