Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT)

Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Contents lists available at ScienceDirect

Mechanical Systems and Signal Processing journal homepage: www.elsevier.com/locate/ymssp

Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT) Lei Liu, Zhijun Li n, Shiyu Liu, Boxi Shen State Key Laboratory of Engines, Tianjin University, 92 Weijin Road, Tianjin 300072, China

a r t i c l e i n f o

abstract

Article history: Received 12 October 2015 Received in revised form 14 December 2015 Accepted 19 December 2015

Based on pervious experimental research on the application of Exhaust Gas Recirculation (EGR) and Lean NOx Trap (LNT) with its effects on NOx emission control and secondary development of CHEMKIN software, an integrated NOx purification chemical kinetics mechanism including NOx adsorption, NOx desorption and NOx reduction process of LNT was created based on actual exhaust gases of the lean-burn gasoline engine. The effect of exhaust gases on NOx deterioration of LNT was investigated by modifying H2, O2 and overlap phase in mechanism of NOx desorption and NOx reduction process. Research found that the inlet temperature of LNT around 300 °C possesses the best NOx adsorption performance compared with 200 °C and 400 °C. Pt plays an import role in the process of NOx adsorption and NOx reduction. The reductive capability order of complex compound between Pt, and H2, CO and HC is Pt–H2 4Pt–CO4 Pt–C3H6. Both CO2 and H2O(g) could deteriorate NOx purification of LNT. The deterioration caused by H2O(g) is not significant as CO2 but harder to be regenerated. O2 could be beneficial to the NOx adsorption process, but it also could weaken the reductive atmosphere in the process of NOx desorption and NOx reduction. & 2016 Published by Elsevier Ltd.

Keywords: Lean-burn gasoline engine Exhaust Gas Recirculation Lean NOx trap Nitrogen oxides Mechanism

1. Introduction With the aggravated energy crisis and increasing concerns over environmental pollution, energy-saving and emissionreduction have become a worldwide focus. In order to meet present energy crisis and future increasingly stringent engine emission regulations, the application of after-treatment system will be more essential for compensating the higher technical cost caused by increasingly complicated in-cylinder design and optimization in the near future. Compared with all the other kinds of cars, passenger cars have the largest quantity and most of them are being powered with gasoline because of its comfortable driving experience and better emission. Lean-burn technology is the development tendency for future engines to possess a pronounced advantage in fuel economy [1,2], the application of which is especially important for future gasoline engines to improve their competitiveness. The major drawback of lean-burn technology application in gasoline engines is the inefficient NOx conversion of a conventional Three Way Catalyst (TWC) under an ultra-lean Air Fuel Ratio (AFR) condition [3]. At present, the major methods of controlling NOx are in-cylinder combustion process improvement [4–8] and high-efficiency catalytic technology of after-treatment system [9–12]. n

Corresponding author. E-mail address: [email protected] (Z. Li).

http://dx.doi.org/10.1016/j.ymssp.2015.12.029 0888-3270/& 2016 Published by Elsevier Ltd.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

2

Exhaust Gas Recirculation (EGR) is one of the effective methods to lower NOx emission by improving the in-cylinder combustion process [13,14]. However, its tolerance is strongly limited by in-cylinder combustion deterioration, such as misfire phenomenon [15]. As for the NOx reduction technology that differs from in-cylinder combustion control, catalytic technology of after-treatment system mainly consists of NOx Direct Decomposition (NDD) [16–18], Selective Catalytic Reduction (SCR) [19–21] and Lean NOx trap (LNT) [22–24] technology. The application of NDD is usually restricted in stationary source NOx emission control because of the relative not higher enough NOx conversion efficiency [25]. The application of SCR needs many large accessory devices which makes it more suitable for diesel engines because of the down-sizing tendency of gasoline engines in the near future. In comparison with NDD and SCR technologies, LNT can ensure a higher NOx conversion efficiency by adsorbing NOx at lean-mode period of the engine and regenerating NOx at rich-mode period of the engine, which make it attracts more focus because of its higher NOx conversion efficiency and easy mobilization. LNT bench test only can research the effect of the amount of exhaust gases (LNT inlet) with and without EGR on NOx conversion efficiency of LNT. If a theoretical understanding of NOx purification in LNT wants to be achieved, the mechanism of internal reaction of LNT should be taken into consideration [26,27], that is also beneficial to the engine control field [28–31]. Various simulation mechanism models of LNT have been proposed in the past decades. NOx storage model was firstly proposed by Olsson et al. [32], and it was further supplementary developed with propylene-based regeneration model [33]. The major drawback of Olsson's model is the unexplainable Nitric Oxide (NO) direct adsorption in the presence of Oxygen (O2). But the restriction was removed by Lingholm et al. later by assuming NO can be directly adsorbed on Barium Carbonate (BaCO3) and a source of unidentified S3 site [34]. However, unlike the NOx storage models, the NOx regeneration model proposed by Larson et al. [35] was generally highly accepted worldwide. In addition, within the creation of NOx regeneration models, there are many regeneration models being proposed by replacing reductants in recent years.

