Ignition Timing: Description and Operation
Ignition when Starting Engine
When starting the engine, at engine speeds up to approximately 600 rpm, the ignition timing is controlled via the trailing edge of the segments on the flexplate and the CKP sensor (L5). When an engine speed of approximately 800 rpm is reached, a transition from fixed ignition timing to one calculated for momentary operating conditions occurs.
The engine control module processes the following information for this purpose:
- Engine coolant temperature (ECT),
- Engine speed/crankshaft position/ignition circuit recognition,
- Camshaft position, TDC cylinder no.1
Acronyms
CKP = CranKshaft Position
Synchronization of Ignition Sequence
To properly coordinate the ignition timing for each respective cylinder, the ignition firing sequence is synchronized during the engine's first revolutions. The engine control module (N3/4) utilizes the CMP sensor (L5/1) signal to recognize ignition TDC of cylinder 1, and the CKP sensor (L5) (magnet-segment) to recognize ignition circuit 2 (cylinder 2 and 3).
Ignition During Engine Warm-Up
During the warm-up period, the ignition timing is corrected according to engine coolant temperature and engine load.
The engine control module processes the following information for this purpose:
- Engine coolant temperature (ECT),
- Engine speed/crankshaft position/ignition circuit recognition,
- Air mass.
Ignition at Closed Throttle Position (CTP)
With the CTP switch contact closed, two fixed ignition maps are transmitted dependent on the engine coolant temperature (warm or cold). The engine control module processes the following information for this purpose:
- Engine coolant temperature (ECT),
- Engine speed/crankshaft position/ignition circuit recognition,
- Closed throttle position (CTP) recognition.
Acronyms
CMP = Camshaft Position
CKP = Crankshaft Position
Ignition at Wide Open Throttle (WOT)
The engine control module recognizes WOT (full load) above a preset throttle valve angle. The correction angle in direction "advance" from warm-up and heating function are taken into account.
The engine control module processes the following information for this purpose:
- Engine coolant temperature (ECT),
- Engine speed,
- Throttle valve position,
- Air mass.
Ignition During Deceleration Shut-Off
Upon resumption of fuel injection, the ignition timing is retarded by a maximum of 15 °, for a maximum of 15 firings, to avoid a sudden surge in engine torque.
The engine control module processes the following information for this purpose:
- Engine coolant temperature (ECT),
- Engine speed/crankshaft position/ignition circuit recognition,
- Engine operating condition.
Intake Air Temperature (IAT) Correction
The ignition timing is retarded according to IAT and load (engine speed and air mass) by the engine control module.
Ignition timing retard occurs only under high load and begins at an IAT of 35 °C to a maximum of 65 °C.
Example: 35 °C --> 2.5 ° CKA retarded
45 °C --> 3.8 ° CKA retarded
65 °C --> 7.0 ° CKA retarded
(Values depending on full load rpm)
The engine control module processes the following information for this purpose:
- Intake air temperature (IAT),
- Engine speed/crankshaft position/ignition circuit recognition,
- Throttle valve position,
- Air mass.
Knock Sensor System (KSS)
The ignition maps of the HFM-SFI are designed for optimal power. If under certain operating conditions, combustion knock occurs, the KSS control integrated into the engine control module identifies the knocking cylinder and retards the ignition timing.
If, for example, the knocking is due to fuel with a low octane rating, the mechanical oscillations produced in the knock sensor are converted to electrical signals and are sent to the engine control module (N3/4).
The engine control module processes the following information for this purpose:
- Knock sensor (KS) signals,
- Camshaft position, TDC cylinder no.1,
- Engine speed/crankshaft position/ignition circuit recognition,
- Engine coolant temperature (ECT),
- Intake air temperature (IAT).
The engine control module compares these incoming signals with nominal values stored in memory.
If discrepancies exist, then the ignition timing of the knocking cylinder is retarded 3° CKA (Crank Angle) at the next ignition cycle. If the cylinder continues to knock, the ignition timing is retarded an additional 3° CKA. The ignition will continue to retard under knock conditions until an ECT dependent maximum ignition retard adjustment is reached (e.g.: 10° CKA at 80-90 °C ECT). If the combustion knock ceases, then the ignition point of the affected cylinder is advanced in steps of 0.35° CKA until the nominal value is once again attained.
Should one of the following parts fail, the ignition timing is retarded for all cylinders depending on the engine coolant temperature:
- Knock sensor (KS),
- Knock sensor evaluation of KSS control integrated in engine control module,
- Camshaft position (CMP) sensor (L5/1).
Three-Way Catalytic Converter (TWC) Warm-up
In order to bring the TWC to its operating temperature quicker, the exhaust temperature is increased.
If the engine is started at an ECT between approximately + 15 °C and +40 °C, the ignition timing is continuously retarded up to 8° for approximately 30 seconds and the idle speed is increased to 1150 ± 100 rpm by means of the ISC. The idle speed increase will be cancelled if a transmission range is selected . At partial throttle operation, the ignition correction angle is determined by a map according to ECT and load.
The engine control module processes the following information for this purpose:
- Engine coolant temperature (ECT),
- Engine speed/crankshaft position/ignition circuit recognition,
- Air mass,
- Ignition spark count,
- Idle speed recognition,
- Transmission range (TR).
Overheating or Engine Knocking Protection
If the ECT becomes too high, the ignition timing is retarded to protect against overheating and engine knocking.
The engine knocking protection begins at 100 °C ECT and is effective only under load.
Ignition timing retard varies according to engine coolant temperature:
Example: 100 °C --> 0° CKA retarded
105 °C --> 1.4° CKA retarded
120 °C --> 3.2° CKA retarded
(The correction is dependent on engine load, values are valid for full load).
The engine control module processes the following information for this purpose:
- Engine speed/crankshaft position/ignition circuit recognition,
- Air mass,
- Engine coolant temperature (ECT).
IMPORTANT NOTE: The engine knocking protection and IAT correction values are added together, e.g. 120 °C ECT and 50 °C IAT result in a maximum ignition retard of 11° CKA at full load.
Ignition System Assist of Idle Speed Control (ISC)
If a variation of the idle speed is present, the ignition timing is changed to assist the ISC. The ignition timing can be advanced or retarded up to 8°.
Fuel Shut-Off Due to Ignition Failure
To protect the TWC from overheating, the injector of the affected cylinder or those of all cylinders will be switched off in case of ignition failures (50 consecutive ignitions).
If the ignition fault is no longer present, the fuel is switched on again if:
- 255 consecutive ignitions were successful,
- Vehicle was in deceleration mode,
- Engine speed of approximately 2500 rpm has been exceeded.
The following faults are recognized by the engine control module via primary current monitoring:
- Ignition final stage in engine control module defective,
- Ignition coil defective or open circuit,
- Short circuit (also on high voltage side),
- Defective spark plug(s).
The engine control module processes the following information for this purpose:
- Primary voltage (combustion voltage, combustion duration),
- Camshaft position, TDC cylinder no.1,
- Engine speed/ignition circuit recognition.