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Hopper cooling is a simple form of water cooling used for small stationary engines. The defining feature of hopper cooling, amongst other water-cooled engines, is that there is no radiator. Cooling water is heated by the engine and evaporation from the surface of the hopper as steam.
In most familiar engines today, this water is circulated from the hot parts of the engine to a radiator, where it gives up its heat to the air.
In early and low powered engines with hopper cooling there is little circulation. Water is instead slowly boiled off, with the heat of vaporisation needed to boil the water coming from the engine heat. The loss of heat with this departing water vapour is enough to cool the engine.
As the heat of vaporisation (energy needed to vaporise water) is much larger than the specific heat capacity (energy to raise the temperature of water by one degree), relatively little water is required to replace that lost by evaporation. The heat needed to boil water is equivalent to a 540 °C rise in temperature, or about 7 times that needed to raise the temperature of the water from ambient to boiling.
| 4.1813 J·g−1·K−1 |
| 2260 J·g−1 |
A typical small engine would consume a few bucketfuls in a working day.
Although hopper cooling is inefficient, in terms of the amount of heat removed for the size of the water jacket, it does maintain the cylinder temperature at a low temperature. Provided that the hopper does not boil dry, the temperature cannot exceed the 100°C atmospheric boiling point of water. This is both an advantage and a disadvantage: it maintains a low operating temperature, helping to preserve the fragile and of these early engines. However it also limits efficiency of operation, as the engine cannot run at a higher and more efficient operating temperature.
Large engines would use thermosyphon cooling. The hopper on the engine would be supplemented by a large drum of water above the engine. This would give some circulation between the two, but as there was no large surface area for heat transfer to the air, as with a radiator, the eventual cooling would still largely be by evaporation.
To avoid frost damage, the hopper could be drained easily through a large brass tap. This would be done regularly, sometimes after every use, to avoid rust inside the hopper. As the cooling water evaporated continuously when running, no anti-freeze or anti-corrosion additives were used.
A handful of engines, now largely diesels, retained hopper cooling into the 21st century. These were simply constructed engines, still using cast-iron blocks to provide more strength from a crude foundry, as a diesel engine required more strength than unsophisticated aluminium castings could provide. Single cylinder horizontal diesel engines were built in India and China and were briefly imported to the West around 2000, but were outlawed by emission control regulations.
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