Use of EGR to reduce emissions in automotive engineering

For some years, the automotive manufacturers are strongly looking on vehicles pollution. Concerning the diesel engine, one of the most constraining problems is the NO2 emission, which has to drastically reduce.

One manner to reduce this emission is the EGR (Exhaust Gas Recirculation) system. EGR injects a portion of the exhaust gas back into the cylinder, so it mixes with the fuel and air (Note that the exhaust adds to the fuel and air; it does not replace any of it).
The added mass in the cylinder is harder to heat up, so the combustion events have lower temperatures (600oC instead of more than 1300oC with no EGR system).

Considering that above 1300oC oxygen and nitrogen rejoin to make nitrogen oxides (NO, NO2, etc…), the EGR system reduces drastically the NO2 emission.

The main challenge with this system is to control precisely the amount of exhaust gas you reintroduce in the cylinder.
If you reintroduce not enough gas, the temperature combustion is too hot and then NO2 are produced. If you reintroduce too much gas, the emission of carbon monoxide (CO), hydrocarbon (HC) and soot particles raise because of the lack of Oxygen.

EGR system was classically made with hydraulic valve, but to respect the new norm concerning NO2 emission, you need a fast and accurate adjustment of the flow of gas in the EGR circuit. Thus, manufacturers are now working on the research and development of electric valves.