The diesel engines have many advantages for medium-to-heavy ground vehicles such as high thermal efficiency[1–5]. However, the particulate matter (PM) emission from diesel engine has been a huge problem to the environment[6–10]. The PM includes carbon, ash, organic compounds, and sulfate materials[11, 12]. The diesel particulate filter (DPF) is used as the mainstream aftertreatment methods for diesel vehicles[13–15]. The accumulation of PM leads to the increases of pressure drop, and decreases the performance of the engine. The active[16–18] and passive regeneration[6, 19–21] are two commonly strategies for DPF regeneration.
The regeneration and emission performance of DPF has been studied by many research groups. Fang et al. report that it is benefit to emit large diameter particles when the regeneration temperature is higher than 525℃[11]. Meanwhile, increasing the flow rate has negative effect for the maximum temperature, maximum temperature gradient, and regeneration performance ratio. Zhang et al. find out that more than 97.9% of the PN and 95.4% of the PM are reduced by the CDPF, and the reduction efficiency is enhanced by the catalyst loading[22]. Meng et al. conclude that the morphology of soot particles transforms form an initial state of aggregation to a chain-like structure as the engine exhaust passes through the aftertreatment system[23]. Meng et al. study the particle emission characteristics during passive regeneration and conclude that the particles discharged form the engine exhibited a bimodal characteristic of the particle concentration versus particle size profile as 300℃ and 350℃. A single peak characteristic as the temperature as the temperature increases to 400℃ and 450℃[24]. Duan et al. compare the peak regeneration temperature of ash-loaded DPFs moving forward to the fresh DPF and conclude that the peak regeneration temperature of Mg-based DPF reaches 694℃ and moving forward to the axial position of 71.5nm[25]. Wang et al. study the effect of hydrothermal aging to regeneration in CeO2-based CDPF. They conclude that soot oxidation rate of fresh catalyst first increases rapidly at 516K and then starts to slow down gradually at 633K, but for hydrothermal aging catalysts are 601K and 789K, respectively[26]. Lao et al. built a population balance model to describe an experimentally study DPF undergoing active regeneration. The introduction of the extended unit collector description enabled the model to describe both the timing of particle breakthrough and the final steady filtration efficiency of the hot regenerated DPF[27]. The results of partial regions’ effect on regeneration and emission performance can be used to optimization of the diesel exhaust after-treatment requires the detailed information.
In this study, the regeneration test bench is applied to investigate the effect of (1) the single region, (2) double regions, and (3) multiple regions on both regeneration and emission characteristics.
Description of Experiments
1.1 Experiment Material
The DPF is purchased locally, which material is cordierite. Carbon black is supplied by Evonik Industries AG, which has been used as a surrogate for diesel soot[11, 19]. Table 1 lists the physical properties of DPF substrate and Table 2 has the physical properties of carbon black.
Table 1
Physical properties of full size DPF substrate
Diameter(mm)
|
Length(mm)
|
Channel
Density(cpsi)
|
Channel size(mm)
|
Filter wall thickness(mm)
|
Pore Diameter(µm)
|
Porosity(%)
|
144
|
152
|
100
|
2
|
0.35
|
7.6
|
27.9
|
Table 2
Physical properties of carbon black
Diameter(nm)
|
BET(m2.g− 1)
|
Volatile(%)
|
Oil Absorption(g.(100g)−1)
|
Ash content(%)
|
25
|
92
|
5
|
460
|
0.02
|
1.2 Experiment method
Figure 1 is the schematic of DPF regeneration testing bench, including air compressor, air dryer, mass flowmeter, electrical heater (LE 10000 DF HT, LEISTER), pressure transducer, and DPF. The air compressor is used to supply the air and air dryer is used to remove the water in the air. The pressure transducer is used to measure the pressure difference. Figure 2 is the schematic of thermocouples inside DPF. The electrical heater is used to heat the air to the desired temperature. The total mass concentration of the emitted particles and the average diameters are measured directly by Nanomet3. In all the experiments, the regeneration temperature, the flow rate and the regeneration time is 550℃, 5g/L, and 1000s, respectively.
Based on the distribution of thermocouples, DPF substrate is divided into following four regions on the radial direction. The locations of regions 1, 2, 3, and 4 are φ 0-35mm, φ 35-72mm,φ 72-95mm,φ 95-120mm, respectively. When region 1 is loaded, regions 2, 3, 4 are sealed by tapes and the mass of carbon black loading is directly proportional by the area of the region. The different regions of DPF substrate are shown in Fig. 3. For the single region tests, the regeneration and emission characteristics for regions 1, 2, 3, and 4 are tested, respectively. For the double regions tests, the regeneration and emission characteristics for regions 12, 13, 14, 23, 24 and 34 are tested, respectively. For the multiple regions tests, the regeneration and emission characteristics for regions 123, 124, 134, and 234 are tested, respectively.
1.3 Data analysis
T max signifies the maximum temperature and (dT/dx)max denotes maximum temperature gradient inside DPF.
The regeneration efficiency η is calculated by following equation,
1
where M0 is the mass of DPF before loading, M1 is the mass of DPF before regeneration and M2 is the mass of DPF after regeneration.
The regeneration performance ratio ε is calculated by
where Qin is the total energy from the electrical heater and cp is the specific heat capacity. The qm is the mass flow rate of air, and t is the regeneration time. T1 is the regeneration temperature of DPF and T0 is the initial temperature of the incoming flow.