Study on the Optimized Matching of VNT and EGR Systems in a Diesel Engine

An experimental investigation of the coupling relationship between the VNT (Variable Nozzle Turbine) and EGR (Exhaust Gas Recirculation) systems was carried out in this study on a four-valve common-rail diesel engine in bench tests, as well as a transient optimization of the VNT and EGR control strategies. It was found that the overall level of NOX and flue gas emissions decreased with decreasing VNT opening. When the opening of the EGR valve was in the range of 5%-10%, the NOX and soot emissions could be reduced simultaneously by decreasing the VNT opening; the response of the intake volume was greatly improved by modifying the coordinated control strategy by boosting the control strategy under the acute load step condition; the NOX emission was significantly reduced by 6% overall, and the soot was reduced by 22.8% and the CO2 emission was improved in the transient-optimized WLTC test cycle. Therefore, by coupling the VNT and EGR systems and optimizing them transiently, emissions of pollutants during rapid engine acceleration can be decreased.


Introduction
With the stricter emission regulations and fuel efficiency regulations, internal combustion engines are developing in the direction of energy saving and emission reduction [1][2][3].VNT turbocharging technology can adjust the turbine flow ratio into the turbine depending on different working conditions, which can improve the diesel engine's dynamics and economy under certain working conditions [4].In combination with EGR technology, NOx emissions can be significantly reduced.In addition, the transient response of the engine is improved and the emission of pollutants from the engine is correspondingly reduced [5][6].The coupled technology of VNT and EGR is considered to be one of the most important technological routes for obtaining excellent performance in modern diesel engines [7].
When VNT and EGR work simultaneously, they have a strong relationship of mutual understanding through the interaction of boost pressure and turbo head pressure [8].The studies of Tanin et al. found that the VNT turbocharger can significantly increase the boost pressure, and the efficiency of the fuel consumption rate and the smoke emission are significantly improved by the increase of boost tension [9].Zamboni G et al. studied that VNT and EGR with the Zamboni G et al. have investigated the matching of VNT and EGR with a diesel engine [10].The optimized matching results showed that NOX emissions were improved by 58%-60%, BSFC was reduced by 5%-9.5% at low operation speeds.The control strategy of VNT and EGR is the key to ensure the transient performance of the engine.Shin et al. found that a multi-input control arithmetic improves the level of control of the boost pressure and the intake flow rate [11].Modification of the exhaust pressure to the transient control strategy of VNT and EGR by Wijetugne et al. resulted in reduced smoke emissions during acceleration and improved response of the whole system [12].
There are more previous studies on VNT-EGR coupling technology, but fewer studies have been conducted on VNT and EGR control strategies with transient corrections.Therefore, in this study, the effect of VNT-EGR coupling on the power, economy and emission performance of a diesel engine is investigated using a four-cylinder diesel engine as the research subject.Combined with the injection parameters, the VNT-EGR system of the diesel engine was matched and optimized.Transient corrections were also made to the VNT and EGR control strategies to reduce pollutant emissions during rapid acceleration.

Specifications of Test Engine
The test on the bench is carried out on a four-cylinder high-pressure common-rail diesel engine.Figure 1 illustrates the structure of the test engine platform.High pressure EGR unit is used in the engine and the exhaust gas is brought from the front of the turbine and passes through the EGR cooler and EGR valve into the intake manifold.Table 1 shows the main parameters of the diesel engine.Table 2 presents the main instruments and equipment used in the test.

Engine Operating Method and Test Conditions
A distribution of transient operating points on the rotating drum test stand during the WLTC test cycle is shown in figure 2. The whole working range is the main emission control region of the engine and the key optimization region for diesel engine development.In this paper, there are four representative working points selected in this region, which covers different loads at low, medium and high speeds.Table 3 lists the working points taken in the experiment.Figure 3 illustrates the relationship between smoke and specific emissions of NOx.EGR valve openings are 0%, 5%, 10% and 15% from right to left.The smoke emission at the four operating points increases as the NOx specific emission decreases, and it increases sharply when the NOx specific emission is around 3g/(kWh) -1 .At small EGR rates, the combination of exhaust gases is not significantly aggravated by the oxygen-enriched combustion conditions in the cylinder.However, when the EGR rate reaches a specific value, incomplete combustion of fuel in the cylinder begins and smoke emissions increase.At working point B75, when the EGR valve opening was 5% and the VNT opening was decreased from 45% to 25%, the NOX ratio emission and smoke emission were reduced by 15.4% and 41.9%, respectively.By reducing the VNT opening, the air-fuel ratio of the diesel engine can be improved and the fuel will have sufficient combustion.

