Why does egr need to be cooled?

its exhaust gas recirculation . a part (5-15%) of the exhaust gas after cooling to a temperature drop of 200 degree C is circulated back to the inlet manifold along with the fresh charge in order to re duce the oxides of nitrogen.i want a detail desciption of the cooling process with the well defined reason for cooling.what is the need of cooling egr?what will be the consequences if we do not cool the egr before sending it back to the inlet manifold?is there any alternative to cooling egr?what if the egr is not at all used ,how else can the nox be reduced meeting the tier3 norms?|||I believe it's cooled to prevent pre-detonation of the fuel/air charge in the cylinder. So, if you send the hot gasses back into the manifold without cooling, you'll get knocking in the engine.|||About all I can tell you is that it has to be cooled to stop the egr from stopping up with crud. I had to replace a lot of them on the cummins engine until they started putting the insulation on the lines - that seems to help a lot.