2. LNT working principle and mechanism modification 2.1. LNT working principle The actual LNT working process is shown in Fig. 1. The application of LNT technology on lean-burn gasoline engines is firstly proposed by TOYOTA in the 1990s [22]. The application of LNT requires the engine to periodically operate between rich-mode and lean-mode. To be specific, LNT could adsorb NOx when the engine operates at lean-mode for ensuring fuel economy, and the saturated LNT will be regenerated by reductive gases, such as HC, CO and H2 which could generate after the engine switch to the rich-mode. It should be noted that there is an overlap phase including NOx desorption and NOx reduction period simultaneously at the beginning of LNT switching to rich-mode from lean-mode, that overlap phase is very short but possesses a significant effect on the entire regeneration process of LNT. 2.2. LNT mechanism modification The modification parts of mechanism in this research is shown in Fig. 2. Compared with the previous mechanism researches of LNT, this research is mainly based on the catalyst of Platinum/ Barium Oxide/Aluminum Oxide (Pt/BaO/Al2O3) with the following modified aspects. Firstly, in the research method, compared with most of the worldwide research simulating the exhaust gases by configuring the standard gases, this research take the actual exhaust gases of the engine test bench as the inlet gases of LNT. Secondly, compared with most of LNT mechanism mainly focusing on sole nitrate route or sole nitrite route, this mechanism conclude both nitrate route and nitrite route simultaneously. Finally, compared with most of the current research focusing on the sole reductive gas of Hydrogen (H2), Carbon monoxide (CO) or Hydrocarbon (HC), this research takes all the reductive gases generated from the engine into consideration. In addition, the mechanism of NOx adsorption process also consider the amount of O2 correction, and the overlapping phase between the processes of NOx desorption and NOx reduction is also taken into consideration.

Fig. 1. Actual LNT working process.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

3

3. Experimental setup and methodology 3.1. System overview The layout of lean-burn gasoline engine test bench with EGR and LNT after-treatment system is shown in Fig. 3. It should be noted that, a temperature controller was installed in the bench test system for controlling the inlet temperature of LNT. The modified lean-burn CA3GA2 gasoline engine in the bench test system is shown in Fig. 4. The specific engine parameters is shown in Table 1. The concentration of NOx, O2, CO, CO2 and HC in the exhaust is monitored by an exhaust gas analyzer (Manufacturer: HORIBA, Japan, Model: MEXA-7100). The experimental LNT is shown in Fig. 5 and its parameters is shown in Table 2. 3.2. Test matrix Test matrix in this research is shown in Table 3. The concentration of exhaust gases of the lean-burn gasoline engine with an EGR system (inlet gases of LNT) is shown in Table 4. 3.3. Simulation methodology This research is mainly based on chemical reaction kinetic simulation of CHEMKIN software, and take the exhaust gases of the lean-burn gasoline engine with an EGR system which were collected in the previous engine bench test as the inlet gases of downstream LNT for the following NOx purification process simulation.

Fig. 2. Modification parts of mechanism.

Fig. 3. Layout of lean-burn gasoline engine test bench with EGR and LNT after-treatment system.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

4

Fig. 4. Modified lean-burn CA3GA2 gasoline engine.

Table 1 Engine parameters. 12 Valves – 3 cylinder in-line 1-3-2 76
73 1 10 90/(3600–4400) 50/6000

Engine type Combustion order Bore
stroke (mm) Displacement volume (L) Compression ratio Maximum torque (Nm/rpm) Maximum power (kW/rpm)

Fig. 5. LNT.

Table 2 LNT parameters. Carrier catalyst Carrier length (mm) Carrier diameter (mm) cpsi (Cells Per Square Inch) Catalyst length (mm) Catalyst diameter (mm) Horn length (mm)

Pt/Bao/Al2O3 200 110 400 480 50 40

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

5

4. Results and discussion 4.1. Validation The engine bench test of EGR-LNT synergetic control system of lean-burn gasoline engines on NOx emission has been finished in the previous researches. The comparison of LNT outlet NOx emission between engine bench test and chemical reaction kinetics simulation is shown in Fig. 6. It can be seen from Fig. 6 that, both engine bench test and chemical reaction kinetics simulation exist NOx slip phenomenon, which indicates that not all the NOx can be absorbed in LNT and some of them could escape from carrier capture. As for the lean-mode of the engine, that is mainly because the NOx spontaneous desorption caused by thermodynamic and kinetics factors during NOx adsorption process, but the amount of NOx desorption is quite small compared with the amount of NOx adsorption. In addition, NOx spontaneous desorption happens in the entire LNT, although some NOx desorbed in the front of LNT could re-adsorb in the middle or in the rear of LNT, there are still some NOx slip caused by losing adsorption sites during repeatedly adsorption and desorption process. As for the rich-mode of the engine, that is mainly because large amount of instantaneous NOx desorption cannot be in touch with and react with reductants sufficiently, which results in the final NOx slip. NOx slip of LNT from the engine bench test has the same trend with chemical reaction kinetics simulation results, which verified the validity of the mechanism. It should be noted that NOx slip results of chemical reaction kinetics simulation is 120–175 ppm lower than that of the engine bench test. That is mainly because of sampling time delay caused by the distance between NOx sampling probe and NOx sensor in HORIBA. So as for the engine bench test for LNT research in the

Table 3 Test matrix. Speed (rpm)

Load (MPa)

AFR

Flow (g/s)

Inlet Temperature (°C)

trich:tlean

2800

0.3

 Rich 12  Lean 23

 Rich 12.1  Lean 14.4

 200  300  400

7:56

Table 4 Concentration of inlet gases. NOx (ppm)

 Rich 30.4  Lean 1199

CO (ppm)

O2 (ppm)

CO2 (ppm)

HC (ppm)

 Rich 30180  Lean 14.05

 Rich 27400  Lean 68,400

 Rich 110000  Lean 106000

 Rich 3590  Lean 5890

Fig. 6. Comparison between experimental results and simulation results.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

6

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Fig. 7. BaO–NO2 site fraction (300 °C).

