Pressure Injection Carburetors and Automatic Mixture Control

Pressure Injection Carburetors

Pressure injection carburetors are distinctly different from float-type carburetors as they do not incorporate a vented float chamber or suction pickup from a discharge nozzle located in the venturi tube. Instead, they provide a pressurized fuel system that is closed from the engine fuel pump to the discharge nozzle. The venturi serves only to create pressure differentials for controlling the quantity of fuel to the metering jet in proportion to airflow to the engine.

Typical Injection Carburetor

The injection carburetor is a hydromechanical device employing a closed feed system from the fuel pump to the discharge nozzle. It meters fuel through fixed jets according to the mass airflow through the throttle body and discharges it under a positive pressure.

Aircraft engine pressure injection carburetors
Figure 1. Pressure-type carburetor

The illustration in Figure 1 represents a pressure-type carburetor simplified so that only the basic parts are shown. Note the two small passages, one leading from the carburetor air inlet to the left side of the flexible diaphragm and the other from the venturi throat to the right side of the diaphragm.

When air passes through the carburetor to the engine, the pressure on the right of the diaphragm is lowered because of the drop in pressure at the venturi throat. As a result, the diaphragm moves to the right, opening the fuel valve. Pressure from the engine-driven pump then forces fuel through the open valve to the discharge nozzle, where it sprays into the airstream. The distance the fuel valve opens is determined by the difference between the two pressures acting on the diaphragm. This difference in pressure is proportional to the airflow through the carburetor. Thus, the volume of airflow determines the rate of fuel discharge.

The pressure injection carburetor is an assembly of the following units:
  1. Throttle body
  2. Automatic mixture control
  3. Regulator unit
  4. Fuel control unit (some are equipped with an adapter)

Throttle Body

The throttle body contains the throttle valves, main venturi, boost venturi, and the impact tubes. All air entering the cylinders must flow through the throttle body; therefore, it is the air control and measuring device. The airflow is measured by volume and by weight so that the proper amount of fuel can be added to meet the engine demands under all conditions.

As air flows through the venturi, its velocity is increased, and its pressure is decreased (Bernoulli’s principle). This low pressure is vented to the low pressure side of the air diaphragm [Figure 2 chamber B] in the regulator assembly. The impact tubes sense carburetor inlet air pressure and direct it to the automatic mixture control, which measures the air density. From the automatic mixture control, the air is directed to the high pressure side of the air diaphragm (chamber A). The pressure differential of the two chambers acting upon the air diaphragm is known as the air metering force which opens the fuel poppet valve.

Engine pressure injection carburetors
Figure 2. Regulator unit

The throttle body controls the airflow with the throttle valves. The throttle valves may be either rectangular or disk shaped, depending on the design of the carburetor. The valves are mounted on a shaft, which is connected by linkage to the idle valve and to the throttle control in the cockpit. A throttle stop limits the travel of the throttle valve and has an adjustment which sets engine idle speed.

Regulator Unit

The regulator is a diaphragm-controlled unit divided into five chambers and contains two regulating diaphragms and a poppet valve assembly. [Figure 2] Chamber A is regulated air-inlet pressure from the air intake. Chamber B is boost venturi pressure. Chamber C contains metered fuel pressure controlled by the discharge nozzle or fuel feed valve. Chamber D contains unmetered fuel pressure controlled by the opening of the poppet valve. Chamber E is fuel pump pressure controlled by the fuel pump pressure relief valve. The poppet valve assembly is connected by a stem to the two main control diaphragms. The purpose of the regulator unit is to regulate the fuel pressure to the inlet side of the metering jets in the fuel control unit. This pressure is automatically regulated according to the mass airflow to the engine.

The carburetor fuel strainer, located in the inlet to chamber E, is a fine mesh screen through which all the fuel must pass as it enters chamber D. The strainer must be removed and cleaned at scheduled intervals.

Referring to Figure 2, assume that for a given airflow in lb/hr through the throttle body and venturi, a negative pressure of 1⁄4 psi is established in chamber B. This tends to move the diaphragm assembly and the poppet valve in a direction to open the poppet valve permitting more fuel to enter chamber D. The pressure in chamber C is held constant at 5 psi (10 psi on some installations) by the discharge nozzle or impeller fuel feed valve. Therefore, the diaphragm assembly and poppet valve moves in the open direction until the pressure in chamber D is 51⁄4 psi. Under these pressures, there is a balanced condition of the diaphragm assembly with a pressure drop of 1⁄4 psi across the jets in the fuel control unit (auto-rich or auto-lean).

