Impact of Electrically Assisted Turbocharger on the Intake Oxygen Concentration and Its Disturbance
7. Conclusions
In this paper, experimental assessment on the impact of EAT on the air system control was conducted, based on a 6.7 L diesel engine. A disturbance rejection controller for the EAT air system was proposed and validated in simulation.
(1) At 265 Nm engine load and 1400 rpm engine speed condition, up to 7.8% FE benefit can be achieved with the assist function of EAT(assuming the electrical energy is free in the driveline electrical regeneration during vehicle braking), through the reduction of pumping loss from wider VGT vane open, relative to the nominal system. Further FE benefit is limited by the HP-EGR deficit from over-reducedp3(low HP-EGR flow capability).
(2) Transient boost response with EAT assist is improved by 52.1% relative to the nominal system.
The new dynamics of p2 introduced by EAT must be considered to avoid intake oxygen concentration overshoot and NOx spike. The EGR requirements should be reconsidered because of the excess fresh air from a fast boost response.
(3) The EGR flow-based control objective should be transformed into the intake oxygen concentration-based control objective, which aligns well with the EAT system and the associated control design. EAT assist improved the volumetric efficiency of the engine (by up to 13.6%
with 1.01 kW E-assist power at 265 Nm engine load and 1400 rpm engine speed in this paper), thereby increased the engine mass flow rate. This leads to HP-EGR gas dilution. EGR dilution and enhanced boost response makes the conventional EGR rate-based air system control not applicable to EAT system. Experimental results confirmed thatXoimworks better as the control output for EAT system than EGR rate for NOx control.
(4) In order to attenuate the disturbances from EAT onXoim, a disturbance rejection based controller was proposed and validated in a high-fidelity GT-SUITE model. Results showed over 36%
improvement in settling time and over 43% improvement in recovery time, relative to the conventional PID controller.
Author Contributions:C.W. and S.L. carried out the engine test, and set up the simulation platform. K.S. and C.W. worked together on the experimental data analysis,Xoimcontroller design and paper writing. H.X. provided a lot of valuable insights on the data analysis and controller validation.
Funding:This research was funded by Ford Motor Company.
Acknowledgments: The authors would like to thank Leon Hu, of Ford Motor Company for several fruitful discussions and his useful insights during the bi-weekly meeting. The authors would like to thank Devesh Upadhyay, Miachiel Van Nieuwstadt, Eric Curtis, and James Yi for their insights. A special thank you to Tongjin Wang from Tianjin University for assisting us in the management and maintenance of the test bench.
Conflicts of Interest:The authors declare no conflict of interest.
Nomenclature Symbols
J Rotational inertia
m. Mass flow rate
P Power
R Ideal gas constant
T Temperature
V Volume
Xoair Oxygen concentration in the air
Xoim Oxygen concentration at the outlet of intake manifold Xocyl Oxygen concentration in the cylinder after
combustion
Xocyl_out Oxygen concentration at the exhaust port
Xoegr Oxygen concentration in the recirculated exhaust gas
η Efficiency
XEGR EGR rate
A Cross-sectional area
cp,c Specific heat capacity of air
cp,e Specific heat capacity of exhaust gases Subscripts
C Compressor
Eng Engine
HP−EGR High pressure exhaust gas recirculation
T Turbine
TEMG Turbocharger shaft mounted electrical motor/generator
TC Turbocharger
vol Volumetric
1 Pre-compressor
2 Post-compressor
3 Pre-turbine
4 Post-turbine
max Maximum value
im Intake manifold
em Exhaust manifold
cyl Cylinder
des Desired value
Appendix A
ForXoimobservation, the existed observer, as developed by Park [23], was adopted. For completeness, it is explained briefly here.
Appendix A.1 Lyapunov-Based Observer Design
The state estimation observer is designed by rearranging the mean-value models of the oxygen concentration in the intake manifold and the exhaust manifold into the state-space form, as shown by:
.ˆ
X= aa11 a12
21 a22
Xˆ+ ww1
2
(A1)
ˆ
y= [ 0 1 ]Xˆ (A2)
where ˆX=% Xˆoim Xˆoegr
&T
,a11=pR2VTˆ2im(−mˆ.Eng),a12=pR2VTˆ2immˆ.HP−EGR,a21=pˆR3VTˆ3em(mˆ.Eng−m.f uelAFR), a22=pˆR3VTˆem3 (−mˆ.Eng−m.f uel),w1=pRˆ2VT2imm.CXoair,w2=0.
The form of the oxygen concentration observer is designed as:
.ˆ
X = aa11 a12
21 a22
Xˆ+ ww1
2 + ll1
2 (y−yˆ)
=AXˆ+W+L(y−yˆ) (A3)
wherel1andl2are the observer gains, which are designed on the basis of the Lyapunov stability theorem.
Appendix A.2 Xoim Observer Validation
TheXoimobserver was firstly built in Matlab/Simulink, followed by the automatic code generation using the Target-link from dSPACE Inc. Then the validation on the test bench was carried out to evaluate the real-time performance. The engine speed was fixed at 1400 rpm, with a 30% VGT vane open. A step change of HP-EGR valve position was manipulated from 25% to 15%, as shown in FigureA1.
After preliminary calibration of the model corresponding to the test bench, the average relative error of the estimated oxygen concentration in the intake manifold and exhaust manifold are 0.23% and 0.86% respectively.
The detailed performance assessment are available in TableA1.
Table A1.Case setup and results for the validation ofXoimobserver on the test bench.
Test Case Fuel Injection Rate uHP-EGR uVGT Xoim Xoegr
Avg.
Xerror
Max.
Xerror
Avg.
Xerror
Max.
Xerror
- mg/cycle % % % % % %
Case 1 18 25→15 30 0.23 2.53 0.86 3.15
Figure A1.Experimental validation of oxygen concentration observer.
From the above experimental results, it can be seen that the oxygen concentration of intake manifold and exhaust manifold estimated by the observer can approximate the actual oxygen concentration accurately, which lays the foundation for the control of oxygen concentration in the intake manifold of diesel engine equipped with EAT.
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