Pressure carburetor

Last updated

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.

Contents

Concept

Most aircraft of the 1920s and 1930s had a float-type carburetor. They are adequate for civil aircraft which normally fly upright, but present a problem for aircraft which fly upside-down or are subject to a negative g-force, especially military fighters and aerobatic aircraft. A float carburetor uses the venturi effect to supply fuel into the engine intake; this depends upon a constant level of fuel in the float bowl to maintain the desired fuel/air mixture. The float operates a valve which keeps the fuel level in the carburetor consistent despite varying demands by means of a linked float valve. As the fuel level increases, the valve closes slowing or stopping the flow into the bowl. However, since the float depends on gravity to function, it is ineffective when the aircraft is inverted. During inversion, fuel is delivered to the float bowl as fast as the fuel pump is capable resulting in an extremely rich mixture stopping the engine almost instantly.

The problem was keenly felt by the RAF during the first years of World War II, because the Rolls-Royce Merlin engines fitted to Hurricanes and Spitfires suffered from the problem, unlike the direct fuel injection engines of their German counterparts. It was largely solved by installing a flow-restricting washer that allowed just enough fuel into the carburetor for the engine to develop maximum power (the R.A.E. restrictor was known as "Miss Shilling's orifice"). However, it was only a stopgap solution.

The pressure carburetor solved the problem. It operates on pressure alone, meaning that gravity no longer has any effect. For that reason, the pressure carburetor operates reliably when the plane is 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 into 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, downstream 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 into 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

Schematic of a pressure carburetor. Pressure Carburetor.svg
Schematic of a pressure carburetor.

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 through a tapered needle valve. The needle valve is controlled by an aneroid bellows, causing a leaning 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 cut off from 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 takeoff 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 (anti-detonant injection) 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% methanol, 49% water and 1% oil), 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. Pratt & Whitney R-4360 Wasp Major 1.jpg
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 allied 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 into 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.

See also

Related Research Articles

<span class="mw-page-title-main">Fuel injection</span> Feature of internal combustion engines

Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines.

<span class="mw-page-title-main">Carburetor</span> Component of internal combustion engines which mixes air and fuel in a controlled ratio

A carburetor is a device used by a gasoline internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the Venturi tube in the main metering circuit, though various other components are also used to provide extra fuel or air in specific circumstances.

<span class="mw-page-title-main">Aircraft engine controls</span>

Aircraft engine controls provide a means for the pilot to control and monitor the operation of the aircraft's powerplant. This article describes controls used with a basic internal-combustion engine driving a propeller. Some optional or more advanced configurations are described at the end of the article. Jet turbine engines use different operating principles and have their own sets of controls and sensors.

<span class="mw-page-title-main">Fuel pump</span> Pump

A fuel pump is a component used in many liquid-fuelled engines to transfer the fuel from the fuel tank to the device where it is mixed with the intake air.

<span class="mw-page-title-main">Inlet manifold</span> Automotive technology

An inlet manifold or intake manifold is the part of an internal combustion engine that supplies the fuel/air mixture to the cylinders. The word manifold comes from the Old English word manigfeald and refers to the multiplying of one (pipe) into many.

Dieseling or engine run-on is a condition that can occur in spark-plug-ignited, gasoline-powered internal combustion engines, whereby the engine keeps running for a short period after being turned off, drawing fuel through the carburetor, into the engine and igniting it without a spark.

Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.

<span class="mw-page-title-main">Ram-air intake</span> An intake design which uses air pressure from vehicle motion to increase static air pressure

A ram-air intake is any intake design which uses the dynamic air pressure created by vehicle motion, or ram pressure, to increase the static air pressure inside of the intake manifold on an internal combustion engine, thus allowing a greater massflow through the engine and hence increasing engine power.

