LEARNING OUTCOME 2

Ignition System

The ignition system's primary function is to generate a high-voltage spark at the spark plugs, igniting the air-fuel mixture in the engine's cylinders at precisely the right moment. This ensures efficient combustion and optimal engine performance.

Conventional Ignition System

Construction:

• Battery: Provides the initial low-voltage power.
• Ignition Switch: Controls the flow of power to the ignition system.
• Ignition Coil: A transformer that steps up the battery voltage to a high voltage (typically 12,000 to 45,000 volts).
• Distributor (Older Systems): Contains a mechanical breaker point assembly (or electronic pickup) and a rotor. It distributes the high voltage spark to the correct spark plug at the correct time.
• Spark Plugs: Create the spark gap in the combustion chamber.
• Wiring: Connects all the components.

Operation:

• Primary Circuit: When the ignition switch is turned on, battery voltage flows through the primary winding of the ignition coil.
• Breaker Points (or Electronic Trigger): In older systems, the distributor's breaker points open and close, interrupting the primary circuit. In electronic systems, a trigger mechanism (inductive, Hall effect, or optical) performs this function.
• Induction: When the primary circuit is interrupted, the magnetic field in the coil collapses.
• Secondary Circuit: The collapsing magnetic field induces a high voltage in the secondary winding of the coil.
• Distributor (or Direct Ignition): The distributor's rotor directs the high voltage spark to the appropriate spark plug. In modern direct ignition systems, each spark plug has its own coil.
• Spark Plug Ignition: The high-voltage spark jumps the gap between the spark plug electrodes, igniting the air-fuel mixture.

Ballast Resistor:

• Purpose: A ballast resistor is used to limit the current flowing through the primary winding of the ignition coil.
• Operation (Without Ballast Resistor): In a system without a ballast resistor, the full battery voltage is applied to the primary winding. This can cause the coil to overheat and fail.
• Operation (With Ballast Resistor): The ballast resistor reduces the voltage applied to the coil's primary winding during normal operation. During engine cranking, the ballast resistor is bypassed, providing a higher voltage to the coil for a stronger spark. This is required because the battery voltage drops during cranking.
• Bypassing the ballast resistor during cranking: This is often done by a connection from the starter solenoid that provides full battery voltage to the coil during engine cranking.

Triggering Methods

Inductive Type (Magnetic Pickup):

• Construction: Consists of a reluctor (a toothed wheel) mounted on the distributor shaft and a magnetic pickup coil.
• Operation: As the reluctor teeth pass the pickup coil, they induce a voltage pulse in the coil. The pulses are then used to trigger the ignition module, which controls the primary circuit of the ignition coil.
• Explanation: The reluctor teeth alter the magnetic field of the pickup coil as they pass. This change in magnetic flux induces a voltage in the coil. The frequency of the pulses is proportional to the engine speed.

Hall Generator (Hall Effect Sensor):

• Construction: Consists of a rotating trigger wheel with slots or vanes and a Hall effect sensor.
• Operation: The Hall effect sensor generates a voltage when a magnetic field is applied perpendicular to its current flow. The trigger wheel interrupts the magnetic field as it rotates, creating voltage pulses. These pulses are used to trigger the ignition module.
• Explanation: The Hall effect sensor uses a semiconductor material that generates a voltage when placed in a magnetic field. When the trigger wheel rotates, the slots or vanes interrupt the magnetic field, causing the sensor to produce voltage pulses. These pulses are very precise.

Optical Pulse (Optical Sensor):

• Construction: Consists of a rotating trigger wheel with slots or holes, an LED (light-emitting diode), and a phototransistor.
• Operation: The LED emits a light beam that is interrupted by the slots or holes in the trigger wheel. The phototransistor detects the light pulses and converts them into electrical pulses. These pulses are used to trigger the ignition module.
• Explanation: The optical sensor uses light to detect the position of the trigger wheel. The LED emits a light beam, and the phototransistor detects the presence or absence of light. As the trigger wheel rotates, the slots or holes interrupt the light beam, creating pulses. These pulses are very accurate and immune to electromagnetic interference.

