Bendix-Stromberg pressure carburetor

Bendix-Stromberg Pressure Carburetor
Type
National origin	United States
Manufacturer	Bendix Corporation

A pressure carburetor is a type of aircraft fuel control that provides very accurate fuel delivery based on real-time pilot commands and environmental factors, and prevents fuel starvation during negative "G" and inverted flight, by eliminating the customary float-controlled fuel inlet valve. Pressure carburetors were found on almost all allied and axis high-performance aircraft engines during World War II.

Design and development

The Bendix Corporation marketed three types of aircraft fuel systems under the Bendix-Stromberg name. Low performance aircraft engines, and almost all aircraft engines produced before 1940 were typically equipped with conventional float-type carburetors. After 1940 high performance aircraft engines were equipped with pressure carburetors, especially those used in combat aircraft. In the last years of World War II, aircraft engines that exceeded a specific horsepower of greater than 1.0, direct injection became the fuel system of choice. These fuel control devices were individually sized and calibrated for almost all piston aircraft engines used by both civil and allied military aircraft made in the 1940s through the 1980s.

The problem: gravity and inertia

Float type carburetors work best when the engine on which they are mounted is in a stable condition. Once the engine is subjected to a change away from that stable condition, the float is influenced by both gravity and inertia, resulting in inaccurate fuel metering and a reduction in engine performance as the fuel to air ratio becomes either too lean or too rich.

Float type carburetors are able to compensate for these unstable conditions through various design features, within reason. Once the float type carburetor is under negative G conditions, such as a rapid nose down flight path, the float lifts toward the top of the fuel chamber, closing the fuel inlet valve and starving the engine of fuel to the point that the engine will not produce power, or when in inverted flight, the float lifts toward the bottom of the fuel chamber, forcing the fuel inlet valve fully open, flooding the engine with fuel to the point that the engine will not produce power.

The solution: remove the float

Bendix-Stromberg overcame the problems found with float-type carburetors 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.

Carburetor components

The pressure carburetor consists of four major portions. Military carburetors may have a fifth portion, depending on engine and application.

The largest portion is the carburetor body which contains the throttle plates used by the pilot to control air flow into the engine and the throat through which all of the air flows on its way to the engine. All of the remaining portions are remotely mounted or are attached to the body, and are interconnected with internal or external passages.

The boost portion measures air density, barometric pressure, and air flow into the carburetor. It is mounted directly in the airflow at the inlet to the throat.

The fuel control portion is used by the pilot to either manually or automatically adjust fuel flow to the engine. It has either three or four positions: idle-cutoff, which stops fuel flow, auto lean that is used for normal flight or cruise conditions, auto rich that is used for takeoff, climb and landing operations, and on some carburetors, military which is used for maximum engine, albeit life shortening, performance.

The fuel metering portion takes input signals from various sources to automatically control fuel flow to the engine. It is comprised 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. The outer end of the rod connects to the fuel metering valve that moves open or closed as the rod is moved by the forces measured within the four pressure chambers.

The fuel delivery portion is either remotely mounted at the eye of the engine supercharger or at the base of the carburetor body. The fuel is sprayed into the air stream entering the engine through one or more spring controlled spray valves that open or close as the fuel flow changes, thereby holding fuel delivery pressure constant.

An accelerator pump portion 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 pressure change when the throttle is opened. Either way, it ejects a measured amount of extra fuel into the air stream to allow smooth engine acceleration.

Some pressure carburetors use an anti-detonation injection (ADI) system. This consists of a control valve in the fuel control portion, a storage tank for the ADI fluid, a pump, a regulator valve that injects a specific amount of ADI fluid based on the fuel flow present, and a spray nozzle that is mounted in the air stream.

Theory of operation

The fuel metering valve responds to pressure differentials across two diaphragms that separate the four pressure chambers of the fuel regulator to control fuel flow into the engine, under all flight conditions. The four chambers are contained in the fuel regulator portion of the carburetor and are referred to by letters A, B, C, and D.

The diaphragm located closest the the carburetor body is the air metering diaphragm. It measures the difference in air pressure taken from two locations within the carburetor. Chambers A and B are on opposite sides of the air metering diaphragm. The mass of the air entering the carburetor was measured by placing a number of pickup tubes directly in the airflow, generating a pressure higher than atmospheric pressure, that changed with the density of the air. The impact tube pressure is connected to "Chamber A" on the side of the air metering diaphragm closest to the carburetor body. As the air pressure in chamber A is increased, the diaphragm is moved away from the carburetor body toward the fuel metering valve. Chamber A also contains a spring that creates a force toward the fuel metering valve.