If nozzle pressure (chamber C pressure) rises to 51⁄2 psi, the diaphragm assembly balance is upset, and the diaphragm assembly moves to open the poppet valve to establish the necessary 53⁄4 psi pressure in chamber D. Thus, the 1⁄4 psi differential between chamber C and chamber D is re-established, and the pressure drop across the metering jets remains the same.

If the fuel inlet pressure is increased or decreased, the fuel flow into chamber D tends to increase or decrease with the pressure change causing the chamber D pressure to do likewise. This upsets the balanced condition previously established, and the poppet valve and diaphragm assembly respond by moving to increase or decrease the flow to re-establish the pressure at the 1⁄4 psi differential.

The fuel flow changes when the mixture control plates are moved from auto-lean to auto-rich, thereby selecting a different set of jets or cutting one or two in or out of the system. When the mixture position is altered, the diaphragm and poppet valve assembly repositions to maintain the established pressure differential of 1⁄4 psi between chambers C and D, maintaining the established differential across the jets. Under low power settings (low airflows), the difference in pressure created by the boost venturi is not sufficient to accomplish consistent regulation of the fuel. Therefore, an idle spring, shown in Figure 2, is incorporated in the regulator. As the poppet valve moves toward the closed position, it contacts the idle spring. The spring holds the poppet valve off its seat far enough to provide more fuel than is needed for idling. This potentially overrich mixture is regulated by the idle valve. At idling speed, the idle valve restricts the fuel flow to the proper amount. At higher speeds, it is withdrawn from the fuel passage and has no metering effect.

Vapor vent systems are provided in these carburetors to eliminate fuel vapor created by the fuel pump, heat in the engine compartment, and the pressure drop across the poppet valve. The vapor vent is located in the fuel inlet (chamber E) or, on some models of carburetors, in both chambers D and E.

The vapor vent system operates in the following way. When air enters the chamber in which the vapor vent is installed, the air rises to the top of the chamber, displacing the fuel and lowering its level. When the fuel level has reached a predetermined position, the float (which floats in the fuel) pulls the vapor vent valve off its seat, permitting the vapor in the chamber to escape through the vapor vent seat, its connecting line, and back to the fuel tank.

If the vapor vent valve sticks in a closed position or the vent line from the vapor vent to the fuel tank becomes clogged, the vapor-eliminating action is stopped. This causes the vapor to build up within the carburetor to the extent that vapor passes through the metering jets with the fuel. With a given size carburetor metering jet, the metering of vapor reduces the quantity of fuel metered. This causes the fuel/air mixture to lean out, usually intermittently.

If the vapor vent valve sticks open or the vapor vent float becomes filled with fuel and sinks, a continuous flow of fuel and vapor occurs through the vent line. It is important to detect this condition, as the fuel flow from the carburetor to the fuel supply tank may cause an overflowing tank with resultant increased fuel consumption.

To check the vent system, disconnect the vapor vent line where it attaches to the carburetor, and turn the fuel booster pump on while observing the vapor vent connection at the carburetor. Move the carburetor mixture control to auto-rich; then return it to idle cutoff. When the fuel booster pump is turned on, there should be an initial ejection of fuel and air followed by a cutoff with not more than a steady drip from the vent connection. Installations with a fixed bleed from the D chamber connected to the vapor vent in the fuel inlet by a short external line should show an initial ejection of fuel and air followed by a continuing small stream of fuel. If there is no flow, the valve is sticking closed; if there is a steady flow, it is sticking open.

Fuel Control Unit

The fuel control unit is attached to the regulator assembly and contains all metering jets and valves. [Figure 3] The idle and power enrichment valves, together with the mixture control plates, select the jet combinations for the various settings (i.e., auto-rich, auto-lean, and idle cutoff).

Engine pressure injection carburetors
Figure 3. Fuel control unit

The purpose of the fuel control unit is to meter and control the fuel flow to the discharge nozzle. The basic unit consists of three jets and four valves arranged in series, parallel, and series-parallel hookups. [Figure 3] These jets and valves receive fuel under pressure from the regulator unit and then meter the fuel as it flows to the discharge nozzle. The manual mixture control valve controls the fuel flow. By using proper size jets and regulating the pressure differential across the jets, the right amount of fuel is delivered to the discharge nozzle, giving the desired fuel/air ratio in the various power settings. It should be remembered that the inlet pressure to the jets is regulated by the regulator unit and the outlet pressure is controlled by the discharge nozzle.

