Evaporative Emissions System: Testing and Inspection
4 EVAPORATIVE SYSTEM DIAGNOSIS4.1 Leakage check
4.1.1 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.
4.1.2 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.
4.1.3 Flow chart leakage check
4.2 Purge Check
4.2.1 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.
4.2.2 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.
4.2.3 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
4.2.4 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.
4.2.5 Flow charts
For details of the Mixture check and the Air check see the following two flowcharts.
4.2.6 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