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Abstract: An experimental and model-based study of the effect of rich air/fuel ratios (AFRs) and temperature on the NO _x slip of a lean NO _x trap (LNT) was conducted in a lean-burn gasoline engine with an LNT after-treatment system. The emissions of the engine test bench and the inlet temperature of the LNT were used as the major inlet boundary conditions of the LNT. The engine periodically operated between a constant lean AFR of 23 with alterable rich AFRs of 10, 11, 12, 13, and 14. A decrease in the rich AFR of the engine strengthened the desorption atmosphere in the LNT, an effect closely related to the number of reductants, and further heightened the NO _x desorption of the LNT, but with a penalty in fuel consumption. To eliminate that penalty, the inlet boundary conditions of the LNT were varied by adjusting the inlet temperature within a range between 200 °C and 400 °C. An increase in inlet temperature heightened the NO _x desorption of the LNT, and a NO _x breakthrough occurred after the inlet temperature exceeded 390 °C. To control NO _x breakthrough, the inlet temperature can be adjusted to offset the strong desorption atmosphere in the LNT commonly created by a rich AFR.

1.  Introduction

 With the impending energy crisis and increasing concerns over environmental pollution, energy-savings and emission-reductions have become a worldwide focus. At present, the gasoline engine is the major power source of passenger cars. To meet present energy demands and increasingly stringent engine emission regulations, it is essential to enhance the economy of gasoline engines and reduce their emissions (Guo et al., 2006; Martínez-Morales et al., 2013). Lean-burn technology is being developed for future engines to achieve a pronounced improvement in fuel economy (Schwarz et al., 2006; Baecker et al., 2007). The major drawback of lean-burn gasoline engines is the inefficient NO _x conversion of the conventional three way catalyst (TWC) under ultra lean air/fuel ratio (AFR) conditions (König et al., 1996). At present, the major methods of NO _x reduction are in-cylinder combustion process improvement (Stokes et al., 1994; Iwamoto et al., 1997; Lumsden et al., 1997) and the highly efficient catalytic technology of after-treatment systems (Hoffmann et al., 1997; Brogan et al., 1998; Gregory et al., 1999).

 Catalytic technology of after-treatment systems consists mainly of NO _x direct decomposition (NDD) (Tadao et al., 2003), lean NO _x traps (LNTs) (Tabata et al., 1995; Gregory et al., 1999), and selective catalytic reduction (SCR) (Gieshoff et al., 2001). Compared with NDD and SCR, LNTs attract more interest because of their higher NO _x conversion efficiency. LNT technology of lean-burn gasoline engines was first proposed by Toyota in the 1990s (Miyoshi et al., 1995). A lean-burn gasoline engine with a coupled LNT needs to operate periodically between rich-mode and lean-mode. NO _x emission is adsorbed in an LNT during the lean-mode of the engine. The engine needs to switch to rich-mode when the LNT is close to the saturation point of NO _x storage. After that, large reductants (e.g., H₂, HC, and CO) are generated from the engine and react with the NO _x which is adsorbed in the LNT during the lean-mode of engine. NO _x is then converted to N₂, accompanied by small amounts of NH₃ and N₂O. The reductants are oxidized to H₂O and CO₂ simultaneously. The rich-mode period is shorter than the lean-mode period, so as to lower the fuel consumption of the engine. The storage period of an LNT (the engine in lean-mode) includes only the process of NO _x adsorption, while the regeneration period (the engine in rich mode) includes the processes of NO _x desorption and reduction. Optimization of the LNT in the regeneration period is very challenging because of the short time and multi-process operation. The process of NO _x desorption is more meaningful because it occurs prior to the process of NO _x reduction and determines the number of NO _x which will be reduced by subsequent reductants.

 Various simulation models of LNTs have been proposed in the past decade. A NO _x storage model first developed by Olsson et al. (2001) was later supplemented with a propylene-based regeneration model (Olsson et al., 2002). The major drawback of Olsson’s model is that it cannot account for NO adsorption in the presence of O₂. This limitation was removed by Lindholm et al. (2008), assuming that NO can be directly adsorbed on BaCO₃ and unidentified S₃ sites. Unlike the storage models, the regeneration model proposed by Larson et al. (2008) is highly globalized. There are also many regeneration models being proposed based on variable reductants (Koci et al., 2009; Digiulio et al., 2012).

