Real-time particulate emissions rates from active and passive heavy-duty diesel particulate filter regeneration

Real-World Particle Emissions from a Modern Heavy-Duty Diesel Vehicle during Normal Operation and DPF Regeneration Events: Impacts on Disadvantaged Communities

We assessed the real-world particulate emissions of a goods movement diesel vehicle, with an emphasis on total particle number and solid particle number emissions at different cutoff sizes. The vehicle was tested on routes in the South Coast Air Basin (SCAB) of California, representative of typical goods movement operation between the ports to warehouses and logistic centers with a mixture of urban and highway driving, as well as elevation change. We evaluated emissions during normal vehicle operation and diesel particulate filter (DPF) active regeneration events. Results revealed small variations in particle emissions between the routes, with particles below 23 nm and even 10 nm being abundant in the exhaust. Both total and solid particle number emissions were about 3 to 246 times higher during DPF regeneration compared to normal vehicle operation, with higher fractions of sub-10 nm solid particles. We showed that typical daily routes for goods movement operation in SCAB, especially the more urban routes, mostly occurred within disadvantaged communities, with minority populations and high indices for poverty, unemployment, and poor education. Our results indicated the vehicles spent a higher fraction of their total time within these areas at low speed and idling conditions, resulting in disproportionately higher exposures to ultrafine particles.

Experimental study on gas and particle emission characteristics of carbon black oxidation process in the presence of water and catalysts

The study of oxidation characteristics of carbon black particle is the basis to investigate the regeneration process and characteristics of diesel particulate filter (DPF). Based on the fixed-bed test bench, the gas and particle emission characteristics of carbon black oxidation process in the presence of water are investigated under different temperatures, Printex-U (PU) masses, and catalysts. The experimental results show that the rise of temperature and PU mass increases the emissions of CO, CO2 and the total average particle number (PN). The oxidation efficiency (η) increases with temperature, but decreases with PU mass. The addition of catalysts promotes PU oxidation, and reduces CO emission. Due to the influence of particle diffusion, CeO2 has slightly lower efficiency than Pt/Al2O3 in the same ratio (1:1), but it is beneficial to significantly reduce particle emission, especially as the ratio increases (1:5). Water decreases CO and the η in PU oxidation, and the negative impact is gradually reduced after 3 % water concentration; However, the PN significantly increases, and expands the particle size range, particularly at high temperature and adding Pt/Al2O3 (from about 10 nm to 6- 30 nm, and a large number of particles with 30- 100 nm are produced). Additionally, the CO2/CO ratio of carbon black oxidation gradually increases with water concentration. Controlling DPF regeneration needs to strike a balance between the benefits on increasing oxidation efficiency and the potential negatives on particulate and harmful gas emission.

Effect of regeneration method and ash deposition on diesel particulate filter performance: a review

As countries around the world pay more attention to environmental protection, the corresponding emission regulations have become more stringent. Exhaust pollutants cause great harm to the environment and people, and diesel engines are one of the most important sources of pollution. Diesel particulate filter (DPF) technology has proven to be the most effective way to control and treat soot. In this paper, we review the latest research progress on DPF regeneration and ash. Passive regeneration, active regeneration, non-thermal plasma-assisted DPF regeneration and regeneration mechanism, DPF regeneration control assisted by engine management, and uncontrolled DPF regeneration and its control strategy are mainly introduced. In addition, the source, composition, and deposition of ash are described in detail, as well as the effect of ash on the DPF pressure drop and catalytic performance. Finally, the issues that need to be further addressed in DPF regeneration research are presented, along with challenges and future work in ash research. Over all, composite regeneration is still the mainstream regeneration method. The formation of ash is complex and there are still many unanswered questions that require further in-depth research.

An experimental investigation of particle and NOx emissions for a non-road diesel engine equipped with an integrated DOC + CDPF + SCR aftertreatment system during different operations

An integrated aftertreatment system consisting of diesel oxidation catalyst (DOC), catalytic diesel particulate filter (CDPF), and selective catalytic reduction (SCR) is an effective way of reducing both NOx and particulate matter (PM). In this paper, the effect of DOC + CDPF + SCR on NO_X and particle emissions is investigated during different operations to assess applicability of this aftertreatment for meeting more rigorous non-road emissions standard. Meanwhile non-negligible issue about regeneration emission is studied. The results show that the DOC + CDPF have no significant effect on NOx but increase the NO₂/NOx ratio which is correlated with load. SCR is the main NOx reduction device with average efficiency of 86.5% for steady-state operations. Meanwhile, NH₃ slip is lower than 16 ppm. During cold and hot non-road transient cycles (NRTC cycles), average NOx efficiencies are 56.7% and 57.8%, respectively, along with NH₃ slip below 10 ppm. DOC + CDPF + SCR maintain filtration efficiency over 97% and 99% for PM and particle number (PN) for either steady-state operation or NRTC cycle, but particle size distributions change. Compared with the original emissions, NOx, PM, and PN emission factors are all below non-road China-IV limit after equipping with DOC + CDPF + SCR. However, during regeneration the aftertreatment does not maintain a high filtration performance but becomes particle generator. The penetration of nuclear particles and decomposition of agglomerated particles lead to high CDPF-out PN of 1.5 × 10⁷ #/cm³-3.5 × 10⁷ #/cm³. During regeneration, accumulated NOx is negligible, but the PM is 121.6 and 44.5 times higher than cold and hot NRTC cycles, respectively. In summary, DOC + CDPF + SCR is excellent way to improve non-road emissions but low SCR efficiency at low-temperature and high accumulated PM during regeneration process should be further addressed.

