Reduce NOx Emissions by Adsorber-Reduction Catalyst on Lean Burn Engine

The effect of a new catalyst system composed of traditional three way catalyst converter and adsorber-reduction catalysis converter on the emission characteristics and BSFC (Breake Specific Fuel ConsumptionBSFC)of a lean burn gasoline engine operated were investigated in this paper under different schemes of catalyst converter arrangement and different speeds and loads. The results show that the position of Three Way Catalyst is before the NOx adsorber Catalyst was the best scheme of catalyst converter arrangement. Which has the highest converter efficiency of reduction NOx emission in lean burn gasoline engine. The effects of speed on the exhaust emission and BSFC were also related to the ratio of lean burn time to rich burn time and the absolute value of both time of the adsorber-reduction catalyst converter. The load of the engine was the main influential factor to the exhaust emission characteristics and BSFC of lean burn gasoline engine, and the more load of the engine was, the more NOx emission, the less NOx conversion rate (CNOx) and the better BSFC were.


Introduction
As the emission regulation becomes more and more strict, it's too hard to reduce exhaust emissions only depending on improving internal operation process of engine.Consequently, many countries have developed the exhaust after-treatment technology to reduce exhaust emissions, meanwhile, retain other performance of the engine [1].
Traditional three-way-catalyst (TWC) synchronously reduces CO, HC and NOx effectively only when the gasoline engine is working under stoichiometric air/fuel ratio condition [2].Excess air ratio of lean burn is more than one, so NOx emission can not be reduced by three-way-catalyst.Currently, the main technologies of reducing NOx emission of lean burn gasoline engine are EGR [3][4][5][6][7] and catalyze [8][9][10][11][12][13].But the high EGR rate decreases the velocity of flame diffusion, and consequently worsens the BSFC.Although the Ricardo developed an variable tumble CCVS system [3] and good exhaust stratified combustion was obtained with high EGR rate of 70%, The structure of this system was complicated and NOx emission was also hard to satisfied the request of stricter emission regulation.For the methods of NOx decomposing [14] and selective catalyst reduction (SCR) [15] using for vehicle exhaust, the NOx catalyst conversion rate and the thermal stability of the catalyst are all hard to satisfied the practical request.Compared with these methods of reducing lean burn NOx, The NOx adsorber reduction catalyst combined with TWC can purify the NOx high efficiently within wide temperature scope.With this method, the BSFC of lean burn gasoline engine is deteriorated little, while the catalyst conversion rate of NOx CNOx can be 97% during the short time of fuel rich condition.The Authors using a modified 16 valves EFI Quasi-Homogenous lean burn gasoline engine studied the effect of different schemes of arrangement NOx adsorber converter and TWC, different loads and speeds on exhaust emission characteristics and BSFC with the combination of NOx adsorber catalyst and TWC.

Research Method
Experimental research on the effect of different combination schemes of lean burn adsorber catalyst and TWC, loads and speeds on exhaust emission characteristics and BSFC were conducted on a modified 16 valves EFI Quasi-Homogenous lean burn gasoline engine with the lean burn gasoline engine electronic control system developed by the authors colleagues [8]. Figure 1 shows the schematic of exhaust pipeline design of the lean burn NOx adsorber catalyst experiment system.Ci, Ti represent the concentration and temperature of exhaust emission at the i place respectively.The parts 2 and 3 in the pipeline can be changed each other.There are three scheme of the parts arrangement in the exhaust pipeline.In the scheme 1, the parts sequence is 1-5-2-4 with NOx adsorber but without TWC.In the scheme 2, the parts sequence is 1-5-2-3-4, that is, the NOx adsorber is set before TWC.On the contrary, in the scheme 3, the NOx adsorber locates after the TWC with the sequence of 1-5-3-2-4.
The original engine sensor and wires were preserved, and the original ECU was replaced by the lean burn gasoline engine control ECU which can regulate the A/F ratio easily.The electronic control throttle and linear A/F sensor were also used for the special demand of the lean burn NOx adsorber-reduction catalyst and lean burn gasoline engine operation.Set the value of A/F and operating time in advance according to the demand of experiment.Throttle and the injection pulse can be modified by lean burn gasoline engine ECU to ensure the required A/F and a steady output power.Linear A/F sensor keep the engine working in the right range of A/F via feedback signal.

