Ignition System: Description and Operation
Function Diagram:
The ignition portion of the HFM-SFI system is essentially identical to the known EZL distributor ignition system (EZL-DI) with a notable exception. The new electronic ignition system no longer distributes high-voltage by mechanical rotation via a distributor, but utilizes three twin circuit ignition coils to distribute high voltage in a static state.
Ignition Coil:
Ignition Coils
The ignition coils (T1) have two high-voltage connections (4a, 4b). Each connection supplies one spark plug with high-voltage.
The engine control module limits the primary current to approximately 7 A. The maximum ignition voltage is approximately 32 kV.
WARNING: The primary connections deliver voltage up to 400 v. Guide sleeve (w) must always be connected to vehicle ground.
Ignition Coil, Cross Section View:
Design and Function
The ignition coil is constructed much like a transformer, where the primary and secondary windings are physically separated from one another. Both ends of the secondary winding are equipped with a high-voltage connection which form, together with two spark plugs, a closed circuit. The secondary winding produces high-voltage during ignition.
The ignition spark occurs at both spark plugs at the same time, whereby one spark jumps from the center to the ground electrode and the other from the ground to the center electrode. As a result, constant positive and negative high-voltage to ground is present at the secondary connections. The ignition spark on the exhaust stroke requires very little energy, so that nearly all the energy stored in the coil is available for the power stroke.
Ignition Coil, Circuit Diagram:
Since the secondary winding is switched in series with both spark plugs, it has the effect of an open circuit (e.g.: on the spark plug connector) on both ignition sparks.
The resistance of the secondary winding must be checked between both high-voltage connections:
Nominal value: 5.2 - 8.5 kohms
Resistance of primary winding between connection 1 and 15:
Nominal value: 0.3 - 0.4 Ohms
Advantages of static high-voltage distribution:
- Reduced electromagnetic noise level (no open sparks),
- No rotating parts,
- Noise reduction,
- Fewer high-voltage connections.
Ignition Coils (T1/1, T1/2, T1/3):
Location of Ignition Coils and Distribution of High-voltage
The ignition coils are located on the valve cover. Each ignition coil provides high-voltage simultaneously to two spark plugs.
Ignition coil Cylinders
T1/1 2 and 5
T1/2 3 and 4
T1/3 1 and 6
Ignition Firing Chart:
The engine control module (N3/4) controls three individual ignition coils. Each coil is connected to battery voltage and the engine control module switches each coil to ground. The high-voltage output of each coil is sent simultaneously to two spark plugs, each assigned to a different cylinder. When fired, one cylinder is in its power stroke and the other is in its exhaust stroke, i.e. 360 crank angle (CKA) apart. The cylinder which is in its exhaust stroke has no combustible mixture, therefore the spark has no effect. One crankshaft rotation later, the cylinders reverse their roles.
The other two ignition coils operate in the same manner, however ignition firing is offset by 120 crank angle (CKA).
Ignition Coil:
The ignition coils (T1/1, T1/2, T1/3) are connected, via the secondary output (4a), directly to a spark plug with a spark plug connector. The secondary output (4b) travels over a ignition wire to the other cylinder assigned to that coil. The guide sleeve (W) also serves as the ground connection for the ignition coil.
Ignition When Starting Engine
Upon 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 crankshaft position sensor (L5). When an engine speed of approximately 800 rpm is reached, 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,
- Engine speed/crankshaft position/ignition circuit recognition,
- Camshaft position.
Multi-point Ignition
To improve engine starting at coolant temperatures below 0°C (32°F), the ignition is fired repeatedly in succession at the ignition point, up to 10° TDC and an engine speed of 600 rpm.
Synchronization of Ignition Sequence
To properly coordinate the ignition timing for each respective cylinder, the ignition firing sequence is synchronized during the engine first revolution. The engine control module (N3/4) utilizes the camshaft position sensor (L5/1) signal to recognize ignition TDC of cylinder 1, and the crankshaft position sensor (L5) to recognize ignition circuit 1 (cylinders 2 and 5).
Ignition During Engine Warm-up
The ignition timing is corrected according to engine coolant temperature and engine load in order to obtain engine operating temperature as quickly as possible.
The engine control module processes the following information for this purpose:
- Engine coolant temperature,
- Engine speed/crankshaft position/ignition circuit recognition,
- Air mass.
Ignition at Idle
With the closed throttle position (CTP) switch closed, four engine coolant temperature-dependent fixed ignition maps are transmitted.
The engine control module processes the following information for this purpose:
- Engine coolant temperature,
- Engine speed/crankshaft position/ignition circuit recognition,
- Closed throttle (idle) recognition (via CAN).
Ignition at Wide Open Throttle (WOT)
The engine control module recognizes WOT (full load) above a preset throttle valve angle. Advanced ignition timing for warm-up and heating functions is recognized when calculating ignition timing at WOT.
The engine control module processes the following information for this purpose:
- Engine coolant temperature,
- Engine speed,
- Throttle valve position (via CAN),
- Air mass.
Ignition During Deceleration Shut-off
Upon resumption of fuel injection, the ignition timing is retarded 3° for 0.5 seconds to avoid a sudden surge in engine torque.
The engine control module processes the following information for this purpose:
- Engine coolant temperature,
- Engine speed/crankshaft position/ignition circuit recognition,
- Engine operating condition (deceleration shut-off recognition).
Intake Air Temperature Correction
The ignition timing is retarded according to intake air temperature and load (engine speed and air mass).
Ignition timing retard occurs only under high load and begins at an intake air temperature of 30°C (86°F) to a maximum of 70°C (158°F).
