Bendix-Stromberg pressure carburetor: Difference between revisions

Bendix-Stromberg Pressure Carburetor
Pressure carburetor for a Pratt & Whitney R-2000 radial engine
Type	Bendix-Stromberg model PD12-F13
National origin	United States
Manufacturer	Bendix

Of the three types of carburetors used on large, high-performance aircraft engines built by 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 Evans Control Systems (CECO) and Holley Carburetor Company. Both of these types of carburetors had far too many internal parts, and in the case of the Holey Carburetor, there were problems with internal air leakage due to difficulty sealing its "moving venturi" design.

A pressure carburetor is a type of aircraft fuel control that provides very accurate fuel delivery 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 fuel system is under pressure from the fuel pump to the spray nozzle. In 1936, the first Bendix-Stromberg pressure carburetor (a model PD12-B) was installed on an Allison V-1710-7.

Background

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, not much different except for size, than those found on automobiles of that time.

After 1938 high performance aircraft engines were equipped with pressure carburetors, especially those used in combat aircraft. These carburetors were a big step forward in technology, and could be looked upon as mechanical counterparts of today's electronic fuel control computers. These pressure carburetors are the topic of this article.

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 only in that it injected the fuel directly into the cylinder head, much like a diesel engine fuel system. 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 post war era. These fuel injection systems are found on high performance general aviation piston engines that continue flying into the 21st century.

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 finely atomized fuel into a given amount of air that is entering the engine. To be burnable, the air to fuel ratio must be within the stoichiometric range of between nine and sixteen pounds of air to one pound of fuel. 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 but 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.

Last, it is also a given that it takes exactly seven pounds of air passing through an engine to create one horsepower. It therefore takes 7,000 pounds of air to create 1,000 horsepower in a given engine. That 7,000 pounds of air requires at a minimum of 437.5 pounds of fuel to a maximum of 777.8 pounds of fuel to be within the burnable range. 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.

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 problem: gravity and inertia

Float type carburetors work best when in a stable operating condition. General aviation aircraft operate in a range of conditions not much different than 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.

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.

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.

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: remove the float

Bendix-Stromberg engineers 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.

There is 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 portions. Military carburetors may have a fourth portion, depending on engine and application.

The throttle body is the main portion of the carburetor. This portion 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 portions are attached to the body, and are interconnected with internal or external passages and tubes or hoses.

Boost bar with AMC showing impact tubes and boost venturi

The fuel control portion 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.

The fuel regulator portion 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 portions 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 portion 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 portion 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 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 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.

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

Theory of operation

Fuel regulator A and B chambers and diaphragm

There are four chambers in the fuel regulator portion of the carburetor. They are referred to by letters A, B, C, and D, with the A chamber closest to the throttle body. The fuel metering servo valve responds to pressure differentials across the diaphragms. The resulting diaphragm movement controls fuel flow into the engine under all flight conditions.

Chamber A

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 impact tubes directly in the airflow, generating a pressure higher than atmospheric pressure that represents the real-time air density. 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 when the air flow is absent.

Chamber B

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.

Fuel regulator C and D chambers and diaphragm, and servo valve to the right

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

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

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 close the servo valve.

The air metering force from chambers A and B apply a force to open the servo valve, and is opposed by the fuel metering force from chambers C and D which apply a force to close the servo valve. These two forces combine into movement of the servo valve to adjust the fuel flow to the precise amount required for the needs of the engine, and the needs of the pilot.

Operation

To start the engine, the mixture lever is placed in idle-cutoff and the fuel pump, ignition and ignition boost are all turned on, once fuel pressure reaches the normal operating pressure, the starter is engaged, rotating the engine. The prime pump is operated until the engine starts. When the engine starts on the prime fuel, the mixture lever is placed in the auto rich position and the prime pump is stopped.

When the engine started, air began flowing through the boost venturi, causing the pressure in the venturi yo drop according to Bernoulli's principle. This causes the air pressure in chamber B to drop.

Fuel regulator diaphragms and servo valve connected by operating rod

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 entering the engine. The difference in pressure between chamber A and chamber B creates the air metering force which opens the servo valve allowing the fuel into the fuel regulator.

The fuel pressure from the fuel pump pushes the diaphragm in chamber D toward the carburetor body, closing the servo valve until the pressures in the four chambers reaches a balanced state.

