Internal combustion engines such as diesel engines, gasoline engines, and gaseous fuel-powered engines exhaust a complex mixture of air pollutants as byproducts of the combustion process. These air pollutants are composed of gaseous compounds including, among other things, the oxides of nitrogen (NOx). Due to increased attention on the environment, exhaust emission standards have become more stringent and the amount of NOx emitted to the atmosphere from an engine can be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been utilized by engine manufacturers to comply with the regulation of engine emissions has been to implement exhaust gas regeneration (EGR). EGR systems recirculate exhaust gas to mix with combustion air supplied to an engine. The recirculated exhaust gas reduces a concentration of oxygen and increases a thermal mass within the engine's cylinders, thereby lowering a resulting combustion temperature. The lowered combustion temperature slows the chemical reaction of the combustion process and decreases the formation of NOx.
Although successful at reducing NOx, the effectiveness of an EGR system can be influenced by an amount of heat contained within the exhaust gas that is recirculated. That is, the exhaust gas that is recirculated back into the engine can contain large amounts of energy in the form of waste heat, which can increase the in-cylinder combustion temperature and counteract the desired NOx reduction. In addition, the excess heat also taxes the engine's cooling system and may represent an unutilized source of energy that, if harnessed, could increase engine efficiency.
An exemplary system implementing EGR is disclosed in U.S. Pat. No. 6,216,458 of Alger et al. that issued on Apr. 17, 2001 (“the '458 patent”). Specifically, the '458 patent discloses an exhaust gas recirculating (EGR) system for a turbocharged internal combustion engine. The EGR system includes a recirculation conduit for recirculating a volume of exhaust gas from the exhaust manifold to the intake manifold, a cooling air conduit for transporting a flow of cooling air, and a gas-to-air heat exchanger disposed in operative association with the recirculation conduit and the cooling air conduit. The heat exchanger is adapted for cooling the volume of the recirculated exhaust gas in the recirculation conduit.