Function Description
Function Description of Sub-systems
Monitoring the efficiency of the catalytic converter
TWC is OK:
In this process the oxygen storage capacity of the TWC (three-way catalytic
converter) is determined by comparing the oxygen sensor amplitudes ahead of and after the TWC. If the oxygen storage capacity of the TWC is greatly reduced due to aging, voltage amplitudes also occur at the oxygen sensors after the TWC, The amplitude of the sensor ahead of the TWC acts as a reference value and is therefore determined by the same signal path.
The quotient of the rear and front amplitudes, the amplitude ratio, is used as the measurement of quality. The appearance of amplitudes after the TWC is dependent on the momentary load of the storage capacity of the TWC. This load can vary with the engine load and rpm, which is why a load and rpm-dependent (tl/n) evaluation is performed.
A: Sensor amplitude ahead of TWC
B: Sensor amplitude after TWC
TWC is OK
TWC is NOT OK:
A: Sensor amplitude ahead of TWC
B: Sensor amplitude after TWC
X: Delay due to gas running time:
TWC is not OK
NOTE: The relationship between the two sensor voltages is the measurement for the efficiency of the TWC.
Oxygen sensor monitoring
The oxygen sensors are monitored for:
- Operational readiness
- Connectivity
- Short circuit to B+
- Short circuit to ECM ground
- Open circuit
- Aging of oxygen sensors via the control frequency and control position
Function of oxygen sensor diagnosis
General
To diagnose the oxygen sensor signals, the DME control module uses the form and frequency of the respective signal as a measuring value.
The data to be calculated for the DME control module are:
- Time of sensor voltage between positive and negative edges
- Maximum recognized sensor voltage value
- Minimum recognized sensor voltage value
- Oxygen sensor value towards rich
- Oxygen sensor value towards lean
- Sensing limits of oxygen sensing system
- Period duration of sensors ahead of TWC
- Sensor voltage ahead of TWC
Function
1. Determination of maximum and minimum sensor voltage
After starting the engine, the values for the previous maximum and minimum are erased. When driving, the absolute minimum and maximum are formed in an engine speed and load range predetermined for diagnosis. These values are recorded by the ECM memory. If the sensor voltage is higher than the presently stored maximum value, it will be overridden by the current sensor voltage. The same applies to the minimum value.
2. Calculation of time between positive and negative edges
If the sensor voltage exceeds the sensing limits, the measurement of the time between the positive and negative edges of the sensor signal is started. If the sensor voltage is lower than the sensing limit, the time counter is stopped. The time between the starting and stopping of the counter is measured.
Monitoring of Sensor Aging
Contamination or aging of the oxygen sensor affects the sensor voltage or period duration of the sensor signal and therefore has a negative influence on emission values.
Example: Illustration of the affect of the duration of the period regarding emission.:
Illustration of the effect of period duration on emission values.
Monitoring of Oxygen Sensors Ahead of TWC
Sensor readiness
The sensor voltage has to have left the 400 mV to 600 mV range within 200 seconds after the engine is started, and must not be above 1000 mV or below -150 mV.
No relevant interference may be present during this period.
Connectivity (signal wire to signal ground)
When the sensor is cold, the sensor voltage must equal 0 V ± 40 mV. When the sensor has reached operating temperature and the voltage of the sensor after the TWC is greater than 350 mV, the voltage of the sensor ahead of the TWC may lie between 60 mV and 400 mV for no longer than 15 seconds.
Open circuit
After starting the engine and 200 seconds of sensor heating, the sensor voltage may not lie between 400 mV and 600 mV for longer than 5 seconds.
Short circuit to B+
When the heater of the sensor was actuated for more than 200 seconds, the sensor voltage may not exceed 1000 mV.
Short circuit to ground
When the heater of the sensor was actuated for more than 200 seconds, the sensor voltage may not fall below -150 mV. When the sensor is cold, the sensor voltage must not lie in the voltage range from -360 mV to -160 mV.
Monitoring of Oxygen Sensors After TWC
Sensor readiness
The sensor voltage has to have left the 400 mV to 600 mV range within 200 seconds after the engine is started, and must not be above 1000 mV or below -150 mV.