Chamber C and chamber D are connected by a fuel passages which contain the fuel metering jets. When the mixture control lever is moved from the idle-cutoff position, fuel starts to flow through the metering jets and into chamber C where it becomes metered fuel.

As fuel begins to flow, the pressure increases in chamber C, applying a force that moves 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.

Cross section of a Bendix-Stromberg pressure carburetor

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 the automatic mixture control. It operates by bleeding higher pressure air from chamber A into 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.

Once airborne and having reached the cruising altitude, the pilot moves the mixture control from auto rich to auto lean. This reduces fuel flow by closing the passageway through the rich jet. The resulting reduction of flow unbalances the fuel metering diaphragm, causing the fuel metering valve to change position, thereby reducing fuel flow to the auto lean flow setting.

In the event of a combat or emergency situation, the mixture control may be moved to the auto rich position, providing extra fuel to the engine, or to military position, if so equipped. When in the military position, the ADI system is activated, injecting the fluid into the engine intake system. The pressure in the ADI system moves the derichment diaphragm in the fuel control to close off the derichment jet, reducing the fuel flow to compensate for the ADI fluid. This causes the cylinder head temperature to increase to very high levels, which increases the engine horsepower. The engine suffers damage as a result of the heat caused by producing higher power. Once the ADI fluid is exhausted or if the mixture control valve is moved out of the military position, the fuel control derichment diaphragm pressure is lost, and the derichment jet is opened once again for normal fuel flow.

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 double barrel carburetor. Third is the PT triple 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 the actual square inches of the throat area for the rectangular style.

PS style

Single round throat, can be mounted updraft, downdraft and horizontal with slight changes

PS-5, PS-7, PS-9

PD style

Double round throat, can be mounted updraft and downdraft with slight changes

PD-7, PD-9, PD-12, PD-14, PD-16, PD-17, PD-18

PT style

Triple round throat, can be mounted updraft and downdraft with slight changes

PT-13

PR style

Two or four rectangular throats, can be mounted updraft and downdraft with slight changes

PR-38, PR-48, PR-52, PR-53, PR-58, PR-62, PR-64, PR-74, PR-78, PR-88, PR-100

Bendix used a method to identify round carburetor bores. The first inch of bore diameter is given the base number one, then each quarter of an inch increase in diameter adds one to the base number.

Examples:

• a 1-1/4 inch bore would be coded as a size number 2 (Base number 1 + 1 for the 1/4 inch over 1 inch)
• a 1-1/2 inch bore would be coded as a size number 3 (Base number 1 + 2 for the two 1/4 inches over 1 inch),

and so on up to a size 18 (Base number 1 + 17 for the seventeen 1/4 inch increments over the 1 inch base).

• Lastly, 3/16 inch is added to the coded size for the actual finished bore diameter.

Using the size number 18 bore, we can calculate the actual bore size as follows:

• The first inch is represented by the number 1, and we subtract that 1 from 18, leaving 17 one-quarter inch units, or 17/4, which reduces to 4-1/4 inches.
• Adding the 1 inch base number, we now have a 5-1/4 inch bore.
• Last, we add the 3/16 for a grand total of 5-7/16 inch diameter for each of the two bores in the PD-18 carburetor body.

Each carburetor model number includes the style, size 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.

Applications

Generally, the PS style carburetors are used on opposed piston engines found on light aircraft and helicopters. The engine can be mounted in the nose, tail, wing or mounted internally on the airframe. The engine can be mounted vertically as well as horizontally.

PD style carburetors are for inline and radial engines from 900 to 1900 cubic inches.

PT style carburetors are usually found on 1700 to 2600 cubic inch engines

PR style carburetors are used on 2600 to 4360 cubic inch engines

References

Bibliography

• Stromberg carburetor application list, Bendix-Stromberg, undated.
• Thorner, Robert H., Aircraft Carburetion, John Wiley & Sons, New York & London, 1946
• Pressure Injection, Flight, September 11, 1941
• Law, Peter, ADI presentation to AEHS, from AEHS web site
• Stromberg Aircraft Carburation, Bendix Corp undated, but pre 1940
• Bendix Carburetors, Flight,
• Training manual, RSA Fuel Injection System, Precision Airmotive Corp. January, 1990
• Bendix PS Series Carburetor Manual, April 1, 1976