Part 2

4 EVAPORATIVE SYSTEM DIAGNOSIS Continued.

4.2 Tank Leak Monitoring with the Tank Leak Diagnostic Module (DM-TL)

Applicable only for test groups






4.2.1 General Description
The tank leak diagnostic module is employed to monitor the evaporative system for very small leaks (> 0.508 mm, i.e. > 0.02 in.) and for small leaks (> 1.016 mm, i.e. > 0.04 in.). The module consists of an electrically operated air pump, a change-over valve and a reference orifice of 0.508 mm (0.02 in.) used for the reference measurement (see Figure 16). The leak integrity of the tank system is determined by comparing the actual current consumption of the air pump motor to the current consumption when the system is operated with a standardized reference opening.






When the air pump is switched off, air is supplied to the fuel tank through the charcoal canister, the change-over valve and the air filter. For purging the evaporative system and charcoal canister, the purge control valve is opened and fresh air passes through the air filter, the change-over valve and the charcoal canister into the intake manifold (see Figure 17).






For a reference current measurement, the air pump motor is briefly enabled with the change-over and purge control valves closed (Figure 18), and the current consumption of the air pump motor is measured while filtered fresh air is pumped through the reference orifice of 0.508 mm (0.02 in.). Additionally, also the pump frequency is determined during the reference measurement from the induced high-frequency oscillations of the current signal.






In monitoring mode (Figure 19), the change-over valve inside the tank leak diagnostic module is opened with the purge control valve closed. The current consumption of the pump motor drops to the no-load level. The current drop that results from the difference in current flow between the reference leak measurement and the no-load measurement is an important reference value for the subsequent monitoring. Following the determination of the no-load current, fresh air is pumped through the charcoal canister into the tank. The current consumption of the pump is approximately proportional to the pressure build-up in the tank and is evaluated.










Typical Enable Conditions for all sub-monitors
- ECM keep-alive time is active
- Engine speed < threshold value
- Altitude < calibrated value
- Engine coolant temperature at engine start > calibrated value
- Ambient temperature within calibrated range
- Difference between engine start temperature and ambient air temperature at engine start < threshold value
- Time after engine start in the preceding trip ≥ calibrated value
- Vehicle speed = 0 (optional)
- ECM keep-alive time > threshold value
- Battery voltage fluctuations since start of monitoring within calibrated range
- Brief changes in the pump current within calibrated range (no fueling, no fuel vapor degassing)
- Airbag not deployed

4.2.2 Detection of a Small Leak ( > 1.016 mm, i.e. > 0.04 in.), P0442

Monitoring Strategy
The tank pressure is compared to a threshold value over a period of time.

Specific Enable Conditions for Detecting a Small Leak
- None

Malfunction Criteria
The tank pressure is modeled from the relative pump current increase after the no-load phase relative to the current drop previously determined from the difference in current flow between the reference leak measurement and the no-load measurement. This happens on the basis of the pump's delivery rate determined from the pump frequency during the reference leak measurement, taking the ambient pressure, the ambient air temperature and the pump characteristic curve into account.
If the tank pressure reaches a calibrated threshold value, no small leak is present (less than 1.016 mm). If this threshold value is not reached within a calibrated time interval, or if before that the pressure has stabilized at a lower value following a minimum waiting time, a small leak is detected (> 1.106 mm).










In-Use Monitor Performance Ratio

Incrementing the numerator
The numerator is incremented only if the monitor has been executed and if
- either a small leak has been detected or
- no small leak has been detected and an existing leak or an existing fault could have been detected.

The latter is the case when a sufficient amount of time has elapsed with "engine OFF" AND "ignition OFF" in order to cover the longest possible monitoring time period.

Incrementing the denominator
The evaporative system denominator is incremented when all additional physical conditions for incrementing are fulfilled.Additional physical conditions for incrementing this denominator, besides those of the "general denominator," are:
- An engine cold start must have occurred, which is when the engine coolant temperature at the engine start is greater than or equal to 40 °F (approx. 4.44 °C) and less than or equal to 95 °F (35.0 °C), and when the difference between the engine coolant temperature and the ambient air temperature at engine start is less than or equal to 12 °F (approx. 6.67 °C).
- The ambient temperature must be between 40 °F (approx. 4.44 °C) and 95 °F (35.0 °C) for a cumulative time of 600 s.

4.2.3 Detection of a Very Small Leak ( > 0.508 mm, i.e. > 0.02 in.), P0456

Monitoring Strategy
From the currently measured curve of the pump current a leak size is determined, and compared to a threshold value.

