Pressure carburetor

A pressure carburetor is a type of fuel metering system for piston aircraft engines manufactured by the Bendix Corporation starting in the 1940s. It is recognized as an early type of mechanical fuel injection and was developed to prevent fuel starvation during inverted flight.

Concept

Most aircraft of the 1920s and 1930s had a float carburetor. The float operates a needle 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, especially military fighters. If an airplane with a float carburetor is flown under zero-G or negative-G conditions for more than a few seconds, the engine will quit.

The pressure carburetor solves the problem by taking gravity out of the system and 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 chamber or needle valve. Instead, it has four chambers in a row separated by flexible diaphragms. The diaphragms are attached concentrically to a shaft which operates a ball-shaped servo valve. The 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.

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 normal ambient air pressure at the carburetor inlet. Chamber B contains the lower air pressure from the venturi. The combination of the two chambers together 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. Combining the two fuel pressures together 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. 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 a metering jet. 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.

Fuel mixture is controlled by bleeding higher pressure air from chamber A to the lower pressure in the venturi by flowing it though a needle valve. The needle valve is pilot-controlled in manual-mixture models, and controlled by an aneroid bellows in automatic systems. Since chamber A contains a spring to hold the servo valve open, the mixture control cannot shut the system off simply by opening the mixture needle valve. Instead, when the fuel mixture is moved to the idle cutoff position, a cam operated linkage manually pushes the servo valve closed, shutting off fuel to the entire system and killing the engine.

Applications

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 PT 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 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.

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