Reducing HC emissions from a gasoline engine at the starting conditions through activated carbon adsorption

Accepted Manuscript Research Paper Reducing HC emissions from a gasoline engine at the starting conditions through activated carbon adsorption Changwei Ji, Puyan Xu, Shuofeng Wang, Yu Feng, Teng Su, Menghui Yu, Xiaoyu Cong PII: DOI: Reference:

S1359-4311(16)32492-9 http://dx.doi.org/10.1016/j.applthermaleng.2016.10.107 ATE 9309

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

20 April 2016 15 October 2016 17 October 2016

Please cite this article as: C. Ji, P. Xu, S. Wang, Y. Feng, T. Su, M. Yu, X. Cong, Reducing HC emissions from a gasoline engine at the starting conditions through activated carbon adsorption, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.10.107

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Reducing HC emissions from a gasoline engine at the starting conditions through activated carbon adsorption Changwei Ji*, Puyan Xu, Shuofeng Wang, Yu Feng, Teng Su, Menghui Yu, Xiaoyu Cong College of Environmental and Energy Engineering, Key Laboratory of Beijing on Regional Air Pollution Control and Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing University of Technology, Beijing 100124, China

Abstract: The general purpose of this work is to examine the adsorption characteristics of activated carbon materials for reducing HC emissions from the gasoline engine under starting conditions. Three kinds of activated carbon materials were selected to adsorb HC emissions within 80 s from the onset of starting at different coolant temperatures of 25, 35, 45 and 55 ℃, respectively. The experimental results showed that the total HC adsorption rates of activated carbon materials declined with the increase of coolant temperatures. The total HC adsorption rates of activated carbon honeycomb, foams and fibers were 68.6%, 66.1% and 53.5% under the coolant temperature of 25 ℃. The early desorption was found for activated carbon foams, which was getting severe with the increase of coolant temperatures. Then, the adsorption performance of activated carbon honeycomb was further investigated at the repeated engine starting conditions. Under the repeated starting conditions, the total HC adsorption rates of activated carbon honeycomb decreased with the increase of adsorption times in the test. The total HC adsorption efficiencies of the first three times under repeated starting conditions were 76.4%, 63.7% and 54.0%, respectively. The results showed that the adsorption processes could be divided into three phases based on the adsorption characteristics, which were rapid adsorption phase, interim adsorption phase and slow adsorption phase, respectively. Key words: Adsorption; Emission control; Activated carbon; Spark-ignition engine; Gasoline *Corresponding author. Tel.: +86 1067392126; Fax: +86 1067392126 E-mail address: [email protected] (C.Ji )

1. Introduction In recent years, reducing hydrocarbon emissions (HC) emitted from automotive vehicles has attracted more and more attentions [1-3]. For gasoline engines, most of HC emissions could be converted to harmless products effectively by an on-board Three Way Catalyst (TWC) while the exhaust temperature exceeding its light-off temperature of 200 to 300 ℃ [4-5]. However, as the TWC could not achieve the light-off temperature quickly [6-7], the engine may expel large amounts of HC emissions during the cold [8] and even warm starting conditions. It was reported that, during the New European Driving Cycle (NEDC) cycle, 70% to 90% HC emissions were resulted from the engine starting due to the inefficient TWC conversion and rich combustion [9-11]. So far, many studies have been carried out on different approaches for reducing HC emissions produced during the starting period [12-16]. Ji et al. tried to improve the engine performance by the addition of alternate fuel such as hydrogen, Dimethyl ether (DME) and methanol. The experimental results showed that the addition of alternate fuel helped improve thermal efficiency and reduce HC emissions during the cold start. However, the engine produced more NOx emissions because of the increased H/C ratio and elevated combustion temperature [18-20]. Kato et al. studied the performance of an electrically heated TWC. They found that the electrically heated TWC could be more effective on reducing HC emissions during the cold start, because heating the TWC by electrical equipment could help it quickly achieve the light-off temperature [21-22]. It is seen from above investigations that HC emissions during the cold could be reduced by either improving the fuel quality [23-24] or heating the TWC by electrical methods. However, for mono-fuel gasoline engine, it is hard to reduce HC emissions by adopting dual fuel strategy which generally requires the recalibration of control unit and the addition of secondary fuel supply system. Heating TWC before starting is effective on improving the TWC conversion efficiency by enabling it to quickly reach the light-off temperature, however, this method would lead

