Part 2

Function

Throttle control




To ensure that the correct throttle angle is reached, the engine control module (ECM) controls the throttle shutter in the throttle unit (6/120), mainly using the signal from:
- accelerator pedal (AP) position sensor (7/51)
- clutch pedal sensor (7/123) via central electronic module (CEM) (4/56)
- stop lamp switch (3/9)
- the throttle position (TP) sensor on the electronic throttle unit (6/120)
- brake pedal sensor (7/124) via brake control module (BCM) (4/16).
Additional signals and parameters are used to ensure optimum throttle control. By example by compensating for:
- the load from the air conditioning (A/C) compressor
- the load from the transmission depending on the selected gear mode (automatic)
- engine coolant temperature (ECT).
In a combustion engine, the difference between the minimum and maximum airflow is considerable. The smaller air flows need more thorough regulation (for example during idle air trim), so the throttle position (TP) sensor signal 1 is amplified approximately 4 times in the engine control module (ECM) before it reaches the Analog/Digital converter in the engine control module (ECM). This means that there are three, two real and one fictitious, input signals available to the engine control module (ECM). These signals are used to determine the position of the throttle disc and to control the throttle motor to the correct throttle angle.
Because the signal is amplified, it reaches its maximum value at approximately a quarter of maximum deployment.
The engine control module (ECM) primarily uses the signal from throttle position (TP) sensor 1 as a measurement of throttle opening. The signal from throttle position (TP) sensor 2 is mainly to check that throttle position (TP) sensor 1 is working. The engine control module (ECM) then uses the signal to calculate a throttle angle (actual value). This is the actual throttle angle. The value for the actual throttle angle is used by those functions in the engine control module (ECM) which depend on this information so that the throttle can be correctly regulated.
There is an adaptation (learning) in the engine control module (ECM) so that the control module can calculate how the damper motor needs to be controlled. See "Adaptation of the electronic throttle unit" below. This adaptation occurs automatically when necessary. The engine control module (ECM) moves the throttle disc to the different positions and reads off and registers the actual values from the throttle position (TP) sensors.
The throttle angle is regulated so that the actual angle (actual value) is the same as the angle calculated by the engine control module (ECM) (desired value). The engine control module (ECM) also uses the values that were stored during adaptation of the throttle angle, and the actual signals from the throttle position (TP) sensor.
The damper motor is deployed by the integrated power stage in the engine control module (ECM) using a pulse width modulation (PWM) signal. The torsion from the opening and return springs in the electronic throttle unit is also used. If the engine control module (ECM) detects a fault in the electronic throttle unit so that the throttle disc cannot be controlled, the springs in the throttle unit will turn the throttle disc to the limp home position (return position). This return position is calibrated to provide a throttle angle large enough to allow the car to be driven to a workshop, although with considerably reduced driveablity.

Throttle angle
The engine control module (ECM) also monitors the throttle unit signals from the throttle position (TP) sensors, to ensure that these signals are within the parameters and correspond to the same throttle angle.
If the difference between the signals exceeds a set limit the maximum value is selected as the actual value (which means that the throttle disc is controlled down). If the engine control module (ECM) detects a fault in both throttle position sensors, the electronic throttle unit power stage is switched off. The throttle switches to limp home mode (return position). Diagnostic trouble codes (DTC) are stored in the engine control module (ECM) if faults are detected in the throttle position (TP) sensors.

Adaptation of the electronic throttle unit
Adaptation of the electronic throttle unit is carried out automatically when requested by the engine control module (ECM). Adaptation is carried out to check the function of the electronic throttle unit, and, if necessary, to update the values obtained from the throttle position (TP) sensors etc. during regulation, as these values can change somewhat over the service life of the throttle unit. Diagnostic trouble codes (DTC) are stored in the engine control module (ECM) if faults are detected in the electronic throttle unit.

Fuel trim





Overview
Fuel trim reduces exhaust emissions. Fuel trim reduces nitrous oxides (NOx), carbon monoxide (CO) and hydrocarbon (HC) emissions.
Theoretically, if the correct amount of oxygen is added during combustion, fuel can be converted to water (H2O) and carbon dioxide (CO2). Emissions would then be completely safe.
In practice considerable amounts of hydro-carbons (HC) and varying amounts of carbon monoxide (CO) and carbon dioxide (CO2) remain.




Due to the high temperature and pressure, nitrous oxides such as NO and NO2 are also formed. The common designation for these gases is nitrous oxides NOx.




By speeding up the reaction between the remaining reactive components using a catalytic converter, these can be converted to water (H2O), carbon dioxide (CO2) and nitrogen (N2).
However this can only happen if the balance of hydro-carbons (HC), carbon monoxide (CO), oxygen (O2) and nitrous oxides (NOx) is exactly right in the exhaust. This happens when the fuel air mixture before combustion is 14.7 kg of air per kg of fuel. The Lambda value is then said to be one, (lambda=1).




