Bendix-Stromberg pressure carburetor: Difference between revisions
Buster40004 (talk | contribs) →Design and Development: added details and re-worded section |
Buster40004 (talk | contribs) →Design and Development: added details and re-worded section |
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:;Third, a way to detect what engine operation is requested by the aircraft's pilot. |
:;Third, a way to detect what engine operation is requested by the aircraft's pilot. |
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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. |
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. |
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===The problem: gravity and inertia=== |
===The problem: gravity and inertia=== |
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| ⚫ | |||
Float type carburetors work best when the engine they are mounted on is in a stable operating condition. Small aircraft operate in a range of conditions not much different than an automobile. Large, fast aircraft are a whole different matter, especially when considering [[fighter aircraft]] that may fly inverted, through a series of high g turns, climbs and dives, all at a wide range of speeds and altitudes, and in a very short period of time. |
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| ⚫ | 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 [[Air-fuel ratio]] becomes either too lean or too rich.<ref name="Stromberg3">Stromberg Aircraft carburetors pp 16-17</ref> |
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Float type carburetors are able to compensate for these unstable conditions through various design features, within reason. Once the float type carburetor is under [[g-force|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.<ref name="Stromberg4">Stromberg Aircraft carburetors p 18</ref> |
Float type carburetors are able to compensate for these unstable conditions through various design features, within reason. Once the float type carburetor is under [[g-force|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.<ref name="Stromberg4">Stromberg Aircraft carburetors p 18</ref> |
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Revision as of 00:08, 20 August 2011
| 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.[1]
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, first with distributed injection and later direct injection, 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 inject the fuel directly into the cylinder head much like a diesel 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 engines that continue flying into the 21st century.[2]
Design and development
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 entering the engine. To be burnable, the air to fuel mixture must be within the range of nine to sixteen pounds of air to one pound of fuel. It is also a given that within that range of possible mixtures, there is but one that is ideal, given the throttle position set by the pilot. in summary, it can be said that the carburetor must provide the engine with the correct mixture ratio required by the engine under all of its operating conditions.[3]
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 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.[4]
For a carburetor to deliver the exact amount of fuel, there must be a way to provide the carburetor with three things:
- First, the exact weight of air flowing through it,
- Second, it must have a way to determine what air-fuel ratio is needed by the engine operating condition,
- Third, a way to detect what engine operation is requested 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 the engine they are mounted on is in a stable operating condition. Small aircraft operate in a range of conditions not much different than an automobile. Large, fast aircraft are a whole different matter, especially when considering fighter aircraft that may fly inverted, through a series of high g turns, climbs and dives, all at a wide range of speeds and altitudes, and in a very short period of time.
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 Air-fuel ratio becomes either too lean or too rich.[5]
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.[6]
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 throttle 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 regulator portion takes input signals from various sources to automatically control fuel flow to 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. 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 derichment 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 entering the supercharger.
Theory of operation
The fuel metering servo valve responds to pressure differentials across two diaphragms that separate the four pressure chambers of the fuel regulator, controlling 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.[7]
- 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.[7]
- 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.[7]
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.[7]
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.[7]
- 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.[7]
- 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.[7]
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.[7]
Operation
To start the engine, the mixture lever is placed in idle-cutoff and the fuel pump, ignition and ignition boost are 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.[7]
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 into the fuel regulator.[7]
The 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 come into a balanced state.
Chamber C and chamber D are connected by a fuel passage which contains 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 as metered fuel.[7]
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.[7]
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.[7]
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.[7]
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, reducing fuel flow to the auto lean flow setting.[7]
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 equipped. When in 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 that closes off the derichment jet, reducing the fuel flow to compensate for the ADI fluid. This causes the cylinder head temperature to climb from normal to very high levels, indicating the engine is suffering damage as a result of producing extra 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.[8]
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.[9]
- 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.[9]
PD style carburetors are for inline and radial engines from 900 to 1900 cubic inches.[9]
PT style carburetors are usually found on 1700 to 2600 cubic inch engines[9]
PR style carburetors are used on 2600 to 4360 cubic inch engines[9]
References
Notes
- ^ Stromberg Aircraft carburetors p 16
- ^ Stromberg carburetor application spreadsheet, author's collection
- ^ Thorner pp 46-47
- ^ Thorner p 47
- ^ Stromberg Aircraft carburetors pp 16-17
- ^ Stromberg Aircraft carburetors p 18
- ^ a b c d e f g h i j k l m n o Pressure Injection, by Charles A. Fisher, AMIMech.E, MIAE in Flight, September 11, 1941 pp 149-152
- ^ ADI, Pete Law presentation
- ^ a b c d e CarbApps05.xls spreadsheet, author's collection
Bibliography
- Stromberg carburetor application list, Bendix-Stromberg, undated.
- Pressure Injection, Flight, September 11, 1941
- ADI presentation to AEHS, Pete Law, 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