Oxygen Enhanced Exhaust Gas Recirculation for Compression Ignition Engines

The benefits of oxygen enhancement in conjunction with EGR on emissions were investigated in a single-cylinder direct injection diesel engine. Cylinder pressure, NO_x, and particulate were measured for EGR sweeps with and without oxygen enhancement. In all cases, the total flow of oxygen to the cylinder was maintained constant. This was achieved by increasing cylinder pressure for typical EGR (N-EGR) and by adding oxygen to the intake stream for oxygen-enhanced EGR (O-EGR). The results show that O-EGR produced a substantially better combination of NO_x and particulate than N-EGR. In the N-EGR cases, the EGR dilutes the oxidizer causing lower NO_x and higher particulate. In O-EGR, flame temperature reduction leading to lower NO_x is achieved by a combination of higher molar specific heats of CO₂ and H₂O and dilution. Particulate emissions decreased or remain constant with increasing O-EGR. In addition to the obvious challenge of providing a source of oxygen to an engine, two operational challenges were encountered. First, as O-EGR was increased, the ratio of specific heats (C_p/C_v) of the cylinder intake charge decreased and decreased the compression temperature, causing significant changes in ignition delay. These changes were compensated for in the experiments by increasing intake temperature but would be challenging to manage in transient engine operation. Second, the increased water concentration in the exhaust created difficulties in the exhaust system and was suspected to have produced a water emulsion in the oil.