Quote:
Originally Posted by phillip
Externally cooled Exhaust Gas Recirculation lowers cylinder temperatures pre- (and therefore post-) combustion. It is used primarily to reduce nitrogen oxide (NOx) emissions, which are created when Nitrogen and Oxygen (main components of air) interact at high temperatures. Because it's replacing air/fuel to be combusted, it can result in lower power efficiencies if applied incorrectly, but the cooling effect can sometimes be significant enough to cancel that out (less heat absorbed by the engine = more work produced by the piston).
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Errr the point of EGR is to reduce emissions and pumping loss by putting mostly inert spent gas back into the intake, and this should only be happening at part load. Under full load you need all the air you can get. The reason you want cooled EGR is because uncooled EGR will raise cylinder temperature and reduce efficiency (by lower heat capacity ratio). Lean burn is better for fuel economy because recirculated exhaust slows down combustion much more than fresh air does, but lean burn causes a major NOx problem.