Example: 30°C (86°F) -> 1.5° CKA retarded
50°C (122°F) -> 2.0° CKA retarded
70°C (158°F) -> 3.5° CKA retarded
CKA = Crankshaft Angle
The engine control module processes the following information for this purpose:
- Intake air temperature,
- Engine speed;crankshaft position/ignition circuit recognition,
- Throttle valve position (via CAN),
- Air mass.
Anti-knock Control
The ignition maps are designed for optimal power. If under certain operating conditions, combustion knock occurs, the anti-knock 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 signal,
- Camshaft position TDC cylinder 1,
- Engine speed/crankshaft position/ignition circuit recognition,
- Engine coolant temperature,
- Intake air temperature.
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 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 engine coolant temperature dependent maximum ignition retard adjustment is reached (e.g.: 12° CKA at 80 to 90°C (176 to 194°F) engine coolant temperature).
If the combustion knock ceases, then the ignition point of the affected cylinder is advanced 0.35° CKA per ignition cycle until the nominal value is once again obtained. Ignition timing retard based on engine coolant temperature takes place as a precaution if one of the following components fails:
- Knock sensors,
- Knock sensor evaluation of anti-knock control integrated in engine control module,
- Camshaft position sensor (L5/1).
Catalyst Warm-up
In order to bring the catalyst to its operating temperature quicker, the exhaust temperature is increased.
If the engine is started at engine coolant temperatures between 15 and 40 °C (59 to 104°F), the ignition timing is continuously retarded for approximately 30 seconds with the engine at idle and the transmission in range "P" or "N"; the idle speed is increased to 1150 ± 100 rpm by means of the idle speed control. The idle speed increase will be canceled if the transmission is engaged in gear. In partial throttle, the ignition correction angle is determined by a map according to engine coolant temperature and load.
The engine control module processes the following information for this purpose:
- Engine coolant temperature,
- Engine speed/crankshaft position/ignition circuit recognition,
- Air mass,
- Ignition spark count,
- Idle speed recognition (via CAN),
- Transmission range (via CAN).
Overheat Protection
If the engine temperature becomes too high, the ignition timing is retarded to prevent damage from overheating.
The overheat protection begins at 100°C (212°F) engine coolant temperature and is effective only under load.
Ignition timing retard varies according to engine coolant temperature:
Example: 100°C (212°F) -> 1.5° CKA retard,
110°C (230°F) -> 3° CKA retard,
120°C (248°F) -> 4° CKA retard.
The engine control module processes the following information for this purpose:
- Engine speed/crankshaft position/ignition circuit recognition,
- Air mass,
- Engine coolant temperature.
The overheat protection and intake air temperature correction values are cumulative, i.e. they are added to one another.
E.g.: 120°C (248°F) engine coolant temperature and 50°C (248°F) intake air temperature results in a maximum ignition retard of 6° CKA.
Fuel Shut-off Due to Ignition Failure
To protect the catalyst from overheating, the injector of the affected cylinder or those of all cylinders will be switched off in cases of ignition failure.
The following failures are recognized by the engine control module (N3/4) via primary current monitoring:
- Ignition final stage in engine control module defective,
- Ignition coil defective or open circuit,
- Short circuit (also on the 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 1,
- Engine speed/crankshaft position/ignition circuit recognition.
Idle Speed Control (ISC)
Idle speed control is performed by the electronic accelerator control module (N4/1) or cruise control/idle speed control module (N4/3). The data required for idle speed control is transmitted from the engine control module (N3/4) to the respective control module (N4/1 or N4/3) via CAN.
Transmission Overload Protection
To protect the transmission shift mechanism from thermal overload during load shifts in the upper engine speed range, transmission overload protection is incorporated into the engine control module.
The engine control module processes the following information for this purpose:
- Transmission overload protection switch (S65) signal,
- Engine speed,
- Air mass.
During the 1-2 and 2-3 upshift, the transmission overload protection retards ignition timing 5° CKA before TDC for 400 ms (reducing engine torque).
Since the ignition timing retard also improves shift quality, this measure is also used during WOT (full load) 3-2 downshift. WOT = Wide Open Throttle
Ignition timing retard occurs if the following conditions are met simultaneously:
- Engine speed greater than 4000 rpm,
- Vacuum in intake manifold below 300 mbar,
- Shift signal received from transmission overload protection switch (S65).
Fixed Operating Mode, Transmission Overload Protection
If the engine control module does not receive a shift signal from the transmission while driving, whether due to a defective overload protection switch (S65) or its wiring, the control module will switch into a fixed operating mode. In the fixed operating mode, the overload protection's effectiveness is limited.
If the shift signal is not present, the control module recognizes the beginning of a shift through a specific change in engine speed.
Operation of the transmission overload protection in the fixed operating mode is recognizable through brief ignition timing retard at high vehicle speeds.
NOTE: In case of complaints of misfiring at high vehicle speed, the transmission overload protection switch (S65) and its wiring must be checked.
Torque Reduction (104.942)
Torque reduction is performed by the engine control module which retards the WOT (full load) ignition angle according to an engine speed dependent map. The following conditions must be met:
- Vehicle speed less than 75 mph,
- Engine speed greater than 3400 rpm,
- WOT (full load),
- Altitude less than 5250 tt. (1600 m) above sea level.
The engine control module processes the following information for this purpose:
- Engine speed,
- Air mass,
- Vehicle speed signal (via CAN),
- WOT (full load) (via CAN),
- Altitude above sea level.