Low Pressure EGR: Notes
The known EGR system has been expanded by the low pressure EGR on the E70. This system offers advantages particularly at high loads and engine speeds. This is why it is used in the heavier E70 as it is often driven in the higher load ranges.
The advantage is based on the fact that a higher total mass of exhaust gas can be recirculated. This is made possible for two reasons:
- Lower exhaust gas temperature - The exhaust gas for the low pressure EGR is tapped off at a point where a lower temperature prevails than in the high pressure EGR. Consequently, the exhaust gas has a higher density thus enabling a higher mass.
In addition, the exhaust gas is added to the fresh intake air before the exhaust turbocharger, i.e. before the intercooler, where it is further cooled. The lower temperature of the total gas enables a higher EGR rate without raising the temperature in the combustion chamber.
- Recirculation before the exhaust turbocharger - Unlike in the high pressure EGR where the exhaust gas is fed to the charge air already compressed, in this system the exhaust gas is added to the intake air before the exhaust turbocharger. A lower pressure prevails in this area under all operating conditions.
This makes it possible to recirculate a large volume of exhaust gas even at higher engine speed and load whereas this is limited by the boost pressure in the high pressure EGR.
Low pressure EGR system
The following graphic shows the control of the EGR system with low pressure EGR:
| Index | Explanation |
|---|---|
| 1 | No EGR |
| 2 | Only high pressure EGR is active |
| 3 | High and low pressure EGR are active |
As already mentioned, the low pressure EGR has the greatest advantage at higher loads and is therefore activated, as a function of the characteristic map, only in this operating mode.
The low pressure EGR, however, is never active on its own but rather always operates together with the high pressure EGR.
Added to this, it is only activated at a coolant temperature of more than 55°C. The low pressure EGR valve is closed as from a certain load level so that only the high pressure EGR valve is active again. This means the EGR rate is continuously reduced.
The low pressure EGR system is located on the right-hand side on the engine directly next to the diesel particulate filter and the low pressure stage of the turbo assembly. The exhaust gas is branched off directly after the diesel particulate filter and fed to the intake air before the compressor for the low pressure stage.
| Index | Explanation |
|---|---|
| 1 | DPF |
| 2 | Turbocharger assembly |
| 3 | Exhaust turbocharger, low pressure stage |
| 4 | Low pressure EGR |
| 5 | Exhaust system |
The following graphic shows the components of the low pressure EGR:
| Index | Explanation |
|---|---|
| 1 | Temperature sensor - LP EGR |
| 2 | LP-EGR valve |
| 3 | Connection for positional feedback |
| 4 | Vacuum unit for LP-EGR valve |
| 5 | Coolant infeed |
| 6 | Coolant return |
| 7 | LP-EGR cooler |
| 8 | Sheet metal gasket with filter |
There is a fine meshed metal screen filter located at the exhaust gas inlet from the diesel particulate filter to the low pressure EGR system. The purpose of this filter is to ensure that no particles of the coating particularly in a new diesel particulate filter can enter the low pressure EGR system.
Such particles would adversely affect the compressor blades of the exhaust turbocharger.
The metal screen filter must be installed when fitting the low pressure EGR cooler to the diesel particulate filter otherwise there is a risk of the turbocharger being damaged.
| Index | Explanation |
|---|---|
| 1 | Cleaned blow-by gas |
| 2 | Ventilation, naturally aspirated operation |
| 3 | Intake manifold |
| 4 | Clean-air pipe |