1 The Effect of Different Combination Schemes of NOx Adsorber-reduction Catalyst and Three Way Catalyst on Exhaust Emission Characteristics
The effects of catalyst system arrangement of different scheme on NOx emission and NOx conversion rate C NOx were shown in Figure 2 and Figure 3.The x-axis denotes the ratio of the absolute lean burn operating time tlean and the absolute rich condition operating time trich.The y-axis denotes the concentration of NOx emission (1) and the NOx conversion rate C NOx (%) respectively.
As showed in Figure 2, all of the three different catalyst arrangements can reduce NOx emission of lean burn gasoline engine.The NOx emission is about 800 x 10 -6 before catalyst.After conversion, the NOx emissions reduce significantly no matter the ratio of t lean to t rich .For example, when the ratio of t t lean to t rich is 200s:20s, NOx emission scheme 1 and scheme 2 was reduced to about 300 x 10 -6 , lower 62.5%.NOx emission of scheme 3 reduced to 50 x 10 -6 , lower 93%.The scheme 3 is better than the scheme 1 and 2. And as the absolute time of t lean /t rich decreased from 200s: 20s to 40s: 4s, NOx emission after catalyzed also reduced from ISSN: 1693-6930  Reduce NOx Emissions by Adsorber-Reduction Catalyst on Lean Burn Engine (Dongpeng Yue) 483 300 x 10 -6 to 50 x 10 -6 for scheme 1 and 2, and reduced from 50 x 10 -6 to 15 x 10 -6 for scheme 3. The above experiment results illuminate that catalyst arrangement as three scheme can achieve better effect of NOx emission reduction, further more, as the absolute time of t lean /t rich decreased, NOx emission reduced.These results can be explain as: when the three ways catalyst is located before the lean burn NOx adsorber-reduction catalyst, the exhaust emission enters three ways catalyst at first, and reacts with the reducing agent rhodium.Although the oxygen is rich in the exhaust gas, the conversion process of NOx is restricted to some extent, and the NOx conversion rate C NOx is decreased.But this arrangement can reduce the concentration of O 2 , HC, CO and the absolute value of NOx emission entering the lean burn NOx adsorber-reduction catalyst, so as to reduce the vacancy of oxygen adsorbed ,alleviate the load of lean burn NOx adsorber-reduction catalyst, and prolong the time before the catalyst's saturation.While t lean /t rich is constant, as the absolute time of t lean /t rich diminishing, on the one hand, the absolute quality of NOx emission is small, and the time is short, so the possibility of NOx overflowing is little; on the other hand, there is a process of NOx reduction in every short time, which make the possibility of NOx overflowing smaller.In the process of NOx reduction, all the NOx adsorbed is almost deoxidized.When the engine turns to lean burn condition again, the adsorber capacity of catalyst is enhanced.Thus, the NOx emission consequentially reduces.In other words, as the absolute time of t lean /t rich diminishing, the NOx emission of the catalyst system reduces.As shown in Figure 3, the NOx conversion rate C NOx of catalyst arrangement of scheme 1 and 2 is almost equal, but both less than the NOx conversation rate C NOx of scheme 3.Not only such, as the absolute time of t lean /t rich diminishing, the NOx conversion rate C NOx of the three scheme all increase, the NOx conversion rate C NOx scheme 1 enlarges from 61.9% (200s: 20s) to 83.5% (40s: 4s); the NOx conversion rate C NOx of scheme 2 enlarges from 70% (200s: 20s) to 85.2% (40s: 4s); and the NOx conversion rate C NOx x of scheme 3 enlarges from 94% (200s: 20s) to 97.3%(40s: 4s).The NOx conversion rate C NOx of scheme 3 is better than the other two schemes; it is mainly because of the different arrangement of the catalysts.

