8.3 Pressure Injection Carburetors & Anti-Detonation Injection

8.3 Pressure Injection Carburetors & Anti-Detonation Injection

Quick Answer: A pressure injection carburetor is "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." Instead it provides a pressurized fuel system that is closed from the engine fuel pump to the discharge nozzle, and 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. The assembly consists of a throttle body, an automatic mixture control, a regulator unit, and a fuel control unit. The regulator holds a constant 1/4 psi pressure drop across the metering jets, and the pressure differential across the air diaphragm between chambers A and B is the air metering force which opens the fuel poppet valve. Because fuel is discharged under pressure on the engine side of the throttle valve, the danger of fuel vaporization icing is practically eliminated.

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Why the ACS Still Lists Pressure Carburetors

FAA-H-8083-32B states that pressure carburetors "have been replaced mostly by fuel injection systems and have limited use on modern aircraft engines." They remain in the Powerplant ACS as a named knowledge element because the surviving fleet — large radials, warbirds, agricultural aircraft, and older transports — is maintained by the same A&P population, and because the pressure carburetor is the conceptual bridge between the float carburetor of Section 8.1 and the continuous-flow injection of Section 8.4. Every force balance you learn here reappears in the RSA servo injector.

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The Two Defects of a Float Carburetor That Drove the Design

Section 8.1 covered the float-type carburetor in detail. FAA-H-8083-32B names its shortcomings directly:

1. Maneuvering sensitivity. A vented float chamber depends on gravity holding a fuel level against a needle valve. Abrupt maneuvers, negative g, and rough air upset the float action and therefore the metering.
2. Low-pressure discharge and poor vaporization. The float carburetor "must discharge fuel at a point of low pressure, [so] the discharge nozzle must be located at the venturi throat, and the throttle valve must be on the engine side of the discharge nozzle." That geometry puts the evaporative temperature drop inside the venturi, and "as a result, ice readily forms in the venturi and on the throttle valve." It also makes discharging fuel into some supercharged induction systems difficult.

The pressure carburetor fixes both. Its fuel chambers "remain filled under all operating conditions," so maneuvering effects are negligible. And because it "discharges fuel into the airstream at a pressure well above atmospheric," the nozzle can sit on the engine side of the throttle valve, where "the drop in temperature due to fuel vaporization takes place after the air has passed the throttle valve and at a point where engine heat tends to offset it."

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The Four Assemblies

1. Throttle Body — the Air Measuring Device

"All air entering the cylinders must flow through the throttle body; therefore, it is the air control and measuring device." It contains the throttle valves, main venturi, boost venturi, and the impact tubes, and it measures airflow by volume and by weight so the correct fuel quantity can be added under all conditions.

• As air accelerates through the venturi its pressure decreases (Bernoulli's principle), and this low pressure is vented to the low-pressure side of the air diaphragm, chamber B.
• The impact tubes sense carburetor inlet air pressure and direct it to the automatic mixture control, from which the air is directed to the high-pressure side of the air diaphragm, chamber A.
• Throttle valves may be rectangular or disc shaped, are mounted on a shaft linked to the idle valve and the flight-deck throttle, and a throttle stop with an adjustment sets engine idle speed.

The boost venturi is the detail candidates miss. It is a small venturi mounted in the throat of the main venturi. Because it sits where velocity is already high, it multiplies the pressure depression available for metering, which is what makes accurate scheduling possible at low airflow.

2. Regulator Unit — Five Chambers and a Poppet Valve

"The regulator is a diaphragm-controlled unit divided into five chambers and contains two regulating diaphragms and a poppet valve assembly." Its purpose is "to regulate the fuel pressure to the inlet side of the metering jets in the fuel control unit," automatically, "according to the mass airflow to the engine."

Air metering force. "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." More airflow means a deeper depression in chamber B, a larger A-minus-B differential, and a wider-open poppet valve admitting more fuel to chamber D. That is the whole scheduling principle in one sentence.

The 1/4 psi Metering Differential — Worked Through

FAA-H-8083-32B works a numerical example that is worth reproducing because it is exactly the kind of reasoning an AMP question tests.

1. For a given airflow, a negative pressure of 1/4 psi is established in chamber B. This moves the diaphragm assembly and poppet valve in the opening direction, admitting more fuel to chamber D.
2. Chamber C is held constant at 5 psi (10 psi on some installations) by the discharge nozzle or impeller fuel feed valve.
3. The assembly therefore moves open until chamber D reaches 5-1/4 psi. That is the balanced condition, with a pressure drop of 1/4 psi across the jets in the fuel control unit.
4. Now suppose nozzle pressure (chamber C) rises to 5-1/2 psi. Balance is upset, and the assembly opens the poppet valve until chamber D reaches 5-3/4 psi, re-establishing the same 1/4 psi differential between C and D. The drop across the metering jets is unchanged.
5. The same self-correction handles a change in fuel inlet pressure: chamber D pressure tries to follow the inlet change, and the poppet valve repositions to restore the 1/4 psi differential.
6. Changing the mixture control from auto-lean to auto-rich selects a different set of jets. The diaphragm and poppet valve reposition to maintain the same 1/4 psi differential across the newly selected jets.

