By using turbochargers, the temperatures in the engine's combustion chambers rise with the torque and output.

This increases the 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 partial 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. The proportion of water in the recirculated exhaust gas also reduces the combustion temperatures.

These effects lower the proportion of NOX and also reduce the amount of exhaust gas emitted.

The quantity of exhaust gas to be recirculated is accurately determined by the PCM.

An excessive EGR rate would lead to an increase in diesel particulate, CO (carbon monoxide) and HC emissions due to lack of air. Furthermore, combustion would become unstable due to a lack of O2 (oxygen).

Modern EGR systems consist of the following components:

MAF sensor,
Actuator motor-controlled EGR valve with
integrated position sensor.

Actuator motor

The actuator motor itself is actuated by the PCM as required. The opening cross-section of the EGR valve depends on the PCM control current (PWM).

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 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. It is therefore a closed loop.

Position sensor

In the face of increasingly stringent emission standards, EGR control via the MAF sensor alone is reaching its limits.

The position sensor in the EGR valve actuator motor unit supplies a further signal to the PCM for calculating the EGR rate.

This signal also allows light soiling on the EGR valve seat to be compensated. An accurate EGR rate, close to the limit of running smoothness, is therefore guaranteed.

Intake manifold flap

NOTE: Throttling of the intake air is only supported with certain versions.

A further step towards minimising NOX is the restriction of intake air via the intake air valve.

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.

Effects of faults

The majority of malfunctions in the EGR system are barely perceived by the driver. The only thing that can happen is increased combustion noise during idling.

If, however, the EGR valve jams open or if excessive metering by the PCM takes place, the following symptoms can occur:

rough engine running,
poor engine performance,
increased emissions of black smoke.

Diagnosis

Regulation of the EGR works as a system. The interaction of individual components is monitored.

Malfunctions lead to increased exhaust emissions which exceed the EOBD limits. Serious faults will also lead to the EGR system being switched off.

Since this is an emissions-related system, malfunctions are indicated by the MIL (malfunction indicator lamp).