Experimental Study on a Diesel Particulate Filter with Reciprocating Flow.

In this article, a new diesel particulate filter (DPF) system with reciprocating flow is proposed, and an experimental study on the characteristics of the active-passive component regeneration of the DPF system is carried out. Several control parameters such as temperature distribution, pressure difference, and pollution emissions of the DPF system are measured for different reciprocating cycles. The mechanism of reciprocating flow regeneration of the DPF system and the effects of the reciprocating flow cycle on the performance of the DPF system are analyzed. Results show that (1) the DPF system can use a tiny amount of extra fuel to maintain the chemical reaction, which in turn realizes the regeneration of the catalyzed DPF because of its properties of heat recovery and reverse blowing of ash; (2) with the increase in the reciprocating flow cycle, the temperature profile moves toward the downstream side of the DPF system and the fluctuation amplitudes of the components of CO, NO, and NO2 increase; (3) if reasonable temperature distribution is formed in the DPF system for a certain reciprocating cycle, the regeneration efficiency can be obviously improved and the average content of particulate matter emission can be kept at quite a low level.

Introduction

The influence of particulate matter (PM) emission from diesel engines on the human health and environment has received wide attraction in recent years.[1,2] Thus, many countries have established strict statutes to limit PM emission of engines. These increasingly stringent regulations are driving manufacturers to develop efficient after-treatment techniques which can remove PM emission from diesel engines.[3,4] It is recognized that the most widely used approach to remove PM emission from diesel engines is a diesel particulate filter (DPF).[5]

Although a typical wall-flow DPF can remove PM in excess of 95% over a wide range of engines,[6] the DPF is designed to keep a certain quantity of PM. The overloaded PM would create a blockage to the flow through the DPF, eventually produce an excessively high pressure drop of the flow through the DPF and prevent the engine operation.[7] Therefore, the DPF needs to be designed to provide a reliable way of removing PM accumulated in the filter in order to restore its PM collection capacity.[8] Removing PM in the filter is known as the regeneration of DPF, which is a main technical challenge in the practical application of the DPF.

There are mainly two types of regenerations of DPF: active regeneration and passive regeneration. Active regeneration periodically removes the PM trapped in a DPF through controlled oxidation with O2 at 550 °C or higher temperatures.[9] In this case, the heat must be supplied from outside sources, such as an electric heater, a microwave heater, and a flame-based burner.[10] The external energy used for heating would increase the cost of a DPF system because of complex supplements.[11] On the other hand, passive regeneration utilizes an ongoing catalytic reaction at the exhaust gas temperature to oxidize the PM trapped in a DPF without additional fuel. Diesel oxidant catalyst (DOC)-assisting DPF regeneration is typical of the passive regeneration method which is also named the continuously regenerating trap (CRT) system. The CRT system is installed with a DOC where NO is preferentially converted to NO2 before a DPF and NO2 is used to oxidize PM trapped in the filter below 300 °C.[12] In order to achieve the best performance, however, the CRT system should satisfy two conditions: the temperature should be in the range 250–450 °C and the NO/soot ratio should be adequately high. Otherwise, the produced NO2 will be too low to oxidize soot. It should be noticed that both hydrocarbon (HC) and CO would be the secondary pollutants from incomplete oxidation conversion of the soot in the CRT system.

It needs to be mentioned that both active regeneration and passive regeneration of the DPF cannot keep the filter clean for a long term. In order to overcome this problem, the active–passive component regeneration of the DOC-assisting DPF is a traditional option.[13] A certain amount of the fuel is injected into the cylinder in the late stage of the combustion process, or a certain amount of the fuel is injected into the exhaust pipe at the upstream of a DOC.[14−16] The DOC is used to oxidize the fuel, heat the downstream DPF, and promote the passive regeneration of the DPF. In-cylinder injection of the fuel has an advantage that no additional fuel injection device is needed, whereas a main disadvantage is that the lubricating oil of the diesel engine will be diluted by the fuel. In-exhaust-pipe direct injection can avoid oil dilution; this becomes the main technical approach of the active–passive component regeneration of the DOC-assisting DPF.

