Exhaust gas recirculation

Car manufacturers have been forced to lower exhaust emissions even further as a result of increasingly stringent legislation. This applies to both diesel and gasoline engines. The exhaust gas recirculation system helps to reduce nitrogen oxide emissions. On gasoline engines, the system also reduces fuel consumption when the engine is operated under partial load.

If the combustion temperature is high, nitrogen oxides are produced in the engine's combustion chamber. The combustion temperature in the combustion chamber is reduced by recirculating a portion of the exhaust emissions back to the fresh intake air. The lower combustion temperature results in less nitrogen oxide.
The exhaust gas recirculation rate for diesel and gasoline engines is shown in the following table:

A distinction must be made here between two types of exhaust gas recirculation: "internal" and "external" exhaust gas recirculation.

With the internal exhaust gas recirculation, the exhaust emissions and the fresh mixture are mixed inside the combustion chamber. On all four-stroke engines, this is achieved by the system-related overlap between the inlet and exhaust valves. The design means the exhaust gas recirculation rate is very low and can only be influenced to a limited extent. It is only since the development of variable valve control that it has been possible to actively influence the recirculation rate in a load dependent and rpm dependent manner.

External exhaust gas recirculation is carried out using an additional line between the exhaust manifold/exhaust pipe and the intake manifold as well as the EGR valve.

In order to influence the exhaust gas recirculation rate to a greater extent, pneumatic check valves and pressure limiting valves were installed, as were delay valves. Some systems also factor in the exhaust gas back pressure as a control pressure for the vacuum cell. In some operating states, the exhaust gas recirculation is switched off entirely. This is enabled by the installation of electrical change-over valves in the control line. Despite these possibilities for influencing the recirculation rate, the system was always dependent on the engine's load state and the associated induction pipe vacuum for controlling the vacuum cell.

On account of the high loads it has to withstand, the EGR valve is doubtless the biggest source of faults. Oil mist and soot from the exhaust emissions impede the valve, and the cross section of the valve opening reduces over time until it completely closes up. This causes a steady reduction in the recirculated quantity of exhaust emissions, and this is reflected in the exhaust emission performance. The high thermal loads further facilitate this process. In many cases the hose system for the vacuum is also a cause of faults. The vacuum required for the EGR valve can be lost due to leaks, meaning the valve no longer opens. If the EGR valve is not working due to a lack of vacuum, this may, of course, also be caused by a faulty pressure transducer or a thermovalve that is not working properly.

There are several ways of checking the exhaust gas recirculation system. Which method you choose depends on whether or not the system is capable of running self-diagnostics. Systems which are not capable of running self-diagnostics can be checked using a multimeter, a manual vacuum pump, and a digital thermometer.

However, before you start running complex tests, conduct a visual inspection of all system-related components. This means checking the following:

• Are all vacuum lines leak-tight, correctly connected, and routed without kinks?
• Are all electrical connections on the pressure transducer and change-over switch correctly connected? Are the cables OK?
• Are there any leaks on the EGR valve or the connected lines?

If no defects are found during the visual inspection, check the system using other tests and measurements.

The following procedure applies when checking vacuum-controlled EGR valves:

Some EGR valves come with a potentiometer to report back the position of the valve. These EGR valves are checked as described above. When checking the potentiometer, proceed as follows:
Remove the 3-pin connector and measure the total resistance at pin 2 and pin 3 of the potentiometer using a multimeter. The measured value must be between 1,500 ohms and 2,500 ohms. To measure the resistance of the slideway, connect the multimeter to pin 1 and pin 2. Using the manual vacuum pump, slowly open the valve. The measured value should begin at around 700 ohms and increase to 2,500 ohms.

During this check, the manual vacuum pump is not used to generate the vacuum, but instead is used as a manometer. Remove the vacuum hose from the pressure transducer to the EGR valve on the pressure transducer and connect the vacuum pump. Start the engine and slowly move the pressure transducer linkage. The display on the vacuum pump manometer must change accordingly.

Electric change-over valves have three vacuum connections. If only two connections are assigned, a non-sealing cap is fitted to the third connection.

To run the check, the manual vacuum pump can be used to perform a continuity test on the output lines of the change-over valve. To do so, connect the vacuum pump to an output line. If a vacuum can be generated, the change-over valve must be supplied with voltage.

To check the thermovalves, remove the vacuum hoses. Connect the manual vacuum pump to the centre connection. When the engine is cold, there must be no passage at the thermovalve. When the engine is at operating temperature, the valve must open the passage. In order to be independent of the engine temperature, the thermovalve can be removed and heated up in a water bath or using a heat gun. When doing so, constantly monitor the temperature to find out what the switching points are.

All the test values listed here are approximate values. Vehicle-specific connection diagrams and test values must be provided in order to obtain precise data.