Concept
Most aircraft of the 1920s and 1930s had a float-type carburetor. The float operates a valve which keeps the fuel level in the carburetor consistent despite varying demands. However, since the float is dependent on gravity to function, a float carburetor will fail to flow any fuel if the aircraft is flying under zero or negative-G conditions. This is not a problem for civil aircraft which normally fly upright, but it presents a problem for aerobatic aircraft which fly upside-down or otherwise are subject to negative G, especially military fighters. If an airplane equipped with a float-type carburetor is flown under zero-G or negative-G conditions for more than a few seconds, the engine runs out of fuel, and it stops running. The problem was keenly felt by the RAF during the first years of the war, as the Rolls-Royce Merlin equipped Hurricanes and the Spitfires suffered this effect, unlike the direct fuel injection engines of their German counterparts. The problem was solved by installing a flow-restricted orifice that opened only when flying inverted or under negative-G conditions (the R.A.E. restrictor was known as "Miss Shilling's orifice"), but this was only a stopgap solution.
The pressure carburetor solves the problem by taking gravity out of the system as it operates on pressure alone. For this reason, the pressure carburetor will operate reliably in any flight attitude. The fact that a pressure carburetor operates on the principle of fuel under positive pressure makes it a form of fuel injection.
Read more about this topic: Pressure Carburetor
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