An Assessment of the Impact of Exhaust Turbine Redesign, for Narrow VGT Operating Range, on the Performance of Diesel Engines with Assisted Turbochargers 2019-01-0326
The regenerative electrically assisted turbocharger (REAT) allows simultaneous improvement of boost response and fuel economy in boosted engines. This is achieved by the optimal scheduling of electrical assist (or regeneration) and vane position of the variable geometry turbine. Results show that these optimal vane positions are limited to a narrow operating band. Hence the exhaust turbine, associated with the REAT system, operates within a narrow band of exhaust flow incident angles relative to the nominal, unassisted, turbocharger. This, in turn, implies that the exhaust turbine of an assisted turbocharger can be redesigned for improved turbine efficiency. This suggests a departure from the standard practice of using a conventional turbine design, that must operate with a full range of vane positions, for an assisted turbocharger. However, this raises a critical design question related to the impact on the overall system performance when using such a limited, high efficiency, turbine design for an assisted turbocharging system.
In this paper, the impacts of the improved turbine efficiencies, of the REAT system, on the performance of a 6.7L Diesel engine are investigated via a high fidelity model. Results are compared against the performance of a similarly designed, high efficiency, conventional turbocharger. Results from a first principles fundamental analysis show that higher turbine efficiency reduces the pre-turbine pressure and hence the engine pumping loss. The benefit from turbine efficiency alone, however, decreases with increasing electrical assist levels. This is because the electrical assist has an effect similar to turbine efficiency improvement. In addition, a REAT system with a high-efficiency turbine also improves the electrical energy balance or state of charge (SOC), since the electrical energy demand reduces from an improved ability of the turbine to transfer work. On the other hand, REAT with a high-efficiency turbine is more prone to deficits in high-pressure exhaust gas recirculation relative to the nominal system.
Kang Song, Devesh Upadhyay, Liangjun Hu, Hui Xie
Tianjin Univ., Ford Research Laboratory, Ford Motor Company