In the context of automobile engines, overflow bottles and expansion tanks are separate entities. Expansion tanks have been the norm on most vehicles since the 1970s, but overflow bottles were an important part of both the philosophy and engineering that led to their introduction.
The differences between overflow bottles and expansion tanks is muddied by a variety of imprecise colloquial ways of referring to both; they are often wrongly called radiator overflow tanks, coolant recovery tanks and coolant reservoirs. A working knowledge of coolant system function is vital to understanding the differences.
Traditional internal combustion engine cooling systems consisted of a sealed system. The radiator had a filler neck at the top, usually to one side or the other rather than in the center, fitted with a filler cap. The system was filled -- properly referred to as charging -- through the filler neck after the cap had been removed. Most manufacturers recommended coolant be topped off at approximately 1 inch below the bottom of the filler neck; this allowed for expansion when the coolant was heated by the running engine. The only way for the engine to vent excess pressure was through the filler cap. Different caps had different pounds-per-square-inch capacities that they could resist; when the limit was exceeded, steam was vented through an overflow vent beneath the cap to protect the engine.
In most traditionally cooled vehicles, when the cap’s maximum pressure capacity was exceeded, the coolant was ejected into the atmosphere or dumped onto the ground. When, in the 1970s, environmental awareness began to affect vehicle design and driver attitude, overflow bottles were added to the system to trap vented fluid as it was ejected. The system did nothing more than prevent excess coolant -- which typically has at least some measure of toxic antifreeze in its mixture -- from poisoning the environment. The trapped fluid was not returned to the engine when the coolant temperature, and thus the volume, decreased.
Expansion tanks are an improvement over overflow bottles in that they do allow the expelled coolant to return to the main cooling system as temperatures decrease. They are an integral part of a sealed radiator system, not an aftermarket add-on. A sealed radiator is just that; it does not have a filler neck and a cap designed to be removed for frequent fluid level checks and refilling. Cooling systems with a sealed radiator are filled at a remote location, often either through a port in the thermostat housing or into the expansion tank itself. Excess pressure is still vented from the engine at high working temperatures, but the expansion tank that it enters is more than simply a catchment reservoir: The coolant is drawn back into the engine by the vacuum created as temperatures decrease.
An Added Bonus
The use of an expansion tank with a sealed radiator system has a fortunate side effect: Siphoning expelled coolant into and back out of the tank removes bubbles created when filling or topping up; bubbles were a problem caused by pouring coolant into traditional systems directly through the filler neck. Air does not transfer heat as efficiently as coolant fluid, so bubbles reduced the overall efficiency of the system.
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