M — Powerplant (Groupe motopropulseur)Chapter 4 · 52 practice questions

Chapter 4: Reciprocating Engine — Fuel Metering (Carburetor & FADEC)

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Reciprocating Engine — Fuel Metering (Carburetor & FADEC)

Overview

This chapter covers the principles, components, and troubleshooting of fuel metering systems used in reciprocating aircraft engines. The two primary systems discussed are float-type carburetors and continuous-flow fuel injection systems (both Bendix and Continental types), along with an introduction to Full Authority Digital Engine Control (FADEC). Understanding how these systems meter fuel under varying engine operating conditions is essential for diagnosing performance issues and performing correct adjustments.

Diagram — Reciprocating Engine — Fuel Metering (Carburetor & FADEC) RECIPROCATING ENGINE FUEL METERING SYSTEMS FLOAT-TYPE CARBURETOR Air Venturi Throttle Float Bowl Float Needle Fuel In Main Jet Idle Passage Idle Mix Fuel-Air to Engine Key: Float/Needle Idle Sys FADEC — FULL AUTHORITY DIGITAL ENGINE CONTROL ECU Electronic Control Unit Tach MAP EGT CHT Sensors → Fuel Metering Unit Ignition Timing Propeller Control Key: Sensors Actuators SYSTEM COMPARISON & TROUBLESHOOTING CARBURETOR FAULTS • Clogged main jet → Lean mixture at high power • Float valve sticking → Flooding / lean cut • Idle mixture screw maladjustment → Rough idle • Venturi ice formation → Power loss • Air leak past throttle shaft → Lean idle Troubleshoot: Fuel flow & pressure FADEC FAULTS • Sensor failure (Tach, MAP, EGT) → Limp mode • ECU software error → Incorrect metering • Actuator jam → Fuel flow disruption • Wiring harness damage → Signal loss • Power supply interruption → System reset Troubleshoot: Diagnostic codes TEA Training — Fuel Metering Fundamentals

Key Concepts Explained in Detail

Float-Type Carburetor Fundamentals

Float carburetor circuits Float Carburetor Circuits Constant Level Chamber Float Needle Inlet Const. level Idle Circuit Throttle Jet idle Mixture Active at idle Main Circuit Jet main Venturi Active at high power Accelerator Pump Pump Diaphragm Injection Sudden throttle opening → extra fuel Power Enrichment Econ. valve Opens at high power Prevents detonation Mixture Control Mixture RICH IDLE CUTOFF IDLE CUTOFF = complete shutdown Quick Diagnosis • Clogged main jet → lean at high power • Air leak downstream of throttle → affects idle • High fuel pressure → rich at all speeds Idle Adjustment Procedure 1. Warm up the engine 2. Adjust mixture screw (lean roll/drop method) 3. Adjust throttle stop screw (specified speed) Legend Idle circuit Main circuit Accel. pump Enrichment The idle circuit is independent of the main circuit. An idle problem does not necessarily affect cruising. Rich mixture = excess fuel (black smoke) | Lean mixture = excess air (high EGT, risk of knocking).
Air-fuel mixture window Air-Fuel Mixture Window Air-Fuel Ratio vs Engine Performance Power / EGT 0 50% 100% ← Rich (excess fuel) — Lean (excess air) → Rich Lean Power EGT Peak RPM (best lean power) RICH Zone LEAN Zone Optimal zone RICH Mixture • Reduced power • Black exhaust smoke • Low EGT • Carbon deposits • High fuel consumption • Fouled spark plugs • Unstable idle • Hard starting LEAN Mixture • High EGT • Knocking / detonation • Risk of engine damage • Power loss • Cylinder overheating • Misfiring • Burned valves • Slow response time Note: A lean mixture raises EGT and can cause damage. Peak RPM indicates the optimal lean mixture. Power EGT Peak RPM Optimal zone Rich Lean

The float-type carburetor operates on the principle of maintaining a constant fuel level in the float bowl. A float and needle valve assembly regulates fuel entry, while the venturi creates a pressure differential that draws fuel from the bowl through the main metering jet into the airstream.