Detailed Characterization of Solid and Volatile Particle Emissions of Two Euro 6 Diesel Vehicles

The solid particle number emissions of Diesel vehicles are very low due to the particulate filters as exhaust aftertreatment devices. However, periodically, the trapped particles are oxidized (i.e., active regeneration) in order to keep the backpressure at low levels. The solid particle number emissions during regenerations are only partly covered by the regulations. Many studies have examined the emissions during regenerations, but their contribution to the overall emissions has not been addressed adequately. Furthermore, the number concentration of volatile particles, which is not included in the regulations, can be many of orders of magnitude higher. In this study, the particulate emissions of two light-duty Euro 6 vehicles were measured simultaneously at the tailpipe and the dilution tunnel. The results showed that the weighted (i.e., considering the emissions during regeneration) solid particle number emissions remained well below the applicable limit of 6 × 1011 #/km (solid particles > 23 nm). This was true even when considering solid sub-23 nm particles. However, the weighted volatile particle number emissions were many orders of magnitude higher, reaching up to 3 × 1013 #/km. The results also confirmed the equivalency of the solid particle number results between tailpipe and dilution tunnel locations. This was not the case for the volatile particles which were strongly affected by desorption phenomena. The high number of volatiles during regenerations even interfered with the 10 nm solid particle number measurements at the dilution tunnel, even though a catalytic stripper equipped instrument was also used in the dilution tunnel.

Potential hazards associated with interactions between diesel exhaust particulate matter and pulmonary surfactant

Long term exposure to diesel exhaust particulate matter (DEPM) can induce numerous adverse health effects to the respiratory system. Understanding the interaction between DEPM and pulmonary surfactant (PS) can be an essential step toward preliminary evaluation of the impact of DEPM on pulmonary health. Herein, DEPM was explored for its interaction with 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), the major component of PS. The results indicated that the surface pressure-area (π-A) isotherms of DPPC monolayers shifted toward lower molecular areas and the compression modulus (C_S^-1) reduced in the presence of DEPM. Atomic force microscopy image showed that DEPM can disrupt the ultrastructure of DPPC monolayers along with the direction of lateral compression. In addition, DPPC can in turn condition the surface properties of DEPM, permitting its agglomeration in aqueous media, which was attributed to the adsorption of DEPM to DPPC. Furthermore, the particle-bound polycyclic aromatic hydrocarbons (PAHs) could be desorbed from DEPM by the solubilization of DPPC and it was positively correlated with the hydrophobicity of PAHs. These findings revealed the toxicity of DEPM-associated PAHs and the role of DPPC in facilitating the removal of the inhaled particles, which can provide a new insight into the potential hazards of airborne particles on lung health.

Impact of drivers on real-driving fuel consumption and emissions performance

Eco-driving has attracted great attention as a cost-effective and immediate measure to reduce fuel consumption significantly. Understanding the impact of driver behaviour on real driving emissions (RDE) is of great importance for developing effective eco-driving devices and training programs. Therefore, this study was conducted to investigate the performance of different drivers using a portable emission measurement system. In total, 30 drivers, including 15 novice and 15 experienced drivers, were recruited to drive the same diesel vehicle on the same route, to minimise the effect of uncontrollable real-world factors on the performance evaluation. The results show that novice drivers are less skilled or more aggressive than experienced drivers in using the accelerator pedal, leading to higher vehicle and engine speeds. As a result, fuel consumption rates of novice drivers vary in a slightly greater range than those of experienced drivers, with a marginally higher (2%) mean fuel consumption. Regarding pollutant emissions, CO and THC emissions of all drivers are well below the standard limits, while NO_x and PM emissions of some drivers significantly exceed the limits. Compared with experienced drivers, novice drivers produce 17% and 29% higher mean NO_x and PM emissions, respectively. Overall, the experimental results reject the hypothesis that driver experience has significant impacts on fuel consumption performance. The real differences lie in the individual drivers, as the worst performing drivers have significantly higher fuel consumption rates than other drivers, for both novice and experienced drivers. The findings suggest that adopting eco-driving skills could deliver significant reductions in fuel consumption and emissions simultaneously for the worst performing drivers, regardless of driving experience.

