Of the three types of

carburetor A carburetor (also spelled carburettor) is a device used by an internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the venturi tube in the main meteri ...

s used on large, high-performance

aircraft engines An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years man ...

manufactured in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

, the Bendix-Stromberg pressure carburetor was the one most commonly found. The other two carburetor types were manufactured by Chandler Groves (later

Holley Carburetor Holley Performance Products is an automotive performance company based in Bowling Green, Kentucky. It was founded in 1896 in Bradford, Pennsylvania by George Holley and Earl Holley. When the company was based in Michigan it was a major producer o ...

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

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 The Allison V-1710 aircraft engine designed and produced by the Allison Engine Company was the only US-developed V-12 liquid-cooled engine to see service during World War II. Versions with a turbocharger gave excellent performance at high ...

-7.

Background

The

Bendix Corporation Bendix Corporation is an American manufacturing and engineering company which, during various times in its existence, made automotive brake shoes and systems, vacuum tubes, aircraft brakes, aeronautical hydraulics and electric power systems, ...

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 Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines. All com ...

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

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 General aviation (GA) is defined by the International Civil Aviation Organization (ICAO) as all civil aviation aircraft operations with the exception of commercial air transport or aerial work, which is defined as specialized aviation services ...

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 Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...

, 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. To be burnable, the

air to fuel ratio The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...

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 A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term ''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 General aviation (GA) is defined by the International Civil Aviation Organization (ICAO) as all civil aviation aircraft operations with the exception of commercial air transport or aerial work, which is defined as specialized aviation services ...

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 Fighter aircraft are fixed-wing military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air superiority of the battlespace. Domination of the airspace above a battlefield ...

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 In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

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 18 The

inverse Inverse or invert may refer to: Science and mathematics * Inverse (logic), a type of conditional sentence which is an immediate inference made from another conditional sentence * Additive inverse (negation), the inverse of a number that, when a ...

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. 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. Boost01

: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 {{Short pages monitor {{Aircraft piston engine components Engine fuel system technology Aircraft fuel system components Carburetor manufacturers Bendix Corporation