2 The Effect of Different Speeds on Exhaust Emission Characteristics and BSFC
As above, scheme 3 is the best scheme among scheme 1 to scheme 3.Under scheme 3, the author studied the effect of different operations on BSFC and exhaust emission characteristics.Figure 4 and Figure 5 show the effect of exhaust emission and the NOx conversion rate C NOx in scheme 3 on different speeds (1500 rpm, 1800 rpm, 2500 rpm) under Operating conditions and mode of the lean burn gasoline engine are: load (PMEP) Pe= 0.2 MPa, speed are 1500 rpm,1800 rpm, and 2500 rpm respectively.A/F in lean burn is 21, and that of rich burn is 12 , t lean /t rich = 10.As shown in Figure 4, when the ratio of t lean / t rich and the absolute time of t lean / t rich keep steady, as the speed of gasoline engine increased ( Pe= 0.2 Mpa, the speed are 1500 rpm, 1800 rpm, and 2500 rpm respectively), in the exhaust emission of the lean burn gasoline engine, the CO,HC reduce and NOx increase slightly.But with the absolute time of t lean / t rich decreased.The CO and HC increased but NOx decreased slightly.This is mainly because the speed of the lean burn gasoline engine increased, which made the oxygenous condition in the cylinder better, that's better to the sufficient oxidation of CO and HC, so the CO and HC emission of the lean burn gasoline engine decreased.Because the engine adopt the the lean burn NOx adsorber-reduction catalyst system, the concentration of CO and HC in the emission of the lean burn gasoline engine is a little higher than that of pure lean burn gasoline engine.After the speed of engine increase, in the adsorber-reduction process of lean burn gasoline engine, because the time ratio of lean burn and rich burn t lean / t rich is a constant, the times of engine operation cycle in the same period increased, which will make the temperature inside the cylinder a little higher and the concentration of NOx increased.But with the absolute time of t lean / t rich decreased, the times of A/F transformed process in the same period increased and the total time of engine operation in transitional period increased, so the CO and HC emission of the engine increased.As the decrease of t lean and t rich improved the conversion rate of NOx C NOx in adsorber-reduction catalyst system, so the NOx emission decreased with the decrease of the absolute time of t lean and t rich .
As shown in Figure 5, when the ratio of t lean / t rich keep steady ,the conversion rate of NOx C NOx reached 94.2% when t lean / t rich = 200: 20(n= 1500 rpm).With the increase of engine speed (C NOx = 94% when n= 1800 rpm, and C NOx = 93.8%when n= 2500 rpm), the conversion rate of NOx C NOx changed little.This is mainly because the NOx will increase with the speed of engine increased, but the temperature of exhaust into the catalyst system rise what is the better for improve the oxygenous condition of the catalyst system and the worse for the restore of NOx emission.When the ratio of t lean / t rich keep steady, the conversion rate of NOx C NOx rise with the ).This is because that the decrease of t lean / t rich absolute time reduced the leak of NOx through lean burn adsorber-reduction catalyst system and improved the adsorbility of NOx emission.The NOx emission of lean burn gasoline engine which has rise slightly due to the increase of speed can be improved by shorten the t lean / t rich absolute time.So if the t lean / t rich absolute time keep steady, the conversion rate of NOx C NOx in adsorber-reduction catalyst system reduce a little with the speed of engine rise.On the contrary, the conversion rate of NOx C NOx in adsorberreduction catalyst system will rise a little if decrease the t lean / t rich absolute time.That is means that the increasing NOx exhaust emission with the rising of speed can be improved by decrease of t lean / t rich absolute time.As shown in Figure 6, when t lean / t rich and the absolute time of t lean / t rich keep steady, the speed of engine rise(from 1500 rpm to 2500 rpm), BSFC be improved.In other words, the speed rised.But the BSFC will rise a little with the absolute time of t lean / t rich reduce.Analyzing this phenomenon, we deem that : when t lean / t rich keep steady, if the absolute time of t lean / t rich stay the same, the turbulence intensity inside cylinder of the lean burn gasoline engine will largen and the combustion condition will be better what lead to the smaller BSFC.But if the speed of engine is much too high, the BSFC will grow the bigger because that the strengthen movement of air inside the cylinder lead to the transferred loss of the quantity of heat inside the cylinder.The range of speed in this experiment belong to the middle and lower range of the engine speed, so the rise of speed lead to the BSFC decreased.As the absolute time of t lean / t rich decrease, because t lean / t rich stays at a constant that the percentage of the total time of the same lean burn gasoline engine operating in lean burn and rich burn condition stays at a constant but the times of A/F transforming process in the same period increased and the total time of engine operate in transitional period increased what large the BSFC of lean burn gasoline engine a little although not notable.
These result can be explain as (compared to following load factor): the impact of the increase of lean burn gasoline engine speed on the exhaust emission and BSFC of lean burn gasoline engine have to do with the ratio and absolute time of t lean / t rich.When t lean / t rich keep steady, if the absolute time of t lean / t rich stay the same, with the speed of engine increase, the concentration of CO and HC will decrease and that of NOx increase a little.But with the absolute time of t lean / t rich decrease, the concentration of CO and HC will increase and that of NOx decrease a little.When t lean / t rich keep steady, if the absolute time of t lean / t rich stay the same, with the speed of engine increase(from 1500 rpm to 2500 rpm), the conversion rate of NOx C NOx in adsorber-reduction catalyst system decreased little, although it's not notable.And the BSFC of lean burn gasoline engine turn better what means that the speed increased and the BSFC of lean burn gasoline engine decreased.If the absolute time of t lean / t rich decreased, both the

