Experimental response surface study of the effects of low-pressure exhaust gas recirculation mixing on turbocharger compressor performance

Abstract In Low-Pressure Exhaust Gas Recirculation (LP-EGR), clean exhaust gas is extracted downstream of the after treatment, and reintroduced upstream of the turbocharger compressor. A major pathway for engine fuel economy improvement, by employing LP-EGR, is the enhancement of compressor and turbine efficiencies by increased flow rates which moves the operating points towards higher efficiencies. However, what is often overlooked in the literature is the influence of LP-EGR/air mixing flow field on the compressor performance. Here, we systematically study this effect on a turbocharger unit for a diesel engine on a hot gas stand using response surface methodology. In addition, the mixing flow field of LP-EGR and air upstream of the compressor was scanned using a 3-dimensional directional probe. A reconfigurable T-junction mixer geometry was used, enabling the study of major mixing parameters such as mixing length, and EGR introduction angle. The mixing flow field showed a strong dependence on EGR-to-air momentum ratio, significantly affecting the EGR uniformity and axial velocity uniformity. Furthermore, multiscale stream-wise vortices were generated in the mixing section, with an intensity that increased with momentum ratio, and decreased with mixing length. In addition, by eccentric LP-EGR introduction a bulk swirl was generated in the mixing section similar to the flow field obtained using an intake guide vane. A decrease in compressor efficiency with LP-EGR introduction, compared to baseline compressor map, was observed. The efficiency degradation was larger at higher EGR momentum ratios and flow rates. In the range of mixing lengths limited by engine packaging, up to length-to-diameter ratio (L/D) = 2.5, the flow perturbations were not damped, and a decline in both compressor pressure ratio and efficiency was observed. Efficiency degradation mechanism is found to be the formation of strong vortices in the mixing zone upstream of the compressor, which are advected to the compressor impeller inlet and perturb the local incidence angle. In contrast, the EGR and axial velocity non-uniformities did not show a negative impact on the compressor. This study identifies the major flow parameters that cause significant degradation of compressor performance, and proposes a figure of merit for design of efficient LP-EGR mixers to benefit from the fuel economy advantages of LP-EGR architecture in diesel and gasoline direct injection engines.