Bendix-Stromberg pressure carburetor
Encyclopedia
Of the three types of carburetor
s 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 "variable 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.
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.
, 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:
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.
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.
-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.
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.
Military carburetors may have a 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.
Chamber A:
Chamber B:
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:
Chamber D:
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.
When the engine started, air began flowing through the boost venturi, causing the pressure (referred to as a partial vacuum as it is lower than atmospheric pressure, but not a full vacuum) in the venturi to drop according to Bernoulli's principle
. This causes the air pressure in chamber A to drop in proportion with the partial vacuum in the boost venturi.
At the same time, air entering the carburetor compresses the air in the impact tubes, generating a positive pressure in chamber B that is proportional to 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 pressure of fuel from the fuel pump pushes against the diaphragm in chamber C, closing the servo valve. The fuel also flows to the mixture control valve, which is closed when in the idle-cutoff position and open in all other positions.
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 D where it becomes metered fuel.
As fuel begins to flow, the pressure increases in chamber D, applying a force to the fuel diaphragm that moves the fuel metering valve open. The fuel in chamber D flows on to the fuel discharge valve. The pressure drop across the metering jets creates the fuel metering force on the fuel diaphragm. This force acts to open the servo valve until a balance is reached with the combined force from the un-metered fuel pressure and the air metering force.
The discharge valve is spring-loaded to a preset pressure discharge pressure, acting as a variable size restriction to hold a constant pressure in chamber D, despite varying fuel flow rates. The valve is opened as the discharge fuel pressure increases above the force from the spring, thereby lowering the fuel pressure to maintain a balanced position with the spring force.
The fuel mixture is automatically altitude-controlled by the automatic mixture control. It operates by bleeding higher pressure air from chamber B into chamber A 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 in military aircraft, into military position, if the aircraft is so equipped. When in the military position, the Anti-Detonation Injection (ADI) system is activated, injecting the ADI 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 a very lean mixture which raises the mean effective pressure
, producing higher engine horsepower. This causes the cylinder head temperature to increase to a very high level, which results in increases engine horsepower. Adding the ADI fluid controls the temperature to an acceptable level, but The engine still 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.
There are four styles:
Each of these styles is available in a number of sizes, using measurements of the area of the bore on a rectangular bore, or a special system for circular bores, and the actual square inches of the throat area for the rectangular style.
PS style
PD style
PT style
PR style
Bendix used a special method to identify round carburetor bores. The first inch of bore diameter is used as the base number one, then each quarter of an inch increase in diameter adds one to the base number.
Examples:
and so on up to a size 18 (Base number 1 + 17 for the seventeen 1/4 inch increments over the 1 inch base).
Using the size number 18 bore as an example, we can calculate the actual bore size as follows:
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.
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
Carburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
s used on large, high-performance aircraft engines built by the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, 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 "variable venturi" design.
A pressure carburetor
Carburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
is a type of aircraft fuel control that provides very accurate fuel delivery and prevents fuel starvation
Fuel Starvation
Fuel starvation and fuel exhaustion are problems that can affect internal combustion engines fuelled by either diesel, kerosene, petroleum or any other combustible liquid or gas. If no fuel is available for an engine to burn, it cannot function...
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
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...
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 CorporationBendix Corporation
The Bendix Corporation was an American manufacturing and engineering company which during various times in its 60 year existence made brake systems, aeronautical hydraulics, avionics, aircraft and automobile fuel control systems, radios, televisions and computers, and which licensed its name for...
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
Power density
Power density is the amount of power per unit volume....
of greater than 1.0, were equipped first with distributed fuel injection
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
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
Poppet valve
A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
. The direct-injection systems differed only in that it injected the fuel directly into the cylinder head, much like a diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
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
General aviation
General aviation is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...
piston engines that continue flying into the 21st century.
Design and development
Starting with the basics of fuel combustionCombustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
, 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
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
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 aviationGeneral aviation
General aviation is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...
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
Fighter aircraft
A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...
may fly inverted
Aerobatics
Aerobatics is the practice of flying maneuvers involving aircraft attitudes that are not used in normal flight. Aerobatics are performed in airplanes and gliders for training, recreation, entertainment and sport...
, 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
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...
, 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
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
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
Inverse (logic)
In traditional logic, an inverse is a type of conditional sentence which is an immediate inference made from another conditional sentence. Any conditional sentence has an inverse: the contrapositive of the converse. The inverse of P \rightarrow Q is thus \neg P \rightarrow \neg Q...
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 servoServomechanism
thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...
-operated poppet
Poppet
The word poppet is an older spelling of puppet, from the Middle English popet, meaning a small child or doll. In British Dialect it continues to hold this meaning. Poppet is also a chiefly English term of endearment.-Folk magic:...
