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Review

Recent Advances in SCR Systems of Heavy-Duty Diesel Vehicles—Low-Temperature NOx Reduction Technology and Combination of SCR with Remote OBD

by
Zhengguo Chen
,
Qingyang Liu
,
Haoye Liu
* and
Tianyou Wang
State Key Laboratory of Engine, Tianjin University, Tianjin 300350, China
*
Author to whom correspondence should be addressed.
Atmosphere 2024, 15(8), 997; https://doi.org/10.3390/atmos15080997
Submission received: 26 July 2024 / Revised: 13 August 2024 / Accepted: 19 August 2024 / Published: 20 August 2024
(This article belongs to the Special Issue Recent Advances in Mobile Source Emissions (2nd Edition))
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Abstract

:
Heavy-duty diesel vehicles are a significant source of nitrogen oxides (NOx) in the atmosphere. The Selective Catalytic Reduction (SCR) system is a primary aftertreatment device for reducing NOx emissions from heavy-duty diesel vehicles. With increasingly stringent NOx emission regulations for heavy-duty vehicles in major countries, there is a growing focus on reducing NOx emissions under low exhaust temperature conditions, as well as monitoring the conversion efficiency of the SCR system over its entire lifecycle. By reviewing relevant literature mainly from the past five years, this paper reviews the development trends and related research results of SCR technology, focusing on two main aspects: low-temperature NOx reduction technology and the combination of SCR systems with remote On-Board Diagnostics (OBD). Regarding low-temperature NOx reduction technology, the results of the review indicate that the combination of multiple catalytic shows potential for achieving high conversion efficiency across a wide temperature range; advanced SCR system arrangement can accelerate the increase in exhaust temperature within the SCR system; solid ammonium and gaseous reductants can effectively address the issue of urea not being able to be injected under low-temperature exhaust conditions. As for the combination of SCR systems with remote OBD, remote OBD can accurately assess NOx emissions from heavy-duty vehicles, but it needs algorithms to correct data and match the emission testing process required by regulations. Remote OBD systems are crucial for detecting SCR tampering, but algorithms must be developed to balance accuracy with computational efficiency. This review provides updated information on the current research status and development directions in SCR technologies, offering valuable insights for future research into advanced SCR systems.