3 The Effect of Different Loads on Exhaust Emission Characteristics and BSFC
Figure 7, Figure 8, and Figure 9 shows the effect of exhaust emission CO, HC, NOx, the conversion rate of NOx C NOx and the BSFC at the speed of 1800 rpm in scheme 3 on different loads (pe= 0.2 MPa, 0.3 MPa, and 0.4 MPa). the A/F ratio of lean burn is 21, the A/F of reduction process is 12, t lean / t rich = 10.As shown in Figure 7, when t lean / t rich keep steady, if the absolute time of t lean / t rich stay the same, with the load of engine increase(0.2MPa, 0.3 MPa, and 0.4 MPa), the CO and HC will decrease and that of NOx increase.But with the absolute time of t lean / t rich decrease, the CO and HC will increase and that of NOx decrease a little.This is mainly because that the increase of engine heighten the temperature inside the cylinder of the lean burn gasoline engine what enhance the fuel oxygenation competently, and the increase of cylinder inner wall temperature decreased the thickness of the layer attached to the wall of cylinder, so the amount of HC adsorbed by the layer attached to the wall of cylinder decreased.At the same time, the increase of load, decrease the unfired of fuel mixture in the cylinder.As a result, the CO and HC will reduce when the load increase, meanwhile, the increase of load and temperature inside the cylinder were favorable to the formation of NOx, so as to NOx increase ; if the load keep steady, when the absolute time of t lean / t rich decrease, the CO and HC will increase and that of NOx decrease.This is because the decrease of the absolute time of t lean / t rich shorten the length of lean burn time that the lean burn catalyst system undergo(maybe the catalyst system hasn't adsorb to saturation), after that the restore and regeneration process in rich burn atmosphere started, the time of the engine operated in the transition condition during the same time-lag increased ,which made the concentration of CO and HC a little bigger, but the NOx concentration became smaller, it's because that under favor of the decrease of the absolute time of t lean / t rich , the NOx which created in the process of lean burn(which has absorbed in the catalyst by the alkaline earth in the lean burn NOx adsorber and restore catalyst system and storaged in the form of nitrate) converted to N 2 absolutely in the restore atmosphere, and in the alkaline earth of the lean burn NOx adsorb and restore catalyst system had no NOx left.All of these increased the adsorb capability of lean burn catalyst for NOx in the lean burn condition, that means the conversion rate of NOx C NOx in adsorber-reduction catalyst increased.As shown as Figure 8, the conversion rate of NOx C NOx in adsorber-reduction catalyst system have relation with not only the absolute time of t lean / t rich but also the load of the engine in the adsorber-reduction catalyst system.The bigger the load, the higher the NOx emission, and the lower the conversion rate of NOx C NOx .whent lean / t rich keep steady, if the absolute time of t lean / t rich stay the same, with the load of engine increase(n= 1800 rpm, pe is 0.2 MPa, 0.3 MPa, and 0.4 MPa respectively), the conversion rate of NOx C NOx decrease.When load is 0.2 MPa, the C NOx is 94%; when load is 0.3 MPa, the C NOx is 85%; when load is 0.4 MPa, the C NOx is only 49% (when t lean / t rich = 200: 20).Compared with the increase of the C NOx caused by the decrease of the adsorber-reduction catalyst system's absolute time of t lean / t rich , the load of lean burn gasoline engine have much more influence on the C NOx .The bigger the load, the smaller the C NOx .Fortunately, lean burn condition mainly work in the operating condition with middle and low speed and part load.
As shown in Figure 9, the load of engine has more influence on the BSFC.The bigger the load, the better.This is because with the load of engine increase, the combustion temperature in the cylinder rise, on the one hand, the accretion of load improve the fuel pulverization what improve the combustion condition; on the other hand, the accretion of load decrease the engine relatively cooling lose and pumping lose, the thermal efficiency of engine rise.Besides, the accretion of load improves the mechanical efficiency of engine.The accretion of load improved the thermal efficiency and the mechanical efficiency, that made the BSFC better .When t lean / t rich keep steady, if the load stay the same, with the absolute time of t lean t rich decrease, BSFC rise a little, what due to that the decrease of the absolute time of t lean / t rich largen the total time of engine works in transitional operating condition in the same time-lag.

