An expansion tank for an engine cooling system comprises a housing forming a main chamber and a swirl chamber, the swirl chamber being defined by a cylindrical wall. An inlet connection discharges through an inlet orifice towards a collector duct having an entrance. From the inlet orifice the coolant is directed as a stream or jet along the adjacent surface of the cylindrical wall towards the entrance of the collector duct, guided by circumferential ribs. The kinetic energy of the stream is converted into pressure energy so that the pressure delivered from the outlet connection is slightly above the pressure at the top of the swirl chamber, above the level of liquid. This swirl chamber pressure is set by the relief pressure allowed by a filler cap, the gain in pressure helping to avoid cavitation in a coolant circulation pump.
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