Detecting the Onset of Nucleate Boiling in Internal Combustion Engines

The use of an electric pump instead of the standard crankshaft-driven one in Internal CombustionEngines, gives the possibility of controlling the coolant flow rate independently of engine speed, allowingtherefore, the use of much lower coolant flow rates than usually adopted and the development of nucleateboiling flow regimes within the engine cooling system. In order to take advantage of nucleate boilingand of the associated high heat transfer coefficients, the onset of this heat transfer regime must be correctlyidentified. This work presents the results of an experimental campaign, which was carried out on asmall displacement spark ignition engine (1.2 dm3, 60 kW) with the aim of detecting the occurrence ofnucleate boiling within the engine cooling system. The test rig was properly instrumented in order tomeasure coolant temperatures at engine inlet and outlet, coolant pressure at several locations in the circuit,coolant flow rate and engine metal temperatures. Operating conditions involving different coolantflow rates were selected in order to enforce both the usual single-phase heat transfer regime and nucleateboiling conditions. Several experimental quantities were analyzed with the aim of establishing the coolantflow rates ranges where the nucleate boiling occurs. The agreement in the trend of coolant temperatureand pressure and engine wall temperature provides hints to identify experimentally the onset ofnucleate boiling.