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to the increase in energy consumption of the engine. Besides, the engine starting time would be prolonged because it costs time to heat the TWC. When the fluid contacts the porous solid, one or multiple components of the fluid could be adhered to the surface of the solid due to the molecular inter-atomic forces. This phenomenon is called adsorption. Adsorption is present in many natural, physical, biological and chemical systems, which is widely used in industrial applications such as the activated charcoal, capturing and using waste heat to provide cold water for air conditioning and other process requirements [25]. Generally, for a specified material, the adsorption could be occurred under high pressure and low temperature conditions, whereas desorption could be taken place under low pressure and high temperature conditions. The above characteristics of absorber makes it could efficiently adsorb HC emissions to the surface of sorbent. Compared with adopting TWC, reducing HC emissions by absorber could be accomplished at low temperatures. These make the absorber is more practical to be used for controlling HC emissions during the cold start. Westermann et al. studied the relative ability of some well-selected zeolitic materials for reducing HC emissions during the simulated cold start conditions. The results showed that even in the presence of some potential inhibitors such as water, the adsorption capacities of zeolitic materials at low temperatures were overall increased with the acidity of zeolites [26]. Yamazaki et al. developed an active-type HC adsorption system and tested its performance. The results showed that HC emissions in the Federal Test Procedure (FTP） testing cycle were reduced by more than 70% as compared to a system without the HC adsorption system [27]. Mukai et al. studied the adsorption characteristics of zeolite based absorber to control HC emissions from a gasoline engine by using model gas of N2, C3H6, C7H8 and H2O. According to the test results, approximately 90% of the total amount of HC modeling gas was immediately stored in the zeolite based absorber after the cold start [28]. Activated carbon materials are widely used [30-31] in the field of gas or liquid purification because of its high

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specific surface, developed pore structure, stable surface chemical properties, strong adsorption and so on. These characteristics also make the activated carbon become an effective and feasible adsorption material for reducing HC emissions at the engine starting condition. However, present researches on the applications of adsorption to reduce HC emissions during cold start are mainly focused on the surface modification of sorbent or the simulation of adsorption process. Few studies reported the activated carbon on adsorbing HC emissions under the real engine conditions. As the starting process of internal combustion engine is quite complex [32], which is hard to be simulated under non-engine conditions, it is of necessity to experimentally investigate the performance of activated carbon on HC adsorption in real engine conditions. In this investigation, three kinds of representative activated carbon materials of activated carbon honeycomb, activated carbon fibers and activated carbon foams were selected to examine the ability of adsorbing HC at different coolant temperatures. The material with the best adsorption performance was further investigated under the repeated engine starting conditions to check its durability in adsorbing HC emissions.

2. Experimental setup and procedure 2.1 Activated carbon materials Three kinds of active carbon are adopted in this test, which are activated carbon honeycomb, activated carbon foams and activated carbon fibers, respectively. The pictures of activated carbon materials used in this experiment are shown in Fig. 1. The shape and density of the selected activated carbon materials are different. Amounts of adsorption materials are selected to keep the relative exhaust back pressure around 14 kPa which is equivalent to the exhaust pressure of the original engine with the TWC. Under this relative pressure, volumes of activated carbon honeycomb, foams and fibers are 100*100*100, 100*100*150 and 100*100*6 mm, respectively. The specifications of activated carbon materials are listed in Table 1.