Conclusion
Applying the new catalyst system composed of traditional three way catalyst converter and adsorber-reduction catalyst converter which is designed by author studied the effect of different schemes of catalyst converter arrangement and different speeds and loads on the exhaust emission characteristics and BSFC of a lean burn gasoline engine, we got the main conclusions below: 1.Under the condition of these experiments, catalyst converter arrangement scheme 3 which is designed by author can achieve the lowest NOx emission of 50 10 -6 and the conversion rate of is the highest to 97.3%.Therefore, when we use lean burn adsorber catalyst combined with three ways catalyst to reduce NOx emission in lean burn S.I. engine, the scheme that the lean burn NOx adsorber-reduction catalyst is arranged after the TWC should be adopt.2. When t lean / t rich keep steady, if the absolute time of t lean / t rich stay the same, as the speed of gasoline engine increased, in the emission of lean burn gasoline engine, the CO, HC reduce and NOx increase slightly, the BSFC be improved.But the change of the conversion rate of NOx is not notable.With the absolute time of t lean / t rich decreased, the concentration of NOx emission decreased a little, and the conversion rate of NOx C NOx largen, so is the BSFC .3. When t lean / t rich keep steady, if the absolute time of t lean / t rich stay the same, with the load of engine increases, the CO and HC will decrease and that of NOx increase.But with the absolute time of t lean / t rich decrease, the CO and HC will increase and that of NOx decrease a little, the conversion rate of NOx C NOx increased and the BSFC be go to bad. 4. The C NOx have relation with not only the absolute time of t lean / t rich but also the magnitude of the engine load.The bigger the load, the higher NOx emission, and the lower the conversion rate of NOx C NOx , but the BSFC be improved in lean burn NOx adsorber-reduction catalyst system.

Figure 1 .
Figure 1.Schematic of experimental system in lean NOx adsorber catalyst in exhaust pipe

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ISSN: 1693-6930 TELKOMNIKA Vol.11, No. 3, September 2013: 481 -488 484 the load being constant.The x-axis denotes the ratio of the absolute lean burn operating time t lean and the absolute rich condition operating time t rich .The y-axis denotes the concentration of exhaust emission include CO, HC and NOx (1 x 10 -6 ), the NOx conversion rate C NOx (%) and the BSFC respectively.

Figure 4 .
Figure 4. Effects of different speeds on exhaust emission (Pe= 0.2 MPa) (a) Effects of different speeds on CO emission, (b) Effects of different speeds on HC emission (c) Effects of different speeds on NOx emission

Figure 7 .
Figure 7. Effects of different loads on exhaust emission (n= 1800 rpm) (a) Effects of different loads on CO emission, (b) Effects of different loads on HC emission (c) Effects of different loads on NOx emission