It is known that the active–passive component regeneration of the DOC-assisting DPF should maintain the temperature in the DOC and DPF within a suitable range. Only the temperature in the DOC is kept in the range of 250–450 °C; NO can be preferentially converted to NO2. The temperature in the DPF should not only be high enough to burn the soot accumulated in the DPF but also be kept below a certain threshold to prevent the DPF from being damaged. As a result, this regeneration method will be facing a challenge on how to control the temperature in the DOC and DPF. This is due to the fact that several factors need to be concerned; they are the wide range of engine operations, thermal inertia of the DOC and DPF, the complexities of the reactions in the DOC and DPF, and the injection model of the fuel injectors.[17,18] In addition, all forms of regeneration can only remove combustible constituents of the PM trapped in the DPF and cannot remove incombustible constituents remaining behind as ash. After a long period of operation, the DPF still needs to be disassembled to clean up the residues in the filter.[19−21]

Reciprocating flow regeneration (RFR) has also been identified to be an effective means to realize active–passive component regeneration of the DPF utilizing a tiny amount of the additional fuel.[22,23] RFR exploits heat recovery properties to provide a high-temperature region that maintains the regeneration of the filter. However, the temperature gradients at the inlet and outlet of the DPF not only defer the regeneration process but also result in additional thermal stress. To address these issues, Zheng et al. proposed an RFR structure in which two inert monolith blocks with a flow-through passage are installed at both the ends of a DPF.[24] The monolith blocks can enhance heat recuperation, reduce the temperature gradient, and maintain a more stable temperature distribution in the DPF. Furthermore, Zheng et al. improved the RFR configuration with two DOCs and a noncatalytic DPF.[25] Each DOC is placed at a location where the catalytic activity can be maintained and the catalytic light-off temperature can be kept in RFR operation. The improved RFR device has a significant advantage in supplemental energy saving when compared to the standard DOC and DPF configuration under the same operation conditions.

RFR is a highly complex physical and chemical reaction process that combines periodically reciprocating flow, PM separation, multiphase reactions, and heat and mass transfer over a range of temporal and spatial domains. To the best of authors’ knowledge, there are few published reports on applying the concept of a periodically reciprocating flow to the exhaust after the treatment system of the diesel engine. There is also no literature on investigating the mechanism of the new DPF system combining the RFR with the component regeneration of the DOC-assisting DPF. There is still much room to be investigated further in this research area.

In the current work, a new DPF system based on the concept of combining the RFR with the active–passive component regeneration of the DOC-assisting DPF is proposed and designed. The proposed new DPF system consists of a catalyzed DPF (cDPF) and two DOCs. There is a space interval between every DOC and the cDPF. The new DPF system uses periodically reciprocating flow, chemical reaction, releasing heat and storing heat to form a stable temperature distribution that a high-temperature region is located in central DPFs and two low-temperature regions are located in the DOCs on both sides. The new DPF system uses a small amount of extra fuel to maintain the oxidation conversion of the soot trapped, uses a downstream DOC to oxidize HC and CO from incomplete oxidation conversion of soot, and utilizes the reverse flow to blow away the ash accumulated in the filter. Furthermore, we will experimentally study the temperature distribution, the treatment of pollution, the mechanism of the RFR of the new DPF system, and the effects of the reciprocating flow cycle on the performance of the DPF system.

Experimental Apparatus and Method

Experimental Apparatus

As shown in Figure , the experimental system mainly consists of a diesel engine, exhaust pipelines, a DPF system, and a measurement system.

Figure 1

Schematic diagram of the experimental setup.

The diesel engine is a six-cylinder in-line with water cooling, turbo-charging and intercooling, four strokes direct injection heavy-duty diesel engine. The main specifications of the engine are given in Table . The diesel engine burns 0# diesel oil (National V emission standard) and is operated under ideal conditions.

Table 1

Diesel Engine Characteristics

compression ratio	16:1
cylinder diameter × stroke	105 × 130 mm
number of cylinders	6
total piston displacement	6.75 L
rated speed	1500 rpm
calibration power	110 kW
full load exhaust temperature	≤540–600 °C

The DPF system is composed of two DOCs, a cDPF, and flow pipelines as shown in Figure . Two DOCs are symmetrically arranged on both sides of the cDPF and a structure of DOC + cDPF + DOC is formed. Two pairs of solenoid butterfly valves in the flow pipelines use synchronous opening and closing to control the periodically reciprocating flow of the airflow in the DPF system.

Figure 2

Schematic diagram of the DPF system.

Each DOC is a cordierite ceramic monolith that has a honeycomb structure with parallel channels. Al2O3 and Ce(Zr)O2 catalyst carriers coat the channel walls and support bimetallic Pt/Pb catalysts. As shown in Figure , the filter of the cDPF is a wall-flow filter with the alternate channels plugged at one end and open at the opposite end. The cordierite particles sintered together form the porous matrix of the internal wall between the alternate channels. The wall-flow filter is also coated by a layer of Al2O3 and Ce(Zr)O2 as catalyst carriers, and the catalyst carriers support bimetallic Pt/Pb catalysts. The characteristics of the DOC and the cDPF are given in Table . The ratio of Pt to Pb is 4:1 in the catalysts used in the DOC and cDPF. Pt in the catalysts can enhance NO oxidation to NO2. A small amount of Pb can improve the thermal stability of the catalysts and reduce the light-off temperature of soot, CO, and HC oxidation.

