1 EGR solenoid valve
2 MAF sensor
3 PCM
4 Oxidation catalytic converter
5 Turbocharger(s)
6 EGR valve
7 Vacuum pump
8 Position sensor in EGR valve
9 Intercooler (not on all versions)
10 EGR cooler (not on all versions)
When turbochargers are used (they are deployed on all the diesel engines described here), the temperatures in the combustion chamber rise along with compression and combustion power.
Combustion temperatures are increased even further by the use of the direct injection process. Both result in the increased formation of NOX in the exhaust gas. In order to keep this NOX content in the exhaust gas within required limits, an EGR system is used.
In the part load range, exhaust gas recirculation is achieved by mixing the exhaust gases with the intake air. This reduces the oxygen concentration in the intake air. In addition, exhaust gas has a higher specific heat capacity than air and the proportion of water in the recirculated exhaust gas also reduces the combustion temperatures.
These effects lower the combustion temperatures (and thereby the proportion of NOX) and also reduce the amount of exhaust gas emitted. The quantity of exhaust gas to be recirculated is precisely determined by the PCM. An excessive exhaust gas recirculation rate would increase soot, CO and HC emissions because of the lack of air.
For this reason, the PCM requires feedback on the recirculated amount of exhaust gases. Three different systems are used which differ in terms of the following components:
โข Position sensor in the EGR valve (on engines with a wastegate-controlled turbocharger, emission standard III)
โข MAF sensor (on engines with variable geometry
turbocharger, emission standard IV).
โข MAF sensor plus a position sensor in the EGR valve (on engines with a variable geometry turbocharger, emission standard IV)
On all three systems the EGR valve is vacuum-actuated by the EGR solenoid valve. The duty cycle with which the EGR solenoid valve is actuated by the PCM determines the vacuum applied at the EGR valve.
System with position sensor in the EGR valve
The position sensor in the EGR valve signals the current position of the EGR valve to the PCM. From this, the PCM can determine the instantaneous quantity of recirculated exhaust gas depending on the MAP, thus forming a closed control loop.
System with MAF sensor
The quantity of exhaust gas recirculated when the EGR valve opens has a direct influence on the MAF sensor measurement.
During exhaust gas recirculation, the reduced air mass measured by the BARO sensor corresponds exactly to the value of the recirculated exhaust gases. If the quantity of recirculated exhaust gas is too high, the intake air mass drops to a specific limit. The PCM then reduces the proportion of recirculated exhaust gas, thus forming a closed control loop.
System with MAF and position sensor
Vehicles which meet emission standard IV use a combination of two sensors (MAF and position sensor). Here, the position sensor serves as an additional correction parameter for the quantity of recirculated exhaust gas. This means that the quantity of exhaust gas can be metered even more accurately.
This way, it is possible to get even closer to the operating limit with a greater quantity of exhaust gas, as a result of which NOX emissions can be reduced further.
Diagnosis
To check the EGR system, various prerequisites must be satisfied:
โข Engine running under certain operating conditions, depending on engine temperature, intake manifold pressure and engine speed.
โข Required operating conditions must be maintained for a certain time span.
During this time it is checked whether the required EGR rate is within the limits.
If the required operating conditions are no longer met, the monitoring is stopped. The data collected so far is frozen. After reaching the operating conditions again the test is continued.
If monitoring has been completed after the specified period of time and no faults have occurred, further monitoring of the EGR system does not take place until the next drive cycle.
Faults in the EGR system have no serious effects on exhaust gas emissions and are thus not MIL active.