Abstract
The electrically assisted turbocharger (EAT) shows promise in simultaneously improving the boost response and reducing the fuel consumption of engines with assist. In this paper, experimental results show that 7.8% fuel economy (FE) benefit and 52.1% improvement in transient boost response can be achieved with EAT assist. EAT also drives the need for a new feedback variable for the air system control, instead of the exhaust recirculation gas (EGR) rate that is widely used in conventional turbocharged engines (nominal system). Steady-state results show that EAT assist allows wider turbine vane open and reduces pre-turbine pressure, which in turn elevates the engine volumetric efficiency hence the engine air flow rate at fixed boost pressure. Increased engine air flow rate, together with the reduced fuel amount necessary to meet the torque demand with assist, leads to the increase of the oxygen concentration in the exhaust gas (EGR gas dilution). Additionally, transient results demonstrate that the enhanced air supply from the compressor and the diluted EGR gas result in a spike in the oxygen concentration in the intake manifold (Xoim) during tip-in, even though there is no spike in the EGR rate response profile. Consequently, there is Nitrogen Oxides (NOx) emission spike, although the response of boost pressure and EGR rate is smooth (no spike is seen). Therefore, in contrast to EGR rate, Xoim is found to be a better choice for the feedback variable. Additionally, a disturbance observer-based Xoim controller is developed to attenuate the disturbances from the turbine vane position variation. Simulation results on a high-fidelity GT-SUTIE model show over 43% improvement in disturbance rejection capability in terms of recovery time, relative to the conventional proportional-integral-differential (PID) controller. This Xoim-based disturbance rejection control solution is beneficial in the practical application of the EAT system.
Highlights
Modern diesel engines are normally equipped with a variable-geometry turbocharger (VGT) or fixed geometry turbocharger (FGT) [1,2]
At 265 Nm engine load and 1400 rpm engine speed condition, up to 7.8% fuel economy (FE) benefit can be achieved with the assist function of electrically assisted turbocharger (EAT)(assuming the electrical energy is free in the driveline electrical regeneration during vehicle braking), through the reduction of pumping loss from wider
Transient boost response with EAT assist is improved by 52.1% relative to the nominal system
Summary
Modern diesel engines are normally equipped with a variable-geometry turbocharger (VGT) or fixed geometry turbocharger (FGT) [1,2]. For a tip-in maneuver with the EAT system, a fast boost response can be achieved in assist mode without the need for an equivalent increase in exhaust turbine power. EAT,vane fast boost response leads to increased fresh flow relative the and wider open position improves the engine volumetric efficiency forair increased engineto mass nominal system, and wider open VGT vane position improves the engine volumetric efficiency for increased engine mass flow rate. Those two facts lead to increased dilution of the HP-EGR gas relative to the nominal system, which distinguished from conventional VGT-EGR engine. For EAT-assisted diesel engines, the transient response of the air system is much faster relative to the conventional VGT-EGR system.
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