Evaluation of heavy-duty vehicle emission controls with a decade of California real-world observations

Over the past decade, efforts to reduce emissions of particulate matter (PM) and oxides of nitrogen (NO + NO₂, or NO_x) from heavy-duty diesel vehicles (HDDVs) have led to the widespread adoption of both Diesel Particulate Filters (DPFs) to control PM and Selective Catalytic Reduction (SCR) to control NO_x. We evaluated the performance of DPFs and SCR with 13,327 real-world fuel-based Black Carbon (BC) and NO_x emission factors from 9,167 unique heavy-duty vehicles (primarily HDDVs) measured at four sites in California (two ports, two highways) from 2011 to 2018. BC emission factors have decreased by 90% during the past decade. At the same time, BC distributions have become increasingly skewed toward "high-emitters" - e.g., the portion of the HDDV fleet responsible for half of all BC emissions has decreased from ~16% to ~3%. NO_x emission factors have also decreased over the past decade, but by only 31%. They remain roughly five times greater than in-use thresholds.We examined changes in BC and NO_x emissions with engine age. BC emissions from DPF-only trucks decreased slightly but insignificantly, by 6 ± 15 mg/kg fuel per year, while for DPF+SCR trucks they increased by 5 ± 3. These changes are less than 5% of in-use thresholds. The annual increase in NO_x emissions with age was much greater: 1.44 ± 0.28 g/kg for older SCR trucks without on-board diagnostic (OBD) capabilities and 0.48 ± 0.35 for newer trucks with OBD, roughly 20- 50% of in-use thresholds. Paired t-tests on the over 600 vehicles that were observed in multiple campaigns were consistent with these results. Observed changes in BC emissions with age were best fit with a "gross emitter" model assuming an annual DPF failure rate of 0.83 ± 0.01% for DPF-only trucks and 0.56 ± 0.01% for DPF+SCR trucks.Implications: These observations of real-world HDV emission factors have several major implications for regulatory efforts to reduce them. The increasing importance of a relatively small number of high BC emitters suggests that widespread sampling of the on-road fleet will be necessary to identify these vehicles. On the other hand, the much more ubiquitous deterioration in NO_x control measures may be better addressed by incorporating on-board diagnostic systems, with telematic data transfer when possible, into inspection and maintenance programs. These NO_x observations also highlight the need for strengthening heavy-duty SCR durability demonstration requirements.

Influence of Different Thermal Aging Conditions on Soot Combustion with Catalyst by Thermogravimetric Analysis

Diesel particulates are deposited in the diesel particulate filter and removed by the regeneration process. The Printex-U (PU) particles are simulated as the diesel soot to investigate the influence of thermal aging conditions on soot combustion performance with the addition of catalysts. The comprehensive combustion index S, combustion stability index Rw and peak temperature Tp are obtained to evaluate the combustion performance. Compared with the PU/Pt mixtures of different Pt contents (2 g/ft3, 3.5 g/ft3, and 5 g/ft3), the 10 g/ft3 Pt contents improve soot combustion with the outstanding oxygen absorption ability. When the weight ratio of PU/Pt mixture is 1:1, the promoted effect achieves the maximum degree. The S and Rw increase to 8.90 × 10-8 %2min-2°C-3 and 39.11 × 105, respectively, compared with pure PU. After the thermal aging process, the PU/Pt mixture with a 350 °C aging temperature for 10 h promotes the soot combustion the best when compared to pure PU particles. It is not good as the PU/Pt mixture without aging, because the inner properties of soot and Pt/Al2O3 catalyst may have been changed. The S and Rw are 9.07 × 10-8 %2min-2°C-3 and 38.39 × 105, respectively, which are close to the no aging mixture. This work plays a crucial role in understanding the mechanism of the comprehensive effect of soot and catalyst on soot combustion after the thermal aging process.

Effect of Different Aging Conditions on the Soot Oxidation by Thermogravimetric Analysis

Diesel particulate filter is an effective device to reduce diesel particulate emission. The particles in diesel particulate filter are usually affected by the aging of high-temperature exhaust gas before the regeneration process. In order to investigate the effect of aging conditions on the soot oxidation process, the effect of aging temperature and aging time on the oxidation process of carbon black (Printex-U, PU) and the PU/catalyst/ash mixture are studied by thermogravimetric analysis. The aging PU particles have lower starting temperature, peaking temperature, ending temperature, and activation energy. Compared with the particles without aging, the PU particles with a 400 °C aging temperature and 20 h aging time are able to reduce the activation energy from 191.2 to 158 kJ/mol. Low aging temperatures (200-300 °C) and the catalyst have a certain synergistic effect on the improvement of PU oxidation activity. The PU/CeO2 mixture with a 300 °C aging temperature and 20 h aging time decreases the activation energy from 178.4 to the lowest 113.6 kJ/mol. The addition of CaSO4 in PU particles cannot stop the improvement of its oxidation activity by aging, but it reduces the effect of aging. This work is helpful to reveal the mechanism of aging on PU and the PU/catalyst/ash mixture in air environment.