Figure 3

Schematic diagram of the wall-flow filter.

Table 2

Characteristics of the DOC and cDPF

	DOC	cDPF
pore density/cpsi	300	200
main material	cordierite	cordierite
porosity/%	35–40	50–60
wall thickness of channel/mm	0.25	0.3
average pore diameter/mm	3–7	8–20
noble metal catalysts	Pt, Pd	Pt, Pd
catalyst load/(g m–3)	706.2	353
catalyst carriers	Al2O3 + Ce(Zr)O2	Al2O3 + Ce(Zr)O2
length/mm	152.4	254
diameter/mm	240	240

The exhaust pipelines extend from the engine exhaust pipe to the DPF system, through passing a main line or a bypass line. Supplemental energy will be applied to increase the DPF substrate temperature in order to initiate thermal regeneration. When the exhaust gas flows into the DPF system through the bypass line, a 30 kW electrical heater is used to heat the exhaust gas. When the exhaust gas flows into the DPF system through the main line, a tiny amount of additional fuel is introduced at the inlet of the DPF system. Pure propane gas (99.9%) is chosen as the additional fuel in order to facilitate measuring and ensure the measurement accuracy.

The flow rate of the exhaust gas is measured using a vortex flow meter, and the flow rate of the propane gas is measured using a glass rotameter. The upstream and downstream pressure of the DPF system, the upstream pressure of the exhaust pipelines, and the ambient pressure are measured using piezoresistive pressure transmitters. The temperature at different locations of the DPF system, the upstream temperature of the exhaust pipelines, and the ambient temperature are measured using K-type thermocouples. The thermocouples are also arranged along the axis of the DPF system in order to measure the temperature of the channel walls in the axis of the DOCs and cDPF.

The concentrations of PM emission in the exhaust gas are measured using a SMG smoke meter, and the components of gaseous pollutants are measured using a Testo 350 flue gas analyzer. The measuring positions of the components of the exhaust gas are set at the measurement hole A and measurement hole B, as shown in Figure . An Agilent 34970A data acquisition instrument is used to acquire the experimental data and transmit the data into a personal computer.

Experimental Method

At the start of the DPF system, the main line is closed while the bypass line is opened. The electrical heater is used to heat the exhaust gas to the temperature starting the DPF system. Meanwhile, a certain amount of the propane gas is introduced at the inlet. As a reasonable temperature distribution in the DPF system is established, the main line is opened and the bypass line is closed. When the temperatures periodically fluctuate and the fluctuating amplitude are nearly equal at every measuring position, the DPF system is considered to be under stable operation conditions, and the experimental parameters including temperature, pressure, pollution emission concentrations, and PM concentrations are measured accordingly.

During the experiment, the reciprocating flow cycle in the DPF system is separately set at 30, 50, 70, and 90 s, the volume flow of the exhaust gas is kept at 248.3 m3/h, the volume flow of the propane gas introduced at the inlet is 0.18 m3/h, and the exhaust temperature is 396 K at the inlet.The components of the exhaust gas at the inlet of the DPF system are listed in Table .

Table 3

Components of the Exhaust Gas at the Inlet

O2/%	CO/ppm	NO/ppm	NO2/ppm	NOx/ppm	SO2/ppm	PM/mg/m3	HC/ppm
17.9	394	208	41.2	249.2	88	256	195

The mean concentration of PM, CO emission, HC emission, and the mean pressure difference at the outlet of the DPF system is the average value of the parameters measured in 30 reciprocating flow cycles as the DPF system is in a stable operation status.

Uncertainty Analysis of Experimental Results

The uncertainties of the measurements in the experiment are dependent on the experimental conditions and the measurement instruments. The specifications of the measurement instruments used in the experiment are listed in Table .

