An assessment of performance trade-offs in diesel engines equipped with regenerative electrically assisted turbochargers

Abstract

The regenerative electrically assisted turbocharger offers performance benefits, such as reduced turbo-lag, over the conventional turbocharger. However, regenerative electrically assisted turbocharger introduces additional control degrees of freedom as well as new causalities in the air-path dynamics of boosted engines. This is because the electrical power applied to (or removed from) the turbocharger shaft disrupts the natural coupling between the engine exhaust, and the turbocharger operation found in conventionally boosted systems. The ideal performance objective of regenerative electrically assisted turbocharger is to achieve fast boost response with improved fuel economy and minimal impact on engine-out emissions. These performance criteria, as we show in this article, drive performance trade-offs that must be considered for optimal system performance. In this article, we investigate these trade-off relationships based on a high fidelity GT-SUITE engine model for a heavy-duty diesel engine. An optimal control law is used in conjunction with a control-oriented plant model. Results from load step tests and from the federal test cycle (FTP-75) indicate that using a properly designed optimal controller, it is possible to manage these trade-offs, and to simultaneously achieve benefits in boost response, FE as well as engine-out emissions.

An assessment of the impacts of low-pressure exhaust gas recirculation on the air path of a diesel engine equipped with electrically assisted turbochargers

“…Simulation results for this test case are shown in Figure 8. In keeping with our earlier findings in Song et al, 30 it is observed that increasing assist levels allow operation with wider open VGT vanes thereby reducing pumping losses (lower p 3 ) and improving BSFC as well as Eq_BSFC. Increasing electrical assist levels, however, also increase the intake oxygen (X Oim ) surplus, making it necessary to increase HP-EGR flow via wider EGR valve openings: Note that in the simulation studies related to Figure 8, the LP-EGR valve was held at 80% open when LP-EGR was necessary.…”

“…It is also seen that for the REAT-HP-EGR system operating at high-load operating conditions (1500 r/min and 800 Nm, 2800 r/min and 800 Nm), it is beneficial to exploit the regeneration function to recover the exhaust energy as this reduces the Eq_BSFC. Detailed discussions about the benefits of electrical regeneration are provided in our previous work in Song et al 30 In this work, it is observed that using LP-EGR can provide minor additional FE benefits relative to the REAT-HP-EGR system. This is related to the increased mass flow rate through the turbine, as discussed earlier in Eq_BSFC: equivalent brake-specific fuel consumption; HP-EGR: high-pressure exhaust gas recirculation; DL-EGR: dual-loop exhaust gas recirculation.…”

Impact of Electrically Assisted Turbocharger on the Intake Oxygen Concentration and Its Disturbance Rejection Control for a Heavy-duty Diesel Engine

“…Turbo-lag is observed through the slow boost pressure (p 2 ) and torque responses, leading to undesired transient emissions spikes and fuel penalty. In addition, the poor transient boost response, together with smoke limited fueling, also leads to poor drivability performance [3]. Moreover, part of the exhaust energy is also wasted, either through the exhaust bypass valve or widely opened VGT vane, to avoid over-boost at high engine load conditions.…”

“…Therefore a higher HP-EGR flow rates may be of necessity to keep the intake oxygen concentration well matched with any hard constraints on engine out emissions. In other words, the EGR rate (X EGR ), as popularly used in the conventional VGT-EGR system, might not be an appropriate output for control for EAT [3]. So far no experimental assessment results are reported on the selection of control output variables for EAT assisted engines, according to the authors' best knowledge.…”

“…It should be noted that equivalent brake-specific fuel consumption (Eq_BSFC), as defined in Equation (2), can also be used to evaluate the efficiency of the system when the cost of assist power has to be taken into consideration [3]. In Equation (2) The increase in-cylinder charge amount caused by increased vol η , together with reduced fuel amount (shown in Figure 5), elevates the oxygen concentration in the HP-EGR gas (EGR dilution), i.e., the increasing Xoegr as shown in Figure 6.…”