Electronic Ignition Systems

Distributor Type Electronic Ignition:

• Construction:
  • Retains a distributor for spark distribution.
  • Uses an electronic trigger (inductive, Hall effect, or optical) instead of mechanical breaker points.
  • An ignition module controls the primary circuit of the coil.
  • High voltage coil.
• Layout:
  • The electronic trigger is located inside the distributor.
  • The ignition module is typically mounted on the distributor or elsewhere in the engine compartment.
  • The coil is connected to the distributor and the ignition module.
• Operation:
  • The electronic trigger generates pulses based on the distributor shaft's rotation.
  • The ignition module receives these pulses and controls the primary circuit of the ignition coil.
  • When the primary circuit is interrupted, the coil produces a high voltage spark.
  • The distributor rotor distributes the spark to the appropriate spark plug.
• Explanation: This system eliminates the wear and maintenance associated with mechanical breaker points, providing more reliable and consistent spark timing.

Distributorless Ignition System (DIS):

• Construction:
  • Eliminates the distributor.
  • Uses a crankshaft position sensor and a camshaft position sensor to determine engine position.
  • Uses a coil pack with multiple coils, each serving two cylinders.
  • An engine control unit (ECU) controls the ignition timing.
• Layout:
  • Sensors are located on the crankshaft and camshaft.
  • The coil pack is mounted on the engine.
  • The ECU is located in the engine compartment or under the dashboard.
• Operation:
  • The sensors provide engine position information to the ECU.
  • The ECU calculates the optimal ignition timing and triggers the appropriate coils in the coil pack.
  • Each coil fires two spark plugs simultaneously, one during the compression stroke and the other during the exhaust stroke (wasted spark system).
• Explanation: DIS systems offer improved ignition timing accuracy and reliability compared to distributor-type systems.

Direct Ignition System (DIS or Coil-on-Plug):

• Construction:
  • Eliminates the distributor and coil pack.
  • Uses individual ignition coils mounted directly on each spark plug.
  • An ECU controls the ignition timing.
  • Crankshaft and camshaft sensors.
• Layout:
  • Sensors are located on the crankshaft and camshaft.
  • Coils are mounted directly on the spark plugs.
  • The ECU is located in the engine compartment or under the dashboard.
• Operation:
  • The sensors provide engine position information to the ECU.
  • The ECU calculates the optimal ignition timing and triggers each coil independently.
  • Each coil fires only one spark plug.
• Explanation: Direct ignition systems provide the most precise ignition timing and are highly reliable. They eliminate high voltage wires, which reduces voltage loss.

Capacitor Discharge Ignition (CDI):

• Construction:
  • Uses a capacitor to store electrical energy.
  • Uses a trigger mechanism (inductive, Hall effect, or optical) to control the discharge of the capacitor.
  • Uses a step-up transformer to produce a high-voltage spark.
• Layout:
  • The capacitor, trigger mechanism, and transformer are typically housed in a single unit.
• Operation:
  • The capacitor is charged by a charging circuit.
  • When the trigger mechanism activates, the capacitor discharges rapidly through the primary winding of the transformer.
  • The transformer steps up the voltage, producing a high-voltage spark.
• Explanation: CDI systems produce a very fast rise time spark, which is beneficial for high-RPM engines. They are commonly used in motorcycles and small engines.

Energy Transfer Ignition:

• Construction:
  • Uses two coils: a primary coil and a secondary coil.
  • The primary coil stores energy, and the secondary coil transfers it to the spark plug.
  • Triggering mechanism.
• Layout:
  • The coils and trigger mechanism are typically housed in a single unit.
• Operation:
  • The primary coil stores energy when the primary circuit is closed.
  • When the primary circuit is opened, the energy is transferred to the secondary coil.
  • The secondary coil produces a high-voltage spark that is delivered to the spark plug.
• Explanation: Energy transfer ignition systems provide a high-energy spark and are less sensitive to spark plug fouling. They are used in some automotive and industrial applications.

