Of the three types of carburetors used on large, high-performance aircraft engines manufactured in the United States during World War II, the Bendix-Stromberg pressure carburetor was the one most commonly found. The other two carburetor types were manufactured by Chandler Groves (later Holley Carburetor Company) and Chandler Evans Control Systems (CECO). Both of these types of carburetors had a relatively large number of internal parts, and in the case of the Holley Carburetor, there were complications in its "variable venturi" design.
A floatless pressure carburetor is a type of aircraft fuel control that provides very accurate fuel delivery, prevents ice from forming in the carburetor and prevents fuel starvation during negative "G" and inverted flight by eliminating the customary float-controlled fuel inlet valve. Unlike the float-type carburetor fuel system that relies on venturi suction to draw fuel into the engine, a pressure carburetor only uses the venturi to measure the mass airflow into the engine and manages the flow of fuel that is continuously under pressure from the fuel pump to the spray nozzle. In 1936, the first Bendix-Stromberg pressure carburetor (a model PD12-B) was installed and flown on an Allison V-1710-7.
Background
The Bendix Corporation marketed three types of aircraft fuel systems under the Bendix-Stromberg name:
The first type was manufactured for low performance aircraft engines and virtually all aircraft engines produced before 1938. These were typically conventional float-type carburetor that were not much different than those found on automobiles or farm tractors of that time, except for size.Stromberg Aircraft carburetors p 16
After 1938, high performance aircraft engines were equipped with floatless pressure carburetors, especially those used in combat aircraft. The floatless pressure carburetor was the progenitor of today's single-port fuel injection, and was a big step forward in fuel delivery technology. It could be looked upon as the mechanical counterpart of today's electronic fuel control system. These floatless pressure carburetors are the topic of this article.Schlaifer, Chapter XVIII, pp 509-546
In the last years of World War II, aircraft engines that exceeded a specific horsepower of greater than 1.0, were equipped first with distributed fuel injection and later with direct injection, which became the fuel system of choice. Using the same principles as the pressure carburetor to measure air flow into the engine, the distributed fuel injection system used individual fuel lines to each cylinder, injecting the fuel at the intake port. The direct-injection systems differed from a pressure carburetor in that the fuel is introduced just up stream from the intake valve in the inlet port in each individual cylinder head in the direct fuel injection system, as opposed to the pressure carburetor where the fuel is introduced at the carburetor. These fuel control devices were individually sized and calibrated to fit almost all piston aircraft engines used by both civil and allied military aircraft made in the post war era. These fuel injection systems are found on high performance general aviation piston engines that continue flying into the 21st century.Stromberg carburetor application spreadsheet, author's collection
Design and development
Starting with the basics of fuel combustion, no matter what type of fuel system is used on a given engine, the carburetor's sole job is to provide exactly the correct amount of fuel into a given amount of air that is entering the engine.Schlaifer, p 509 To be burnable, the air to fuel ratio must be within the flammability limits of between of air to of fuel (for gasoline engines). Above or below this ratio, the fuel will not burn.
Next, it is also a given that within that range of acceptable mixtures, there is only one ratio that is the ideal air-fuel ratio at that time, given the throttle position set by the pilot. In summary, it can be said that the ideal carburetor provides the correct air-fuel mixture ratio, as required by the engine, under all of its operating conditions.Thorner pp 46-47
Lastly, the exact amount of fuel needed changes between the overly-lean lower limit of 16:1 and the overly-rich upper limit of 9:1 as the engine operating condition changes.Thorner p 47
To summarize, for a carburetor to deliver the exact amount of fuel required, it is necessary to provide the carburetor with three things:
First, the exact weight of the air flowing through it, Second, what air-fuel ratio is needed for the engine's operating condition, Third, what engine operation is sought by the aircraft's pilot.
Once these three things are delivered to the carburetor, a well designed carburetor will provide the engine with the exact, correct, fuel flow at all times. Any well-designed carburetor does this routinely, no matter what type or size engine is used. Aircraft carburetors on the other hand, operate under extraordinary conditions, including violent maneuvers in three dimensions, sometimes all at the same time.
The problems: ice, gravity and inertia
When fuel is vaporized, it cools the surrounding air due to the refrigeration effect as the fuel absorbs heat when it changes state from a liquid to a gas.Schlaifer, p 515 This may result in the air dropping below freezing, causing water vapor contained in the air to first change state from a gas to a liquid, which then becomes ice. This ice forms on the throttle plate, which is located "down stream" of the fuel nozzle. The ice also forms on the inner walls of the carburetor, sometimes to such a degree that it blocks airflow to the engine.