future, the sampling time delay should be considered, that is also very important for ensuring less NOx slip during switching process between lean-mode and rich-mode of the engine. 4.2. Contrast between nitrate route and nitrite route Although NO occupies a completely majority position in NOx compared with NO2, NO2 that indeed exists in the engine emissions, so it is not easy to distinguish nitrate route and nitrite route clearly in NOx adsorption process. This research divides nitrate route and nitrite route separately by assuming all NOx is NO and NO2 is 0. In the NOx adsorption process, BaO–NO2 is the mainly intermediate product in the nitrite route. Its site fraction is shown in Fig. 7. BaO–NO2 shows a periodical trend in the switching process between lean-mode and rich-mode of the engine. Its site fraction in the first cycle is higher than the rest of cycles. BaO–NO2 site fraction in the second cycle is lower than the first cycle but higher than the rest of cycles. That is mainly caused by the initialization of the first cycle in simulation, which is the state of LNT with no NOx adsorbed, and the rest of cycles are in a relatively stable state. In those stable cycles, there is a small fluctuation in the switching process between rich-mode and lean-mode. It increases firstly and then starts to decrease. The small fluctuation is a “crossroad” between nitrate route and nitrite route for NOx adsorption, the fluctuation degree is decided by the amount of instantaneous selection of nitrate route and nitrite route. After that “crossroad”, BaO–NO2 decreases with the increase in lean-mode time, that is the process of Ba(NO3)2 formation from BaO–NO2. In addition, as for the following BaO–NO2 site fraction, it shows a generally similar trend, which indicates that the amount of nitrate route selection and nitrite route selection is decided by the fixed ratio between NO and NO2 at the inlet of LNT, and some other inlet boundary conditions. It can be found from Fig. 8 that, similar with BaO–NO2 site fraction, Ba(NO3)2 site fraction also shows a periodical trend, but the trend is opposite to BaO–NO2 site fraction. The increase or decrease of them are both opposite to each other. The specific trend of them is shown in Fig. 9. Ba(NO3)2 shows a gradually reduction during rich-mode, but BaO–NO2 could accumulate because it is not a reductive target. That explains the higher BaO–NO2 site fraction formation in nitrite route at the beginning of NOx adsorption process. Then BaO–NO2 shows a decrease trend with the increase in lean-mode time, that is mainly because of the formation of Ba (NO3)2 from intermediate products, which also explains the increase trend in Ba(NO3)2. As for the situation in the research that all NOx is NO, total Ba(NO3)2 site fraction contribution from nitrite route is 12%, then the contribution of 13% from nitrate route can be deduced. Thus, nitrate route and nitrite route possess the equivalent probability and it also could conclude that nitrate route will occupy a majority position in actual emissions which include both NO and NO2. As for the LNT which possesses a fixed ratio between NO and NO2 at inlet, because its internal catalyst possesses some noble metal, so the NOx adsorption process also will be influenced by inlet temperature. In order to analyze the specific reason, additional inlet temperature of 200 °C and 400 °C was taken into consideration. Fig. 10 shows the Ba(NO3)2 site fraction at the LNT inlet temperature of 200 °C, BaO–NO2 shows periodical changes, but it does not reproduce the previous decease trend. This indicates that BaO–NO2 formed from nitrite route does not further form Ba(NO3)2 and it is terminated in the intermediate-process. In addition, it also should be noted that the maximum site fraction of BaO–NO2 is just 18% which is far lower than that at 300 °C. It verifies that inlet temperature has an important influence on nitrate route selection and nitrite route selection in LNT, which is mainly due to the restriction of temperature activation window of noble metal. Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Fig. 8. Ba(NO3)2 site fraction (300 °C).

Fig. 9. Specific trend of BaO–NO2 site fraction and Ba(NO3)2 site fraction (300 °C).

Fig. 10. BaO–NO2 site fraction (200 °C).

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

7

8

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

As it is also shown in Fig. 11, the maximum Ba(NO3)2 site fraction in LNT at inlet temperature of 200 °C is only 0.1%. It almost seems to be no generation if the deviation effect is taken into consideration. That is mainly caused by the intermediate-process termination which has been discussed above. The other reason is that nitrate route is easily influenced by the inlet temperature of LNT. As it is shown in Fig. 12, when inlet temperature of LNT is 400 °C, BaO–NO2 also shows periodic changes between leanmode and rich-mode, its site fraction is pronounced higher than that at 200 °C and 300 °C. All the above discussions confirmed that the inlet temperature has an influence on BaO–NO2 site fraction in LNT, and with the results at inlet temperature of 200 °C taken into consideration. It can be concluded that as for the nitrite route, BaO–NO2 site fraction increases with the increase in inlet temperature. Of course, the activity of Pt should not be neglected. It should be noted that as a contrast to that at inlet temperature of 300 °C, BaO–NO2 does not show a decreasing trend after the small rebound. It shows a slow decrease at once after the instantaneous increase at the beginning, that is the process of further oxidation of BaO– NO2 to form Ba(NO3)2. The instantaneous increase of BaO–NO2 is because of the unstable Ba(NO3)2 caused by high temperature. So, as for BaO–NO2, that is the reason why it stays in the intermediate-process without converting to a large amount of Ba(NO3)2. As it is shown in Fig. 13, Ba(NO3)2 site fraction increases with the increase in time, but the increasing rate decreases with that. Ba(NO3)2 site fraction in the first cycle is lower than that in the following cycles, that is because the Ba(NO3)2 formation in the first cycle is less stable compared with the following cycles. After that, Ba(NO3)2 could be stabilized at 20%, less than that at 300 °C but higher than that at 200 °C, which concludes that the best activity temperature window for NOx adsorption is around 300 °C. The relative lower NOx adsorption activity at higher temperature is restricted by Pt activity temperature, and it is also restricted by the unstable Ba(NO3)2 which is just formed.

