M — Powerplant (Groupe motopropulseur)Chapter 3 · 16 practice questions

Chapter 3: Reciprocating Engine — Ignition & Starting

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Chapter: Reciprocating Engine — Ignition & Starting

Overview

This chapter covers the ignition and starting systems of reciprocating aircraft engines. The ignition system is responsible for generating and delivering a high-voltage spark to ignite the fuel-air mixture in the cylinders at the correct moment. The starting system provides the initial mechanical rotation required to begin the engine's operating cycle. Understanding the components, operation, timing, and troubleshooting of these systems is critical for safe and efficient engine operation.

Diagram — Reciprocating Engine — Ignition & Starting RECIPROCATING ENGINE IGNITION & STARTING SYSTEM IGNITION SYSTEM MAGNETO Left (Engine) High-Tension MAGNETO Right (Engine) High-Tension Self-contained generator Independent of aircraft electrical Self-contained generator Independent of aircraft electrical Ignition Leads Ignition Leads CYLINDER (Typical — 4 to 6 per engine) PISTON SP SP Top Plug Bottom Plug COMBUSTION CHAMBER Fuel-Air Mixture Inlet / Exhaust Outlet STARTING SYSTEM STARTER MOTOR Electric or Manual Provides initial rotation Mechanical Drive ENGINE CRANKSHAFT Rotates pistons to begin cycle Rotation Connecting Rod / Linkage DUAL IGNITION SYSTEM — KEY ADVANTAGES REDUNDANCY Two independent magnetos If one fails, engine runs on the other system EFFICIENCY Two spark plugs per cylinder More complete combustion Better fuel economy TIMING Precise spark timing Relative to piston position Optimized for all conditions System Components Ignition Path Starting Path Spark / Timing | TEA Training — Chapter: Ignition & Starting

Key Concepts Explained

Ignition System Fundamentals

High-Tension Magneto — Principle and Components High Tension Magneto — Principle and Components SCHEMATIC CROSS-SECTION N S Rotating Magnet (Rotor) Secondary Coil Primary Coil Breaker Points Capacitor Distributor (Rotor Arm) High Tension Lead Spark Plug IGNITION SEQUENCE: 1. Variable magnetic field (Rotor) 2. Primary break → Field collapse TRANSFORMATION: Mutual induction in secondary coil generates ~20,000 Volts. DISTRIBUTION: Distributor rotor routes HV to spark plug under compression.

The ignition system in a reciprocating aircraft engine must produce a spark of sufficient energy to ignite the fuel-air mixture, and it must do so at precisely the correct time relative to piston position. Modern aircraft engines typically use a dual ignition system with two independent magnetos, each supplying spark to one set of spark plugs per cylinder. This redundancy ensures continued operation if one system fails and improves combustion efficiency.

Magnetos

A magneto is a self-contained electrical generator that produces the high voltage required for spark ignition. It operates independently of the aircraft's electrical system, making it reliable even during electrical failure. The two main types are:

  • High-tension magnetos: Generate the high voltage directly within the magneto and deliver it to the spark plugs via ignition leads. This is the most common type on modern aircraft reciprocating engines.
  • Low-tension magnetos: Generate a lower voltage that is stepped up by a transformer near the spark plug. These are less common but offer advantages in certain high-altitude or high-compression applications.

Magneto Components and Operation

A magneto consists of:

  • Rotating magnet: A permanent magnet that rotates within the magneto housing.
  • Primary coil: A low-voltage winding (typically 200-300 turns of heavy wire).
  • Secondary coil: A high-voltage winding (typically 15,000-20,000 turns of fine wire) wound around the primary coil.
  • Breaker points: Mechanical switches that open and close the primary circuit.
  • Condenser (capacitor): Connected across the breaker points to absorb energy and prevent arcing.
  • Distributor: Routes the high-voltage pulse to the correct spark plug lead.