Non-Volatile Particle Number Emission Measurements with Catalytic Strippers: A Review

Vehicle regulations include limits for non-volatile particle number emissions with sizes larger than 23 nm. The measurements are conducted with systems that remove the volatile particles by means of dilution and heating. Recently, the option of measuring from 10 nm was included in the Global Technical Regulation (GTR 15) as an additional option to the current >23 nm methodology. In order to avoid artefacts, i.e., measuring volatile particles that have nucleated downstream of the evaporation tube, a heated oxidation catalyst (i.e., catalytic stripper) is required. This review summarizes the studies with laboratory aerosols that assessed the volatile removal efficiency of evaporation tube and catalytic stripper-based systems using hydrocarbons, sulfuric acid, mixture of them, and ammonium sulfate. Special emphasis was given to distinguish between artefacts that happened in the 10–23 nm range or below. Furthermore, studies with vehicles’ aerosols that reported artefacts were collected to estimate critical concentration levels of volatiles. Maximum expected levels of volatiles for mopeds, motorcycles, light-duty and heavy-duty vehicles were also summarized. Both laboratory and vehicle studies confirmed the superiority of catalytic strippers in avoiding artefacts. Open issues that need attention are the sulfur storage capacity and the standardization of technical requirements for catalytic strippers.

Particle Number Measurements Directly from the Tailpipe for Type Approval of Heavy-Duty Engines

The type approval of heavy-duty engines requires measurement of particulates downstream of a proportional to the exhaust flow partial flow dilution system. However, for particle number systems, which measure in real time, this is not necessary and a fixed dilution could be used. In order to assess this dilution possibility, an inter-laboratory exercise was conducted, where a “Golden” system measuring directly from the tailpipe with “hot” (150 °C) fixed dilution was compared with the laboratory regulated systems. Additional “Golden” counters were measuring from 10 nm, below the current cut-off size of 23 nm defined in the regulation, in order to collect data below 23 nm and to confirm that the direct sampling is also possible for smaller sizes. Seven diesel engines and two CNG (compressed naturals gas) engines were used in six laboratories. The results of the “Golden” instruments were within 25% in most cases, reaching 40% in two laboratories for both >23 nm and >10 nm. The repeatability of the measurements (10% to 40%) remained the same for both systems with both cut-off sizes. One test with regeneration showed clear difference between the 10 nm systems, indicating that the thermal pre-treatment only with evaporation tube might not be adequate. Another system measuring from the tailpipe with a fixed “cold” (at ambient temperature) dilution gave differences of up to 50% in most cases (on average +26%). Dedicated tests with this system showed that the differences were the same with fixed or proportional dilution, indicating that it is not the concept that resulted in the overestimation, but the calibration of the system. The main conclusion of this study is that direct sampling with fixed dilution from the tailpipe can be introduced in the future regulation.

Impact of Particulate Size During Deep Loading on DPF Management

Wall-flow particulate filters are a required emission control device to abate diesel emission in order to comply with current regulations. DPFs (diesel particulate filters) are characterized by high filtration efficiency—but in order to avoid deterioration of power and performance, they are required to cause low values of backpressure. The periodical oxidation of collected particle allows for the reestablishment of the ideal flow conditions. Studies highlighted that the regeneration event has an important impact on engine emission, since it is responsible for the emission of a large number of smaller particles. From these considerations, the importance of optimizing the DPF management for what concerns both filtration and regeneration mechanisms arises. The present paper focuses on the loading process of the filter. A filtration model was implemented, based on the ‘unit-collector’ and fluid-dynamic approaches, known as valid modelling techniques. The model was used to predict trapped mass and filter backpressure evolution with time during loading processes, in which soot particle sizes varied, with the aim to analyze how particulate size affects the filter pressure drop rise. A wall-flow filter was investigated, and the behavior of clean material was evaluated by a parametric analysis in which particle diameter varies in the filed 20–1000 nm, that is the typical range of soot sizes in diesel engine exhaust. The results demonstrate that soot size has a great influence on the initial deep bed loading process. Moreover, it defines the value from which the linear pressure drop shape during cake filtration starts, not only when the initial loaded is completed, but also each time the regeneration event is concluded. This outcome provides an important guideline to define the most appropriate strategy for the initial DPF loading in order to establish the regeneration event based on the estimation of trapped mass accounting for the filter backpressure and on the time interval between two successive regeneration.