Fig. 11. Ba(NO3)2 site fraction (200 °C).

Fig. 12. BaO–NO2 site fraction (400 °C).

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

9

4.3. The function of Pt The noble metal Pt plays an important role in both nitrate route and nitrite route. As for the NOx adsorption process. Nitrate Route: NOx þBaO/BaCO3-Ba(NO3)2

(1)

Nitrite Route: NOx þBaO/BaCO3-Ba(NO2)2

(2)

NOx þBa(NO2)2-Ba(NO3)2

(3)

Actually, the equations of (1) and (2) (3) are simplified routes. The adsorption of NOx on BaO and BaCO3, and final formation of Ba(NO3)2 are all attributed to catalytic action of Pt. Al2O3 also plays a role in the process of NOx adsorption, but the amount of its participation is negligible, it mainly helps to distribute noble metals and capture more NOx. So, in this mechanism, the role of Al2O3 is neglected. The most important role of Pt in NOx adsorption process is forming Pt–O, which further oxidizes Pt–NO to Pt–NO2. Pt– NO2 also participates in the oxidation process of intermediate BaO–NO2, BaO–NO3 and BaO–O, which contributes to the final product of Ba(NO3)2.

Fig. 13. Ba(NO3)2 site fraction (400 °C).

Fig. 14. Pt–O site fraction.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

10

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Fig. 15. Pt–NO site fraction.

Fig. 16. Pt–NO2 site fraction.

Pt–O, Pt–NO and Pt–NO2 in NOx adsorption process is shown in Figs. 14–16. As it is shown in Fig. 14, during the lean-mode of engine, Pt–O still maintains a relative higher site fraction after ensuring NOx adsorption, which indicates that the oxygen concentration under the selected lean-AFR is enough for NO oxidation and NOx adsorption. In addition, under the rich-mode of the engine, Pt–O also maintains a small site fraction even though O2 stays at a lower level in LNT, and that will have an influence on NOx regeneration. The existence of O2 in LNT during the richmode of the engine would weaken the reductive atmosphere of NOx desorption and NOx reduction created by reductive gases such as H2, CO and HC. This affects the NOx desorption and NOx reduction process, and further affects the adsorption sites and the amount of NOx adsorption. Finally affecting the overall NOx emission control effect of LNT. As it is shown in Fig. 15, Pt–NO maintains a small site fraction of 0.005–0.011% in LNT under the lean-mode and richmode of engine. It illustrates that except small part of non-oxidized Pt–NO, all the other Pt–NO are oxidized into Pt–NO2 and participate into the reactions related with intermediate products, which finally form into Ba(NO3)2. Of course, there are also some part of them that are converted into other intermediate products. As it is shown in Fig. 16, Pt–NO2 maintains a small site fraction of 0–0.0018% in LNT under the lean-mode and rich-mode of the engine, it also maintains a lower level. It illustrates that, same as NO–Pt, except small part of Pt–NO2 which do not participate the reaction of Ba(NO3)2 formation, almost all the other Pt–NO2 participate in the reaction of oxidizing BaO–NO2 into Ba(NO3)2. As for the purification reactions in LNT, Pt does not only participate in NOx oxidation reaction and further NOx adsorption reaction in NOx adsorption process. Pt also plays an import role in NOx regeneration. For the surface reaction kinetics, direct reactions between NOx, and H2, CO and C3H6 without Pt are all inefficiency. However, NOx regeneration efficiency could be improved under the catalytic action of Pt with the adjustment of the catalytic activity temperature window. Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

11

In the NOx regeneration process in LNT, the intermediate products with Pt participated are mainly Pt–H2, Pt–CO and Pt– C3H6. Pt–C3H6 site fraction is shown in Fig. 17. Pt–C3H6 increases with the increase in rich-mode time and shows a spurt at the beginning of each rich-mode, it reaches the peak value around 2.3% at the end of all cycles. Compared with that at lean-mode, Pt–C3H6 site fraction during richmode can reach a higher level. That is mainly because H2, CO and HC increases more obviously in the incomplete combustion of in-cylinder at instantaneous rich-mode, and it reaches the highest value at the end of rich-mode cycle. The accumulative trend of Pt–C3H6 is firstly because Ba(NO3)2 site fraction which could be reduced during rich-mode decreases with the rich-mode time. Secondly it is because its relative weakness reducibility compared with the other two reductive gases. Finally it is because with the increase in rich-mode and lean-mode cycles, internal LNT exist some catalyst degradation phenomenon, which is so-called deactivation state, and the catalyst cannot be effective touched and further reduced to adsorption empty sites. As it is shown in Fig. 18, with the increase in rich-mode time, Pt–CO site fraction shows a regular trend and is smaller than Pt–C3H6 site fraction. That is mainly because the reductive capability of Pt–CO is stronger than Pt–C3H6, so the reaction priority of Pt–CO is higher than that of Pt–C3H6, which finally results in the Pt–CO site fraction smaller than Pt–C3H6 site fraction in rich-mode. In addition, different from the increase in Pt–C3H6 site fraction, with the increase in rich-mode time, Pt–CO site fraction is stabilized at around 0-0.125%. It indicates that, at each rich-mode in LNT, the consumption of Pt–CO is dynamic equalized with the amount of inlet CO and the Pt–CO site fraction.

Fig. 17. Pt–C3H6 site fraction.

Fig. 18. Pt–CO site fraction.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

12

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Fig. 19. Pt–H2 site fraction.