Connectivity (signal wire to signal ground)
When oxygen sensing is activated and the sensor heating was triggered for 200 seconds and if no interference exists such as secondary air, tank ventilation or fault conditions, the sensor voltage must not remain in the range from -40 mV to +60 mV for longer than 200 seconds.
Open circuit
After starting the engine and 200 seconds of sensor heating, the sensor voltage must have passed through the 400 mV to 500 mV range within 400 seconds.
Short circuit to B+
When the heater of the sensor was actuated for more than 200 seconds, the sensor voltage may not exceed 1000 mV.
Short circuit to ground
When the heater of the sensor was actuated for more than 200 seconds, the sensor voltage may not fall below -150 mV. When the sensor is cold, the sensor voltage must not lie in the voltage range from -360 mV to -160 mV.
Oxygen Sensor Heating
A resistor (shunt) is provided in the heating circuit for each oxygen sensor pair ahead of the TWC and for each sensor pair after the TWC. This is located in the DME control module. The monitoring of the heating is accomplished by measuring the sensor heating current (depending on the voltage drop at the shunt) and the voltage for the sensor heating. The DME control module uses these values to calculate the heating resistance.
For right-left recognition, a check is carried out during a cold start to determine whether the sensor readiness of a sensor pair is within a certain time span. If, for example, the left sensor is operationally ready much later than the right sensor, we can conclude that the left sensor has a heating malfunction.
Monitoring of Combustion Misfire
Combustion misfiring at one or more cylinders increases emission values considerably.
If the misfire rate lies above the threshold value which exceeds the increase in emissions by 1.5 times the permissible limit value during two successive journeys, the Check Engine MIL must be switched on.
A misfire rate of slightly above 2% is sufficient in the example shown here to reach a high HC value and so to exceed the diagnostic threshold.
If the misfire rate is above a threshold value that could lead to permanent damage to the TWC, the Check Engine MIL will come on immediately and must flash.
The illustration shows an example of misfire rates in the rpm/engine load map of a gasoline engine, in which the temperature of the TWC exceeds 1000° C and permanent damage to the TWC may occur.
In order to monitor combustion misfire, the DME control module uses the rough running of the engine as a measuring variable, i.e. each misfire leads to a negative change in rpm which is interpreted as rough running.
The engine rpm is recorded by an inductive sensor located above a ring gear on the flywheel. In a 6-cylinder engine, three power strokes take place per crankshaft revolution. With each stroke, the crankshaft is accelerated. Using the input signals from the speed sender, the DME control module measures the revolutionary speed (time) of the crankshaft from one stroke to the next and can therefore determine whether or not the crankshaft is accelerating at the appropriate point. In conjunction with the camshaft and crankshaft position sensor signals, the DME control module can recognize cylinder 1 ignition TDC when the engine is started and can therefore allocate the individual strokes to the respective cylinder. In this way, in the event of misfiring the DME control module can assign the negative acceleration of the crankshaft to the appropriate cylinder.
To obtain a clear statement as to whether operation is tree from misfiring or misfiring is occurring, a rough running threshold value must be available for each rpm/engine load point. This threshold must be low enough to recognize single misfires, but far enough removed from the "engine specific" running characteristics to avoid fault detection.
When driving, the following operating conditions lead to a black-out of the
monitoring:
- Dropping below or exceeding an rpm limit (stalling)
- High rpm jumps (shifting)
- Time factor after starting engine (cold start)
- Falling below a load threshold (driving resistance)
- Recognition of poor road (pot hole)
- Idle speed
The rough running threshold is adapted when the engine is decelerating
(coasting/inertia fuel shutoff). Since the combustion strokes have no influence here, a uniform rpm signal is obtained which adapts the existing rpm data in the computer of the DME control module.
Storing, confirmation and fault healing of an emission relevant combustion misfire
Emission relevant fault occurs for the first time
- Fault entry takes place, i.e. one or more cylinders with an above average misfire contribution are recorded as fault-causing cylinders.
- Fault window tl/n is recorded.
