Function Description of Sub-Systems
Function Description of Sub-systems
Misfire Detection
Legal requirements
Californian exhaust emissions laws require the monitoring of combustion misfiring, as the latter leads to an increase in noxious emissions and to TWC damage.
A distinction must be made between emission relevant misfiring after start-up and during the journey and misfiring which is damaging to the TWC.
Emission relevant faults exist if the limits of the FTP emissions test have been exceeded 1.5 times.
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 threshold.
Misfiring damaging to the TWC is present if the temperature of the TWC reaches over 1000 °C.
The drawing shows what misfire rate (in %) - as a function of the load/speed range - allows the temperature of the TWC to climb to approx. 1000 °C.
Emission relevant misfiring
To detect emission relevant faults, the misfires are counted within a cycle of 1000 crankshaft revolutions. The first 1000 crankshaft revolutions after the engine has been started are "emission relevant after start-up". After this, the misfires become "emission relevant during the journey". At the end of the cycle, the counter is reset to zero.
A fault is stored:
- The first time the threshold is exceeded in the case of emission relevant misfiring after start-up
- The fourth time the threshold is exceeded in the case of emission relevant misfiring during the journey.
TWC damaging misfiring
To detect TWC damaging faults, the misfires are counted within a cycle of 200 crankshaft revolutions. A fault is stored as soon as a given threshold - dependent upon the load/speed range - is exceeded, and the Check Engine MIL (Malfunction Indicator Light) flashes for the duration that TWC damaging misfiring is present.
Fuel injection cutoff
If TWC damaging misfiring occurs over two consecutive cycles, up to two fuel injectors can be cut off.
Detection of misfiring
To detect misfiring, the rough running of the engine is evaluated. To this aim, the flywheel (1 crankshaft revolution) is divided into three segments.
The rpm/crankshaft position sensor measures the period of time which passes until the segment has moved past the rpm/crankshaft position sensor.
If combustion misfiring occurs, the drive torque is reduced briefly. This leads to an interruption in the rpm and therefore to a longer segment time, or to negative angular acceleration. The rough running of the engine is proportional to the change in angular acceleration. At constant rpm or constant acceleration, rough running equals zero.
Marked changes in rpm also occur during dynamic engine operation, and these must be distinguished from combustion misfiring. It is possible to distinguish between them, as the change in rpm extends over more revolutions than in the case of combustion misfiring.
Sensor wheel adaptation
When the segment times are measured disturbances occur which are eliminated by means of adaptation, thereby permitting more precise detection of misfiring.
The following disturbances may occur:
- Mechanical tolerance of tooth flanks
- Damage to tooth flanks
- Centre point and centre of rotation of flywheel are not identical
- Torsional vibrations of the crankshaft
- Different charge/carburetion from cylinder to cylinder.
Adaptation can take place in three load and eight speed ranges, i.e. in a total of 24 different ranges. Adaptation is performed in both fuel off and fuel on modes.
The dominant range of adaptation is fuel off. Adaptation must take place in this range first; only then can further adaptation procedures be performed.
Other thresholds are used, depending upon learning progress.
Diagnosis of Fuel Tank Ventilation System in RoW Vehicles
The fuel tank ventilation system undergoes both passive and active diagnosis. First of all, passive diagnosis is performed, followed by active diagnosis if the first result is not OK.
Passive diagnosis
During a mixture adaptation phase, the system recognizes that there are no faults in the anticipatory mixture control and the oxygen sensor is stable at around 1.0. If high absorption of the EVAP canister is detected in a subsequent tank ventilation phase, and purging takes place with a high proportion of fuel, this is a sure sign that the fuel tank ventilation system is in good working order.
Active diagnosis
Monitoring of the fuel tank ventilation system is based on the principle that the oxygen sensor in the DME control module adapts the mixture towards lean via the open EVAP canister purge valve when additional fuel flows from the EVAP canister, and enriches the mixture in the case of fresh air coming through the purge air line of the EVAP canister. The duty cycle of the Idle Air Control (IAC) Valve is also changed.
The oxygen sensor operates during fresh air supply up to the diagnostic threshold in the direction of enrichment or in the case of fuel vapors up to the diagnostic threshold in the direction of lean operation.
By reaching the diagnostic threshold within a predetermined time the opening of the EVAP canister purge valve is detected and the diagnosis is therefore successfully concluded.
