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A fundamental vehicle study for a new engine cooling system, the so called NUCLEATE BOILING ENGINE COOLING SYSTEM, has been tested under real and critical conditions in a climate controlled wind-tunnel. Using this cooling system requires a profound knowledge of the cooling circuit and all cooling components such as: condenser, waterpump, liquid/vapor-separator, expansion tank, fan control unit. After preliminary experimental and theoretical studies, two types of cooling circuits were realized and tested in a car: Completely filled nucleate boiling cooling system Partially filled nucleate boiling cooling system Both types of circuits were sealed systems. This meant there was no evaporation or loss of liquid into the environment. By using the nucleate boiling cooling system, the heat of the cylinder head was dissipated while the boiling process change of liquid phase to vapor at constant boiling temperature.

This paper deals with engine evaporative cooling on the VW TDI diesel engine at high heat rejection running points. Engine tests and thermohydraulic 3D computations inside the engine head are used. First, the basic engine is studied. Then, the flow rate corresponding to evaporation is determined. Around this flow rate, the influence of inlet coolant temperature and circuit pressure are studied. The engine tests and 3D calculations give hydraulic ways of improvements consisting in balancing the flows around the different cylinders. Geometrical modifications of the inside engine circuit (gasket....) are then tested to achieve these improvements.