- Engine temperature is recorded.
Suspected fault is not confirmed
With every engine start an area window tl/n is opened. If this area window coincides with the fault window during misfire-free operation and the engine temperature is within the area committed to memory, the fault will be erased from memory.
Suspected fault is confirmed
If an emission relevant fault is recognized during the second trip, final entry of the fault takes place.
The Malfunction Indicator Lamp (MIL) is switched on.
TWC damaging misfire rate
When TWC damaging misfire occurs, the MIL lights up immediately and starts to flash. When the vehicle leaves this load/rpm range, the status of the Check Engine MIL changes. The flashing light stays on continuously.
Fault healing
The Check Engine MIL may be switched off after three successive warm-up cycles when the area window coincides with the fault window, operation is without misfire and the engine temperature corresponds with the area committed to memory.
Monitoring the Fuel Tank Ventilation System
System Overview:
1 - EVAP canister purge valve
2 - EVAP canister
3 - Purge air
4 - Tank
5 - Tank pressure sensor
6 - Shutoff valve
7 - Operating purge valve
8 - To intake manifold
9 - Vacuum control valve
System Description
In order to prevent fuel vapors from entering the atmosphere, an EVAP canister is connected to the tank ventilation line.
Fuel vapors are collected in the EVAP canister. As the EVAP canister can only accommodate a certain amount of fuel vapors, the fuel vapors must be taken in and combusted during driving.
For this to occur, the EVAP canister purge valve (1) opens. Due to the vacuum predominating in the intake manifold, air is sucked up via the purge air line (3) through the EVAP canister. The air sucked up in this way draws the collected fuel vapors with it.
American law requires the tank ventilation system to be checked for leaks. For this purpose, the tank pressure sensor (5) and shutoff valve (6) are required.
Function of Tank Ventilation Diagnosis
In order to check the system for leaks, vacuum is built up in the tank, the system sealed and a test performed to see how quickly the vacuum diminishes again. Pressure conditions are influenced by the vaporization of fuel. Therefore, prior to diagnosis the pressure increase due to fuel vaporization is examined.
Tank ventilation diagnosis is divided into three steps:
1. Measuring the pressure increase due to fuel vaporization.
The EVAP canister purge valve (1) and shutoff valve (6) are closed. The increase in pressure is measured by the tank pressure sensor (5).
2. Vacuum buildup in the system.
The EVAP canister purge valve is opened and the shutoff valve remains closed. This causes vacuum to build up in the tank. If no vacuum builds up, a large leak is present, e.g. the tank cap is not screwed on.
3. Vacuum reduction in the system.
At a particular vacuum (approx. 8 mbar), the tank vent valve is closed once more. The system is now sealed. Depending upon the speed at which vacuum is reduced, leaks in the system may be determined.
Diagnosis of Secondary Air System
Through the secondary air system, additional oxygen is introduced behind the exhaust valves in order to reduce harmful emissions. This occurs after the engine is started, within a defined engine temperature range and for a predetermined time.
Whilst secondary air injection is active, passive diagnosis of the system is performed.
During passive diagnosis of the secondary air system, the additional air behind the exhaust valves leads to a surplus of oxygen in the oxygen sensors and therefore to a voltage reduction in oxygen sensors ahead of the TWC.
Oxygen Sensor Voltage Without Secondary Air:
X = Oxygen sensor voltage
t = Time
Oxygen sensor voltage without secondary air
The diagnosis checks in a predetermined rpm load and temperature range whether the sensor voltage is too low due to secondary air or too high because of a lack of secondary air.
Oxygen Sensor Voltage With Secondary Air:
X = Oxygen sensor voltage
t = Time
A = OK counter
Oxygen sensor voltage with secondary air
If passive diagnosis is interrupted, e.g. due to excessive rpm, then active diagnosis is performed at idle speed.
During active diagnosis, the DME switches the secondary air pump on when the engine has reached operating temperature. Here, the deviation of the oxygen sensor is observed and, in this way, the secondary air system assessed.