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The schematic of experimental setup is shown in Fig. 2. It is seen from Fig. 2 that the adsorption device is connected parallel with the exhaust pipe in order to preserve the pore structures of activated carbon materials from damages caused by the high exhaust temperature. When the engine starts, the Electronic Control Unit （ECU）opens valve B and C and closes valve A to make exhaust gases pass the adsorption device along the red arrow direction. Thereby, HC emissions produced during the starting period could be adsorbed. When the TWC achieves its light-off temperature, the ECU opens valve A and closes valve B and C. The exhaust gases are controlled to pass the exhaust pipe along the black arrow direction to enable the harmful emissions to be converted effectively by the TWC. The engine used in this experiment is a 1.5 L spark-ignition engine made by Beijing Automotive Industry Corp (BAIC), whose specifications are listed in Table 2. The engine is loaded by a CAC (a type of AC electrical dynamometer) eddy current dynamometer manufactured by Power link and the uncertainty is ± 1 rpm in engine speed detection. The coolant and ambient temperatures are measured by a FC2210 data acquisition unit manufactured by Power link and the uncertainty is ± 1℃. HC emissions are recorded by a Horiba 584L emissions analyzer which sensitivity is 1 ppm. HC emissions are sampled and measured on both sides of the adsorption device. The adsorption ability is can be shown by the difference in data. In the tests, each adsorption material is used only once (the material used in the adsorption test of the consecutive engine starting is exceptional).

2.2 Experimental procedure During the experiment, the ambient temperature and pressure were 25 ℃ and 101 kPa, respectively. The engine was started under different coolant temperatures of 25, 35, 45 and 55 ℃ to explore the effects of different activated carbon materials on HC adsorption under the starting conditions. According to the adsorption abilities of activated carbon materials, material with the best adsorption performance was further investigated

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under the repeated starting conditions at a given starting coolant temperature of 15 ℃ and pressure of 101 kPa to check its adsorption efficiency under frequently starting conditions. In the adsorption test of repeated engine starting, the engine was started consecutively seven times to observe the variations of HC emissions with the increase of starting times.

3. Results and discussion 3.1 The analysis of adsorption processes for different activated carbon materials Fig. 3 plots the variations of the exhaust temperatures and HC concentrations with time for the unadsorbed and adsorbed conditions under different starting coolant temperatures of 25, 35, 45 and 55 ℃, respectively. L1 is the original HC emissions which are sampled before the adsorption device. L2, L3 and L4 are HC emissions after the adsorption by activated carbon fibers, activated carbon honeycomb and activated carbon foams, which are sampled after the adsorption device. It is seen from Fig. 3 that, HC emissions of the original engine in L1 are raised steeply and reached the peak at around 10 s after the engine starting. This is because the ECU needs to increase the amount of fuel injection quantity to enable the engine start successfully. As the fuel injection duration is prolonged, the unburned fuel is prone to be emitted to exhaust pipe in the form of HC emissions. In addition, HC emissions in the exhaust could not be converted effectively by the TWC because of the low exhaust temperature. This makes HC emissions are increased rapidly in the first few engine cycles when the engine starts. Then, HC concentrations in L1 are declined rapidly and tended to be stabilized after around 40 s. The reason for this is that the engine is warmed up gradually with the stoichiometric air-to-fuel ratio. With the adoption of stoichiometric mixtures and improved thermal conditions, the fuel could be burnt more completely after the engine starting. This helps HC emissions decrease continuously with time after the engine starting. The other reason for the reduction of HC emissions is that the light-off temperature of TWC is achieved after the engine

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started for 40 s. These reasons above result in HC emissions in L1 finally achieve relatively stable and low values. As the raised coolant temperature avails reducing the fuel-film effect and improve the combustion completeness, the non-adsorbed HC emissions in L1 are decreased with the increase of coolant temperature. Furthermore, Fig. 3 also demonstrates that the trends of variations in HC emissions with time after the adsorption in L2, L3 and L4 are similar to those of original HC emissions in L 1. HC emissions after the adsorption are increased rapidly at the beginning. Then, the concentrations are declined and become stabilized finally. It is seen from Figs. 3a to 3c that the peak HC concentrations gain the highest values for L 2 and lowest values for L4. The primary reason could be attributed to the fact that activated carbon foams have larger volume compared with other activated carbon materials under the similar exhaust back pressure. Namely, the surface area of HC adsorption is larger for activated carbon foams. Thereby, more HC emissions could be adsorbed by activated carbon foams. The surface area of activated carbon honeycomb is a little bit smaller than activated carbon foams. Thereby, its ability for adsorbing HC emissions is slightly worse than that of activated carbon foams. As the activated carbon fibers gains the smallest surface area, the activated carbon fibers acquires the lowest adsorption ability in the test. Fig. 3 also indicates that, the exhaust temperatures are increased after the engine starting. The higher coolant temperature is, the faster exhaust temperature could be gained. In addition, at around 40 s after the engine starting and the exhaust temperature of about 50 to 60℃, HC concentrations in L4 begin to rise again. The reason for the rising of HC concentrations in L4 could be ascribed that adsorption ability is restricted by the raised exhaust temperature. It could be deduced that desorption temperature of activated carbon foams may be lower than other activated carbon materials. Therefore, although activated carbon foams adsorbed the largest amounts of HC emissions, the low desorption temperature makes HC emissions to be early desorbed for activated carbon foams. With the increase of time, because of the increased exhaust temperature, desorption efficiency of