1. Introduction

Diesel vehicles are a major source of automotive air pollution. Although heavy-duty diesel trucks account for less than 5% of the total number of vehicles, they are responsible for approximately 80% of vehicle nitrogen oxide (NOx) emissions [1]. Effectively controlling NOx emissions from heavy-duty diesel vehicles is crucial for improving air quality and managing environmental pollution.
Selective Catalytic Reduction (SCR) technology is a widely used NOx aftertreatment technology for diesel vehicles, capable of significantly reducing NOx emissions [2]. The basic principle involves using a reducing agent to convert NOx in the exhaust into nitrogen and water through a catalytic reduction process. SCR technology was first developed in the 1950s and began to be applied in NOx emission control from power plant combustors in the 1970s. Early SCR systems typically used vanadium oxide as a catalyst and required a large amount of ammonia to achieve effective NOx reduction at high temperatures [3]. In the 1990s, with increasingly stringent vehicle emission regulations, SCR technology was introduced into diesel engine aftertreatment systems to control NOx emissions, using urea as the reductant carrier [4]. Compared to power plant combustors, diesel engines exhibit much lower exhaust temperatures and experience more variable operating conditions, making it more challenging to achieve high NOx conversion efficiency throughout the vehicle lifecycle.
In recent years, major heavy-duty vehicle markets (such as the United States, China, and Europe) have further tightened heavy-duty vehicle emission regulations. California Air Resources Board (CARB) has outlined a two-phase plan to regulate NOx emissions of heavy-duty vehicles. In Phase 1, from 2024 to 2026, they reduced the Federal Test Procedure (FTP) certification NOx limit from 0.27 g/kWh to 0.07 g/kWh, introduced a Low Load Cycle (LLC) for low-load NOx emissions, with a limit of 0.27 g/kWh, and made idling NOx limits mandatory at 10 g/hr. In Phase 2, from 2027, they implemented stricter NOx limits aimed at a 90% reduction in emissions from heavy-duty vehicles, that is, they required certification values to be between 0.02 and 0.04 g/kWh. Additionally, CARB requires heavy-duty vehicles to be equipped with advanced remote On-Board Diagnostics (OBD) systems to monitor SCR system performance and ensure long-term compliance with emission standards [5]. The recently enacted Euro VII emission regulations [6] also impose stricter NOx emission limits compared to the Euro VI regulations. Under the World Harmonized Steady-state Cycle (WHSC) (which applies only to compression-ignition engines), NOx emission limits are tightened by 50%, and, under the World Harmonized Transient Cycle (WHTC), NOx emission limits are tightened by 56%. New limits have also been introduced for other pollutants, including ammonia (NH3) with a limit of <60 mg/kWh, and greenhouse gas nitrous oxide (N2O) with a limit of <200 mg/kWh. The Euro VII emission regulations require separate evaluation of Real Driving Emissions (RDE) for heavy-duty vehicles, and the RDE test conditions include a significant number of low-load scenarios, which increases the difficulty of reducing NOx emissions [7]. It also emphasizes the need for heavy-duty vehicles to be equipped with an OBD system to identify potentially NOx-noncompliant vehicles [8]. China VI emission standards have also required heavy-duty vehicles to be equipped with remote OBD systems to monitor NOx emissions [9].
New regulations imposed higher requirements for achieving high NOx conversion efficiency in diesel engine SCR systems under low load and low exhaust temperature conditions and emphasized the need for integrating SCR with remote OBD technology to monitor NOx emissions throughout the lifecycle of a vehicle. The integration of OBD systems with SCR systems allows for real-time monitoring of vehicle emissions, providing continuous information on NOx emission levels and the operational status of the SCR system. However, there are still several issues in the implementation of this technology, such as whether the information provided by the OBD system is sufficiently accurate; the discrepancy between the operating conditions for which the OBD system provides data and the conditions stipulated by regulations; what algorithms can process the data stream to better assess whether the vehicle is emitting excess pollutants; and which data items and algorithms are accurate and feasible for detecting vehicle tampering. However, there is a lack of specific reviews on recent advances in heavy-duty diesel vehicle SCR technology regarding the low-temperature NOx reduction technologies and the combination of SCR and OBD technologies even though the significance has become larger and larger. This review will fill the gap regarding the above two aspects, providing important references for selecting research directions for advanced SCR system technologies and for technical route development for related professionals. In this review, first, the working principle of SCR systems will be introduced, then, the low-temperature NOx reduction technologies for diesel engine SCR systems will be reviewed, and, lastly, the relevant research on the combination of SCR and OBD technologies will be reviewed.

2. Working Principle of SCR System

In diesel vehicles, urea SCR is currently the most predominant technology route. Urea solution serves as a carrier for ammonia, effectively mitigating potential issues related to ammonia storage, toxicity, and safety, as urea is in liquid form at room temperature and pressure. The use of alternative reductants in automotive SCR systems is still under research [10]. The working principle of urea SCR technology is illustrated in Figure 1.
In a urea SCR system, firstly, a 32.5% urea solution is injected into the high-temperature exhaust gas upstream of the SCR catalyst [11]. The urea solution then undergoes thermal decomposition (Reaction (1)) or hydrolysis (Reaction (2)) at high temperatures, forming gaseous ammonia and carbon dioxide. The main reactions involved are as follows:
Thermal decomposition:
CO(NH2)2 → 2NH3 + CO2
Hydrolysis:
CO(NH2)2 + H2O → 2NH3 + CO2
The generated ammonia reacts with NOx in the exhaust gas within the SCR system, converting NOx into nitrogen (N2) and water (H2O). During this process, the supply of urea solution must be effectively matched to the amount of NOx in the exhaust to achieve efficient NOx conversion and minimize ammonia slip [12]. NOx in the high-temperature exhaust undergoes three main reduction reactions: the standard SCR reaction (Reaction (3)), the fast SCR reaction (Reaction (4)), and the slow SCR reaction (Reaction (5)) [13]. The dominant reaction type depends on the ratio of NO to NO2 in the NOx mixture. In diesel engine NOx emissions, the ratio of NO is usually much higher than that of NO2, so the standard SCR reaction is the primary reaction type. In diesel engine aftertreatment systems, the upstream diesel oxidation catalyst (DOC) converts NO to NO2. As the proportion of NO2 increases and approaches 50%, the fast SCR reaction gradually becomes dominant, leading to a rapid increase in overall NOx reduction efficiency. Under certain engine operating conditions, if the NO2 proportion upstream of the SCR exceeds 50%, the slow SCR reaction becomes the dominant process, resulting in a sharp decrease in NOx conversion efficiency.
Standard SCR reaction:
4NH3 + 4NO + O2 → 4N2 + 6H2O
Fast SCR reaction:
2NH3 + NO + NO2 → 2N2 + 3H2O
Slow SCR reaction:
4NH3 + 3NO2 → 3.5N2 + 6H2O

