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Previous work on the cooling jackets of the Cummins L10 engine revealed flow separation, and low coolant velocities in several critical regions of the cylinder head. The current study involved the use of detailed cooling jacket temperature measurements, and finite element heat transfer analysis to attempt the identification of regions of pure convection, nucleate boiling, and film boiling. Although difficult to detect with certainty, both the measurements and analysis pointed strongly to the presence of nucleate boiling in several regions. Little or no evidence of film boiling was seen, even under very high operating loads. It was thus concluded that the regions of seemingly inadequate coolant flow remained quite effective in controlling cylinder head temperatures. The Cummins L10 upon which this study has focused is an in-line six cylinder, four-stroke direct injection diesel engine, with a displacement of 10 liters.

An experiment which simulates operating conditions in a diesel engine cylinder head which are favorable for initiation of nucleate boiling is described and the relationship between heat flux and coolant passage wall temperature is presented. Measurements of temperatures in the head between the flame face and the coolant passages indicated nucleate boiling may initiate within the valve bridge at a wall temperature of approximately 393 K; however, forced convection was the dominant mode of heat transfer. The results compare favorably with J. C. Chen's heat transfer correlation which accounts for forced convection and nucleate boiling.