Thermo syphon was the term applied to the cooling system of the low powered vehicle engines preceding WW2, and immediately afterwards. Being of low specific power output, (Think Austin 7, Austin 10, Ford 8, Ford 10 etc).there was little need for a high volume (mass flow) of coolant to be forced around the engine, so there was no coolant pump. The coolant flow was derived from convection. The coolant, heated within the cylinder block and cylinder head, rose to the top of a tall radiator, where it was cooled by the air flowing around the tubes and fins. As it cooled, it increased in density, and fell to the bottom of the radiator. Convection then drew it back into the bottom of the cylinder block, ready to abstract heat and so cool the engine. And so the coolant circulated, slowly and steadily.
As engines became more sophisticated and and increased in power for their size, the greater heat rejection required higher mass flows, so a coolant pump became necessary, to prevent local hot spots, and to carry away the greater quantity of heat from the engine.
In VERY round figures, whatever power is available at the flywheel, a similar quantity goes into the cooling system, and has to be dissipated.
Howard
typo corrected
Edited By Howard Lewis on 19/08/2018 17:56:13