2.3 Ignition Timing, Position Sensors, & Control Modules

2.3 Ignition Timing, Position Sensors, & Control Modules

Quick Summary: Ignition timing determines the exact instant in crankshaft rotation that the spark plug arcs relative to Top Dead Center (TDC) of the compression stroke. Because an air/fuel mixture takes approximately 1.5 to 2.0 milliseconds to burn completely, the spark must fire before TDC so that peak cylinder combustion pressure occurs between 10° and 15° After Top Dead Center (ATDC). The PCM constantly recalculates optimal spark advance using inputs from position sensors, load sensors, temperature sensors, and knock sensors.

---

1. Flame Propagation & Peak Cylinder Pressure

Ignition is not an instantaneous explosion; it is a controlled flame front propagating outward from the spark plug gap.

• If the spark fires too late (retarded timing), peak pressure occurs far ATDC, resulting in sluggish acceleration, high exhaust gas temperatures, and poor fuel economy.
• If the spark fires too early (excessive advanced timing), combustion pressure peaks before TDC, fighting the rising piston. This causes extreme cylinder pressures, destructive heat spikes, and engine detonation (spark knock).

   PEAK COMBUSTION PRESSURE TIMING CURVE

   Cylinder
   Pressure
     ^                  Peak Pressure
     |                  (10° - 15° ATDC)
     |                     /\
     |                    /  \
     |     Spark Fires   /    \
     |     (25° BTDC)   /      \
     |         *       /        \
     |        / \     /          \
     +-------+---+---+------------+------> Crank Angle
            TDC BTDC 0° ATDC     10° ATDC

---

2. Engine Position Sensors: CKP vs. CMP

The PCM relies on two critical engine position sensors to synchronize primary coil firing:

1. Crankshaft Position (CKP) Sensor:
   • Measures crankshaft rotational speed (RPM) and exact crankshaft position.
   • Senses a multi-toothed target wheel (e.g., 36-1 or 60-2 reluctor wheel) mounted on the crankshaft or flywheel.
   • Primary Function: Determines base ignition timing and calculates engine misfire events based on instantaneous crankshaft acceleration.
2. Camshaft Position (CMP) Sensor:
   • Senses the rotation of the camshaft target wheel (rotating at 1/2 crank speed).
   • Primary Function: Identifies whether Cylinder #1 is on the compression stroke or exhaust stroke. This allows the PCM to synchronize sequential fuel injection and single-coil COP firing order.

---

3. Position Sensor Technologies & Testing

Technicians encounter three main sensor designs on modern ASE A8 certification tests:

---

4. Electronic Ignition Timing Modifiers

The PCM calculates total spark advance using the following core formula:

$\text{Total Spark Advance} = \text{Base Timing} + \text{Engine Speed Advance} + \text{Engine Load Advance} \pm \text{Correction Factors}$

Key Input Variables

• Engine Load (MAP / MAF): Under high engine load (wide-open throttle), cylinder density is high, causing faster flame propagation. The PCM retards timing slightly to prevent detonation. Under light engine load (cruising vacuum), cylinder density is low and flame burn is slow, requiring the PCM to advance timing.
• Engine Coolant Temperature (ECT): During cold engine start, fuel vaporization is poor. The PCM advances timing to ensure complete combustion.
• Intake Air Temperature (IAT): High intake air temperatures increase the risk of auto-ignition. The PCM retards timing if IAT exceeds calibrated thresholds (e.g., above 115°F / 46°C).
• Knock Sensor (KS): Serves as the ultimate engine protection feedback loop.

---

5. Spark Knock (Detonation) vs. Pre-Ignition & Knock Sensor Operation

Understanding abnormal combustion is vital for A8 diagnosis:

• Pre-Ignition: The air/fuel mixture ignites before the spark plug fires, caused by glowing carbon deposits, overheated spark plug electrodes (wrong heat range), or sharp hot spots in the combustion chamber. Pre-ignition damages pistons rapidly (melted piston crowns, burned valves).
• Detonation (Spark Knock): The spark plug fires normally, initiating a flame front. However, unburned end-gases ahead of the flame front reach critical temperature and pressure, auto-igniting spontaneously. This produces colliding pressure waves that ring against cylinder walls at 5.0 to 8.0 kHz.

+-------------------------------------------------------------------+
|                     KNOCK SENSOR CONTROL LOOP                     |
|                                                                   |
|  [ Piezoelectric KS ] ---> [ Detects 5-8 kHz ] ---> [ Sends AC    |
|  [ Mounted on Block ]      [ Vibration Noise ]      [ Voltage ]   |
|                                                          |        |
|                                                          v        |
|  [ Restores Advance ] <--- [ Retards Timing  ] <--- [ PCM Recvs   |
|  [ Gradual Step-Up  ]      [ 1° - 3° Steps   ]      [ Knock Signal|
+-------------------------------------------------------------------+

Knock Sensor Construction & Feedback Logic

The knock sensor contains a piezoelectric crystal element weighted by an internal mass. When engine detonation occurs, the 5–8 kHz vibration causes the crystal to flex, outputting a small AC voltage signal to the PCM.

• Upon receiving the knock signal, the PCM instantly retards ignition timing in calibrated increments (e.g., 1° to 3° per knock event, up to 15° max retard).
• Once knock ceases, the PCM slowly re-advances ignition timing back toward the optimal timing limit.

---

6. Primary Driver Triggering & Ignition Feedback (IGT / IGF)

In modern Asian and European control systems, primary ignition relies on two distinct communication lines between the PCM and the ignition module or COP internal driver:

1. IGT (Ignition Trigger Signal):
   • A 0-to-5 volt digital square wave sent from the PCM to the ignition module driver.
   • Rising Edge (0V to 5V): Commands the internal driver transistor to turn ON, beginning primary coil dwell charging.
   • Falling Edge (5V to 0V): Turns the driver transistor OFF, collapsing the primary field and triggering secondary spark.
2. IGF (Ignition Feedback Signal):
   • A pulse returned from the ignition driver back to the PCM every time the primary winding successfully discharges.
   • Generated by sensing primary voltage spike (CEMF) or primary current drop inside the module.

Critical Diagnostic Rule: If the PCM outputs an IGT command but fails to receive an IGF feedback signal (due to an open IGF wire or dead coil driver), the PCM sets a Diagnostic Trouble Code (such as P1300 / P1305) and disables the fuel injectors for that cylinder within 2 to 3 seconds to prevent unburned raw fuel from overheating and destroying the catalytic converter.