Float type carburetors work best when in a stable operating condition. General aviation aircraft operate in a range of conditions not much different from that of an automobile, so a float type carburetor may be all that is needed. Large or fast aircraft are a different matter, especially when considering that fighter aircraft may fly inverted, or through a series of high g turns, climbs and dives, all at a wide range of speeds and altitudes, and in a very short time.Thorner pp 129-130
Once the carburetor leaves a stable condition, the float is influenced by both gravity and inertia, resulting in inaccurate fuel metering and a reduction in engine performance as the air-fuel ratio changes, becoming either too lean or too rich for maximum engine performance, and in some cases, stopping the engine.Stromberg Aircraft carburetors pp 16-17
Float type carburetors are able to compensate for these unstable conditions through various design features, but only within reason. For example, once the float type carburetor is under negative g conditions, such as a rapid nose down attitude, the float lifts toward the top of the fuel bowl as the float becomes weightless when the aircraft descends faster than the float and the fuel. The float is lifted upward by inertia, closing the fuel inlet valve as if the fuel bowl was full of fuel. Cutting off the fuel supply causes the fuel-air ratio to become greater than sixteen to one, which is then too lean for combustion to take place, stopping the engine.Stromberg Aircraft carburetors p 18Schlaifer, p 514
The inverse is also true when the aircraft is in inverted flight. The float becomes submerged as the fuel is pulled downward by gravity to the top of the fuel bowl. The float lifts upward toward the bottom of the inverted fuel bowl. With the float at the bottom of the fuel bowl, the fuel inlet valve opens, as it would when there is not enough fuel in the fuel bowl. With the fuel inlet valve open, the fuel pump continues pumping fuel into the fuel bowl, where the resulting excess fuel causes the fuel-air ratio to become lower than nine to one, which is then too rich for combustion to take place, stopping the engine.
The solution: move the fuel nozzle and remove the float Bendix-Stromberg engineers overcame the problems found with float-type carburetors by moving the fuel discharge nozzle to the carburetor adapter or in some cases at the "eye" of the supercharger, both below the throttle plates and by eliminating the float from the fuel metering system. The new "pressure carburetor" design replaced the float-operated fuel inlet valve with a servo-operated poppet-style fuel metering valve.Schlaifer p. 522
There are however, either one or two small floats in the fuel regulator air bleed system. These floats have nothing to do with the air-fuel ratio, as their only purpose is to allow any entrained air that may have become trapped in the fuel regulator to return to the fuel tank where it will be vented to the atmosphere.
Carburetor components
The pressure carburetor consists of three major components.
The throttle body is the main component of the carburetor. It contains one or more bores through which all of the air flows into the engine. Each bore contains a number of throttle plates which are used by the pilot to control the air flow into the engine. A venturi is also installed in each bore. The impact tubes are mounted in each venturi, placing them directly in the path of the incoming air. All of the remaining main components are attached to the body, and are interconnected with internal passages or external tubes or hoses.
The fuel control component is used by the pilot to adjust fuel flow into the engine. It contains a number of jets that control fuel pressures within the fuel control. It has a rotating plate-type valve with either three or four positions: idle-cutoff, which stops all fuel flow, auto lean which is used for normal flight or cruise conditions, auto rich which is used for takeoff, climb and landing operations, and on some carburetors, military which is used for maximum, albeit life shortening, engine performance.Thorner pp 70-71
The fuel regulator component takes input signals from various sources to automatically control fuel flow into the engine. It consists of a number of diaphragms sandwiched between metal plates, with the center of the roughly circular diaphragms connected to a common rod, forming four pressure chambers when assembled. The outer end of the rod connects to the fuel metering servo valve that moves away from the throttle body to open, allowing more fuel flow or toward the throttle body to close, reducing the amount of fuel to flow. The rod is moved by the forces measured within the four pressure chambers.
The smaller components of the carburetor are either attached to, are a part of the major portions, or are remotely mounted, depending on the engine application.
The boost component is mounted on the inlet side of the throttle body. It measures air density, barometric pressure, and air flow into the carburetor. It is mounted directly in the air flow at the inlet to the throat. The automatic mixture control, if equipped, is mounted either on the boost portion for throttle bodies with two or more throats, or on the throttle body itself for the single throat models.
The fuel delivery component is either remotely mounted at the "eye" of the engine's supercharger or at the base of the carburetor body. The fuel is sprayed into the air stream as it enters the engine through one or more spring-controlled spray valves. The spray valves open or close as the fuel flow changes, holding a constant fuel delivery pressure.
An accelerator pump is either remotely mounted or mounted on the carburetor body. The accelerator pump is either mechanically connected to the throttle, or it is operated by sensing the manifold pressure change when the throttle is opened. Either way, it injects a measured amount of extra fuel into the air stream to allow smooth engine acceleration.