3. Low-Temperature NOx Reduction Technologies

The effective enhancement of NOx conversion efficiency in SCR systems under low exhaust temperature conditions, particularly during low-load and cold-start conditions, remains a significant challenge for SCR systems. The low NOx emission conversion efficiency of SCR systems at low exhaust temperatures is mainly due to two reasons: 1. urea cannot process the thermal decomposition/hydrolysis reactions at low temperatures, and 2. the activity of the SCR catalyst at low temperatures is poor. However, the current new regulations have all emphasized the significance of low NOx emissions at low-load, low-exhaust temperature conditions for heavy-duty vehicles. For example, CARB introduced an LLC for low-load NOx emissions, with a limit of 0.27 g/kWh, and made idling NOx limits mandatory at 10 g/hr for heavy-duty vehicles. The exhaust temperature in the WHTC/WHSC test cycles stipulated by the current Euro VI and China VI emission standards is relatively low [14]. The Euro VII emission regulations have introduced the Real Driving Emissions (RDE) test into the assessment scope for heavy-duty engines for the first time. Compared to traditional laboratory conditions, the RDE conditions involve the engine operating more frequently under low temperatures and cold starts, which increases the necessity to improve the SCR system NOx conversion efficiency at low temperatures [15]. In this context, new technologies, related to low-temperature catalyst optimization, advanced SCR system arrangement, and new SCR reductants, are reviewed.