Operation sequence:

  1. As the rotating magnet turns, it induces a voltage in the primary coil.
  2. When the breaker points close, current flows through the primary coil, building a magnetic field.
  3. At the E-gap angle — the position where the magnetic flux in the coil is maximum — the points open.
  4. The sudden collapse of the magnetic field induces a high voltage in the secondary coil (typically 15,000-25,000 volts).
  5. This high voltage is directed by the distributor to the appropriate spark plug.

E-Gap Angle

E-Gap Angle and Breaker Point Timing E-gap angle and points opening Magnet position: Maximum flux (E-gap) Coil core Primary coil Secondary coil HV N S E-gap Max. flux Rotation Points opening at E-gap moment Primary circuit Primary coil L₁ fixed moving Breaker points Capaci- tor C₁ Ignition switch (grounding) OFF Ground At the E-gap angle, the magnetic flux is maximum. The points opening interrupts the primary current → rapid field collapse → high voltage induced in the secondary coil (spark at the spark plug). Magnetic flux lines High voltage (HV) Opening here = E-gap

The E-gap angle is a critical timing parameter. It is the rotational position of the magnet where the magnetic flux through the coil is at its peak. The breaker points must open precisely at this point to achieve the maximum rate of flux change, which produces the highest voltage spark. Incorrect E-gap adjustment reduces spark energy and can cause misfiring.

Impulse Coupling

Impulse Coupling — Retard and Snap Mechanism Impulse coupling: retard and strong spark 1. Starting position — retarded timing MAGNETO ROTOR slow rotation cam pawl spring (compressed) RETARDED TIMING Spark after TDC (avoids kickback) Starting speed: ~40-80 rpm 2. Triggering — rapid rotor rotation MAGNETO ROTOR RAPID rotation spring (released) STRONG SPARK High voltage generated at the E-gap moment 3. Centrifugal disengagement at idle weight weight centrifugal force spring pawl (retracted) At idle > 500 rpm The coupling disengages Operating sequence 1. Starting: pawl holds the cam → rotor blocked, spring compressed 2. Triggering: pawl releases the cam → spring spins the rotor abruptly 3. Idle: centrifugal force spreads the weights → pawl retracted, coupling inactive Note: The spark occurs at the E-gap moment during rapid rotation

An impulse coupling is a mechanical device attached to the magneto drive shaft. Its purposes are:

  • Retard timing for starting: During engine cranking, the impulse coupling holds the magneto rotor back, delaying the spark until the piston is near top dead center (TDC). This prevents kickback and makes starting easier.
  • Snap action for strong spark: A spring-loaded mechanism releases suddenly, spinning the magneto rotor at high speed even though the engine is turning slowly. This produces a hot, reliable spark during low-RPM starting.

Once the engine starts and reaches idle speed, a centrifugal mechanism disengages the impulse coupling, allowing normal magneto timing.

Ignition Switch Positions

Ignition Switch Positions — State Diagram Ignition Switch Positions (OFF, R, L, BOTH, START) OFF R L BOTH START Ignition Switch 5 positions — rotary LEFT MAGNETO Primary circuit L primary coil P P M L Ground RIGHT MAGNETO Primary circuit R primary coil P P M R Ground L SPARK Plugs L1–L4 ACTIVE R SPARK Plugs R1–R4 ACTIVE Circuit status OFF L: Ground R: Ground R L: Ground R: Active L L: Active R: Ground BOTH L: Active R: Active LEGEND Active circuit Grounded circuit Spark Magneto check: RPM drop indicates a retarded magneto

The ignition switch controls the grounding of the magneto primary circuits. Typical positions are:

  • OFF: Both magnetos are grounded to the engine. No spark can be produced, and the engine will not run.
  • RIGHT: Only the right magneto is active. The left magneto is grounded.
  • LEFT: Only the left magneto is active. The right magneto is grounded.
  • BOTH: Both magnetos are connected and operating normally. This is the standard position for engine operation, providing redundant spark and optimal combustion.
  • START: Engages the starter and may also provide a retarded spark (via impulse coupling or a separate starting vibrator) for easier starting.

