a deposit effect at high compression ratios
RUMBLE is a type of abnormal combustion which may impose a limit on usable compression ratios if proper attention is not paid to fuel and lubricant factors. It is characterized by a low-frequency noise.
This noise is much more likely to be present in engines which have been operated on light-duty schedules than in those which have been used in heavy-duty operation. Once deposits are present, rumble generally occurs at wide-open throttle and high engine speeds.
Rumble does not appear to be confined to any particular type of combustion chamber, and increasing engine rigidity does not show promise of reducing the problem. Fuels with low deposit-forming tendencies help alleviate the problem. While not a complete cure, phosphorus fuel additives do a good job in reducing the incidence of rumble.
This paper presents the results of work aimed at defining the problem, together with data illustrating the influence of various fuel factors.