-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
Entrainment (engineering)
Entrainment as commonly used in various branches of engineering may be defined as the entrapment of one substance by another substance. For example:* The entrapment of liquid droplets or solid particulates in a flowing gas, as with smoke....
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.- The throttle body is the main portion of the carburetor. This portion contains one or more boresGauge (bore diameter)The gauge of a firearm is a unit of measurement used to express the diameter of the barrel. Gauge is determined from the weight of a solid sphere of lead that will fit the bore of the firearm, and is expressed as the multiplicative inverse of the sphere's weight as a fraction of a pound . Thus...
through which all of the air flows into the engine. Each bore contains a number of throttle platesThrottleA throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
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 passages or external tubes or hoses.
- The fuel control portion is used by the pilot to adjust fuel flow into the engine. It contains a number of jetsNozzleA nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
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 superchargerSuperchargerA supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
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.
Military carburetors may have a anti-detonation injection
Water injection (engines)
In internal combustion engines, water injection, also known as anti-detonant injection, is spraying water into the cylinder or incoming fuel-air mixture to cool the combustion chambers of the engine, allowing for greater compression ratios and largely eliminating the problem of engine knocking...
(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
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 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 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 A" on the side of the air metering diaphragm closest to the carburetor body. As the air pressure in chamber A is decreased, the diaphragm is pulled toward the carburetor body.
Chamber B:
- 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 B" on the side of the air metering diaphragm farthest from the carburetor body. As the air pressure in chamber A is increased, the diaphragm is moved toward 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.
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:
- Chamber C contains unmetered fuel, that is the pressure of the fuel as it enters the carburetor. 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 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 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 (referred to as a partial vacuum as it is lower than atmospheric pressure, but not a full vacuum) in the venturi to drop according to Bernoulli's principle
Bernoulli's principle
In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy...
. This causes the air pressure in chamber A to drop in proportion with the partial vacuum in the boost venturi.
At the same time, air entering the carburetor compresses the air in the impact tubes, generating a positive pressure in chamber B that is proportional to 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 pressure of fuel from the fuel pump pushes against the diaphragm in chamber C, closing the servo valve. The fuel also flows to the mixture control valve, which is closed when in the idle-cutoff position and open in all other positions.
Chamber C and chamber D are connected by a fuel passage which contains the fuel metering jets
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
. When the mixture control lever is moved from the idle-cutoff position, fuel starts to flow through the metering jets and into chamber D where it becomes metered fuel.
As fuel begins to flow, the pressure increases in chamber D, applying a force to the fuel diaphragm that moves the fuel metering valve open. The fuel in chamber D flows on to the fuel discharge valve. The pressure drop across the metering jets creates the fuel metering force on the fuel diaphragm. This force acts to open the servo valve until a balance is reached with the combined force from the un-metered fuel pressure and the air metering force.
The discharge valve is spring-loaded to a preset pressure discharge pressure, acting as a variable size restriction to hold a constant pressure in chamber D, despite varying fuel flow rates. The valve is opened as the discharge fuel pressure increases above the force from the spring, thereby lowering the fuel pressure to maintain a balanced position with the spring force.
The fuel mixture is automatically altitude-controlled by the automatic mixture control. It operates by bleeding higher pressure air from chamber B into chamber A 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
Cruise (flight)
Cruise is the level portion of aircraft travel where flight is most fuel efficient. It occurs between ascent and descent phases and is usually the majority of a journey. Technically, cruising consists of heading changes only at a constant airspeed and altitude...
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 in military aircraft, into military position, if the aircraft is so equipped. When in the military position, the Anti-Detonation Injection (ADI) system is activated, injecting the ADI 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 a very lean mixture which raises the mean effective pressure
Mean effective pressure
The mean effective pressure is a quantity related to the operation of an reciprocating engine and is a valuable measure of an engine's capacity to do work that is independent of engine displacement. When quoted as an indicated mean effective pressure or imep , it may be thought of as the average...
, producing higher engine horsepower. This causes the cylinder head temperature to increase to a very high level, which results in increases engine horsepower. Adding the ADI fluid controls the temperature to an acceptable level, but The engine still 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:
- PS single barrel carburetor
- PD double barrel carburetor
- PT triple barrel carburetor
- PR rectangular bore carburetor
Each of these styles is available in a number of sizes, using measurements of the area of the bore on a rectangular bore, or 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 special method to identify round carburetor bores. The first inch of bore diameter is used as 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 as an example, we can calculate the actual bore size as follows:
- The first inch is represented by the base number one, and we subtract that one from the size number, 18. This leaves 17 one-quarter inch units, or 17/4, which reduces to 4-1/4 inches.
- Adding the one 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