Ignition Harness and Shielding

Spark Plugs — Inspection and Defects Spark Plugs — Inspection and Defects ANATOMY AND SPARK Insulator (Ceramic) Center Electrode Gap (Spacing) Ground Electrode COMMON INSPECTION DEFECTS Carbon Fouling Rich mixture. Bridged gap. (Short circuit). Cause: Misfire Oil Fouling Worn rings or valve guides. Wet appearance Worn Electrodes Rounded, eroded. Increases required voltage (misfire). Overheating White insulator, blistered. Lean mix or timing. INSTALLATION PROCEDURE Torque Spec: Always use a torque wrench. Follow manufacturer specs. ! WARNING: No anti-seize unless specified (alters torque).

The ignition harness consists of high-tension leads that carry the spark from the magneto distributor to each spark plug. These leads are shielded to contain electromagnetic interference (EMI). Without shielding, the high-voltage discharge would radiate radio frequency noise, disrupting aircraft communication and navigation systems. Shielding also provides protection against moisture and physical damage.

Spark Plugs

Spark plugs must withstand extreme temperatures (up to 1,500°F at the firing tip) and pressures (up to 1,000 psi). Key considerations:

  • Torque: Spark plugs must be installed to the manufacturer's specified torque, typically using a calibrated torque wrench. Over-torquing can crack the ceramic insulator or strip cylinder head threads. Under-torquing can cause poor heat transfer and plug overheating.
  • Anti-seize compound: May be recommended by the manufacturer for certain plug types or cylinder materials. Always follow the specific maintenance manual.
  • Gap: The spark plug gap must be set to specification. A bridged gap (carbon fouling) occurs when carbon deposits accumulate and connect the electrodes, causing a misfire. This is often due to rich mixture operation during low-power ground operations.
  • Fouling: Carbon or lead fouling can cause misfiring. Cleaning or replacement is required.

Magneto Timing

Magneto Timing — Internal, External, Early/Late Magneto Timing: Internal, External, Advance/Retard Internal Timing — E-gap Angle N S Coil Pts Cap E-gap Points open at max flux. (E-gap angle) → max voltage. Internal timing adjustment: 1. Rotate rotor to max flux point. 2. Set point gap (0.38–0.48 mm). 3. Align point opening with E-gap mark. 4. Incorrect adjustment → weak or irregular spark. Result: power loss, misfires. External Timing — Magneto vs Crankshaft Cyl. #1 25° BTDC TDC Magneto (mounted on engine case) drive spark External timing procedure: 1. Place piston #1 at 25° BTDC (compression). 2. Rotate magneto until points open. 3. Secure magneto in this position. 4. Verify operation at run-up (RPM drop). ADVANCED ignition (too early) Detonation, knocking, risk of engine damage RETARDED ignition (too late) Power loss, high EGT, overheating

Proper magneto timing ensures the spark occurs at the correct piston position for maximum power and efficiency. Two types of timing are involved:

Internal Timing

Internal timing sets the relationship between the rotating magnet and the breaker points. The points must open at the E-gap angle. This is typically set using timing marks on the magneto itself.

External Timing

External timing synchronizes the magneto's spark output with the engine's piston position. The procedure involves:

  1. Rotating the engine to the specified timing mark (e.g., 25° before TDC).
  2. Rotating the magneto drive until the breaker points just open.
  3. Securing the magneto in that position.

This ensures the spark occurs at the correct crankshaft angle. Late timing (spark occurring after the specified point) reduces power and increases exhaust gas temperatures because combustion continues into the exhaust stroke. Early timing can cause detonation and engine damage.