Specific Enable Conditions for Detecting a Very Small Leak
- Charcoal canister load < calibrated value
- Integrated purge mass since last tank purging stop > 0 g (optional)

Malfunction Criteria
For very small leak monitoring, a leak size is determined from the measured curve of the actual pump current. The tank pressure and the mass air flow delivered by the pump are determined from the actual current curve and the pump frequency (see above). The equivalent leakage area and the free gas volume are determined from the tank pressure and the mass air flow by means of the general gas laws and Bernoulli's law for the mass flow rate calculation for compressible media. The ambient pressure and ambient temperature are also taken into account.
The resulting system of equations is solved using the recursive least squares algorithm (RLS). Each set of data during the measurement adds a new line to the over-determined set of equations, which is solved based on the result of the preceding calculation step to provide a new estimate for the gas volume and leak size. With the variance, the algorithm also yields a measure of the quality of the model adaptation to the measurement values. If the variance falls below a calibrated value following an adjustable minimum period of time, the calculated leak size is evaluated. If this leak size is greater than a calibratable threshold value, a very small leak is detected. The leak size is also evaluated when the modeled pressure attains a second calibratable threshold, the pump current attains a calibratable multiple of the reference current, or when a calibratable maximum time has elapsed.
The very small leak monitoring starts immediately after the no-load current is determined. However, this monitoring for very small leaks is continued only if the monitoring for small leaks has been successfully concluded and no small leak has been found. If the specific enable conditions for the very small leak monitoring are fulfilled, the pump remains actuated.










In-Use Monitor Performance Ratio

Incrementing the numerator
The numerator is incremented only if the monitor has been executed and if
- either a very small leak has been detected or
- no small leak had been detected, the very small leak detection has been finished, and an existing leak or an existing fault could have been detected.

The latter is the case when a sufficient amount of time has elapsed with "engine OFF" AND "ignition OFF" in order to cover the longest possible monitoring time period.

Incrementing the denominator
The evaporative system denominator is used for this monitor. Incrementing is already described above.

4.3 Leakage check with LDP

Applicable only for test groups






4.3.1 General description
The leakage diagnosis procedure is a pressure check of the EVAP system. In order to perform the check, the EVAP system will be sealed and pressure applied by the leakage diagnosis pump (LDP). The pressure variation time is analyzed by the ECM.

4.3.2 Monitoring function description
The diagnosis procedure consists of the following steps:

1. Tank pressure check
The first step of leakage diagnostics is the pressure check of fuel tank system by testing the reed switch. In case of an open reed switch, the fuel tank system has sufficient pressure for the sealed check and no further pressure has to be supplied to the fuel tank system by the LDP.
The diagnosis is waiting until the EVAP purge valve is opened in order to purge the carbon canister. In case the reed switch remains open or the reed switch stuck open, the reed switch is defective.

In the case the reed switch is closed, the LDP is switched on in order to supply pressure to the fuel tank system and the diagnostic is continued with the step 2 to 3 (as described below).

2. LDP Self-check procedure

Closed check
LDP control is disabled and the reed switch has to be closed otherwise the reed switch is defective.

Close to open check
LDP control is switched on once and the diaphragm has to move to the upper position. The time is measured between closed and open position of diaphragm detected by the reed switch. When the final upper position of diaphragm is reached in a certain time, then the check will be passed.

3. Leak check of EVAP system

Fast pulse
After the self check procedure, the LDP control supplies pressure to the fuel tank system with a pressure dependent number of compression strokes in a certain time. In order to supply pressure to the fuel tank system, the LDP can perform compression strokes in several attempts.

EVAP system sealed check, measure stroke and measure phase
The decrease of fuel tank pressure is measured via time of diaphragm movement followed by a compression stroke. Within a certain time, the LDP control is determined within at least four measurement strokes. The averaged time is a measure for the tightness of fuel tank system.






4.4 Purge Check - flow rate of EVAP system faulty, P0441

Applicable only for test groups






4.4.1 General description
The fuel tank has an integrated ventilation system in order to enable a pressure equalization between the tank and the environment when temperature variations occur or when fuel is added to or extracted from the tank. To prevent fuel evaporation to the atmosphere, a charcoal canister is installed between the tank and the atmosphere, which absorbs discharged hydrocarbons contained in the fuel vapor. As the charcoal canister has a limited storage capacity, it has to be discharged after defined time intervals. This is realized by temporarily opening a connection between the charcoal canister and the intake manifold for a short period of time. After the canister purge valve is opened, ambient air is drawn in through the charcoal canister due to the vacuum in the intake manifold. The stored hydrocarbons are discharged and enter the combustion chamber together with the ambient air. The additional air and fuel charges during the canister purge phases lead to a mixture deviation, which is compensated by the ECM.

