Exhaust gas recirculation (EGR/EGR)

The principle of exhaust gas recirculation (EGR/EGR Exhaust Gas Recirculation) is that a quantity of the exhaust gases from the engine are recirculated through the intake manifold into the combustion chamber. The big disadvantage (mainly with diesel engines) is that the intake system is heavily soiled with soot, the vehicle has less energy and can start to smoke or run erratically when idling.

The exhaust gas is mixed with the incoming air and therefore the oxygen content of the combustion mixture decreases. EGR is used to reduce emissions of nitrogen oxides (NOx) that are produced when fuel is burned in petrol and diesel engines, etc. With exhaust gas recirculation, nitrogen oxides are already reduced during combustion. With exhaust gas aftertreatment measures (catalytic converter) alone, the prescribed emission limit values ​​can be met at great expense by chemically reducing the nitrogen oxides. This has particularly been the case since the introduction of the Euro 6 limits in September 2014.

The use of EGR in diesel engines has always been characterized by the conflicting goals of ensuring low nitrogen oxide emissions while minimizing particle emissions. High exhaust gas recirculation rates result in low nitrogen oxide emissions, but promote the formation of soot particles during combustion. Less NOx leads to more soot and vice versa. Since both are limited by the existing emission standards, it is important to weigh up exactly how much exhaust gas can be fed back into the combustion process. As the engine load increases, the tendency of the diesel engine to emit soot particles increases. In such operating conditions, care must be taken to ensure that the additional soot-promoting high return rate is reduced in order to avoid visible soot emissions - for example when the vehicle accelerates. The engine management is responsible for this. A quick detection of the respective engine operating point and a quick control of the recirculation valve enable a timely adjustment of the recirculation rate.

When using a soot filter (DPF), the larger soot emissions due to the use of EGR are no longer as important because the soot is trapped. This leads to low NOx emissions and there is no need to invest in NOx treatment.

In the case of petrol engines, the objective for using an exhaust gas recirculation system is different. The focus here is not on minimizing pollutant emissions, but on reducing fuel consumption. This is influenced by gas exchange losses, especially at part load.

When a conventional petrol engine is operated under partial load, the throttle valve creates a vacuum in the intake duct, which reduces the intake air mass and the engine output is reduced accordingly. At the same time, the gas exchange work increases. By admixing exhaust gas, the throttling of the clean air is reduced for a given amount of fuel and consequently the associated gas exchange losses are reduced.

In the case of a petrol engine with direct injection into the combustion chamber and charge stratification, the fuel burns from the outset in part-load operation with excess air, i.e. with the throttle valve wide open. As a result, the throttling losses are smaller and consumption is already reduced. An ignitable mixture is only around the spark plug. In these operating states, EGR has a similar effect to that of a diesel engine: the combustion temperature is lowered and nitrogen oxide emissions reduced.