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activated carbon foams gradually becomes higher than the adsorption efficiency. Therefore, more HC emissions are desorbed from activated carbon foams in later part of starting. Moreover, it is also found that, since the coolant temperature directly influences the combustion and exhaust temperatures, the increased coolant temperature also results in the raised exhaust temperatures and desorption efficiency which causes HC to be desorbed from activated carbon foams earlier. The higher coolant temperature is, the faster exhaust temperature could be acquired and the earlier desorption could be occurred for the activated carbon foams. From above discussions, it could be deduced that desorption temperature of activated carbon foams is lower than other activated carbon materials. For the same reason, the HC concentration after adsorption in L3 is lower than that of L4 in Fig. 3d. This proves that activated carbon honeycomb gains the best adsorption ability at high starting coolant temperature. Fig. 4 depicts the variations of instantaneously adsorbed HC emissions with time under different coolant temperatures of 25, 35, 45 and 55 ℃, respectively. M1, M2 and M3 are HC concentrations that reflect differences between HC emissions of original and after adsorption for activated carbon foams, activated carbon honeycomb and activated carbon fibers, respectively. It is seen from Fig. 4 that the trends of variations in HC concentrations in M1 to M3 are similar to those of original HC emissions, which are first quickly increased and achieved the peak at about 10 s. The main reason for this is that the massive micropores of activated carbon materials make HC emissions could be adsorbed quickly and effectively at the beginning of adsorption processes. Then, the adsorbed HC emissions are declined with the decreased original HC emissions. Reasons for the decrease in HC emissions in later part of adsorption could be ascribed to the fact that both the cylinder and exhaust temperatures are increased with time. The increased cylinder and engine temperatures provide better condition for the fuel atomization and complete combustion. The increased exhaust temperature helps the TWC to gradually reach its light-off temperature. Therefore, the higher starting coolant temperature is, the less amount

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of original HC emissions are emitted. Thereby, the peak values in M1, M2 and M3 are decreased with the increase of coolant temperature. Finally, all the instantaneously adsorbed HC emissions in M1, M2 and M3 are become stable values. This is because that most amounts of original HC emissions are effectively converted by the TWC. In addition, the adsorption is suppressed by the rising of exhaust temperature for activated carbon materials. Therefore, the differences between HC emissions of original and after adsorption are declined after the engine starting for about 40 s. It is seen from Figs. 4a to 4c that the peak HC concentrations gain the highest values for M1 and lowest values for M3. As adsorbed HC emissions in M1 gain the highest peak value, it could be concluded that activated carbon foams has the best ability in adsorbing HC emissions. The crucial reason could be that activated carbon foams have a larger surface area. However, M1 is decreased continuously after the engine starting for about 40 s, which even dropped to below zero at later time from the starting due to the increased exhaust temperatures. For the same reason, the adsorbed HC emissions in M 2 are higher than those in M1. Fig. 5 depicts the variations of instantaneous HC adsorption efficiencies at different coolant temperatures of 25, 35, 45 and 55 ℃, respectively. The instantaneous HC adsorption efficiency is defined as the ratio of different values between the original HC concentrations and the HC concentrations after adsorption divided by the original HC concentrations at a given time. N1, N2 and N3 are the variations in instantaneous HC adsorption rates for activated carbon foams, activated carbon honeycomb and activated carbon fibers during the adsorption processes. It is seen from Fig. 5 that all the efficiencies of activated carbon materials are increased rapidly and achieved the peaks at around 10 s. The main reason is the high concentrations of original HC emissions at first about 10 s. Then, all the concentrations are declined continuously at varying degrees. It is seen from Fig. 5 that the instantaneous adsorption efficiencies are suppressed by the rise of exhaust temperature. Among the concentrations in Figs. 5a to 5c, HC concentrations gain the peak values for N1 and lowest values