Diagnosis of Oxygen Sensing System Mixture Adaptation
General
The adaptive oxygen sensing system corrects longer lasting deviations of the fuel/air mixture of lamda = 1 by changing the anticipatory control calculated in the DME control module and the injection timing altered as a result.
Through mixture adaptation , the anticipatory control is multiplied and augmented in such a way that corrections to the oxygen sensor (FR) itself can be kept to a minimum 3 and the mixture remains within the range lamda = 1 even during open-loop (as opposed to closed loop) control.
This process is based on the following assumptions:
Over the life span of the engine and through different tolerances, two basic faults may occur:
The multiplicative fault and the additive fault per time unit (false air). At idle speed the false air fault dominates (low air flow rate, low rpm); with a high air flow rate the multiplicative fault dominates.
A combination of multiplicative and additive faults is compensated correctly when a corrective value is adapted in its dominant area. Therefore, there are two major adaptation areas:
Adaptation in the sector close to idle and adaptation under load.
Area 1: Area close to idle = TRA
Area 2: Load area = FRA
System description
The adaptation values can be read out with the Porsche System Tester 2.
The key to the adaptation values is as follows:
TRA: = Adaptation in area close to idle for cyl. 1 - 3
TRA2: = Adaptation in area close to idle for cyl. 4 - 6
FRA: = Adaptation under load for cylinders 1 - 3
FRA2: = Adaptation under load for cylinders 4 - 6
FR: = Oxygen sensor for cylinders 1 - 3
FR2: = Oxygen sensor for cylinders 4 - 6
Function
After the DME control module has been disconnected from constant B+3 the numerical values are at 1.00 (FRA) or 0 (TRA). After the engine is started, first the programmed map is
displayed.
Example:
Area 1:
TRA : 0
TRA2: 0
After oxygen sensing has been enabled (depending on engine temperature) the oxygen sensor (FR) moves around its mean value (e.g. 0.98-1.03).
Example:
Oxygen sensor:
FR: 1.03
FR2: 1.02
If the engine now runs too lean (e.g. through a 10 % addition of false air) in this operating condition, the FR value (oxygen sensor) will deviate from its mean value by this value (by 1.10), to compensate for the false air. Despite the 10 % false air, the mixture is now once more in the Lamda = 1 range and the oxygen sensor is working at around 1.11.
Example:
Oxygen sensor:
FR: 1.09
FR2: 1.11
Since the range (adjustment) of the oxygen sensor is limited, however, the adaptation must relieve the oxygen sensor after a few minutes so that the full adjustment range is available again.
During this process, the TRA value in area 1 shifts up (when the engine is idling), i.e. time is added to the injection time until the FR once more works at around its mean value. With the IRA value the Porsche System Tester 2 shows the added time in milli seconds.
Example:
Area 1:
TRA: 0.165
TRA2: 0.167
Should the adaptation phase under load (in area 2) come to an end, the FRA value would change. The value is displayed by the Porsche System Tester 2 in %.
Example:
Area 2:
FRA: 1.06
FRA2: 1.07
Since, however, the false air of 10 % has a greater effect at idle than under load, the FRA value only changes until the oxygen sensor fluctuates around its mean value once more.
The changes due to the adaptation in TRA (area 1) or in FRA (area 2) also affect the other adaptation area.
The maximum adjustment of the FR is 25 % towards rich and 25 % towards lean. Therefore, an incorrect fuel/air mixture towards either rich or lean can be briefly corrected by the oxygen sensor. For corrections of longer duration, the adaptation takes care of the correction. This correction factor remains stored in the DME control module until the latter is disconnected from constant B+.
Changes in the fuel/air ratio caused by fuel tank ventilation are corrected by an additional adaptation factor (FTEA).
Therefore, the adaptation value for fuel tank ventilation (FTEA) with high
absorption of the EVAP canister could look like this:
Example:
Tank ventilation:
FTEA: 0.93
Adaptation thresholds:
Rich threshold: TRA = 0.37
FRA = 1.25
Lean threshold: TRA = 0.37
FRA = 0.75
NOTE: Map changes in the full load range (FRA) have a multiplicative effect, map changes in the idle range (TRA) have an additive effect.