Ignition System: Description and Operation

Electronic Ignition System Description

The electronic ignition (EI) system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy, and control of exhaust emissions. This ignition system uses an ignition coil pack assembly. The ignition coil pack is mounted above the cylinders between the camshafts with short integrated boots connecting the coils to the spark plugs. The driver modules within each ignition coil are commanded ON/OFF by the engine control module (ECM). The ECM primarily uses information from the crankshaft position (CKP) sensor and the camshaft position (CMP) sensors to control sequence, dwell, and timing of the spark. The EI system consists of the following components:

Crankshaft Position (CKP) Sensor

The crankshaft position sensor circuits consist of an engine control module (ECM) supplied 5-volt reference circuit, low reference circuit, and an output signal circuit. The crankshaft position sensor is an externally magnetically biased digital output integrated circuit sensing device. The sensor provides a pulse for each magnetic pole of the magnetic encoder wheel on the crankshaft. Each pole on the encoder wheel is spaced at 60-pole spacing, with 2 missing poles for the reference gap. The crankshaft position sensor produces an ON/OFF DC voltage of varying frequency, with 58 output pulses per crankshaft revolution. The frequency of the crankshaft position sensor output depends on the velocity of the crankshaft. The crankshaft position sensor sends a digital signal, which represents an image of the crankshaft encoder wheel, to the ECM as each pole on the wheel rotates past the crankshaft position sensor. The ECM uses each crankshaft position sensor signal pulse to determine crankshaft speed and decodes the crankshaft encoder wheel reference gap to identify crankshaft position. This information is then used to determine the optimal ignition and injection points of the engine. The ECM also uses crankshaft position sensor output information to determine the camshaft position sensor - intake and camshaft position sensor - exhaust relative to the crankshaft, to control camshaft phasing, and to detect cylinder misfire.

Camshaft Position (CMP) Sensor

The CMP sensors are triggered by a notched reluctor wheels which is part of the intake and the exhaust camshafts. Both of the CMP sensors provide four signal pulses every camshaft revolution. The CMP sensor information is used by the ECM to determine the position of the valve train relative to the crankshaft position. By monitoring the CMP and CKP signals, the ECM can accurately trigger the fuel injectors.

Ignition Coils

The ignition coil pack directly supplies voltage to each spark plug. Each ignition coil (IC) contains a solid state driver module as its primary element. The ECM will command the IC circuit ON, this allows the current to flow through the primary coil windings for the appropriate time or dwell. When the ECM commands the IC circuit OFF, this will interrupt current flow through the primary coil windings. The magnetic field created by the primary coil windings will collapse across the secondary coil windings, which induces a high voltage. The secondary coil voltage travels from the coil output terminal, through the spark plug boot, and across the spark plug gap to the engine block. The ignition coil pack is not serviceable and must be replaced as an assembly.

Engine control module (ECM)

The engine control module (ECM) is responsible for maintaining proper spark and fuel injection timing for all driving conditions. The ignition control (IC) is the method the ECM uses to control spark advance. The primary coil ON/OFF is directly controlled by the ECM. To provide optimum driveability and emissions, the ECM monitors input signals from the following components in calculating ignition spark timing:

* The crankshaft position (CKP) sensor
* The engine coolant temperature (ECT) sensor
* The manifold absolute pressure (MAP) sensor
* The intake air temperature (IAT) sensor
* The vehicle speed sensor (VSS)
* Ambient pressure sensor (BARO)
* The knock sensor (KS)
* The vehicle speed sensor (VSS)
* The mass airflow (MAF) sensor
* The camshaft position (CMP) sensors