Table 4

Measuring Instrument Characteristics

instrument	performance parameter	manufacturer and model	precision
vortex flow meter	range 100–1000 m3/h	LUGB-23100C	1.5%
glass rotameter	range 0.01–0.6 m3/h	Propane gas	2.5%
pressure transmitter	range 0–10 kPa	Keller PD-23/8666.1/0.1	linearity ≤0.10% FS
	sensitivity 1.6 mA/kPa
thermocouple	variety Ni–Cr–Si/Ni–Si, Φ0.5 × 500 mm × L1500 mm	WRNK-162	±1.5 K
flue gas analyzer	response time O2 < 20 s	Testo 350	O2, 0.2 vol %
	response time CO < 40 s		CO, 5% of reading
	response time HC < 40 s		HC, 10% of reading
	response time NO < 30 s		NO, 5% of reading
	response time NO2 < 40 s		NO2, 5% of reading
smoke meter	range 5–500 mg/m3	SMG100	5%
	particle size 0.1–10 μm
	response time < 15 s
data acquisition	scan speed 60 channels/s	Agilent 34970A	dc voltage 0.002%
			temperature 1 °C

An uncertainty analysis is performed for the experimental data using the propagation of the error method described by Moffat.[26] The uncertainty of the temperature measurement is ±1.5 K, the uncertainty of the pressure measurement is ±2.0 Pa, the uncertainty of the flow measurement of the exhaust gas is ±3.75 m3/h, the uncertainty of the flow measurement of the propane gas is ±0.005 m3/h, the measurement uncertainty of the emission concentrations is ±10 mg/m3, and the measurement uncertainty of the particulate concentrations is ±10 mg/m3.

Analysis of RFR

As the DPF system utilizes a tiny amount of propane gas as the additional fuel to maintain a stable operation, the DPF system can filter PM, remove the CO and HC emissions, and realize the regeneration of the periodically reciprocating flow, as shown in Figure .

Figure 4

Heat-stored and heat-rejected zone during a cycle.

During the forward flow cycle (Tf), the diesel exhaust gas is introduced in the left DOC, flowed through the cDPF and the right DOC in turns. The conversion of carbon monoxide (CO) can reach 90% and the conversion of hydrocarbon can reach 80% at 623 K in the DOC.[27] Consequently, the propane gas, CO, and HC in the exhaust gas are oxidized to H2O and CO2 by the residual oxygen of the exhaust gas in the left DOC. The hydrocarbons oxidized also include the volatile constituents adsorbed in the PM. The volatile constituents are mainly the soluble organic fraction (SOF) released from the PM at Tsof = 346 K.[6] At the same time, nitrogen monoxide (NO) is partially converted to nitrogen dioxide (NO2) in the exhaust gas in the left DOC.

As the diesel exhaust gas flows through the cDPF, the PM contained in exhaust gas is trapped and deposited on the walls of inlet passages during the Tf, as shown in Figures and 5. After the DPF system is started, the temperatures in the cDPF can be kept higher than 823 K. The temperature of the light off of soot oxidation is Tl2 = 873 K in loose contact between the soot and catalyst, and the temperature of the light off is Tl1 = 693 K in intimate contact for a cDPF.[28−31] The soot particulates on the walls are oxidized into CO2 and the residual ash particles remained on the walls of the inlet passages in the cDPF, as shown in Figures and 5.[32,33] The short residence time results in the incomplete soot oxidation and the rapid CO generation in the cDPF. CO from the cDPF is oxidized into CO2 in the right DOC.

Figure 5

Schematic diagram of filtering particulates.

In the upstream region of the DPF system, the solid frame in the left DOC and the cDPF has a higher temperature than the diesel exhaust gas. The solid frame in the region releases heat to the exhaust gas during the Tf, as illustrated in Figure . In the downstream region of the DPF system, the chemical reaction in the cDPF and the right DOC releases heat to the solid frame and the heat is stored in the solid frame of the heat-storing region. The amount of thermal storage in the solid frame during Tf is described by eq .

During the backward flow half cycle (Tb), the diesel exhaust gas is introduced in the right DOC, flowed through the cDPF and the left DOC in turns. During Tb, the solid frame storing heat during the previous Tf releases heat to the exhaust gas and activates the same chemical reactions in the new upstream region of the DPF system as in the previous Tf, as seen from Figure . The releasing heat in the new heat-releasing region during the Tb is described by eq .

During Tb, the particulates contained in the exhaust gas are trapped and deposited on the walls of the outlet passage of the cDPF, as demonstrated in Figures and 5. The soot particulates are oxidized, and the residual ash particles remained on the walls of the outlet passages. At the same time, the residual ash particles on the walls of the inlet passages during the previous Tf are stirred, blown away, and carried away by the reverse flow, as shown in Figure .

During Tb, the chemical reactions release heat to the solid frame in the new downstream region of the DPF system. The amount of thermal storage of the new heat-storing region during Tb is described by eq .

The same process of heat storing and heat releasing in the same location is repeated in the next reciprocating flow cycle. The DPF system with a reciprocating flow can use a tiny amount of extra fuel to maintain the chemical reaction, which in turn realizes the continuous active–passive component regeneration of the cDPF because of its properties of heat recovery and reversely blowing away the ash.