Fig. 19 shows that compared with Pt–C3H6 site fraction and Pt–CO site fraction, Pt–H2 site fraction is smaller. It is stabilized at a lower level in the whole lean-rich duration because of the stronger reductive capability. Of course, its inherent lower generation amount is also an important factor and should not be neglected, but if taking the order of magnitudes into consideration, it can conclude that the reductive capability is the most important factor resulting in the lower site fraction. In addition, it also should be noted that, different from Pt–C3H6 site fraction change, the decrease in Pt–H2 site fraction and Pt–CO site fraction occurs instantaneously, that is mainly because NOx desorption and NOx reduction coexist with each other at the beginning of regeneration cycle in LNT, and there is NOx breakthrough phenomenon. After that, Pt–CO site fraction and Pt–H2 site fraction decrease to 0 instantaneously, which indicates that the reductive capability of Pt–CO and Pt– H2 are used to the extreme. 4.4. Overall effect of EGR on NOx slip of LNT NOx slip of LNT under upstream EGR rate of 0, 10%, 15%, 20%, 25% and 30% are shown in Fig. 20. There is a NOx slip phenomenon at outlet of LNT through the entire working process at all upstream EGR rate. As for NOx adsorption process in LNT, that is mainly because of the desorption phenomenon caused by thermodynamics and kinetics factors, but the spontaneous NOx desorption is very less compared with NOx adsorption. In addition, NOx slip could be found in all axial direction of LNT. Although some NOx slip which occurred at the front of LNT could be reabsorbed at the end of LNT, NOx slip caused by losing adsorption sites always exist throughout the whole adsorption period because of reiteratively adsorption and desorption. As for the NOx regeneration process in LNT, it is mainly because the NOx breakthrough at the beginning of desorption period could not possess enough time to react with reductive gases and participate in redox reactions, which finally results in NOx slip. NOx slip in LNT increases with the increase in adsorption time. That is mainly because with the time increasing, internal adsorption sites of LNT are occupied with NOx and other gases which could compete with NOx, such as CO2 and H2O(g), which finally result in the decrease in NOx adsorption capability of LNT. In addition, it can also be seen from Fig. 20 that there is a stable NOx slip interval at the end of each adsorption period. That is mainly because LNT possesses more adsorption sites and higher amount of NOx adsorption could adsorb in them at the beginning of adsorption period, so NOx adsorption possesses the priority compared with NOx desorption, which result in lower NOx slip. But with the increase in adsorption time, the quantity of surface adsorption sites start to decrease, NOx adsorption and NOx slip gradually reach a balance state at the end of adsorption period. Under the different upstream EGR rate, the minimum NOx slip which indicates the optimum NOx control effect, and the difference between inlet NOx and maximum NOx slip which indicates the worst NOx control effect are shown in Table 5. And the difference between Inlet NOx and Maximum NOx Slip under different upstream EGR rate is shown in Fig. 21. That difference mainly indicates the deterioration degree of NOx purification of LNT. The difference between inlet NOx and maximum NOx slip decreases with the increase in upstream EGR rate and stabilizes at 11.6–17.0 ppm after upstream EGR rate reaches beyond 20%. That is mainly because the entry amount of CO2 and H2O(g) of LNT increase with the upstream EGR rate, which aggravates the adsorption competition reaction among NOx, CO2 and H2O(g) in LNT, increases the deactivated adsorption sites and further deteriorates the overall NOx adsorption effect and increases NOx slip. There is a small rebound when upstream EGR reaches 20–30%, that is mainly because the adsorption sites reaction competition is aggravated after upstream EGR rate over 20% and reaches the top tolerance that downstream LNT could withstand, which results in an unstable atmosphere of the difference. But that phenomenon cannot influence the overall decrease tendency with the increase in upstream EGR rate. Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

13

Fig. 20. NOx slip of LNT under upstream EGR rate of 0, 10%, 15%, 20%, 25% and 30%. (a) EGR Rate¼ 0 (b) EGR Rate¼ 10% (c) EGR Rate¼15% (d) EGR Rate¼20% (e) EGR Rate¼ 25% (f) EGR Rate¼30%.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

14

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Table 5 The optimum and the worst NOx control effect of the EGR-LNT synergetic control system. Upstream EGR rate

Difference between inlet NOx and maximum NOx Slip

Minimum NOx slip

0 10 15 20 25 30

49.0 49.0 33.0 14.1 17.0 11.6

300 205 138 140 276 311

Fig. 21. Difference between Inlet NOx and Maximum NOx Slip.

Fig. 22. Minimum NOx slip.

The minimum NOx slip under the different upstream EGR rate is shown in Fig. 22. The minimum NOx slip is always the NOx slip at the beginning of LNT working process, which indicates the characteristic of LNT best working state. With the increase in upstream EGR rate, the minimum NOx slip of LNT firstly shows an increase trend and then starts to decrease after that. That is mainly because when EGR rate is 0, 10%, 15% and 20%, the positive effect of EGR-LNT synergetic control system compared with sole EGR and sole LNT increases with the increase in upstream EGR rate. And when upstream EGR rate is 15% and 20%, the positive effect reach the optimum. But when upstream EGR rate further increase to 25% and 30%, excessive CO2 and H2O(g), along with the increase of upstream EGR rate, enter into the internal of LNT and aggravate Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