Main Metering System: At high power settings, fuel flows primarily through the main metering jet. The size of this jet determines the fuel-air ratio at full throttle. A clogged main jet restricts fuel flow, causing a lean mixture and rough running at high power settings, while idle and cruise may remain unaffected since fuel demand is lower.

Idle System: At idle, the throttle plate is nearly closed, creating high manifold vacuum downstream. Fuel is drawn through separate idle passages and the idle mixture screw, bypassing the main jet. This is why idle mixture adjustments are independent of main metering adjustments.

Economizer Valve (Power Enrichment Valve): This valve opens at high power settings to provide additional fuel, preventing a lean mixture that could cause detonation. It enriches the mixture above the main jet's capacity at high manifold pressures.

Accelerator Pump: When the throttle is opened quickly, the accelerator pump delivers a momentary shot of fuel to prevent hesitation. A strong pump stream observed when moving the throttle by hand indicates proper operation. If hesitation persists despite a strong stream, a worn check valve may be allowing fuel to bypass the venturi.

Continuous-Flow Fuel Injection Systems

Continuous fuel injection flow Continuous Fuel Injection FUEL PUMP Engine driven Pressure: 20-25 psi fuel pressurized PRESSURE REGULATOR Maintains constant pressure fuel return (excess) metered fuel FLOW DIVIDER Distributes to each cylinder INJ. 1 sprays INJ. 2 sprays INJ. 3 sprays INJ. 4 sprays manifold manifold manifold manifold MIXTURE CONTROL Rich / Lean IDLE CUTOFF cuts off supply control IDLE RICHNESS SCREW counter-clockwise = richer RECIPROCATING ENGINE Injectors spray into the intake manifolds Air-fuel mixture is drawn into each cylinder Continental System — Continuous injection: pressure is kept constant (20-25 psi), the divider distributes fuel to each injector. LEGEND Pressurized fuel Fuel return Control

Two major types are covered: Bendix (now Precision) RSA series and Continental fuel injection systems. Both use a continuous-flow principle where fuel is delivered to injector nozzles at each cylinder intake port.

System Components:

  • Fuel pump (engine-driven with auxiliary electric boost pump)
  • Fuel pressure regulator (maintains system pressure)
  • Flow divider (distributes fuel equally to cylinders)
  • Injector nozzles (deliver fuel as a fine spray)
  • Mixture control valve (allows pilot to adjust fuel flow)
  • Idle cut-off valve (stops fuel flow for engine shutdown)

Bendix RSA Systems: These use a fuel control unit that integrates the regulator, mixture control, and idle cut-off functions. Fuel pressure is regulated by a diaphragm-type regulator. Low fuel pressure at idle typically indicates a faulty regulator rather than a pump problem.

Continental Systems: These use a separate fuel pressure regulator and flow divider. The idle mixture is adjusted by a dedicated screw; turning it outward (counterclockwise) enrichens the mixture. Direction can vary by system, so manufacturer specifications must always be followed.

FADEC Systems

Sensor-FADEC-actuator loop Sensor-FADEC-Actuator Loop SENSORS Engine Speed Position sensor (RPM) Temperature EGT / Cylinder head (°C) Pressure Manifold pressure (inHg / kPa) Throttle Position Throttle valve (degrees) Additional Parameters Fuel pressure, voltage, etc. FADEC COMPUTER Data Acquisition Sensor signal processing Mixture Calculation Optimal air-fuel ratio Output Commands Signals to actuators ACTUATORS Fuel Servo Fuel flow control (electronic injection) Rich/lean mixture Ignition Ignition timing (electronic timing) Spark plugs Feedback Parameter monitoring in real time Fault codes ⚠ Maintenance Fault codes to investigate No bypass permitted Follow manufacturer instructions Feedback: continuous monitoring and real-time adjustment Sensor signals Actuator commands Feedback Active circulation

Full Authority Digital Engine Control (FADEC) electronically manages fuel metering, ignition timing, and other engine parameters. Sensors monitor engine conditions and the ECU adjusts fuel delivery for optimal performance. When fault codes indicate sensor failure, the correct action is to replace the faulty sensor per manufacturer instructions—bypassing is never permitted.