 Although models have been developed from chemical experiments, few such models have been coupled with a real-time engine test bench. In the present study, the process of NO _x purification (NO _x adsorption, desorption, and reduction) was analyzed separately. The NO _x purification of an LNT was based on the actual exhaust of the engine. In addition, the process of switching from the adsorption to desorption periods was investigated thoroughly. The contrasting effects of rich AFRs and temperature on NO _x desorption in the LNT were analyzed in detail so as to find an innovative technology path to achieve better NO _x desorption of the LNT without a penalty in fuel consumption.

2.  Experimental setup and methodology

 The layout of the lean-burn gasoline engine test bench is shown in Fig. 1. A modified lean-burn gasoline engine CA3GA2 (Tianjin FAW XIALI Automobile Co., Ltd., China) (a port fuel injection (PFI) gasoline engine with three in-line cylinders, 12 valves and a displacement volume of 1 L) was selected. The engine bench tests were carried out at a speed of 2800 r/min with a load of 0.3 MPa. The rich-lean period of the engine was fixed at 7:56. The concentrations of NO _x , O₂, CO, CO₂, and HC in the exhaust were monitored by an exhaust analyzer (model MEXA-7100, Horiba, Japan). The experimental LNT consisted of cylindrical flow-through monolith bricks (110 mm diameter, 200 mm length) containing square channels at a density of 400 channels per square inch (cpsi). The washcost was Pt/BaO/Al₂O₃. The model of the LNT was based mainly on Larson et al. (2008), but revised by removing reduction reactions. The inlet boundary conditions of the LNT consisted mainly of inlet gases (exhaust gases of the lean-burn gasoline engine) and temperature. The inlet gases of the LNT were measured at a lean AFR of 23 and at rich AFRs of 10, 11, 12, 13 and 14 (Table 1). The inlet temperature of the LNT was adjusted between 200 °C and 400 °C.

3.  Results and discussion

3.1.  NO _x slip of LNT

 Fig. 2 shows the NO _x slip in the first to the sixth cycles of the LNT at rich AFRs of 10, 11, 12, 13, and 14 with the same fixed inlet temperature of 300 °C. The trends in NO _x adsorption and desorption at those rich AFRs were similar within each cycle, even in the first cycle, which was likely to show irregular trends. However, the trend in NO _x adsorption and desorption in the second to the sixth cycles was different from that in the first cycle. This was mainly because the LNT was in an initialization state with no NO _x adsorbed before the start of the first cycle.




 Fig. 3 shows the NO _x slip in the first to the fourth adsorption periods at rich AFRs of 10, 11, 12, 13 and 14. NO _x slip in the first adsorption period was markedly lower than that in the second to the fourth adsorption periods. That was due mainly to the initialization of the LNT mentioned above. The increase in adsorption time heightened the NO _x slip. The main reason is that under the same NO _x flow of the LNT inlet, the increase in adsorption time reduces the number of adsorption sites of the LNT. Also, NO _x slip increased with the increase in rich AFR. The different NO _x slips among different rich AFRs reached a maximum at the beginning of the adsorption period and then decreased to near 0 at the end. That was determined mainly by the NO _x adsorption probability. Specifically, the number of inlet NO _x is assumed to be N, and the total number of adsorption sites in the LNT is assumed to be 100. A completely purified LNT is assumed to be vacant100 (V₁₀₀) and a completely saturated LNT is assumed to be vacant0 (V₀). Thus, the NO _x adsorption probability is V_x /N, where x is the number of real-time adsorption sites in the LNT. Also, V_m /N>V_n /N can be assumed at the beginning of the adsorption period, where m is the number of NO _x adsorption sites at a rich AFR of 10, and n is the number at a rich AFR of 14. In addition, the number of adsorption sites in the LNT decreased with the increase in adsorption time, which indicates that V_m /N>V_a /N and V_n /N>V_b /N, where a is the number of adsorption sites in the LNT after a period of adsorption time at a rich AFR of 10, and b is the number at a rich AFR of 14. Note that a completely saturated LNT is an ideal state which cannot be achieved. V_a and V_b are infinitely close to 0. Thus, V_a /N≈V_b /N can be assumed. This explains why the difference in NO _x slip among different rich AFRs decreased to near 0 at the end of adsorption period. But to verify the assumption of V_m /N>V_n /N, the NO _x slips in the first to the fourth desorption periods at rich AFRs of 10, 11, 12, 13, and 14 are enlarged in Fig. 4.