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for N3. It is found that activated carbon foams have the highest adsorption rates. However, sharp drops are found for HC emissions in N1 after achieving the peaks. The raised exhaust temperature makes that the peak value of N2 is higher than that of N1 in Fig. 5d. Just as the drops of N1, the adsorption rates of HC emissions in N3 are also decreased with the increase of temperatures of starting coolant and exhaust. Compared to HC emissions in N1 and N3, the adsorption rates of HC emissions in N 2 for different coolant temperatures are slightly affected by the rise of coolant temperature, which are varied gently with the temperature.

3.2 Different adsorption phases of HC emissions From above results, it can be confirmed that in former part of starting, the adsorption ability for activated carbon foams is the largest, but the early desorption is found at about 40 s after starting. In addition, some of activated carbon foams are damaged during the experiment. Compared with activated carbon foams, the adsorption performance for activated carbon honeycomb is more stable that early desorption is not occurred in the adsorption processes. Moreover, activated carbon honeycomb has higher adsorption efficiencies than activated carbon fibers. In conclusion, activated carbon honeycomb has the best adsorption performance among activated carbon materials in the experiment. It is seen from Figs. 3 to 5 that the HC adsorption processes could be divided into three phases based on the instantaneously adsorbed HC emissions and the instantaneous HC adsorption efficiencies. The first period could be called as the rapid adsorption phase which is defined as the period of adsorption from the engine starting to the peak values of HC emissions. Within this period, both of the instantaneously adsorbed HC emissions and instantaneous HC adsorption efficiencies are increased rapidly, and the total amounts for adsorbed HC emissions are increased fast. The second period is called as the interim adsorption phase which is defined as the period of adsorption from reaching the peak values of HC emissions to the stable values. The instantaneously adsorbed HC

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emissions and instantaneous HC adsorption efficiencies are both decreased continuously, whereas, the total amounts of adsorbed HC emissions are still increased in this period. The third period is called as the slow adsorption phase which is defined as the period of adsorption from becoming the stable values to the end. The effects of HC adsorption are small compared with those of other adsorption phases. Fig. 6 shows the percentage of adsorbed HC concentration of each period to the total absorbed HC for activated carbon honeycomb under different coolant temperatures of 25, 35, 45 and 55 ℃, respectively. The corresponding time for each adsorption period is 0 to 10, 10 to 40 and 40 to 80 s, respectively. The corresponding time of rapid adsorption phase is shortest while that of slow adsorption phase is longest. It is seen from Fig. 6 that in the period of rapid and interim adsorption phases, the percentages are both around 40% while the percentage of slow adsorption phase is lowest and around 15%. These results demonstrate that the optimal adsorption time is the period that includes rapid and interim adsorption phases in the practical applications for HC adsorption. Compared to the previous adsorption results in this experiment, nearly 90% of adsorption efficiency is achieved in about 50% of the adsorption time in the period of rapid and interim adsorption phases.

3.3 The adsorption abilities per unit weight of different adsorption materials Fig. 7 shows the adsorption abilities per unit weight of different adsorption materials under different coolant temperatures of 25, 35, 45 and 55 ℃，respectively. The adsorption ability per unit weight is defined as the ratio of different values between the original HC concentrations and the HC concentrations after adsorption divided by the weight of each adsorption material. It is seen form Fig. 7 that the adsorbed HC concentrations of three activated carbon materials are all decreased with the increased coolant temperature as the adsorption is suppressed by the raised exhaust temperature. The highest adsorbed HC concentrations are found for the activated carbon fibers while the lowest adsorbed HC concentrations are found for the activated carbon

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honeycomb. The main reason for this is that the activated carbon fiber has the lowest weight under the same exhaust back pressure among these three adsorption materials. It leads to the best adsorption ability per unit weight but the lowest total HC adsorption rate. Although the adsorption ability per unit weight of the activated carbon foams are slightly better than that of the activated carbon honeycomb, the early desorption is occurred in the adsorption process for the activated carbon foams due to the raised exhaust temperatures. Thus, according to the foresaid results, the activated carbon honeycomb has the best adsorption performance due to the more stable and efficient adsorption performance under the same exhaust back pressure.