The velocity of the air flow entering the carburetor is measured by placing one or more venturi directly in the airflow. The venturi creates a lower than atmospheric pressure that changes with the velocity of the air. The negative air pressure from the venturi is connected to "Chamber B" on the side of the air metering diaphragm farthest from the carburetor body. As the air pressure in chamber B is decreased, the diaphragm is pulled away from the carburetor body toward the fuel metering valve.

The difference in pressure between the two air chambers creates what is known as the air metering force, which moves the fuel metering valve open when it is greater than the opposing force or closed when it is less than the opposing force.

The second diaphragm is the fuel metering portion of the regulator, and is located farthest from the carburetor body. It measures the difference in fuel pressure taken from two locations within the regulator itself. Chambers C and D are on opposite sides of the fuel metering diaphragm.

Chamber C contains metered fuel, that is fuel that has already passed through the metering valve, but not yet injected into the air stream. The pressure in this chamber moves the metering valve outward when the fuel pressure is higher than the pressure in chamber D, on the opposite side of the diaphragm.

Chamber D contains unmetered fuel, that is the pressure of the fuel as it enters the carburetor. The pressure in this chamber moves the metering valve inward when the fuel pressure is higher than the pressure in chamber C, on the opposite side of the diaphragm.

The difference in pressure between the two fuel chambers creates the fuel metering force, which acts to open or close the servo valve.

Operation

To start the engine, the mixture lever is placed in idle-cutoff and the ignition and ignition boost is turned on, then the starter is engaged, rotating the engine. The prime pump is operated until the engine starts. The mixture lever is then placed in the auto rich position.

When the engine starts, air begins to flow through the venturi, and the pressure in the venturi drops according to Bernoulli's principle. This causes the pressure in chamber B to drop.

At the same time, air entering the carburetor compresses the air in the impact tubes, generating a positive pressure in chamber A based on the density and speed of the air as it enters. The difference in pressure between chamber A and chamber B creates the air metering force which opens the servo valve and allows fuel in.

Chamber C and chamber D are connected by a fuel passage which contains the fuel metering jets.

The pressure from the fuel pump is measured by the force pushing the diaphragm in chamber D toward the closed position. When the mixture control lever is moved from the idle-cutoff position, fuel starts to flow through the metering jets and into chamber C as metered fuel. As fuel begins to flow, the pressure increases in chamber C, moving the fuel metering valve open. The pressure drop across the metering jets create the fuel metering force which acts to close the servo valve until a balance is reached with the pressure from the air metering diaphragm..

From chamber C the fuel flows to the discharge valve. The discharge valve acts as a variable restriction which holds the pressure in chamber C constant despite varying fuel flow rates.

The fuel mixture is automatically altitude-controlled by bleeding higher pressure air from chamber A to the chamber B as it flows though a tapered needle valve. The needle valve is controlled by an aneroid bellows that senses barometric pressure, causing a richening of the mixture as altitude increases.

Variants

Bendix-Stromberg produced a number of pressure carburetor styles and sizes, each of which could be calibrated to a specific engine and airframe.

There are four styles, starting with the PS single barrel carburetor. Next is the PD two barrel carburetor. Third is the PT three barrel carburetor and last, the PR rectangular bore carburetor. Each of these styles is available in a number of sizes, which is a measurement of the area of the bore, with a special system for circular bores, and actual square inches of area for the rectangular style.

PS style

PS5

PS7

PS9

PD style

PD9

PD12

PD14

PD18

PT style

PT13

PR style

PR48

PR58

PR64

PR100

Each model number includes the style and a specific model letter, which may be followed by a revision number.

Each application (the specific engine and airframe combination) then receives a "list number" that contains a list of the specific parts and flow sheet for that application.

Needless to say, there are hundreds of parts list and flow sheets in the master catalog.

Specifications (variant)

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Applications

Pratt & Whitney R-4360 Wasp Major. The pressure carburetor is the black box on top of the crankcase at the rear of the engine.

Pressure carburetors were used on many piston engines of 1940s vintage used in World War II aircraft. They went from being a new design early in the war to being standard equipment on nearly every aircraft engine by the war's end. The largest pressure carburetors were the Bendix PR-100 series which were used on the Pratt & Whitney R-4360, the largest piston aircraft engine to see production.

After the war, Bendix made the smaller PS series which was found on Lycoming and Continental engines on general aviation aircraft. These small pressure carburetors eventually evolved in to the Bendix RSA series multi-point continuous-flow fuel injection system which is still sold on new aircraft. The RSA injection system sprays fuel into the ports just outside the intake valves in each cylinder, thus eliminating the chilling effect of evaporating fuel as a source of carburetor ice -- since the temperature in the intake ports is too high for ice to form.