Results and Discussion

Temperature Profiles during a Reciprocating Flow Cycle

As the reciprocating flow cycle is 50 s, temperature profiles along the axial direction in the DPF system during a reciprocating flow cycle are displayed in Figure . During each half cycle every temperature distribution is a trapezoidal profile which exibits a high-temperature region in the middle (−277.5 mm ≤ X ≤ 63.5 mm) and has high-temperature gradients on both sides. The cDPF is always kept at a higher temperature than the lighting off of soot oxidation. Each DOC has a temperature gradient in a range from 360 to 1080 K. The high-temperature region travels at close to a constant speed from the upstream side of the DPF system toward the downstream side during each half cycle.

Figure 6

Change of temperature profiles during a cycle, (a) forward half cycle and (b) backward half cycle.

At the beginning of a Tf, a temperature maximum can be observed at X = −277.5 mm near the upstream side in Figure a. During the Tf, the temperature in the high-temperature region increases and the temperature near the downstream side increases more rapidly and gradually grows into a new temperature maximum at X = 0 mm. At the beginning of the Tb, after the flow direction switches, the maximum temperature at the end of the previous Tf still remains near the new upstream side as shown in Figure b. During the Tb, the temperature within the high-temperature region decreases and the temperature near the new downstream side decreases at a slower rate. A new temperature maximum at X = −277.5 mm appears gradually. The downstream maximum will become a new upstream maximum after the next change in the flow direction. The changes of temperature profiles during the Tf are not exactly the same with those during the Tb. The main reason could be that an asymmetric temperature distribution along the axial direction of the DPF system is formed in the startup process of the DPF system.

Figure shows a process of storing heat and releasing heat during a reciprocating flow cycle. The temperature within the region of −351.5 mm ≤ X ≤ −273.5 mm decreasing continuously illustrates that the solid frame releases heat to the exhaust gas during the Tf as shown in Figure a. The temperature within the region of −273.5 mm ≤ X ≤ 357.5 mm increasing continuously illustrates that the chemical reaction releases enough heat to heat the solid frame in the region. The heat storage in the solid frame of −273.5 mm ≤ X ≤ 357.5 mm during the Tf is represented by the shadow area in Figure a. The heat storage is described by eq .

Figure 7

Heat-stored and heat-rejected during a cycle, (a) forward half cycle and (b) backward half cycle.

During the Tb, the temperature within the region of −273.5 mm ≤ X ≤ 351.5 mm decreases, and the heat stored during the previous Tf is released to heat the exhaust gas and activate the chemical reactions as shown in Figure b. The releasing heat during the Tb is indicated by the shadow area in the region in Figure b, and its value is described by eq . The temperature within the region of −351.5 mm ≤ X ≤ −273.5 mm increases, and the heat released from the chemical reactions is stored in the solid frame. The heat stored during the Tb is indicated by the shadow area in the region in Figure b, and its value is described by eq .

A dividing point between the endothermic regions and exothermic regions in the DPF system during the Tf and Tb is located in the same position, that is, X3 = X4. The heat stored in the endothermic regions in the Tf is equal to the heat released in the exothermic regions in the Tb within the same regions as shown in Figure a,b. The same process of storing heat and releasing heat in the same location is repeated in the next reciprocating flow cycle.

Chemical Reactions during a Reciprocating Flow Cycle

Figure shows the temperature fluctuation curves at different measuring points in the DPF system as the reciprocating flow cycle is 50 s. Figure shows the fluctuation curves of the components of O2, CO, NO, and NO2 at the outlet with the reciprocating flow in the DPF system. At the beginning of each half cycle, the lower temperature results in the lower CO conversion in a new downstream DOC and the CO components increase rapidly, and then slowly decrease with the temperature growth of the new downstream during the half cycle.

Figure 8

Temperature fluctuation at different measuring points.

Figure 9

Profiles of O2, CO, NO, and NO2 at the outlet.

The variation of NO is associated with NO2, as shown in Figure . At the beginning of a Tf, the downstream DOC is at a lower temperature (400–430 K) and the chemical reaction equilibrium of NO2 and NO is on the NO side, as illustrated in Figures and 9, and then gradually shifts toward the NO2 side with the increasing temperature in the range of 433–573 K, the NO content decreases, and the NO2 content increases during the Tf. When the corresponding conversion is 100% at 573 K, the ratio of NO to NO2 reduces as the temperature further increases. Therefore, with increasing temperature, the NO content first increases and the NO2 content decreases when the temperature is higher than 573 K.