<span class="mw-page-title-main">Back-fire</span> Explosion in the exhaust of an engine

A backfire or afterburn is combustion or an explosion produced by a running internal combustion engine that occurs in the exhaust system, rather than inside the combustion chamber. It is also sometimes referred to as an afterfire, especially in cases where the word backfire is used to mean a fuel burn that occurs while an intake valve is open, causing the fire to move backward through the system and out through the intake instead of the exhaust. When the flame moves backward it may also be called a "pop-back". A backfire can be caused either by ignition that happens with an exhaust valve open or unburnt fuel making its way into the hot exhaust system. A visible flame may momentarily shoot out of the exhaust pipe. A backfire is often a sign that the engine is improperly tuned.

Carburetor heat is a system used in automobile and piston-powered light aircraft engines to prevent or clear carburetor icing. It consists of a moveable flap which draws hot air into the engine intake. The air is drawn from the heat stove, a metal plate around the exhaust manifold.

A nitro engine generally refers to an engine powered with a fuel that contains some portion of nitromethane mixed with methanol. Nitromethane is a highly combustible substance that is generally only used in very specifically designed engines found in Top Fuel drag racing and in miniature internal combustion engines in radio control, control line and free flight model aircraft.

In internal combustion engines with carburetors, a choke valve or choke modifies the air pressure in the intake manifold, thereby altering the air–fuel ratio entering the engine. Choke valves are generally used in naturally aspirated engines to supply a richer fuel mixture when starting the engine. Most choke valves in engines are butterfly valves mounted upstream of the carburetor jet to produce a higher partial vacuum, which increases the fuel draw.

A throttle is a mechanism by which fluid flow is managed by constriction or obstruction.

The Quadrajet is a four barrel carburetor, made by the Rochester Products Division of General Motors. Its first application was the new-for-1965 Chevy 396ci engine. Its last application was on the 1990 Oldsmobile 307 V8 engine, which was last used in the Cadillac Brougham and full size station wagons made by Chevrolet, Pontiac, Oldsmobile, and Buick.

<span class="mw-page-title-main">Miss Shilling's orifice</span> Fuel flow restrictor retro-fitted to Merlin engines

Miss Shilling's orifice was a very simple technical device created to counter engine cut-outs experienced during negative G manoeuvres in early Spitfire and Hurricane fighter aeroplanes during the Battle of Britain. Officially called the R.A.E. restrictor, it was referred to under various names, such as Miss Tilly's diaphragm or the Tilly orifice in reference to its inventor, Beatrice "Tilly" Shilling.

<span class="mw-page-title-main">Reece Fish Carburettor</span> Motor vehicle

The Reece-Fish carburetor was a carburetor used by Mini Se7en racers in the 60s and 70s.

<span class="mw-page-title-main">SU carburettor</span>

The SU carburettor was a constant-depression carburettor made by a British manufacturer of that name or its licensees in various designs spanning most of the twentieth century.

Internal combustion engines come in a wide variety of types, but have certain family resemblances, and thus share many common types of components.

<span class="mw-page-title-main">Bendix-Stromberg pressure carburetor</span>

Of the three types of carburetors used on large, high-performance aircraft engines manufactured in 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 Chandler Groves and Chandler Evans Control Systems (CECO). Both of these types of carburetors had a relatively large number of internal parts, and in the case of the Holley Carburetor, there were complications in its "variable venturi" design.

Manifold injection is a mixture formation system for internal combustion engines with external mixture formation. It is commonly used in engines with spark ignition that use petrol as fuel, such as the Otto engine, and the Wankel engine. In a manifold-injected engine, the fuel is injected into the intake manifold, where it begins forming a combustible air-fuel mixture with the air. As soon as the intake valve opens, the piston starts sucking in the still forming mixture. Usually, this mixture is relatively homogeneous, and, at least in production engines for passenger cars, approximately stoichiometric; this means that there is an even distribution of fuel and air across the combustion chamber, and enough, but not more air present than what is required for the fuel's complete combustion. The injection timing and measuring of the fuel amount can be controlled either mechanically, or electronically. Since the 1970s and 1980s, manifold injection has been replacing carburettors in passenger cars. However, since the late 1990s, car manufacturers have started using petrol direct injection, which caused a decline in manifold injection installation in newly produced cars.

References

  1. Brown, Michael (2023-03-17). "How Does an Airplane Carburetor Work | Angle of Attack". www.angleofattack.com. Retrieved 2024-04-26.