15

the deterioration effect by competing with NOx, and further deteriorate the overall NOx emission control effect of EGR-LNT synergetic control system. The enlarged deterioration effect of LNT caused by excessive upstream EGR rate under the upstream EGR rate of 25% and 30% is specifically shown in Fig. 23. When upstream EGR rate is 25% and 30%, after lean-burn gasoline engine switch to rich-mode from lean-mode, their oscillation effect in internal LNT is much higher than that at upstream EGR rate of 0, 10%, 15% and 20%. The oscillation is the strength of unstable NOx adsorption and desorption phenomenon in surface adsorption sites, which is caused by CO2 and H2O(g). That strength at upstream EGR rate of 30% is higher than that at 25%. In addition, with the increase in lean-mode time, the unstable NOx adsorption and desorption phenomenon shows a strengthen effect at the beginning and a gradually weaken effect after that, and it finally tends to a stable value, which is the final collected NOx emission. In the previous researches, CO2, H2O(g) and O2 were found that they are all the main gases which vary with the upstream EGR rate. But the influence of them are not same. It will be specified in the following discussion. 4.5. Specific effect of CO2, H2O(g) and O2 on NOx slip of LNT 4.5.1. CO2 influence results CO2 could be adsorbed in LNT in the form of BaCO3. BaCO3 site fraction represents the adsorption sites competition between CO2 and NOx. BaCO3 site fraction which vary with upstream EGR rate, and total site fraction of Ba(NO3)2 and Ba (NO2)2 which represent the NOx adsorbed in LNT are shown in Fig. 24.

Fig. 23. Enlarged NOx slip at upstream EGR rate of 25% and 30%.

Fig. 24. BaCO3 site fraction and total site fraction of Ba(NO3)2 and Ba(NO2)2.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

16

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

BaCO3 site fraction decrease to a stable value with the increase in adsorption time. That is mainly because NOx possesses a better adsorption capability than CO2, and part of CO2 adsorbed in adsorption sites is replaced by NOx with the increase in adsorption time. On the contrary to BaCO3 site fraction, total site fraction of Ba(NO3)2 and Ba(NO2)2 increase to a stable level with the increase in adsorption time, the reason is the same with adsorption capability of CO2 and NOx. It also can be seen from Fig. 24 that total site fraction of Ba(NO3)2 and Ba(NO2)2, and site fraction of BaCO3 show a regular variation under the different upstream EGR rate. BaCO3 possesses the highest variation in all Ba related products under the different upstream EGR rate, and total site fraction of Ba(NO3)2 and Ba(NO2)2 possess the secondary position. Total site fraction of them almost reach 70%, which indicates that NOx and CO2 are the major adsorption gases in LNT. In addition, if taking some deactivated adsorption sites into consideration, then it could conclude that there are only a little other gases were adsorbed in adsorption sites, so the influence is also very weak. BaCO3 Site fraction increases with the increase in upstream EGR rates, but total site fraction of Ba(NO3)2 and Ba(NO2)2 decrease with it. It indicates that NOx possesses the better absorbability compared with CO2, but higher CO2 come along with upstream EGR could compensates its absorbability weakness. And as for this case, the amount of CO2 becomes the major factor in influencing Ba related adsorption sites. 4.5.2. H2O(g) influence results H2O(g) enters into internal LNT and competes with NOx for adsorption sites mainly in the form of Pt–OH and Pt–H2O, and the site fraction of Pt–OH possesses the priority position compared with Pt–H2O. As it is shown in Fig. 25, compared with total site fraction of Ba(NO3)2 and Ba(NO2)2, Pt–OH site fraction shows a lower level. In addition, compared with CO2, Pt–OH which comes along with H2O possesses no such significant influence on affecting NOx conversion efficiency. Site fraction of Pt–OH increases with the increase in upstream EGR rate, but it possesses no significant influence on deteriorating NOx conversion efficiency until upstream EGR rate reaches 20%. It indicates that H2O(g) only shows a pronounced influence after upstream EGR rate reaches a certain level. It also can be seen from graph

Fig. 25. Pt–OH site fraction.

Fig. 26. Pt–H2O site fraction.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

17

that Pt–OH site fraction increases with the increase in number of cycles, that is mainly because there is part of Pt–OH in internal LNT which nether participate in the following reactions nor cannot be regenerated, that is the deterioration phenomenon. Although there is also a phenomenon that some BaCO3 cannot be regenerated in internal LNT, its amount is stabilized at 3%. In addition, although the amount of Pt–OH that cannot be regenerated stays at a lower level, its amount could accumulate with the increase in cycles. It indicates that compared with CO2, the deterioration phenomenon in internal LNT caused by H2O(g) is not easy to be compensated. From a long point of view, its deterioration could beyond CO2. Pt–OH is generated from H2O(g) through the intermediate products of Pt–H2O in the mechanism, and finally influences the Ba(NO3)2 generation. As it is shown in Fig. 26, Pt–H2O site fraction stays at a lower level, which indicates that NOx and CO2 restricts the deterioration influence of internal LNT caused by H2O(g). But they almost possess no influence in Pt–OH generation from Pt–H2O(g) under the catalytic effect of Pt. Pt–H2O site fraction is enlarged in Fig. 27 because of the lower site fraction. Although Pt–H2O possesses a lower amount generally, all Pt–H2O site fraction uprush shows up after upstream EGR rate reaches 20%. Which infers that there is still some Pt–OH cannot be generated from Pt–H2O, but the amount is very low. That is beneficial to LNT performance in some degree. 4.5.3. O2 influence results O2 is also a kind of gas which enters into LNT as gases of CO2 and H2O(g) discussed in the above, but O2 cannot participate in adsorption sites competition reactions, the role of it is to react with Pt and further to form Pt–O. Pt–O plays a key role in NOx adsorption, it is mainly used for oxidizing Pt–NO to Pt–NO2 and oxidizing Ba(NO2)2 to Ba(NO3)2. The decrease in O2 could result in NOx adsorption decrease directly. And as a contrast, excessive O2 could deteriorate NOx regeneration process in LNT by weakening reductive atmosphere in internal LNT.

