Pressure Carburetor - Operation

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 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 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 49% 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.