Should the additional fuel/air mixture from the EVAP canister purge valve (fuel vapours and fresh air) be exactly at the ratio of lambda=1, the duty cycle of the IAC valve is reduced. When the diagnostic threshold is reached, here, too, diagnosis is successfully completed.
Diagnosis of Fuel Tank Ventilation System in USA Vehicles
System Overview
System description
In order to prevent fuel vapours from entering the atmosphere, an EVAP canister is connected to the tank ventilation line.
Fuel vapours are collected in the EVAP canister. As the EVAP canister can only accommodate a certain amount of fuel vapours, these 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 vapours 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 vaporisation of fuel. Therefore, prior to diagnosis the pressure increase due to fuel vaporisation is examined.
Tank ventilation diagnosis is divided into three steps:
1. Measuring the pressure increase due to fuel vaporisation.
- 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 build-up 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 EVAP canister purge 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.
Oxygen Sensor Monitoring
The oxygen sensors are monitored for:
- Operational readiness
- Connectivity
- Short circuit to B+
- Short circuit to ground
- Open circuit
- Aging of oxygen sensors via the control frequency and control position
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.
Illustration of the effect of period duration on emission values.
Diagnosis of Oxygen Sensing System Mixture Adaptation
General
The adaptive oxygen sensing system corrects longer lasting deviations of the fuel/air mixture of lambda = 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, and the mixture remains within the range lambda = 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 range. Therefore, there are three major adaptation ranges:
System description
The adaptation values can be read out with the Porsche System Tester 2.
The key to the adaptation values is as follows:
RKAT = Adaptation in range close to idle Cylinders 1 - 3
RKAT2 = Adaptation in range close to idle Cylinders 4 - 6
FRAU = Adaptation in lower load range Cylinders 1 - 3
FRAU2 = Adaptation in lower load range Cylinders 4 - 6
FRAO = Adaptation in upper load range Cylinders 1 - 3
FRAO2 = Adaptation in upper load range 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+, the numerical values are at 1.00 (FRA) or 0 (RKAT). After the engine is started, first the programmed map is displayed.
Example:
Range 1:
RKAT: 0 %
RKAT2: 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 lambda = 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 RKAT value in range 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 RKAT value the Porsche System Tester 2 shows the added time in percent.
Example:
Range 1:
RKAT: 1,5 %
RKAT2: 1,0 %
Should the adaptation phase take place under load, the FRA value would change. The value is displayed by the Porsche System Tester 2 as a factor.
Example:
Range 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 (FR) fluctuates around its mean value once more.
The changes due to the adaptation in RKAT or in FRA also affect the other adaptation range.
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 (FTEAD).
Therefore, the adaptation value for fuel tank ventilation (FTEAD) with high absorption of the EVAP canister could look like this:
Example:
Fuel tank ventilation:
FTEAD: 0.93
Adaptation thresholds:
Rich threshold:
RKAT = 4.5 %
FRAU = 1.32
FRAO = 1.32
Lean threshold:
RKAT = 4.5 %
FRAU = 0.70
FRAO = 0.70
NOTE: Map changes in the load range have a multiplicative effect, map changes in the idle range have an additive effect.
Monitoring the Efficiency of the Catalytic Converter
In this process the oxygen storage capacity of the TWC (three-way catalytic converter) is determined from the signal characteristic of the oxygen sensor 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 appearance of amplitudes after the TWC is dependent on the momentary load of the TWC's storage capacity. This load can vary with the engine load and rpm, which is why a load and rpm-dependent (rl/n) evaluation is performed.
The signal amplitude of the sensor after the TWC indicates its oxygen storage capacity.
Diagnosis of Secondary Air Injection 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.
While the secondary air injection is active, the diagnosis of the system is performed. 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.
NOTE: The running time of the pump depends on the air flow rate.
Diagnosis is interrupted or cancelled when the vehicle leaves a given load/rpm range. No further secondary air diagnosis takes place during this driving cycle.
Secondary air diagnosis is performed in conjunction with oxygen sensing. To this aim, during secondary air injection the oxygen sensing system is operated by the input of a lean nominal oxygen exhaust gas value at which thinning of secondary air is not adjusted. If a rich mixture is measured nonetheless, the system regulates this to = 1. The value required for this purpose is then converted to the actual secondary air mass. The secondary air mass is calculated.
The criterion for a fault is the determined ratio "Actual air mass divided by specified air mass".
In order to compensate the effect of a transient mixture adaptation on secondary air mass measurement, a so-called offset measurement of the secondary air mass is performed directly after secondary air injection when = 1 without secondary air.