Fig. 27. Enlarged Pt–H2O site fraction.

Fig. 28. O2 site fraction.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

18

The amount of O2 under different upstream EGR rate is shown in Fig. 28. The amount of O2 under the different EGR rates on the entire lean-mode period is above 5%, the surplus of it indicates that the amount of O2 is enough for the consumption in LNT. But under the NOx regeneration period, there is still O2 in internal LNT even when upstream EGR rate is 0. And it cannot reach 0 until upstream EGR rate reach around 10%. It indicates that the AFR research selected at lean-mode and rich-mode could ensure a better NOx adsorption and fuel economy. But there is a little higher during rich-mode AFR selection which results in O2 surplus at NOx regeneration process, and that is not beneficial to the reduction atmosphere in LNT created by H2, CO and HC. It can be found from the above discussion that there is a kind of trade-off relationship between amount of upstream EGR rate and efficiency of downstream LNT. CO2 and H2O(g) which come alone with upstream EGR could complete with NOx for the adsorption sites in internal LNT and further deteriorates NOx adsorption efficiency. And the existence of O2 in NOx regeneration process could weaken the atmosphere of NOx desorption and NOx reduction in internal LNT at NOx regeneration process, which would finally deteriorate NOx purification effect.

5. Conclusions An integrated and revised NOx purification chemical kinetics mechanism was investigated in this research. Research found that inlet temperature of LNT as one of its boundary conditions possesses a key influence on NOx adsorption route selection and LNT shows the best NOx adsorption performance at inlet temperature around 300 °C. The drawback at relative lower temperature is the reaction activity inhibition and at relative higher temperature is the Ba(NO3)2 instability. Pt plays an import role in the process of NOx adsorption, NOx desorption and NOx reduction. Coated weight of Pt should be balanced from the view of both NOx purification performance and reasonable system cost. As for NOx regeneration process in LNT, the reductive capability order of complex compound between Pt and H2, CO and HC is Pt–H2 4Pt–CO4 Pt–HC. CO2 in the form of BaCO3 site fraction could influence the site fraction of Ba(NO2)2 and Ba(NO3)2, which further influence NOx adsorption. Deterioration caused by H2O(g) is not significant than that caused by CO2, but it is harder to be regenerated. AFR should be reasonable selected considering both oxidation function of O2 and fuel economy, excessive O2 would deteriorate the NOx regeneration process.

Acknowledgments This study was supported by the National Nature Science Foundation of China (Grant nos. 50276042 and 51576140).

References [1] C. Schwarz, E. Schünemann, B. Durst, J. Fischer, A. Witt, Potentials of the Spray-Guided BMW SI Combustion System, SAE paper no. 2006-01-1265, SAE International, Detroit, MI, USA, 2006. [2] H. Baecker, A. Kaufmann, M. Tichy, Experimental and simulative investigation on stratification potential of spray-guided GDI combustion systems, SAE paper no. 2007-01-1407, SAE International, Detroit, MI, USA., 2007. [3] A. König, T. Richter, E. Jobson, M. Preis, E. Leveroni, B. Krutzsch, R. Noirot, M. Chevrier, Research results on processes and catalyst materials for lean NOx conversion, SAE paper no. 962041, SAE International, Detroit, MI, USA, 1996. [4] J. Stokes, T. Lake, M. Christle, I. Denbratt, Improving the NOx/fuel economy tradeoff for gasoline engine with CCVS combustion system, SAE paper no. 940482, SAE International, Detroit, MI, USA, 1994. [5] Y. Iwamoto, K. Noma, O. Nakayama, T. Yamauchi and H. Ando, SAE paper no. 970541, SAE International, Detroit, MI, USA, 1997. [6] G. Lumsden, D. Eddleston, R. Sykes, Development of gasoline direct injection Engine, SAE paper no. 970505, SAE International, Detroit, MI, USA, 1997. [7] R.K. Maurya, D.D. Pal, A.K. Agarwal, Digital signal processing of cylinder pressure data for combustion diagnostics of HCCI engine, Mech. Syst. Signal Process. 36 (1) (2013) 95–109. [8] N. Togun, S. Baysec, T. Kara, Nonlinear modeling and identification of a spark ignition engine torque, Mech. Syst. Signal Process. 26 (2012) 294–304. [9] M. Hoffmann, M. Hori, T. Kreuzer, E.S. Kreuzer, J. Leyrer, W. Strehlau, Lean NOx catalysis to gasoline fueled European cars, Proc. Inst. Mech. Eng., Part D.: J. Automob. Eng. 105 (2) (1997) 133–135. [10] D. Gregory, R.A. Marshall, B. Eves, M. Dearth, J. Hepburn, M. Brogan, D. Swallow, Evolution of lean NOx-trap on PFI and DISI lean burn vehicles, SAE paper no. 1999-01-3498, SAE International, Detroit, MI, USA, 1999. [11] M. Brogan, A. Clark, R. Brisley, Recent progress in NOx-trap technology, SAE paper no. 980933, SAE International, Detroit, MI, USA, 1998. [12] H. Zhang, J. Wang, Y. Wang, Optimal dosing and sizing optimization for a ground vehicle diesel engine two-cell selective catalytic reduction system, IEEE Trans. Veh. Technol. pp (99) (2015) 1. [13] F. Sarikoc, M. Kettner, A. Velji, U. Spicher, A. Krause, A. Elsaesser, Potential of Reducing the NOx emissions in a spray Guided DI Gasoline Engine by Stratified Exhaust Gas Recirculation (EGR), SAE paper no. 2006-01-1261, SAE International Detroit, MI, USA, 2006. [14] M. Tabata T. Yamamoto T. Fukube Improving NOx and fuel Economy for mixture Injected SI Engine with EGR SAE paper no. 9506841995 SAE International Detroit, MI, USA. [15] S. Arun, S. Gerald, Improving NOx and fuel economy for mixture Injected SI Engine with EGR, SAE paper no. 2003-01-0543, SAE International, Detroit, MI, USA, 2003. [16] T. Nakatsuji, J. Ruotoistenmäki, M. Matsubara, A new NOx direct cata-lytic decomposition on a Rh-based catalyst, SAE paper no. 2003-01-3243, SAE International, Detroit, MI, USA, 2003. [17] M. Iwamoto, H. Hamada, Removal of nitrigen monooxide from exhaust gases through novel catalytic processes, Catal. Today 10 (1) (1991) 57–71. [18] M. Iwamoto, N. Mizuno, NOx emission control in oxygen-rich exhaust through selective catalytic reduction by hydrocarbon, Proc. Inst. Mech. Eng., Part D., J. Automob. Eng. 207 (1993) 23.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i

