I hope this is in the right category! Good education helps good aid in repairs right?

After reading the wikipedia article on EGR systems I was dissatisfied with the explanation and thought I could write a better one to replace it. I hope for it to be factually correct and simple to understand.

Please edit or rewrite my explanation if you think it can be better.

For every cycle of the combustion chamber within a internal combustion engine, a charge of air is drawn in during the induction phase. Without an EGR valve almost all of the oxygen in the air is burned when it is mixed with fuel and ignited. This creates heat, raising the temperature of the combustion chamber. When at peak temperatures the nitrogen and oxygen present in the air of the next induction phaze react causing NOx. An EGR valve aims to reduce the peak temperatures of the combustion chamber by replacing a percentage of the intake air with non combustible (inert) exhaust gasses. Less oxygen, less combustion, less heat, less NOx


1 Answer 1


True, but the temperature problem applies mostly to diesels. Diesels have a NOx problem due to lean air fuel ratio, and a catalytic converter cannot solve it because there is too little CO and HC in the exhaust that could be used to reduce NOx. Yes, diesels have catalytic converters, but they are oxidizing and not three-way: they eliminate what little CO and HC there is in the exhaust. Reducing NOx requires selective catalytic reduction (SCR), which requires a special SCR fluid that needs to be filled often.

Gasoline engines have another reason for EGR: the throttle valve can be held more open, helping to reduce pumping losses. This means that the efficiency of the engine is increased, causing better fuel economy. Remember that a gasoline engine operates at part load most of the time (when not accelerating), so the throttle valve is only partially open and thus causes pumping losses. In gasoline engines, the stoichiometric air fuel ratio obtained by closed loop control of the combustion process and the catalytic converter together eliminate practically all NOx emissions. This would occur quite well even without EGR. As a matter of fact, not all gasoline engines have EGR and they still have perfectly acceptable emissions, but probably efficiency and thus fuel economy are not optimal then.

Also, CO2 has a lower heat capacity ratio than air. Thus, the combustion temperatures are reduced by another way than because of CO2 being inert and thus not taking part in the combustion. So, your explanation for the temperature reduction is only partial, not explaining the full benefit of EGR.

  • Thank you for your input , I will research the points you have mentioned and see if I can incorporate them into my explanation.
    – Tee Jay
    Feb 13, 2017 at 13:51

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