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. It utilizes turbocharging and charge air cooling, at all ratings, and is currently marketed at ratings from 180 to 260 kW. The engine was initially released only at the lower output levels, and during the development of higher power ratings a study was undertaken to ensure that the cylinder head cooling jackets would provide adequate temperature control for the increased power output.The initial phases of the study involved coolant flow visualization in a full-scale transparent model of the jackets ,* and temperature measurements conducted at the Georgia Institute of Technology on a rig utilizing a single cylinder portion of the head . The next section of this paper will summarize the cooling jacket design used in the L10 cylinder head, and the results of the earlier flow study. A review of the heat transfer modes of pure convection, nucleate boiling, and film boiling will then be presented, explaining the rationale for the studies reported here. The final sections will summarize the findings of temperature measurements obtained in the cooling jackets of a test engine, and of finite element analysis techniques used to estimate heat transfer coefficients in various regions of the cylinder head cooling jackets.