Magneto Check (Run-Up)

Magneto Run-Up Check — RPM Drop Interpretation Magnetos Check at Idle (RPM Drop) M-POWERPLANT ch3 — Alternating Engine — Ignition and Starting Tachometer — Engine Speed (RPM) 0 8 16 24 RPM × 100 Normal zone Idle zone (600-1000) Red zone (> 2700) Ignition Selector — Test Positions OFF L BOTH R START Test sequence: 1. BOTH (normal) 2. L (left test) 3. BOTH (return) 4. R (right test) 5. BOTH (return) Interpreting the RPM Drop Position RPM Drop Assessment BOTH (baseline) 0 Baseline speed L (left) 50-100 Normal R (right) 50-100 Normal L or R > 175-200 Excessive Retarded magneto → less power: A larger drop on one magneto indicates it is retarded relative to the other. Retarding the ignition reduces power output. Simplified Ignition Circuit Diagram Magneto Left (L) Magneto Right (R) Selector BOTH/L/R Engine (spark plugs) RPM Drop Thresholds (RPM) 50-100 RPM Normal > 175-200 RPM Excessive Max 50 RPM difference between L and R.

During engine run-up, the magneto check verifies proper operation of each magneto individually. The procedure:

  1. Run the engine at a specified RPM (typically 1,700-2,000 RPM).
  2. Switch from BOTH to LEFT — observe the RPM drop.
  3. Return to BOTH, then switch to RIGHT — observe the RPM drop.

Acceptable limits: A drop of 50-100 RPM is normal. A drop exceeding 175-200 RPM, or a difference of more than 50 RPM between magnetos, indicates a problem. A larger drop on one magneto suggests it is firing later (retarded) relative to the other, producing less power.

Starting Systems

Direct-Cranking Electric Starter

The most common starter on modern aircraft engines is the direct-cranking electric starter. It consists of:

  • A high-torque DC electric motor.
  • A solenoid or relay to engage the starter.
  • A drive mechanism (often a Bendix drive or similar) that engages the engine's starter ring gear.

When the ignition switch is turned to START, the starter motor engages and rotates the crankshaft. The impulse coupling provides retarded timing and a strong spark for ignition.

Inertia Starter

Inertia starters are found on older or large radial engines. They use a hand-crank or electric motor to spin a heavy flywheel to high speed. The stored kinetic energy is then engaged to the engine crankshaft via a clutch, providing a powerful rotation for starting.

Important Procedures and Regulations

Spark Plug Installation

  1. Clean the spark plug hole threads.
  2. Apply anti-seize compound if specified by the manufacturer.
  3. Start the plug by hand to avoid cross-threading.
  4. Torque to the specified value using a calibrated torque wrench.
  5. Connect the ignition lead securely.

Magno-to-Engine Timing Procedure

  1. Set the engine to the specified timing mark (e.g., 25° BTDC).
  2. Ensure the magneto is in the correct position (e.g., E-gap).
  3. Install the magneto and rotate its drive until the points just open (use a timing light or ohmmeter).
  4. Secure the magneto and verify timing with a timing light at cranking speed.

Ignition Harness Inspection

  • Check for worn, cracked, or chafed insulation.
  • Look for signs of arcing or corona discharge.
  • Ensure all connections are secure and clean.
  • Replace any lead with exposed conductor — arcing can cause misfire and ignite fuel vapors.

Common Relationships Between Concepts

  • Faulty condenser → Pitted points: A failed condenser cannot absorb the inductive kick when the points open, causing arcing that erodes the point surfaces.
  • Rich mixture → Carbon fouling: Excess fuel during low-power operation leaves carbon deposits that can bridge the spark plug gap.
  • Retarded timing → Power loss + high EGT: Late spark reduces the time for complete combustion, wasting energy in the exhaust and overheating the engine.
  • Impulse coupling → Retarded spark for starting: The coupling holds the magneto back, then snaps it forward, producing a strong, late spark that prevents kickback.
  • Shielded harness → EMI suppression: The metal braid around ignition leads contains the high-frequency noise generated by the spark discharge.
  • Larger RPM drop on one magneto → Retarded magneto: The magneto that fires later produces less power, causing a greater RPM drop when it is the only active magneto.

Practice this chapter

Reinforce Reciprocating Engine — Ignition & Starting with 16 Transport Canada–style practice questions, matched to your weak areas.

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