Bendix-Stromberg pressure carburetor: Difference between revisions

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

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 inlet valve.

Carburetor construction

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.

Operation

The poppet valve responded to pressure differentials across two diaphragms that separate the four pressure chambers of the fuel regulator portion of the carburetor to regulate fuel flow to the engine, under all flight conditions. The four chambers in the pressure carburetor are contained in the fuel regulator portion of the carburetor and are referred to by letters A, B, C, and D. Chambers A and B are on opposite sides of the air metering diaphragm, which is located closest to the carburetor body.

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. 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. 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 created a lower than atmospheric pressure that changed 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 moved away from the carburetor body.

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 positive or closed when it is negative.

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 portion. 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.

Specifications (variant)

{{Pistonspecs}} {{Jetspecs}}

Concept

Most aircraft of the 1920s and 1930s had a float-type carburetor. The float operates a valve which keeps the fuel level in the carburetor consistent despite varying demands. However, since the float is dependent on gravity to function, a float carburetor will fail to flow any fuel if the aircraft is flying under negative-G conditions. This is not a problem for civil aircraft which normally fly upright, but it presents a problem for aerobatic aircraft which fly upside-down or otherwise be subject to negative G, especially military fighters. If an airplane equipped with a float-type carburetor is flown under zero-G or negative-G conditions for more than a few seconds, the engine runs out of fuel, and it stops running. The problem was keenly felt by the RAF during the first years of the war, as the Rolls-Royce Merlin equipped Hurricanes and the Spitfires suffered this effect, unlike the direct fuel injection engines of their German counterparts. The problem was sought solved by installing a flow-restricted orifice that opened only when flying inverted or under negative-G conditions (the R.A.E. restrictor was known as "Miss Shilling's orifice"), but this was only a stopgap solution.

The pressure carburetor solves the problem by taking gravity out of the system as it operating on pressure alone. For this reason, the pressure carburetor will operate reliably in any flight attitude. The fact that a pressure carburetor operates on the principle of fuel under positive pressure makes it a form of fuel injection.

Construction

Like a float carburetor, a pressure carburetor has a barrel with a venturi inside it through which air flows on its way to the engine cylinders. However, it does not have a float to control the flow of fuel in to the carburetor. Instead, it has four chambers in a row separated by flexible diaphragms. The diaphragms are attached concentrically to a shaft which operates a wedge-shaped servo valve. This valve controls the rate at which fuel can enter the pressure carburetor. Inside the barrel, downsteam of the throttle sits the discharge valve, which is a spring-loaded valve operated by fuel pressure that controls the rate that fuel is discharged in to the barrel.

Some pressure carburetors had many auxiliary systems. The designs grew in complexity with the bigger models used on bigger engines. Many have an accelerator pump, an automatic mixture control, and models on turbocharged engines feature a temperature compensator. The result is that pressure carbureted engines are fairly simple to operate compared to float carbureted engines.

Operation

The four chambers in the pressure carburetor are all in a row and are referred to by letters. Chamber A contains impact air pressure at the carburetor inlet. Chamber B contains the lower air pressure from the throat of the venturi. The difference in pressure between the two air chambers creates what is known as the air metering force, which acts to open the servo valve. Chamber C contains metered fuel, and chamber D contains unmetered fuel. The difference in pressure between the two fuel chambers creates the fuel metering force, which acts to close the servo valve. Since the fuel pressures are naturally higher than air pressure, chamber A contains a spring which makes up the difference in force to create a balance.

When the engine starts and air begins to flow through the venturi, 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 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. As fuel begins to flow, the pressure drop across the metering jet creates the fuel metering force which acts to close the servo valve until a balance is reached with the air pressure and the spring.

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, causing a richening of the mixture as altitude increases.

The fuel mixture is manually controlled by a fuel mixture control lever in the cockpit. The cockpit lever has either three or four detent positions that causes a cloverleaf shaped plate to rotate in the mixture control chamber. The plate covers or uncovers the fuel metering jets as the mixture control lever is moved as follows:

1. Idle-cutoff position, where all fuel flow is cutoff to the metered side of the fuel chamber, thereby closing the servo valve, stopping the engine.
2. Auto-Lean position, where fuel flows through the enrichment and lean fuel metering jets. This is sometimes called the cruise position, as this is the most-used position while in-flight.
3. Auto-rich position, where the fuel flows through the rich, enrichment and lean fuel metering jets. This position is used for take off and landing.
4. War Emergency position (military carburetors only), where fuel flows through the lean and rich fuel metering jets only, but only when there is pressure in the Anti-detonation injection (ADI) system.

The ADI system, an adjunct to the pressure carburetor found on large military piston engines, consists of a supply tank for the ADI liquid (a mixture of 50% alcohol and 50% water), a pressure pump, a pressure regulator, a spray nozzle, and a control diaphragm that moves the carburetor enrichment valve closed when pressure is present.

The ADI system adds cooling water to the fuel-air mixture to prevent pre-ignition (detonation) in the engine cylinders when the mixture is leaned to a more powerful - yet engine damaging - mixture that adds considerable power to the engine. The supply of ADI liquid is limited so that the system runs out of liquid before the engine is damaged by the very high cylinder head temperatures caused by the very lean mixture.

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.