Spark Plugs:

• Spark plugs are critical components of an internal combustion engine's ignition system.
• Their primary function is to ignite the compressed air-fuel mixture within the engine's cylinders, initiating the combustion process.
• They achieve this by generating a high-voltage spark across a gap between two electrodes.
• Beyond ignition, spark plugs also play a role in heat dissipation from the combustion chamber.

Cold vs. Hot Spark Plugs:

Short-Reach vs. Long-Reach Spark Plugs:

Reasons for Differences:

• Heat Range (Cold/Hot): The heat range of a spark plug is determined by its ability to dissipate heat. Cold plugs dissipate heat quickly, preventing pre-ignition in high-performance engines. Hot plugs retain more heat, burning off deposits and preventing fouling in standard engines.
• Reach (Short/Long): The reach of a spark plug is determined by the length of its threaded section. This difference is dictated by the design of the cylinder head. Using the correct reach ensures that the firing tip is positioned correctly within the combustion chamber for optimal ignition.

Spark Plug Condition and Fault Diagnosis

Normal Appearance:

• Appearance: Light tan or gray deposits on the insulator tip and electrodes.
• Interpretation: Indicates proper air-fuel mixture, ignition timing, and engine operation.

Carbon Fouling (Black, Dry Deposits):

• Appearance: Dry, black, sooty deposits on the insulator tip and electrodes.
• Interpretation:
  • Rich air-fuel mixture.
  • Weak spark.
  • Faulty choke.
  • Restricted air filter.
  • Short trips (not allowing the engine to reach operating temperature).
  • Weak compression.
  • Late ignition timing.

Oil Fouling (Wet, Black Deposits):

• Appearance: Wet, oily deposits on the insulator tip and electrodes.
• Interpretation:
  • Worn piston rings or cylinder walls.
  • Faulty valve stem seals.
  • Overfilled crankcase.
  • PCV system problems.

Lead Fouling (Tan or Yellowish-Brown Deposits):

• Appearance: Tan or yellowish-brown deposits on the insulator tip and electrodes.
• Interpretation:
  • Use of leaded gasoline (not common in modern vehicles).
  • Additives in some fuels.

Ash Deposits (White or Gray Deposits):

• Appearance: White or gray, powdery deposits on the insulator tip and electrodes.
• Interpretation:
  • Oil or fuel additives.
  • Coolant leaks into the combustion chamber.

Gap Bridging (Deposits Bridging the Gap):

• Appearance: Deposits bridging the gap between the electrodes.
• Interpretation:
  • Excessive deposits from oil or fuel.
  • Worn spark plugs.

Electrode Wear (Rounded or Worn Electrodes):

• Appearance: Rounded or excessively worn electrodes.
• Interpretation:
  • Normal wear and tear.
  • Extended spark plug service interval.

Damaged Insulator (Cracked or Broken Insulator):

• Appearance: Cracked or broken insulator.
• Interpretation:
  • Physical damage during installation.
  • Thermal shock.
  • Detonation.

Pre-Ignition/Detonation Damage (Melted or Damaged Electrodes):

• Appearance: Melted or severely damaged electrodes, often with signs of pitting or erosion.
• Interpretation:
  • Overly advanced ignition timing.
  • Lean air-fuel mixture.
  • Overheating.
  • Incorrect spark plug heat range (too hot).
  • Low octane fuel.
  • Carbon deposits causing hot spots.

Wet Plug:

• Appearance: The Plug is wet with fuel.
• Interpretation:
  • Flooded engine.
  • No spark.
  • Faulty fuel injector.

Diagnostic Tips:

• Examine all spark plugs to get a comprehensive view of engine condition.
• Compare the condition of each spark plug to a spark plug condition chart.
• Consider the engine's operating conditions and recent maintenance history.
• Use a compression test and other diagnostic tools to confirm findings.
• When removing spark plugs, keep them organized, so you know from which cylinder they came.