3.4 Total adsorption efficiencies of HC emissions The Fig. 8 shows the total adsorption rate of adsorbed HC emissions for activated carbon materials for 80 s. The HC adsorption efficiency is defined as the ratio of difference in values between the total original HC concentrations and the total HC concentrations after adsorption divided by the total original HC concentrations.. With the increase of coolant temperature, the total adsorption efficiencies of activated carbon honeycomb are changed to be 68.5%, 66.4%, 65.1% and 62.2%, respectively. The total adsorption efficiencies of activated carbon foams are changed to be 66.1%, 60.9%, 54.3% and 49.5%, respectively. The total adsorption efficiencies of activated carbon fibers are changed to be 53.5%, 32.7%, 34.4 % and 38.2%, respectively. These above results indicate that the total adsorption efficiencies of activated carbon honeycomb are slightly affected by the increased coolant temperature. The total adsorption efficiencies of activated carbon foams are declined due to the released HC emissions and suppressed HC adsorption capacity by the rise of exhaust temperature.

3.5 Repeated engine starting adsorption experiment The real vehicles may frequently stop and restart while waiting traffic lights, under traffic jams and driving

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short distances. Thus, it is important that the adsorbers could effectively and consecutively adsorb HC emissions in the repeated engine starting conditions. Additionally, the adsorption has to be strong enough in order to prevent the release of adsorbed HC emissions. Thus, activated carbon materials are further investigated under the adsorption test of repeated engine starting. Activated carbon honeycomb is chosen in this test because of the best adsorption performance among all the activated carbon materials during the experiment. Fig. 9 shows HC emissions and efficiencies after adsorption for consecutive seven starting times at a coolant temperature of 15 ℃. It is seen from Fig. 9 that HC emissions after the activated carbon honeycomb adsorption are all increased rapidly at the beginning. Then, the concentrations are decreased continuously and finally become stabilized. Both the peak and stable values of HC emissions are increased with the rise of starting times. The adsorption efficiencies are declined continuously with the starting times. The adsorption efficiencies of first three times are 76.4%, 63.7% and 54.0%, respectively. However, the stable values are even higher than those of original HC emissions after the engine starting for three times. The reason for this could be attributed to the fact that a large volume of micropores leads to high rates for HC adsorption at the first few engine starting times, but the saturation of the micropores results in a consecutive decrease for the volume of unadsorbed micropores. The adsorption could not prevent the adsorbed HC emissions from releasing under this condition. Finally, with the weakening of HC adsorption and the releasing of adsorbed HC emissions, HC emissions after adsorption are increased gradually and become higher than the original HC emissions and the total HC adsorption rates are declined continuously over starting times. According to the results in the repeated HC adsorption experiments, the adsorption ability could be affected greatly by the saturation of micropores. Thus, desorption strategy is significant for the reuse of sorbent. For the adsorption system in this paper, desorption can be realized by the help of exhaust gas with high temperatures. For example, the HC adsorption material is isolated after the adsorption to prevent the adsorbed HC emissions to

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desorb due to the exhaust gas with high temperature. Then, the valves of B and C are opened while the valve A is closed when the second TWC achieves its light-off temperature. Thus, the adsorbed HC emissions can be desorbed by the exhaust gas with high temperature and converted by the second TWC. The adsorption device will be isolated again before the exhaust temperature achieving the limiting temperature of sorbents. The regeneration of sorbent can be realized by this way.