Coordinated Control Strategy of VNT and EGR System
The VNT and EGR strategies are coordinated to include VNT closed-loop control based on boost pressure and EGR closed-loop control based on intake flow, indirectly ensuring proper EGR rates by controlling the boost pressure and intake flow values.At that instance, the amount of exhaust gas into the cylinder can be indirectly determined by controlling the intake air flow.
In equation ( 1),   represents the total intake flow of the engine;  in denotes the cooled intake pressure;   means the total intake flow and   denotes the cooled intake temperature.Equation ( 2) in   indicates the fresh intake air flow and   stands for the amount of exhaust gas.
Figure 4 and figure 5 demonstrate the effects of variations in boost pressure and intake volume on the emission performance of a diesel engine.The specified emission of NOx decreases with the increase in the target value of boost pressure, but the soot emissions increase correspondingly.Reason for this is that the coordinated control strategy increases the boost pressure by decreasing the VNT opening while increasing the EGR valve opening to keep the intake flow.When the objective value of inlet flow increases, the coordination control strategy increases the inlet flow by decreasing the opening of the EGR valve, the VNT opening increases accordingly, and NOX and soot emissions are significantly improved.

Problems and Optimization of Coordinated Control Strategy
Figure 6 and figure 7 show the relationship between boost pressure and intake flow rate for a vehicle during emergency load step conditions, respectively.The increase in boost pressure during the emergency load step situation is lagging due to the presence of turbocharger delay.At that time, when the VNT and EGR valves are not closed in time, the intake flow rate rises slowly at the beginning of acceleration.

Comparison of WLTC Test Cycle Results
The WLTC test cycling was conducted on the vehicles with consistent test boundary conditions both before and after the optimization of the coordinated control strategy.Figure 8    Figure 9 illustrates the comparison of emissions before and after optimization of the WLTC test cycle.Compared with the pre-optimization, the NOX after optimization is reduced by 6%, which is attributed to the fact that the peak NOX emissions in the low-speed section do not significantly decrease and the peak NOX emissions in the medium-speed, high-speed, and ultra-high-speed sections are all reduced.The speed and torque of the low-speed section are smaller, the overall NOX emission level is less, and the difference before and after the optimization is relatively minor.As the speed and load increase, the optimized instantaneous NOX emissions start to improve.At the same time, the cumulative soot emission was reduced by 22.8% after optimization, and the CO, THC and CO2 emissions were reduced by 43.7%, 44.7% and 0.6%, respectively.

Conclusions
To analyze the effect of coupling of VNT and EGR on the performance of a diesel engine, the study conducted a bench test of the setup.The main conclusions drawn from the investigation are as follows: 1) By reducing the VNT opening, the aggregate level of NOX and smoke emissions is decreased.When the opening of the EGR valve is in the range of 5%-10%, a reduction in both NOX and smoke emissions can be achieved by reducing the VNT opening.
2) Variations in the target values of boost pressure and intake volume affect the emission performance of the engine in the coordinated control strategy of the VNT and EGR systems.
3) In the emergency load step condition, the booster pressure corrects the coordinated control strategy and the responsiveness of the intake volume is significantly enhanced.4) Overall NOx emissions were reduced by 6%, soot by 22.8%, and CO2 emissions were improved in the WLTC test cycle after transient optimization.

Figure 1 .
Figure 1.The test bench.Figure 2. Distribution diagram of operating point of WLTC test cycle.

Figure 2 .
Figure 1.The test bench.Figure 2. Distribution diagram of operating point of WLTC test cycle.

Figure 3 .
Figure 3. Trade-off relationship between smoke and NOX emissions.

Figure 4 .
Figure 4. Change in target value of boost pressure.

Figure 5 .
Figure 5. Change in target value of intake flow rate.

Figure 6 .
Figure 6.Change relationship of boost pressure with time.

Figure 7 .
Figure 7. Change relationship of intake flowrate with time.

Figure 8 .
Figure 8. Engine speed curve for WLTC test cycle.

Figure 9 .
Figure 9.Comparison of pollutant emission values before and after WLTC test cycle optimization.

Table 1 .
Technical parameters of the engine.

Table 2 .
Main test equipment.

Table 3 .
Test assess working conditions.
3.1.Effect of VNT and EGR Valve Opening on Diesel Engine Emission performanceVNT and EGR systems are deeply inter-coupled.It is necessary to further analyze the coupling behaviors of the VNT and EGR systems.When their openings change, it will directly affect the engine's boost pressure, and the engine's performance will also vary accordingly.It is essential to further analyze the coupling characteristic of VNT and EGR system.
illustrate the engine speed for the WLTC test cycle.