L. Liu et al. / Mechanical Systems and Signal Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

19

[19] J. Gieshoff, M. Pfeifer, S.A. Sindlinger, P.C. Spurk, G. Garr, T. Leprince, M. Crocher, Advanced urea SCR catalysts for automotive applications, SAE paper no. 2001-01-0514, SAE International, Detroit, MI, USA, 2001. [20] M. Hori, A. Okumura, H. Goto, M. Horiuchi, M. Jenkins, K. Tashiro, Deveelopment of new selective NOx reduction catalyst for gasoline leanburrn engines, SAE paper no. 972850, SAE International, Detroit, MI, USA, 1997. [21] C. Havenith, R. Verbeek, D. Heaton, P. Sloten, Development of a urea DeNOx catalyst concept for european ultra-low emission heavy-duty diesel engines, SAE paper no. 952652, SAE International, Detroit, MI, USA, 1995. [22] N. Miyoshi, S. Matsumoto, K. Katoh, T. Tanaka, J. Harada, N. Takahashi, K. Yokota, M. Sugiura, K. Kasahara, Development of new concept three-way catalyst for automotive lean-burn engines, SAE paper no. 950809, SAE International, Detroit, MI, USA, 1995. [23] M. Brogan, A. Clark, R. Brisley, Recent progress in NOx-trap technology, SAE paper no. 980933, SAE International, Detroit, MI, USA, 1998. [24] J. Theis, J. Ura, J. Li, G. Surnilla, J. Roth, C. Goralski, NOx release characteristics of lean NOx traps during rich purges, SAE paper no. 2003-01-1159, SAE International, Detroit, MI, USA, 2003. [25] N. Imanaka, T. Masui, Advanced materials for environmental catalysts, Chem. Rec. 9 (1) (2009) 40–50. [26] Q.Y. Su, Y.B. Li, S. Wang, C.M. Gao, Detailed mechanism for reduction of N2O by CO over rhodium in automotive exhaust, Top. Catal. 56 (1–8) (2013) 345–351. [27] Q.Y. Su, L. Xie, Y.B. Li, X.Q. Qiao, Detailed kinetic modeling of automotive exhaust NOx reduction over rhodium catalyst, Can. J. Chem. Eng. 92 (9) (2014) 1579–1586. [28] J.C., R.B. Randall, Improved automated diagnosis of misfire in internal combustion engines based on simulation models, Mech. Syst. Signal Process. 64– 65 (2015) 58–83. [29] A. Broatch, C. Guardiola, B. Pla, P. Bares, A direct transform for determining the trapped mass on an internal combustion engine based on the incylinder pressure resonance phenomenon, Mech. Syst. Signal Process. 62–63 (2015) 480–489. [30] X.Y. Zhu, H. Zhang, D.P. Cao, Z. Fang, Robust control of integrated motor-transmission powertrain system over controller area network for automotive applications, Mech. Syst. Signal Process 58–59 (2015) 15–28. [31] H. Zhang, J.M. Wang, State estimation of discrete-time Takagi–Sugeno fuzzy systems in a network environment, IEEE Trans. Cybern. 45 (8) (2015) 1525–1536. [32] L. Olsson, H. Persson, E. Fridell, M. Skoglundh, B. Andersson, Kinetic study of NO oxidation and NOx storage on Pt/Al2O3 and Pt/BaO/Al2O3, J Phys Chem B 105 (29) (2001) 6895–6906. [33] L. Olsson, E. Fridell, M. Skoglundh, B. Andersson, Mean field modelling of NOx storage on Pt/BaO/Al2O3, Catal. Today 73 (3–4) (2002) 263–270. [34] A. Lindholm, N.W. Currier, J. Li, A. Yezerets, L. Olsson, Detailed kinetic modeling of NOx storage and reduction with hydrogen as the reducing agent and in the presence of CO2 and H2O over a Pt/Ba/Al catalyst, J. Catal. 258 (1) (2008) 273–288. [35] R.S. Larson, J.A. Pihl, V.K. Chakravarthy, T.J. Toops, C.S. Daw, Microkinetic modeling of lean NOx trap chemistry under reducing conditions, Catal. Today 136 (1) (2008) 104–120.

Please cite this article as: L. Liu, et al., Effect of exhaust gases of Exhaust Gas Recirculation (EGR) coupling lean-burn gasoline engine on NOx purification of Lean NOx trap (LNT), Mech. Syst. Signal Process. (2016), http://dx.doi.org/10.1016/ j.ymssp.2015.12.029i