By switching to a backward flow, the temperature suddenly approaches to 573 K and the conversion of NO to NO2 nearly reaches 100% in the new downstream DOC at the beginning of the Tb. In this case, the NO content rapidly decreases and the NO2 content rapidly increases at the beginning of the Tb. Then, the NO content increases and the NO2 content decreases with the increasing temperature after the temperature is higher than 573 K in the new downstream DOC. The periodical variation of the O2 content is associated with that of the component contents of CO, NO2, NO, and NO2 as shown in Figure .

Figure shows the fluctuation curves of the pressure difference ΔP between the inlet and the outlet of the DPF system, PM emission, and the temperature at the outlet of the DPF system.

Figure 10

Curves of ΔP, PM, and the temperature at the outlet.

At the beginning of a Tf, the flow passes the inlet passages, the internal wall, and the outlet passages in turns in the cDPF. The velocity of the flow decreases along the inlet passages gradually. A part of soot particles in the flow is heated and oxidized into ashes. The ashes can be directly deposited onto the internal walls and form a very thin ash layer used as the substrate for soot particle deposition. Another part of soot particles in the flow can be deposited onto the thin ash layer and adhered to the internal walls.

Because the temperature within the region of X ≥ −277.53 mm is higher than 623 K in the upstream DOC, the chemical reaction equilibrium of NO2 and NO is on the NO side during the Tf. In addition, the cDPF is always kept at a temperature than 823 K as shown in Figure . Most of the soot particles underlain by the thin ash layer are oxidized into ashes further with O2 alone and without the NO2 catalytic action. Because of the inertia force, the ash layer and the captured soot particles appear first in the rear part of the inlet channel and then gradually move to the middle part of the inlet channel during the Tf. At the end of the Tf, most of the ashes and soot particles incompletely oxidized are accumulated on the internal wall in the rear part of the inlet channel.

By switching to the backward flow, the flow passes the outlet passages, the internal wall, and the inlet passages channel in turns in the cDPF. At the beginning of the Tb, the new upstream DOC is at a lower temperature than 623 K and the chemical reaction equilibrium of NO2 and NO is on the NO2 side. The temperature near the right end of the cDPF is also kept lower than 823 K as shown in Figure . According to the published data in ref (34), the residual particles attached on the walls at the rear part of the inlet passages during the previous Tf are stirred by and are completely oxidized into ashes further with the NO2 catalytic action, rather than being blown away on the wall immediately. Then, the ashes produced are shifted along the flow path gradually and are carried out by the reverse flow at the end of the Tb. However, the content of the ashes produced during every half cycle is too little to influence the PM emission content of the DPF system. Subsequently, the deposition state of the PM in the outlet passages during the Tb is the same as that in the inlet passages during the previous Tf. As switching to next forward flow, the flow resided in the flow pipelines, the DOCs channels and the outlet passages of cDPF reversely flows out the DPF system at the beginning of the Tf. Therefore, the PM content rapidly increases, and then rapidly decreases at the beginning of the Tf, as shown in Figure .

In the next Tf, because of the temperature distribution in the upstream DOC and the cDPF, the residual soot particles attached on the walls at the rear part of the outlet passages during the previous Tb are only incompletely oxidized into ashes with O2 alone. Then, the residual soot particles are shifted along the flow path gradually and are carried out by the reverse flow at the end of the Tf. Hence, the PM content rapidly increases at the end of the Tf. By switching to the next backward flow, the PM content rapidly increases to a peak value, and then rapidly decreases as shown in Figure .

The pressure difference between the inlet and outlet periodically fluctuates and the fluctuating amplitude of the pressure difference is nearly equal as shown in Figure . This illustrates that the accumulation of the PM trapped on the internal wall balances with the soot oxidation and reverses cleaning of the particulates in the cDPF.

Thus, it can be seen that if a reasonable temperature distribution in the upstream DOC and cDPF during a half cycle is formed, the regeneration efficiency of the DPF system can be improved.

Influence of the Reciprocating Flow Cycle

Figure shows the influence of the reciprocating flow cycle on the temperature profiles along the axial direction in the DPF system at the end of the forward half cycle. The experiment conditions including the flow, components and temperature of the exhaust gas, and the flow of the propane gas are the same as in the abovementioned experiment. With the increasing reciprocating cycle, the temperature profile along the axial direction in the DPF system moves toward the downstream and the high-temperature region in the profile slightly becomes wider.

Figure 11

Influence of reciprocating cycles on temperature profiles.

Figure shows the varying curves of CO, NO, and NO2 components at the outlet for different reciprocating cycles. Table shows the mean component of the exhaust gas at the outlet of the DPF system for different reciprocating cycles. The mean concentration of PM emission, CO emission, HC emission, and the mean pressure difference of the DPF system is the average value of the parameters measured in 30 reciprocating flow cycles as the DPF system maintains stable operation at the reciprocating flow cycle set as shown in Table .