 NO _x slips in the second to the fourth desorption periods were markedly lower and more stable than that in the first desorption period. This was caused mainly by the incomplete desorption of NO _x . Some NO _x absorbed in the previous adsorption period, cannot be desorbed in this desorption period and is further reduced by reductants. Those adsorption sites that cannot be regenerated are defined as inactive adsorption sites. The number of inactive adsorption sites increased with the increase in the number of cycles and then gradually stabilized. The NO _x slip of the LNT increased with the increase in rich AFR. This indicates that the number of NO _x adsorption sites decreased with the increase in rich AFR at the beginning of the adsorption period, which justifies the above assumption. In addition, the increased level between rich AFRs of 12 and 13 was higher than that between 13 and 14. The increased level between rich AFRs of 11 and 12 was also higher than that between 10 and 11. This implies that for a lean-burn gasoline engine with a rich AFR of 12, whether aiming to achieve higher NO _x conversion efficiency of the LNT or a lower fuel consumption (fuel consumption decreases with the increase in rich AFR), the increased level in NO _x conversion efficiency of the LNT or decreased level in fuel consumption after a rich AFR plus or minus 1 is higher than that after a rich AFR plus or minus more than 1. Moreover, there were fluctuations of NO _x in the process of switching from the adsorption to desorption periods (Fig. 4). This process in the second cycle is enlarged in Fig. 5.




 Fluctuations in the process of switching from the adsorption to desorption periods occurred at all rich AFRs, but the fluctuation level was very variable. Note that the fluctuation always first decreased to the lowest point and then increased rapidly to a stable level. The moment of minimum NO _x slip signalled the beginning of desorption period. Compared with the theoretical start of desorption period, the actual start point had a time delay. Specifically, the switching time experiment set was 0.1 s (119 s to 119.1 s), while the time delay at rich AFRs of 10 and 14 were 0.07 s and 0.13 s, respectively. The time delay increased with the increase in rich AFR. NO _x slip at the start point of NO _x desorption at a rich AFR of 10 was 160×10⁻⁶ higher than that at a rich AFR of 14, while the time delay between them was only 0.06 s. This indicates that the time delay in the process of switching from the adsorption to desorption period has a pronounced influence on NO _x slip. The start point of NO _x desorption is influenced by the strength of the desorption atmosphere. The decrease in rich AFR heightened the strength of the desorption atmosphere in the LNT, which is always characterized by the concentration of O₂ and the real-time temperature of the LNT.

3.2.  Characterization of the strength of a rich atmosphere: O₂

 As one of the major determinants of the strength of the desorption atmosphere in the LNT, O₂ participates mainly in the process of NO _x adsorption and desorption. There are two main paths in the process of NO _x adsorption, the nitrate path and the nitrite path. In the nitrate path, NO is first oxidized to NO₂. Then NO₂ is adsorbed onto a BaO site and forms nitrate. In the nitrite path, NO is directly adsorbed on BaO and forms nitrite, then the nitrite is oxidized to nitrate. Note that, compared with the nitrite path, the nitrate path is more likely to occur in the LNT. This is mainly because, unlike the nitrate path, the nitrite path occurs only at low temperatures. Also, O₂ mainly weakens the desorption atmosphere in the process of NO _x desorption. The strength of the O₂ atmosphere in the adsorption period is determined mainly by the contrast between the NO and NO₂ slips.

 Fig. 6 shows details of the NO and NO₂ slips in the second desorption period at rich AFRs of 10, 11, 12, 13, and 14. The decrease in the rich AFR reduced the concentration of O₂. This is indicated by the NO _x slip (NO and NO₂ slips) increasing with the decrease in the rich AFR. In addition, note that the decrease in the rich AFR heightened the NO slip but lowered the NO₂ slip. This indicates there was not enough O₂ to oxidize NO to NO₂ after it weakened the desorption atmosphere.

3.3.  Characterization of the strength of a rich atmosphere: temperature

 Compared with the desorption atmosphere created by the decrease in the rich AFR of the engine, the desorption atmosphere created by the internal temperature of the LNT was more efficient, and without a penalty in fuel consumption.

