Hubble, bubble... surface carburetors

Elsewhere on the site, I have detailed the patented carburetor fitted to my 1903 California. Full of wicks, it is an unusual version of what is known as a "surface" carburetor; broadly defined as any carburetor that relies on simple evaporation from the surface of the liquid fuel to form a combustible mixture.

There were many variations on the theme, but arguably the two most successful designs were from De Dion (as fitted to their popular tricycle) and Minerva. The De Dion version was a simple affair, and unless you opted for a long-rage fuel tank as an optional extra, your tank and your carburetor were the same object. Air entering down a "chimney" was directed across the surface of the fuel which evaporated and filled the space at the above the liquid. Subtleties in design allowed the height of the chimney to be adjusted as the fuel level fell, and a gentle warming from the exhaust gases did wonders for the mixture.

The De Dion carburetor

A mixing valve at the top provided a means of diluting the (hopefully over-rich) mixture with air on its way to the motor. In the De Dion version (the Minerva is similar) there is also provision for a throttle valve to control the amount of mixture.

De Dion mixing valve

The Minerva carburetor was of similar design. Unlike the De Dion job however, the fuel tank and carburetor were in separate compartments, with fuel transferred as needed by raising the spring-loaded plunger labelled d on the drawing below. A float gave the rider some idea of the amount of fuel actually in the carburetor, and presumably the controls had to adjusted as the level changed.

Minerva tank and carburetor

The 0.68 specific gravity fuel needed for these carburetors to function was highly volatile stuff. Not only were fires a problem, but if left in the tank or carburetor for more than a day it would "go off" and render the machine unrideable. Much was made of the need to keep your storage cans tightly sealed - no roadside pumps then! - and to try to keep the contents of the carburetor "fresh" there was usually some means of sealing it off from the outside air when the machine was not running. On the Minerva, for example, there was a twist cap h on top of the air-inlet chimney.

In 1902 there was hardly a spray carburetor to be seen (arguably FN had one of the leading examples), but by 1904 the tables had turned entirely and the surface carburetor was essentially extinct.

Copyright Leon Mitchell 1999


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