Pressure carburetor
Encyclopedia
A pressure carburetor is a type of fuel metering system manufactured by the Bendix Corporation
for piston aircraft engines, starting in the 1940s. It is recognized as an early type of throttle-body fuel injection
and was developed to prevent fuel starvation during inverted flight.
. 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 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 operates 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
.
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.
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:
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.
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.
Bendix 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...
for piston aircraft engines, starting in the 1940s. It is recognized as an early type of throttle-body 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 was developed to prevent fuel starvation during inverted flight.
Concept
Most aircraft of the 1920s and 1930s had a float-type carburetorCarburetor
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....
. 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
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. 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
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...
. 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
Rolls-Royce Merlin
The Rolls-Royce Merlin is a British liquid-cooled, V-12, piston aero engine, of 27-litre capacity. Rolls-Royce Limited designed and built the engine which was initially known as the PV-12: the PV-12 became known as the Merlin following the company convention of naming its piston aero engines after...
equipped Hurricanes
Hawker Hurricane
The Hawker Hurricane is a British single-seat fighter aircraft that was designed and predominantly built by Hawker Aircraft Ltd for the Royal Air Force...
and the Spitfires
Supermarine Spitfire
The Supermarine Spitfire is a British single-seat fighter aircraft that was used by the Royal Air Force and many other Allied countries throughout the Second World War. The Spitfire continued to be used as a front line fighter and in secondary roles into the 1950s...
suffered this effect, unlike the direct fuel injection engines of their German counterparts. The problem was 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
Miss Shilling's orifice
Miss Shilling's Orifice was a very simple technical device made to counter engine cut-out in early Spitfire and Hurricane fighter aeroplanes during the Battle of Britain...
"), but this was only a stopgap solution.
The pressure carburetor solves the problem by taking gravity out of the system as it operates 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
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....
.
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 throttleThrottle
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...
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
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
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
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 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
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....
. 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:
- 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.
- 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.
- 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.
- 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
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...
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
Lycoming Engines
Lycoming Engines is a U.S. aircraft engine company, known primarily for its general aviation engines. For most of its history Lycoming has been part of the AVCO group as AVCO Lycoming. In 1987 AVCO was purchased by Textron to become Textron Lycoming...
and Continental engines on 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...
aircraft. These small pressure carburetors eventually evolved in to the Bendix RSA series multi-point continuous-flow 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....
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.