The takeaway the exam wants: the regulator does not schedule fuel flow directly. It schedules a constant pressure drop across a selected jet area, and flow then follows from that fixed drop and the jet size.

The Idle Spring

"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." An idle spring in the regulator solves this: as the poppet valve moves toward closed it contacts the spring, which "holds the poppet valve off its seat far enough to provide more fuel than is needed for idling." The resulting over-rich flow is then trimmed by the idle valve in the fuel control unit, which "restricts the fuel flow to the proper amount" at idle and "is withdrawn from the fuel passage and has no metering effect" at higher speeds.

3. Automatic Mixture Control (AMC)

The AMC consists of a bellows assembly, a calibrated needle, and a seat, and its purpose is "to compensate for changes in air density due to temperature and altitude changes."

• The metallic bellows is sealed at 28 inches of mercury absolute pressure and responds to both pressure and temperature.
• At sea level the bellows is contracted and the needle is not in the atmospheric passage.
• As the aircraft climbs and atmospheric pressure falls, the bellows expands, inserting the tapered needle farther into the atmospheric passage and restricting airflow into chamber A.
• Simultaneously, air leaks slowly from chamber A to chamber B through a small back-suction (mixture control) bleed, at a rate that is about the same at high altitude as at sea level. Restricting the inflow while the bleed keeps leaking lowers chamber A pressure, reduces the air metering force, closes the poppet valve, and leans the mixture.
• The AMC can be removed and cleaned if the lead seal at the point of adjustment is not disturbed. Breaking that seal converts a cleaning into an unapproved adjustment.

4. Fuel Control Unit — Jets and Valves

The fuel control unit "is attached to the regulator assembly and contains all metering jets and valves" and exists "to meter and control the fuel flow to the discharge nozzle." Remember that the inlet pressure to the jets is regulated by the regulator unit and the outlet pressure is controlled by the discharge nozzle.

Three jets:

• Auto-lean jet — meters the basic fuel flow, the amount required to run the engine on a lean mixture.
• 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."
• Power enrichment jet — supplies the additional fuel for high-power operation.

Four valves:

1. Idle needle valve — a contoured needle or cylinder valve in series with all other metering devices, linked to the throttle shaft so it restricts flow only in the idle range.
2. Power enrichment valve — opens at high power to bring the enrichment jet into play.
3. Regulator fill valve.
4. Manual mixture control — a rotary disc valve: a stationary disc ported from the auto-lean jet, the auto-rich jet, and two smaller ventholes, with a cloverleaf-shaped rotating plate held against it by spring tension. In IDLE CUTOFF all ports and vents are closed. In AUTO-LEAN the auto-lean jet port and the two ventholes are open and the auto-rich port stays closed. In AUTO-RICH all ports are open.

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The Vapor Vent System and Its Two Failure Modes

Vapor vents exist "to eliminate fuel vapor created by the fuel pump, heat in the engine compartment, and the pressure drop across the poppet valve." The vent is located in the fuel inlet (chamber E) or, on some models, in both chambers D and E. When air rises to the top of the chamber and displaces fuel, the float drops and pulls the vapor vent valve off its seat, letting vapor escape through the vent line back to the fuel tank.

The vent check. Disconnect the vapor vent line at the carburetor, turn the fuel booster pump on, and observe the connection. Move the mixture control to auto-rich, then return it to idle cutoff. 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.

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Anti-Detonation Injection (Water Injection)

FAA-H-8083-32B covers anti-detonation injection with the pressure carburetor because the two were installed together on large radials. "Water injection was used mostly on large radial engines. The water injection system enabled more power to be obtained from the engine at takeoff than is possible without water injection."

The logic is worth understanding because it ties the whole mixture chapter together:

• At takeoff power the carburetor "delivers more fuel to the engine than it actually needs." Section 8.5 establishes why: the excess fuel is not there to make power, it is there "to prevent overheating and detonation." In fact "a leaner mixture would produce more power."
• Inject an antidetonant fluid — a water-alcohol mixture, with the alcohol present for freeze protection — and "the mixture can be leaned out to that which produces maximum power, and the vaporization of the water-alcohol mixture then provides the cooling formerly supplied by the excess fuel."
• Carburetors equipped for water injection are modified so that selecting ADI resets the mixture schedule to the leaner power setting at the same time the fluid begins to flow.

The failure mode that follows directly from that design: if ADI fluid is selected but does not flow while the carburetor has already gone to its leaner ADI schedule, the engine is running a lean mixture at takeoff manifold pressure with no evaporative cooling — the exact combination Section 2.1 identifies as a primary cause of detonation. That is why the ADI system carries a fluid-pressure interlock and why servicing the ADI tank is a preflight and a maintenance item rather than an optional top-off.

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Independent Prep Note

Independent FAA AMT Powerplant prep by OpenExamPrep. Not sponsored by or affiliated with the Federal Aviation Administration (FAA). Technical data compiled from FAA-H-8083-32B, FAA AC 43.13-1B, and 14 CFR Parts 43 and 65.