Figure 12

Profiles of CO, NO, and NO2 during different cycles.

Table 5

Components of the Exhaust Gas at the Outlet of the DPF System

cycle/s	O2/%	CO/ppm	NO/ppm	NO2/ppm	PM/mg/m3	ΔP/Pa (with C3H8)	ΔP/Pa (without C3H8)
30	17.5	23.1	247.2	18.4	4.7	697.8	278.3
50	17.3	18.2	251.4	24.3	13.2	825.1	278.3
70	17.1	16.3	257.5	24.3	20.1	853.0	283.5
90	17.1	13.3	258.3	23.3	21.0	868.7	296.4

The periodical change regulation of CO, NO, and NO2 components at the outlet with the reciprocating flow is consistent with each other for different reciprocating cycles. For every reciprocating cycle, at the beginning of each half cycle, the lower temperature in the new downstream DOC results in the lower CO conversion and the CO component increases rapidly. Subsequently, the CO conversion increases and the CO component slowly decreases with the temperature growth of the new downstream DOC during the half cycle. Because the temperature profile moves toward the downstream side with the increasing reciprocating cycle, the maximum temperature of the downstream DOC increases and the temperature difference between the downstream DOC and the upstream DOC increases during a half cycle. Therefore, with the increasing reciprocating cycle, the peak value of the CO emission fluctuation increases, the efficiency of CO conversion increases, and the average CO emission decreases at the outlet of the DPF system, as listed in Table .

The periodical variation of NO emission is associated with NO2 emission for every reciprocating cycle. At the beginning of each half cycle, the NO emission decreases and the NO2 emission increases. As the NO emission decreases to the troughs, the NO2 emission increases to the peaks. Subsequently, the NO emission increases and the NO2 emission decreases. With the increasing reciprocating cycle, the fluctuating amplitudes of NO and NO2 components increase. However, there is little change in the mean concentration of the NO and NO2 emissions with the increasing reciprocating cycle.

Figure shows the varying curves of PM emission with and without C3H8 fuel addition at the outlet for different reciprocating cycles.

Figure 13

Profiles of the PM at the outlet for different reciprocating cycles.

As the DPF system is operated without C3H8 fuel addition, the soot particles in the flow through the cDPF cannot be oxidized into ashes. According to the published data in ref (35), during each half cycle, no ashes are produced and deposited onto the internal walls to form an ash layer which is used as the substrate for particle deposition. The particles carried by the flow can be only deposited on the internal walls and hardly be adhered to the internal walls. At the beginning of each half cycle, the flow residing in the flow pipelines, the DOCs channels and the cDPF passages reversely flow out the DPF system for every reciprocating cycle. Most of the residual particles deposited on the internal walls of the cDPF passages during the previous half cycle are stirred, blown away, and carried away by the reverse flow immediately. Only a very small amount of the particles on the internal walls are gradually blown away and carried out in the system by the reverse flow. Therefore, the PM content rapidly increases, and then rapidly decreases at the beginning of each half cycle for every reciprocating cycle. Subsequently, the PM content decreases slowly during the half cycle as shown in Figure . With the increasing reciprocating cycle, the fluctuating range of the PM content remains almost unchanged at the beginning of each half cycle.

As the DPF system is operated with C3H8 fuel addition for the 30 s reciprocating cycle, because the temperature within the region of X ≥ −277.53 mm is higher than 623 K in the upstream DOC, the chemical reaction equilibrium of NO2 and NO is on the NO side during the Tf. In addition, the cDPF is always kept at a higher temperature than 823 K, as shown in Figure S2 in the Supporting Information. Therefore, most of the soot particles deposited onto the internal walls in the inlet channel are oxidized into ashes with O2 alone and without the NO2 catalytic action. At the end of the Tf, the ashes and soot particles incompletely oxidized are accumulated on the internal wall in the rear part of the inlet channel. During the Tb, the upstream DOC is kept at a lower temperature than 623 K and the chemical reaction equilibrium of NO2 and NO is on the NO2 side. Furthermore, the temperature near the right end of the cDPF is also kept lower than 823 K. Both the soot particles remained in the inlet channel during the previous Tf, and the soot particles deposited onto the internal walls in the outlet channel are completely oxidized into ashes with the NO2 catalytic action during the Tb.