3.1. Research on Low-Temperature Catalyst

The catalyst is a core component of the SCR system, and its catalytic activity and stability directly affect the NOx conversion efficiency of the system. SCR catalysts include vanadium (V)-based, copper (Cu)-based, and iron (Fe)-based catalysts [16]. Vanadium-based catalysts, such as V2O5/TiO2 and V2O5-WO3/TiO2, have been used more frequently in early-stage SCR systems due to their low cost [17]. Ngo et al. [18] studied the effects of WO3/TiO2, V2O5/TiO2, and V2O5-WO3/TiO2 catalysts in NH3-SCR for NOx removal under conditions with and without formaldehyde and water using in situ FTIR and operando EPR spectroscopy. They found that the catalytic activity decreases in the following order: V-W-Ti > V-Ti >> W-Ti. Guo et al. [19] prepared V2O5-WO3/TiO2 (VWT) and V2O5-WO3-CeO2/TiO2 (VWCeT) catalysts with different vanadium contents and compared them with Ca-doped catalysts. They found that Ce addition not only enhances NO conversion but also improves alkali resistance. The Ca-doping level on the catalysts is crucial for the SCR reaction; in practical applications, the catalysts are initially more sensitive to SO2 in alkaline flue gas, but the sulfur resistance may increase as calcium doping exacerbates alkali poisoning effects. Martin et al. [20] evaluated the bimodal effect of water on V2O5/TiO2 catalysts with different vanadium species in simultaneous NO reduction and 1,2-dichlorobenzene (o-DCB) oxidation. They found that, in the SCR reaction, water has a detrimental effect at low temperatures due to competitive adsorption with NO and NH3, whereas, at high temperatures, it promotes increased NO conversion and suppresses side reactions, thereby improving selectivity towards N2. In the o-DCB oxidation reaction, the effect of water is the sum of two contributions: one positive, related to the removal of surface-adsorbed harmful species, and one negative, associated with competitive adsorption with o-DCB. However, vanadium-based catalysts have drawbacks, including a narrow temperature window for efficient conversion, lower nitrogen selectivity at high temperatures, and poor thermal stability.
Due to increasingly stringent NOx emission regulations for heavy-duty vehicles, low-temperature molecular sieve catalysts are becoming more widely used recently. Iron-based molecular sieve catalysts exhibit strong resistance to hydrothermal aging, high NOx conversion efficiency at high temperatures, and sulfur tolerance [21]. Compared to iron-based molecular sieves, copper-based molecular sieves typically have better low-temperature catalytic activity and higher ammonia storage capacity. Given the stricter NOx reduction requirements during cold start and at low temperatures, copper-based molecular sieves are widely applied in SCR systems. However, copper-based molecular sieve catalysts tend to generate more N2O (with a greenhouse effect 298 times that of CO2), which is strictly restricted in the Euro VII emission regulations [22].
Given the complementarity between iron and copper catalysts, Girard [23] and Metkar et al. [24] proposed combining copper-based and iron-based catalysts. These methods leverage the efficiency advantages of each type of catalyst within different temperature ranges—copper-based catalysts improve NOx reduction at low temperatures, while iron-based catalysts enhance NOx reduction at high temperatures—thereby extending the overall efficient range of the SCR system. Jung et al. [25] combined vanadium-based catalysts with copper-based catalysts to reduce N2O formation during the SCR reaction process, and found that the combined catalyst in this experiment could be employed as an integration between these two catalysts.
Although copper-based molecular sieve catalysts have significant advantages in low-temperature NOx conversion efficiency, their conversion efficiency is highly susceptible to deactivation caused by various poisons, including hydrothermal aging, sulfur, phosphorus, alkali metals, alkaline earth metals, and heavy metals [16]. Therefore, exploring strategies to enhance the durability of SCR copper molecular sieve catalysts is crucial to ensuring low NOx emissions from diesel vehicles throughout their lifecycle.