 Fig. 7 shows the real-time temperature of the LNT at rich AFRs of 10, 11, 12, 13 and 14 with the same fixed inlet temperature of 300 °C. The real-time internal temperature of the LNT rose from 300 °C to between 511 °C and 601 °C. This increase was mainly because of exothermic reactions in the desorption period. The heat generated from exothermic reactions breaks the bonds between adsorption sites and NO _x , which contributes to the large NO _x desorption and is beneficial for the reduction of NO _x to N₂. The decrease in rich AFR raised the real-time internal temperature of the LNT.




 To compare the effects of inlet temperature and rich AFRs on NO _x desorption of the LNT, inlet temperatures of 200 °C and 400 °C were selected (Fig. 8).




 Fig. 8 shows the NO _x slip in the first to the sixth cycles of the LNT at rich AFRs of 10, 11, 12, 13, and 14 with an inlet temperature of 300 °C, and at inlet temperatures of 200 °C and 400 °C with a rich AFR of 12. The NO _x slips in the adsorption and desorption periods varied greatly among the inlet temperatures of 200 °C, 300 °C, and 400 °C. At an inlet temperature of 200 °C, the LNT reached the saturation point at the beginning of the adsorption period with little NO _x adsorbed. Likewise, little NO _x was desorbed from the LNT in desorption period. However, NO _x adsorption and desorption at an inlet temperature of 400 °C had the opposite effect. In the second to the sixth cycle, NO _x adsorption at a rich AFR of 12 with an inlet temperature of 400 °C was also higher than that at an inlet temperature of 300 °C with rich AFRs of 10, 11, 12, 13, and 14. This is attributable to the better NO _x desorption in the first desorption period at a rich AFR of 12 with an inlet temperature of 400 °C. Note that from the enlarged process of NO _x adsorption and desorption, the start point of NO _x desorption at a rich AFR of 12 with an inlet temperature of 400 °C was earlier than in other cases. This implies that the inlet temperature of the LNT has an obvious effect on the start point of the NO _x slip in desorption period, which is similar to the rich AFR effect. In addition, the NO _x slip increased directly to the highest point at a rich AFR of 12 with an inlet temperature of 400 °C, unlike in other cases where it first decreased to the lowest point and then increased rapidly to a stable level. The NO _x breakthrough, such as happened at a rich AFR of 12 with an inlet temperature of 400 °C, is related to temperature and needs to be controlled within a certain range. Less NO _x breakthrough promotes NO _x desorption. A large NO _x breakthrough cannot be reduced by reductants effectively and quickly, and tends to slip out of the LNT directly without being purified.

 Fig. 9 shows the effect on NO _x slip of inlet temperatures between 300 °C and 400 °C. The actual start point of NO _x desorption at all inlet temperatures had a time delay. An increase in inlet temperature lowered the time delay. In addition, an increase in inlet temperature increased the NO _x slip. Compared with the start point of NO _x desorption at an inlet temperature of 400 °C, the time delay at an inlet temperature of 300 °C was only 0.15 s, while the difference in NO _x slip was 400×10⁻⁶. Note that the maximum difference in NO _x slip between inlet temperatures of 400 °C and 300 °C was 1600×10⁻⁶ (Fig. 9). This was due mainly to the NO _x breakthrough. The lowest temperature which resulted in NO _x breakthrough was 390 °C.

4.  Conclusions

 The optimization of NO _x desorption is beneficial for NO _x reduction and adsorption in the next cycle, and may increase the NO _x conversion efficiency of an LNT. An increase in the rich AFR of the engine was beneficial for NO _x adsorption and desorption in the LNT. NO _x adsorption was greatly influenced by NO _x desorption in the previous cycle. Differences in NO _x slip among different rich AFRs gradually decreased to near 0 at the end of the adsorption period, but remained stable throughout the whole desorption period. The strength of the desorption atmosphere created by a rich AFR can be compensated by adjusting the inlet temperature, without a penalty in fuel consumption. Temperature was the major factor affecting NO _x breakthrough.

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* Project supported by the National Natural Science Foundation of China (Nos. 50276042, 50776062, and 51276128), the National High Technology R&D Program (863) of China (No. 2008AA06Z322), and the Tianjin Research Program of Application Foundation and Advanced Technology (No. 11JCZDJC23200), China