Because of the flow direction periodically switching in shorter time for the 30 s reciprocating cycle, the PM on the internal wall is stirred more strongly and the contact between the soot and catalyst is more intimate on the internal wall. The conversion efficiency of the soot captured on the internal wall is increased. At the beginning of each half cycle, only the suspended PM in the flow residing in the DPF system reversely flow out the outlet. Therefore, the fluctuating range of the PM content with C3H8 fuel addition is lower than that without C3H8 fuel addition for the 30 s reciprocating cycle, as shown in Figure .

Comparing the 30 s reciprocating cycle with the 50 s reciprocating cycle noted above, the regeneration efficiency is improved, the average content of PM emission is only 4.7 mg/m3, and the mean pressure difference of the DPF system is 697.8 Pa. Thus, it can be seen that if a reasonable temperature distribution which corresponds to a reciprocating cycle is formed in the DPF system, the regeneration efficiency can be improved further.

As the DPF system is operated with C3H8 for the 70 and 90 s reciprocating cycle, the process of PM capture and regeneration is basically the same as that of the 50 s reciprocating cycle noted above. Because of the temperature distribution in the upstream DOC, the chemical reaction equilibrium of NO2 and NO is on the NO side during the Tf, as shown in Figures S3 and S4 in the Supporting Information. Most of the soot particles captured on the internal walls in the inlet channel are oxidized into ashes with O2 during the Tf. At the end of the Tf, most of the ashes and soot particles incompletely oxidized are accumulated on the internal wall in the rear part of the inlet channel.

At the beginning of the Tb, for the 70 and 90 s reciprocating cycle, because of the temperature distribution in the upstream DOC, the chemical reaction equilibrium of NO2 and NO is on the NO2 side. The residual partial soot particles attached on the internal walls in the inlet passages during the previous Tf are stirred and are completely oxidized into ashes further with the NO2 catalytic action. At the same time, the soot particles deposited onto the internal walls in the outlet channel are also completely oxidized into ashes with the NO2 catalytic action. When the temperature in the partial region of the upstream DOC is higher than 623 K at the later period of the Tb, only a part of soot particles deposited onto the internal walls in the outlet channel can be oxidized into ashes with O2 alone. At the end of the Tb, the residual soot particles are accumulated on the internal wall in the rear part of the outlet channel. The number of the residual soot particles on the internal wall in the outlet channel increases with the increasing reciprocating cycle. Therefore, the regeneration efficiency of the DPF system decreases and the average content of PM emission increases with the increasing reciprocating cycle, as listed in Table .

As switching to a forward flow for 70 s and 90 s cycle, the suspended PM in the flow residing in the DPF system and a small amount of particles soot incompletely oxidized on the internal wall of the outlet channel reversely flow out the DPF system. Therefore, the PM content rapidly increases, and then rapidly decreases at the beginning of the Tf, as shown in Figure . And then, the fluctuating amplitude of PM emission with C3H8 fuel addition is lower than that without C3H8 fuel addition as switching from a backward flow to a forward flow for the 50, 70, and 90 s cycle.

In the next Tf, because of the temperature distribution in the upstream DOC, the residual soot particles in the outlet passages of the cDPF during the previous Tb are only incompletely oxidized into ashes further with O2 alone. Then, the residual soot particles are shifted along the flow path gradually and are carried out by the reverse flow at the end of Tf. Therefore, the PM content rapidly increases at the end of Tf. By switching to the next backward flow, the PM content continuously increases to a peak value, and then rapidly decreases. Therefore, by switching from the forward flow to the backward flow for the 50, 70, and 90 s cycle, the fluctuating range of the PM content with C3H8 fuel addition is significantly higher than that without C3H8 fuel addition and increases with the increasing reciprocating cycle, as shown in Figure .

The efficiency of removing PM of the DPF system is over 95%, the pressure difference is less than 870 Pa, the CO emission is less than 24 ppm, and the HC emission is near zero for every reciprocating flow cycle set in the experiment, as listed in Table .

Conclusions

An experimental study on the characteristics of removing pollution emissions and regeneration of the DPF system with a reciprocating flow has been carried out. The influence of the reciprocating flow cycle on the regeneration procedure of the DPF system has also been analyzed.

The DPF system can use the pollutants in emissions and a tiny amount of extra fuel to maintain the chemical reaction, periodically releasing heat and storing heat and reversely blowing away ash, which in turn realize the regeneration of the cDPF.

If a reasonable temperature distribution in the upstream DOC and cDPF during a half cycle is formed, the regeneration efficiency of the DPF system can be improved.

The reciprocating cycles can influence the operation performance of the DPF system. With the increasing reciprocating cycle, the temperature profile moves parallel toward the downstream side and the fluctuating range of CO, NO, and NO2 components increases.

If a reasonable temperature distribution which corresponds to a reciprocating cycle is formed in the DPF system, the regeneration efficiency can be improved further.