Important Procedures and Adjustments

Idle Mixture Adjustment Methods

Lean roll and lean drop methods Lean Roll and Lean Drop Methods Lean Roll Method (lean roll) 1. Warm up the engine to operating temperature 2. Set the idle speed (e.g., 800 RPM) 3. Lean the mixture until peak RPM (leanest best power) 4. Enrich slightly until a drop of a few RPM (rich of peak) RPM vs Mixture Peak Leaning → Interpretation RPM ↑ = too rich RPM ↓ immediate = too lean Lean Drop Method (lean drop) 1. Set the idle speed to specification 2. Lean the mixture until the RPM drops (lean drop) 3. Enrich until the RPM rises back to its maximum 4. Final adjustment: slightly rich of peak (stable + cooling) Drop then recovery Drop Enriching → Idle Adjustment Procedure — Correct Sequence 1. Warm up the engine 2. Adjust the idle mixture (mixture screw — appropriate method) 3. Adjust the idle speed (throttle stop screw — specified value) Fuel Injection Systems Continental (continuous injection) • Pump 20-25 psi, pressure regulator • Mixture screw: counter-clockwise = enriches • Mixture control for altitude Bendix RSA • Integrated fuel control unit • Mixture valve to injectors • Separate mixture screw FADEC — Full Authority Digital Engine Control • Sensors: temperature, pressure, RPM, throttle position • Actuators: fuel metering, ignition • Fault codes: investigate systematically • No bypass is permitted

Lean Roll Method:

  1. Warm engine to operating temperature
  2. Set idle speed to specified RPM
  3. Lean mixture until RPM peaks (leanest best power)
  4. If RPM increases when leaning, original mixture was too rich
  5. Enrichen slightly from peak RPM—RPM will drop as mixture becomes richer than best power
  6. Final setting is slightly rich of peak for smooth operation and adequate cooling

Lean Drop Method:

  1. Set idle speed to specification
  2. Lean mixture until RPM drops (lean drop point)
  3. Enrichen until RPM recovers to maximum
  4. Final mixture is slightly rich of peak RPM

Proper Sequence: Always adjust idle mixture first, then set idle speed to specification. Adjusting speed before mixture can result in incorrect settings.

Idle Speed Adjustment

If the idle speed screw has no effect on RPM, the throttle is likely being held open by a binding throttle cable. Inspect for kinks, friction, or improper routing before assuming other causes.

Throttle Linkage Adjustment

When installing a new carburetor, throttle linkage must allow both full open (for takeoff power) and full close (for idle). Failure to achieve either range can result in loss of power or inability to idle.

Fuel Pressure Adjustment

Fuel pressure specifications (e.g., 20-25 psi) must be followed precisely. Setting pressure too high (e.g., 30 psi) causes excessive fuel flow through injectors, resulting in a rich mixture across all power settings, leading to spark plug fouling and reduced power.

Float Level Adjustment

Float level must be set using the manufacturer's specified gauge or tool. Using generic tools may not provide correct measurement. Always follow manufacturer's instructions per AC43.13-1B.

Common Relationships Between Concepts

Rich vs. Lean Mixture Symptoms

Idle vs. Cruise Symptom Patterns

Idle vs Cruise Symptom Patterns — Diagnostic Matrix Idle vs Cruise Symptoms — Diagnostic Matrix Float carburetor — Idle circuit vs main circuit relationships Observed symptom Circuit involved Symptom ↓ / Circuit → Idle mixture screw Air leak downstream of throttle plate Clogged main jet Rich at idle Mixture too rich at idle — black smoke, unstable rpm ✓ Likely cause Screw too rich — turn counter-clockwise to lean out Partial effect An air leak leans out the mixture — may compensate No effect The main circuit is not active at idle Rough idle Fluctuating rpm, tendency to stall, instability Possible cause Incorrect adjustment — check the lean roll procedure ✓ Likely cause Maximum vacuum at idle — parasitic air rushes in No effect The main jet does not feed the idle circuit Power loss at high power Fuel shortage, reduced max rpm No effect The idle screw only acts when the throttle is closed Minor effect At high power, vacuum is lower — reduced impact ✓ Likely cause Maximum flow required — a clogged jet starves the engine Note: An air leak downstream of the throttle mainly affects idle (maximum vacuum); the main circuit is independent of the idle circuit.
  • Rich at idle, normal at cruise: Idle mixture screw set too rich; idle circuit is separate from main metering
  • Rough idle, smooth cruise: Air leak downstream of throttle valve (adds extra air at idle, leaning mixture)
  • Smooth idle, rough at high power: Clogged main metering jet (restriction only affects high fuel flow)
  • Rough at all power settings: Float level, fuel pressure, or general system issue