The evaporative system monitor is carried out once per driving cycle. During part load conditions, the purge valve is commanded open and the response of the lambda control is monitored. If no clear mixture response can be detected after the majority of a defined number of checks has been run, a fault is suspected and an air check is carried out in the next idle speed situation. If no clear response of the idle speed control on the additional air charge occurs after the switch-open command of the canister purge valve at idle speed, a faulty flow rate in the evaporative system is detected and the corresponding entry in the fault memory is set.






4.4.2 Monitoring Strategy
The part load check is the comparison between the mixture deviation that is present during the opening process of the canister purge valve and a threshold value. The idle speed check is the comparison between the ratio of expected mass air flow/actual mass air flow and a threshold value, performed while the purge valve is open.

4.4.3 Typical Enable Conditions
- Canister purge integral greater than threshold value (depending on engine start temperature)
- Integrated air mass greater than threshold value (depending on engine start temperature)
- Engine coolant temperature and limp-home mode temperature greater than threshold value
- Altitude less than 2700 m above sea level
- Ambient temperature greater than threshold value
- Lambda control in a closed control loop condition
- Engine speed deviation less than threshold value

4.4.4 Malfunction Criteria
Depending on the load condition of the engine and the charge state of the charcoal canister, opening the canister purge valve leads to a mixture deviation (lean or rich), which is detected and compensated by the ECM.

If all enable conditions are fulfilled at part load, the current mixture value is stored as a reference value and the canister purge valve is commanded open according to a given characteristic curve. During the opening process, the current mixture value of the lambda control system is continuously compared with the reference value. If the absolute value of the mixture deviation does not reach a defined threshold value, the canister purge valve may be defective, or the mixture composition currently coming from the charcoal canister does not lead to response in the lambda control. Thus, initially a fault suspicion is generated and an air check is requested for the next idle speed situation. To avoid unnecessary idle speed phases in vehicles with a start/stop system, a fault suspicion is generated not before a defined number of mixture checks without a PASS result have been carried out.

At idle speed, the additional air charge caused by the open canister purge valve leads to an increase in engine speed, which is compensated by the idle speed control by means of closing the throttle valve.

At idle speed, the canister purge valve is commanded open according to a given characteristic curve. When a sufficient opening percentage of the purge valve is reached, a relative mass air flow value is generated from the expected mass air flow values without any canister purge valve activity and the actual mass air flow value. If the relative mass air flow value does not reach a defined threshold value, a faulty flow rate in the evaporative system is detected.

4.4.5 In-Use Monitor Performance Ratio

Incrementing the numerator:
The numerator is incremented by one when the diagnostic has ended and a fault could have been detected. The enable conditions are monitored by a shadow counter, which causes the numerator to increment whenever the diagnostic could have run.

Incrementing the denominator:
The denominator is incremented by one when the conditions for incrementing the denominator according to CCR (d) (4.3.2) (D) "Evaporative System Denominator" are fulfilled

4.5 Circuit Monitoring EVAP Purge Valve Power Stage

4.5.1 Short to Battery/Ground or Open Circuit

Monitoring Strategy
For short to ground and open circuit:
The voltage at the output of the EVAP purge valve power stage is continuously monitored by the ECM.
For short to battery:
The current at the output of the EVAP purge valve power stage is continuously monitored by the ECM.

Typical Enable Conditions (Details see Summary Table)
For all monitors:
- Engine speed greater than defined threshold value

For short to ground and open circuit:
- Control signal for the position "Closed" is present at the EVAP purge valve

For short to battery:
- Control signal for the position "Open" is present at the EVAP purge valve

Malfunction Criteria
Short to ground, P0458:
If the voltage at the output of the EVAP purge valve power stage in the ECM is in a defined lower range for a defined time period, a short to ground is detected and a fault is stored in the fault memory.

Short to battery, P0459:
If the current at the output of the EVAP purge valve power stage in the ECM is greater than a defined threshold value for a defined time period, a short to battery is detected and a fault is stored in the fault memory.

Open circuit, P0444:
If the voltage at the output of the EVAP purge valve power stage in the ECM is in a defined upper range for a defined time period, an open circuit is detected and a fault is stored in the fault memory.