4. Conclusion In this paper, three kinds of activated carbon materials were selected to adsorb HC emissions for 80 s during the engine starting. The adsorption material with the best adsorption performance was further investigated in the adsorption test of repeated engine starting. The test results showed that all of the activated carbon materials could reduce HC emissions effectively. The total adsorption efficiencies of activated carbon honeycomb, foams and fibers are 68.6%, 66.1% and 53.5% at the coolant temperature of 25 ℃.The efficiencies of these materials are decreased gradually with the increase of coolant temperature. The early desorption of activated carbon foams is occurred in slow adsorption period. Both of the instantaneously adsorbed HC emissions and the instantaneous HC adsorption efficiencies are increased rapidly at the beginning of the adsorption processes. Then, they are decreased after the engine starting for about 10 s. The total HC adsorption efficiencies for activated carbon honeycomb in the adsorption test of repeated engine starting are declined gradually. The peak values of HC emissions after adsorption are increased with the increase of engine starting times. The processes of HC adsorption could be divided into three phases based on the variations of instantaneously adsorbed HC emissions and instantaneous HC adsorption efficiencies, which are the rapid adsorption phase,

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interim adsorption phase and slow adsorption phase, respectively. The results in the adsorption test of repeated engine starting showed that the total adsorption rate is decreased consecutively in the repeated adsorption test. This indicates that adsorption ability could be affected greatly without desorption. Thus, desorption strategy is required for the applications of HC adsorption.

Acknowledgments This work was supported by National Natural Science Foundation (Grant No.51476002), National Key Basic Research Development Project (973) (Grant No.2013CB228403) and Beijing Municipal Commission of Science and Technology (Grant No. Z141100003814017).

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Fig. 1 Activated carbon materials in the experiment

Fig. 2 Schematics of experimental setup

1. Intake air 2. Air cleaner 3. Engine 4. TWC 5. ECU 6. Valve A 7. Valve B 8. Adsorption Unite 9. Valve C 10. HC sampling points

11. Exhaust gas Fig. 3 HC concentrations after adsorption and the exhaust temperatures versus time at the coolant temperatures of 25 ℃ (a), 35 ℃ (b), 45 ℃ (c), 55 ℃ (d) Fig. 4 Instantaneous HC adsorption abilities versus time at the coolant temperatures of 25 ℃ (a), 35 ℃ (b), 45 ℃ (c), 55 ℃ (d) Fig. 5 Instantaneous HC adsorption efficiencies versus time at the coolant temperatures of 25 ℃ (a), 35 ℃ (b), 45 ℃ (c), 55 ℃ (d) Fig. 6 Different adsorption phases of HC emissions for activated carbon honeycomb at the coolant temperatures of 25 ℃ (a), 35 ℃

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(b), 45 ℃ (c), 55 ℃ (d) Fig. 7 The adsorption abilities per unit weight of different adsorption materials under different coolant temperatures of 25, 35, 45 and 55 ℃ Fig. 8 The total HC adsorption efficiencies of activated carbon materials

Fig. 9 Repeated engine starting HC emissions after adsorption and total adsorption efficiencies for each time of activated carbon

honeycomb

Table 1 Specifications of activated carbon materials

Table 2 Engine specifications

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24

25

26

27

28

Table 1 Specifications of activated carbon materials Iodine

Activated carbon

absorb

Specific surface 2

Adsorption

of

Average

pore

Weight

Porosity

（mg / g）

area（m / g）

benzene（%）

diameter (Å)

(g)

(%)

≥950

950

37

20-50

190

55

≥200

800

25-32

15-50

125

80

≥850

900-1000

30-35

17-20

12

35

honeycomb Activated carbon foams Activated carbon fibers

30

Table 2 Engine specifications #

of

Stroke

cylinders

mm

4

84.8

/

Bore / mm

75

Displacement /

Compression

Rated torque / Nm

Rated power / kW

L

ratio

(at 3800 rpm)

(at 6000 rpm)

1.499

10.5

148

85

31

Highlights > Three kinds of activated carbon materials are selected to adsorb HC emissions. > The adsorption is investigated at different starting coolant temperatures. > The adsorption abilities in the repeated engine starting test are investigated. > The adsorption processes could be divided into three different phases. > HC emissions are obviously decreased by adsorption.

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