Hot Start Difficulties

Vapor lock in fuel lines is the most common cause of hot start problems. Fuel vaporizes in lines due to heat, preventing liquid fuel from reaching injectors. Priming may worsen the condition. Cooling fuel pump or lines may be necessary.

Mixture Control and Idle Cut-Off

Mixture Control and Idle Cut-Off Troubleshooting Mixture control and idle cut-off position Decision tree — Idle fault diagnosis (reciprocating engine) Engine idling Mixture pulled toward IDLE CUT-OFF (mixture control in cut-off position) Does the engine stop? (complete fuel shut-off) YES NORMAL OPERATION Valve closes properly NO FAULT DETECTED Engine keeps running POSSIBLE CAUSES: • Mixture cable misadjusted (incomplete travel to cut-off) • Mixture valve stuck open (fuel still flowing) CABLE MISADJUSTED • Insufficient travel • Stop incorrectly positioned • Excessive cable slack • Ferrule/end fitting adjustment VALVE STUCK • Dirt / debris • Internal corrosion • Broken spring • Damaged seat ADDITIONAL CHECKS • Accelerated idle test • Check idle mixture richness Normal operation Fault Possible causes Corrective actions
  • Purpose of mixture control: Allows pilot to lean/enrichen for altitude changes, takeoff, and cruise
  • Purpose of idle cut-off: Stops fuel flow when mixture control is pulled to idle cut-off position
  • Engine continues running in idle cut-off: Mixture control cable not adjusted to fully close valve; check cable adjustment first
  • Cannot lean at idle: Mixture control valve likely stuck or broken

Fuel Injection System Troubleshooting

  • Low fuel flow at full throttle: Check fuel pump output pressure first
  • Normal pressure but lean at high power: Check for blocked injector nozzles
  • High fuel flow at all power settings: Mixture control cable may be preventing leaning
  • Engine dies unless mixture held full rich: Low fuel pressure insufficient for leaner settings
  • Rich idle after verifying mixture cable: Adjust idle mixture needle per manufacturer procedure
  • Fuel leak at injector nozzle: Replace damaged copper gasket and torque to specification

Accelerator Pump Issues

Carburetor Accelerator Pump Mechanism Carburetor Accelerator Pump LEGEND Fuel Pulsed fuel Air Mixture CARBURETOR CUTAWAY — SUDDEN THROTTLE OPENING BOWL Constant level Float Needle valve Fuel inlet Main jet ACCELERATOR PUMP Diaphragm Spring Inlet valve Outlet valve Fuel supplement Throttle Sudden opening Venturi Air Rich mixture DIAGNOSTIC — WORN CHECK VALVE SYMPTOMS • Acceleration stumble (hesitation) upon sudden throttle opening • Possible backfire into the carburetor • Engine stalls if throttle is opened too quickly CAUSE Check valve worn or stuck in the open position — fuel flows back into the bowl TEST 1. Remove the carburetor 2. Blow into the pump outlet passage WORN VALVE (VIEW) Normal Worn Leak CORRECTION: Replace the check valve and inspect the accelerator pump diaphragm. TECHNICAL NOTE A worn valve causes insufficient instant enrichment during acceleration.

Hesitation on acceleration with a strong pump stream indicates a worn check valve. Backfiring through the intake on acceleration indicates a lean mixture during the transient, often due to faulty accelerator pump. Stalling when throttle is opened indicates the accelerator pump is not delivering the extra fuel needed.

Carbon Buildup on Injector Nozzles

Carbon buildup should be removed using approved methods such as ultrasonic cleaning, followed by flow-checking to ensure proper spray pattern and flow rate. Wire brushes can damage precision orifices.

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Reinforce Reciprocating Engine — Fuel Metering (Carburetor & FADEC) with 52 Transport Canada–style practice questions, matched to your weak areas.

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