General
The exhaust recycling (EGR) primarily serves to reduce the amount of nitrogen oxides to comply with the emission limits even before exhaust aftertreatment. The suctioned in or supercharged fresh air is mixed with exhaust so that the concentration of oxygen in the fuel/air mixture reduces. If the concentration of oxygen reduces the combustion temperature drops which leads to reduced output of nitrogen oxides (NOx).
The fact that exhaust gas recirculation (EGR) is active over the whole rotational speed range means that the ratio between recycled exhaust gas masses and suctioned in or supercharged fresh air mass must be matched exactly and thus exactly regulated. For a fuel/air mixture with an excessively high proportion of exhaust the combustion deteriorates and the emission of soot particles, carbon monoxide (CO) and hydrocarbon increases. On the other hand the emission of nitrogen oxides (NOx) increases if the ratio of fresh or supercharged air is too high.
1 | Exhaust gas recirculation cooler |
2 | EGR line (cold) |
3 | Exhaust manifold |
4 | Charge air pipe |
Y621 | Exhaust gas recirculation positioner |
A | Exhaust from exhaust manifold |
G14.20-A011-76
Function
The exhaust gas recirculation rate refers to the relationship between the quantity of recycled exhaust gas and the quantity of fresh air. It is determined via the engine management control unit (MR) (A6), which compares the quantity of fresh air actually intaken or supercharged with the quantity of exhaust gas actually recycled.
The magnitude of the exhaust gas recirculation rate is calculated by the engine management control unit (MR) on the basis of the information provided by the charge air pressure and temperature sensor (B111).
Regulation of the exhaust gas recirculation rate then takes place via the exhaust gas recirculation positioner, which is actuated by the engine management control unit (MR) using a pulse width modulated signal. The exhaust gas recirculation positioner opens or closes the exhaust gas recirculation line (2) via its throttle valve and thus regulates what quantity of exhaust gas is mixed in.
The branched off exhaust initially flows through the exhaust gas recirculation cooler (1) attached to the coolant circuit. Here, it is cooled from approx. 650 °C to approx. 170 °C.
The exhaust gas recirculation rate can be increased by the cooling, since the density and therefore the quantity of recycled exhaust gas is increased. Furthermore, the added quantity of cooled exhaust causes a reduction in the NOx emissions by the lowering of the combustion temperature.
Once the exhaust gas coming from the exhaust manifold (3) has passed through the exhaust gas recirculation cooler (1), it passes through the exhaust gas recirculation line (2) and into the charge air pipe (4). It is mixed here with the fresh air coming out of the charge air cooler and then led to the individual cylinders for combustion.