Function

Exhaust gas recirculation (EGR) is a tried and tested method of reducing harmful substances. A defined quantity of exhaust gas is removed at the exhaust manifold and mixed back in with the intake air. This reduces the amount of oxygen in the fuel/air mixture, thereby reducing the combustion temperature in the cylinders. As hazardous nitrogen dioxide (NOx) is produced primarily at high temperatures and pressures, exhaust gas recirculation provides a means of reducing the levels of NOx concentration emitted to the environment by up to 50%. 

In diesel engines, exhaust gas recirculation also reduces the formation of soot particles by approx. 10%. 

The amount of exhaust gas recirculated is calculated by the engine control unit and regulated as appropriate for the design and dimensioning of the system by means of various actuators. These include: 

• The EGR valve The EGR valve is responsible for dosing the quantity of exhaust gas recirculated. It is mounted either on the exhaust manifold or in the intake area. In some engines, it is located inside a heat-resistant exhaust gas line which connects the exhaust manifold to the intake area. EGR valves in diesel vehicles have large opening cross-sections due to the high recirculation rates. The cross-sections of EGR valves in petrol engines are much smaller. Pneumatic and electromotive EGR valves are used in passenger car applications. 
  
  Pneumatic EGR valves are actuated via electro-magnetic valves under negative pressure. The negative pressure to control the valves is tapped from the intake pipe or generated with a vacuum pump. Electric or electromotive EGR valves are controlled directly by the control unit; they do not required negative pressure or a solenoid valve. 

• The butterfly valve (diesel) Since in diesel engines the pressure difference between the exhaust gas side and the intake side is not sufficient for high exhaust gas recirculation rates, "butterfly valves" are used in the intake pipe to generate the necessary negative pressure. 
  
  

• Electric changeover valve In simple systems with an electric changeover valve, the EGR valve simply has an open/close function. The negative pressure required to control the valve is tapped from the intake pipe or generated with a vacuum pump.
  
  

• Electropneumatic converter In systems with electropneumatic converters, the EGR valve is infinitely variable. The negative pressure required to control the valve is tapped from the intake pipe or generated with a vacuum pump. 

• EGR lines EGR lines are also a component of exhaust gas recirculation. They connect all of the components for exhaust gas recirculation from the exhaust gas extraction point to the intake area, running via the EGR radiator and the EGR valve. As space is at a premium inside the engine compartment, connection routes often have to be complicated. Both flexible and rigid EGR lines are available. 
  
  Modern EGR lines must meet strict requirements. They have to be able to equalise not only different and varying temperature levels at mounting points but also the mounting tolerances of the components involved, for example, in addition to being resistant to temperature, exhaust gas and corrosion. 
  
  Unfavourable operating conditions such as frequent short trips, non-compliance with service intervals or combustion problems affecting the engine can in some cases lead to EGR lines in which deposits have accumulated deteriorating or coking. EGR valves are at risk of the same problem. This reduces the exhaust gas recirculation rate, in turn creating other problems that can impair optimum engine running. If an EGR valve has to be changed because of clogging or coking, the EGR line connected to it must always be checked and replaced if necessary.

Safety

Faults in the EGR system manifest themselves in the form of bucking, irregular idling or lack of power. They are signalled by the engine indicator lamp lighting up. Vehicles affected by this type of fault will often switch to emergency operation; this changeover manifests itself to the driver in the form of significantly reduced power. The switch to emergency operation protects the engine against damage.

Environmental protection

Exhaust gas recirculation is one of the most important methods to reduce harmful substances. It helps to reduce the concentration of hazardous nitrogen oxides that are emitted into the environment by up to 50%. With diesel engines, exhaust gas recirculation also reduces the formation of soot particles by approx. 10%.

Depreciation

The exhaust gas recirculation system does not require any particular care. However, non-compliance with oil change and maintenance intervals can lead to malfunctions. An exhaust gas recirculation system that is in perfect working order is a prerequisite for passing the exhaust gas test.