1 MAF sensor
2 PCM
3 Oxidation catalytic converter
4 Turbocharger(s)
5 EGR valve servo motor
6 Position sensor (integrated in servo motor)
7 Intercooler
8 EGR cooler
9 Intake manifold flap with servo motor (only in emission standard IV)
By using turbochargers, the temperatures in the combustion chamber rise together with the compression and combustion performance.
In addition, the combustion temperatures are increased by using the direct fuel injection method. 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, the EGR system is becoming increasingly important.
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 lead to an increase in diesel particulate, CO and HC emissions due to lack of air.
For this reason, the PCM requires feedback on the amount of recirculated exhaust gases. This works via the MAF sensor and a position sensor which is integrated into the servo motor of the EGR valve. The servo motor itself is activated by the PCM depending on requirements.
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 MAF 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.
Position sensor
In the face of increasingly stringent emission standards, EGR control via the MAF sensor alone is reaching its limits. For this reason, a position sensor, which is integrated into the EGR valve servo motor, is used in addition to the MAF sensor.
Intake manifold flap
A further step towards minimizing NOX is the restriction of intake air via the intake manifold flap. By partial closing of the intake manifold flap a vacuum is generated behind the intake manifold flap. The vacuum results in the exhaust gases being drawn in more efficiently by the engine via the EGR valve, enabling the EGR rate to be metered more effectively. This combination (MAF sensor, position sensor and intake manifold flap control) allows even more precise metering of the recirculated quantity of exhaust gas. This way, it is possible to get even closer to the operating limit with a greater quantity of exhaust gas. The NOX emissions are thereby reduced to a minimum.
Diagnosis
The EGR control works as a system. The interaction of individual components is monitored. Malfunctions lead to increased exhaust emissions which exceed the EOBD limits. Certain faults also lead to the EGR system being switched off. Therefore, this is a MIL active system. Malfunctions in the EGR system are detected by the MAF sensor. In case of a fault, the EGR system is switched off. In the event of specific faults, the PCM limits the injected fuel quantity (power output reduction).