The jets in the basic fuel control unit are the auto-lean jet, the auto-rich jet, and power enrichment jet. The basic fuel flow is the fuel required to run the engine with a lean mixture and is metered by the auto-lean jet. The auto-rich jet adds enough fuel to the basic flow to give a slightly richer mixture than best power mixture when the manual mixture control is in the auto-rich position.

The four valves in the basic fuel control unit are:
  1. Idle needle valve
  2. Power enrichment valve
  3. Regulator fill valve
  4. Manual mixture control

The functions of these valves are:
  • The idle needle valve meters the fuel in the idle range only. It is a round, contoured needle valve, or a cylinder valve placed in series with all other metering devices of the basic fuel control unit. The idle needle valve is connected by linkage to the throttle shaft so that it restricts the fuel flowing at low power settings (idle range).
  • The manual mixture control is a rotary disk valve consisting of a round stationary disk with ports leading from the auto-lean jet, the auto-rich jet, and two smaller ventholes. Another rotating part, resembling a cloverleaf, is held against the stationary disk by spring tension and rotated over the ports in that disk by the manual mixture control lever. All ports and vents are closed in the idle cutoff position. In the auto-lean position, the ports from the auto-lean jet and the two ventholes are open. The port from the auto-rich jet remains closed in this position. In the auto-rich position, all ports are open. The valve plate positions are illustrated in Figure 4. The three positions of the manual mixture control lever make it possible to select a lean mixture a rich mixture, or to stop fuel flow entirely. The idle cutoff position is used for starting or stopping the engine. During starting, fuel is supplied by the primer.

Engine pressure injection carburetors
Figure 4. Manual mixture control valve plate positions
  • The regulator fill valve is a small poppet-type valve located in a fuel passage which supplies chamber C of the regulator unit with metered fuel pressure. In idle cutoff, the flat portion of the cam lines up with the valve stem, and a spring closes the valve. This provides a means of shutting off the fuel flow to chamber C and thus provides for a positive idle cutoff.
  • The power enrichment valve is another poppet-type valve. It is in parallel with the auto-lean and auto-rich jets, but it is in series with the power enrichment jet. This valve starts to open at the beginning of the power range. It is opened by the unmetered fuel pressure overcoming metered fuel pressure and spring tension. The power enrichment valve continues to open wider during the power range until the combined flow through the valve and the auto-rich jet exceeds that of the power enrichment jet. At this point the power enrichment jet takes over the metering and meters fuel throughout the power range.
  • Carburetors equipped for water injection are modified by the addition of a enrichment valve and a enrichment jet. The enrichment valve and enrichment jet are in series with each other and parallel with the power enrichment jet.

The carburetor controls fuel flow by varying two basic factors. The fuel control unit, acting as a pressure-reducing valve, determines the metering pressure in response to the metering forces. The regulator unit, in effect, varies the size of the orifice through which the metering pressure forces the fuel. It is a basic law of hydraulics that the amount of fluid that passes through an orifice varies with the size of the orifice and the pressure drop across it. The internal automatic devices and mixture control act together to determine the effective size of the metering passage through which the fuel passes. The internal devices, fixed jets, and variable power enrichment valve are not subject to direct external control.

Automatic Mixture Control (AMC)

The automatic mixture control unit consists of a bellows assembly, calibrated needle, and seat. [Figure 5] The purpose of the automatic mixture control is to compensate for changes in air density due to temperature and altitude changes.

Reciprocating engine fuel metering
Figure 5. Automatic mixture control and throttle body

The automatic mixture control contains a metallic bellows, which is sealed at 28 "Hg absolute pressure. This bellows responds to changes in pressure and temperature. In the illustration, the automatic mixture control is located at the carburetor air inlet. As the density of the air changes, the expansion and contraction of the bellows moves the tapered needle in the atmospheric line. At sea level, the bellows is contracted, and the needle is not in the atmospheric passage. As the aircraft climbs and the atmospheric pressure decreases, the bellows expands, inserting the tapered needle farther and farther into the atmospheric passage and restricting the flow of air to chamber A of the regulator unit. [Figure 2] At the same time, air leaks slowly from chamber A to chamber B through the small bleed (often referred to as the back-suction bleed or mixture control bleed). The rate at which air leaks through this bleed is about the same at high altitude as it is at sea level. As the tapered needle restricts the flow of air into chamber A, the pressure on the left side of the air diaphragm decreases. As a result, the poppet valve moves toward its seat, reducing the fuel flow to compensate for the decrease in air density. The automatic mixture control can be removed and cleaned if the lead seal at the point of adjustment is not disturbed.


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