A base program in the engine control module (ECM) calculates the injection period based on data about load, i.e. the measured air mass and engine speed (rpm). The calculated injection time (from the base program) is then modified by a circuit (short-term fuel trim). The signal from the heated oxygen sensor (HO2S) is used to finely adjust the injection period so that lambda=1 is reached. The short-term fuel trim is also a circuit that finely adjusts the injection period so that the fuel air mixture is optimized (lambda=1). The control module also used the signals from the front and rear heated oxygen sensors (HO2S) to correct the front heated oxygen sensor (HO2S) (offset adjustment) and thereby the injection period. This gives a higher degree of accuracy during fuel trim. Fuel trim is a rapid process which may take place several times a second. Adjustment of the calculated injection period calculated in the base program is limited.
The integrator can be read using VIDA.

Adaptive functions




Certain factors, such as deviations in tolerance for certain components such as the mass air flow (MAF) sensor and injectors, intake air leakage, fuel pressure etc, will affect the composition of the fuel air mixture. To compensate for this, the engine control module (ECM) has adaptive (self learning) functions. When the engine is new, the short-term fuel trim is assumed to vary cyclically around a nominal center line (A) 1.00 with, for example, a ± 5% change in the injection period when fuel trim is active.
If there is air leakage the short-term fuel trim will be offset to a new position (B) and will then work for example between 1.10 (+10%) and 1.20 (+20%), although still at an amplitude of 5%, but with an offset in relation to the original center line (A). The injection period has then been increased to compensate the increase in the amount of air.
The adaptive functions will correct the change, so that the short-term fuel trim will work around the new center line (B) where it will again have its full range of control available.
Put simply, fuel trim is a measurement of the difference (C) between the original short-term fuel trim center line (A) and the new center line (B).




The adaptive functions consist of two sections and correspond to the different operating ranges of the engine, load (D) and engine speed (E):
- Additive adaptation (1) is when the engine is idling. This is how the control module adjusts the CO content at idle speed. Long-term fuel trim, idling can be read off using VIDA.
- Multiplicative adaptation (2), carried out at loads and engine speeds above idle. Long-term fuel trim, load can be read off using VIDA.
The adaptive adjustments of the injection period are stored continuously in the control module. This means that under different operating conditions the fuel air mixture is obtained before the heated oxygen sensor (HO2S) is warm enough to function.
A diagnostic trouble code (DTC) will be stored in the control module if any adaptation value is too high or too low. For further information, also see: Heated oxygen sensor (HO2S) diagnostic Heated Oxygen Sensor (HO2S) Diagnostic

Knock control




Knock occurs in the combustion chamber when the fuel and air mixture self ignites. This can occur either before or after the spark plug has produced an ignition spark. In both cases the gas in two or more places ignites in the combustion chamber.
This results in an extremely fast combustion process with flames from several directions. When these flames collide, the pressure in the cylinder increases rapidly and there is a mechanical knocking sound.
If any of the cylinders knock there is a specific type of vibration in the cylinder block. These vibrations are transferred to the knock sensor (KS) which is screwed into place in the cylinder block. The resulting mechanical stress in the piezo electrical material in the knock sensors generates a voltage. The engine control module (ECM) can then determine which cylinder is knocking with the help of the camshaft position (CMP) sensor and the engine speed (RPM) sensor.
The knock sensor (KS) also senses some normal engine sound. The control module is able to recognize the vibrations which correspond to knocking by filtering, amplifying and using software to evaluate the signal.
If the knock sensor (KS) detects knocking in the engine above a certain threshold value, the ignition timing is first retarded and then the fuel/air mixture is enriched to eliminate knocking.

Ignition control




The following components are used for ignition control:
- engine speed (RPM) sensor (7/25)
- camshaft position (CMP) sensor (7/172-7/173)
- mass air flow (MAF) sensor (7/17)
- engine coolant temperature (ECT) sensor (7/16)
- throttle position (TP) sensor on the electronic throttle unit (6/120)
- knock sensor (KS) (7/24)
- transmission control module (TCM) (4/28)
- spark plugs with ignition coils (20/3-20/7)
- brake control module (BCM) (4/16).
The engine control module (ECM) calculates the optimum ignition advance based on the software and information from the sensors. The engine control module (ECM) cuts the current to the ignition coil mounted on the cylinder to be ignited and produces a spark.
During the starting phase the engine control module (ECM) produces a fixed ignition setting. When the engine has started and the vehicle is being driven, the engine control module (ECM) calculates the optimum ignition setting, taking factors such as the following into account:
- engine speed (RPM)
- load
- temperature.
The engine control module (ECM) analyses the signal from the knock sensor (KS) when the engine reaches operating temperature. If any of the cylinders knock, the ignition is retarded for that specific cylinder until the knocking ceases.
The ignition then advanced to the normal position or until the knock recurs.
Before the transmission control module (TCM) changes gear, it sometimes transmits a torque limiting request to the engine control module (ECM). The engine control module (ECM) then retards the ignition momentarily to reduce the torque, resulting in smoother gear changes and reducing the load on the transmission. There are different ignition retardation levels depending on the signals from the transmission control module (TCM). The return signal from the engine control module (ECM) to the transmission control module (TCM) confirms that the signal reached the engine control module (ECM). The Brake Control Module (BCM) transmits information to the engine control module (ECM) about deviations in the drive line. The signal is used to stop the diagnosis. For further information, also see: Misfire diagnostic Misfire Diagnostics
The engine misfires if the fuel does not ignite correctly. For further information, also see: Misfire diagnostic Misfire Diagnostics