3.2. Research on Advanced SCR System Arrangement

Currently, diesel engines use urea solution as a reductant. In the exhaust system, the urea solution must first be evaporated and then decomposed into ammonia (NH3) and carbon dioxide (CO2). The thermal decomposition of urea only occurs at higher temperatures, which means that urea injection cannot begin until the exhaust temperature is sufficiently high. On the other hand, urea solution is prone to crystallization at low temperatures, which can lead to crystallization in the pipes and nozzles of the SCR system, thus affecting the normal operation of the SCR system. Therefore, in general, the urea solution in the SCR system needs to start injection only when the exhaust temperature reaches 180 °C. Accelerating the exhaust temperature rise during low-temperature engine operation to meet the urea injection requirements has become a crucial research focus [26].
Thermal management is an important method for improving the low-temperature conversion efficiency of catalysts. Techniques such as using burners, reformers, and electrically heated catalysts allow for flexible adjustment of heat injection locations and intensities, effectively reducing heat loss in the exhaust gases, although this requires additional equipment. Another issue with these methods is that they consume additional energy, increasing engine fuel consumption. According to the research by Lee et al. [27], using a 3 kW electric heating device to preheat the exhaust gas in a diesel engine with a rated power of 110 hp can effectively reduce NOx emissions during cold starts, but it also increases fuel consumption by 3%. Alternatively, adjusting engine combustion control parameters can increase exhaust temperatures without consuming extra heat, but this often results in combustion deviating from the optimal range, leading to increased fuel consumption. Additionally, thermal storage materials can be used within the post-processing system [28]. Thermal management is not limited to SCR systems but is a general approach. A detailed discussion is not provided in this review. Besides thermal management, optimizing the configuration of an SCR system is a more energy-efficient approach.
The Close-Coupled SCR (cc-SCR) technology addresses this challenge by placing the SCR system as close as possible to the engine exhaust outlet, thereby increasing the exhaust temperature within the SCR system and shortening the urea injection start time during cold starts. Typically, the cc-SCR is used in conjunction with a downstream main SCR. Figure 2 shows a compact SCR diesel after-treatment system designed by Southwest Research Institute. This system includes an upstream cc-SCR and a downstream main SCR. The cc-SCR primarily handles NOx emissions under low exhaust temperature conditions, while the downstream main SCR addresses NOx emissions under high exhaust temperature conditions [29]. Research indicates that, the shorter the distance between the cc-SCR and the engine, the higher the exhaust temperature [30]. Configuring the cc-SCR after the turbocharger is more practical in real applications, so most cc-SCR systems currently use this configuration [31].
Han et al. [32] compared cc-SCR systems with traditional SCR systems and found that the cc-SCR system improved NOx conversion efficiency up to 89.2% and 99.3% during cold and hot start WHTC cycles, 5.4% and 3.0% higher than those of the traditional SCR systems, respectively. Li et al. [33] found that using a cc-SCR system could reduce the urea injection start time during the cold start WHTC cycle from 560 s to 72 s. They also noted that adding a cc-DOC in front of the cc-SCR did not improve the cold start performance of the cc-SCR. However, the cc-SCR configuration further increases the difficulty in developing SCR catalyst models and control strategies. In particular, since the cc-SCR is located upstream in the aftertreatment system, its high temperature and high-temperature rise rate may reduce the ammonia storage capacity [26], and excess ammonia may escape downstream and be oxidized to N2O in the DOC [34], which is difficult to eliminate with the aftertreatment system. Currently, there is limited research on control strategies for complex configurations.
The Catalyzed Diesel Particulate Filter (CDPF) is a diesel exhaust after-treatment technology that coats the SCR catalyst on the wall of the DPF. In traditional aftertreatment configurations, the DPF is generally located upstream of the SCR system, meaning the exhaust gas has already experienced a certain degree of temperature drop by the time it passes through the DPF. By combining SCR and DPF, SDPF can reduce the thermal mass of the after-treatment substrate, thus speeding up SCR heating and reducing urea injection start time [35]. The wall-flow structure of the SDPF differs significantly from the traditional flow-through SCR design. To achieve similar NOx conversion efficiency, the catalyst loading in SDPF needs to be about three times that of traditional SCR [36]. This requires using DPF substrates with higher porosity and larger dimensions, which increases the thermal and mechanical strength of the substrate [37]. Furthermore, rapid SCR reactions quickly consume NO2, which is a key oxidant for soot regeneration in the DPF. This leads to faster accumulation of soot in the SDPF, requiring more frequent regeneration, which adversely affects engine back pressure and SCR catalyst stability [38]. Additionally, SDPF technology faces issues such as ash clogging, catalyst poisoning, and increased N2O (Nitrous Oxide) generation at low temperatures [39]. Therefore, the reliability of CDPF technology remains an urgent problem to solve before it becomes widely commercially available.

3.3. Research on Low-Temperature SCR Reductants

A urea solution cannot effectively decompose into ammonia at low temperatures, meaning that SCR systems typically only start functioning when the diesel engine’s exhaust temperature reaches above 160 °C. This is one of the fundamental reasons for the low NOx emission treatment efficiency of SCR systems under low-temperature conditions. Additionally, the evaporation of urea solution absorbs heat, further lowering the exhaust temperature, which affects NOx conversion efficiency at low temperatures. Therefore, selecting a reductant that can directly perform the reduction reaction at lower temperatures can fundamentally address these issues.
Solid ammonium SCR technology has become a possible solution as a low-temperature SCR reductant. The key difference between solid ammonium SCR and urea SCR technology lies in the source of ammonia. In solid ammonium SCR technology, gaseous ammonia is generated from solid ammonium compounds using exhaust heat or external energy, then stored in the injection system and directly injected upstream of the SCR system. Due to the use of gaseous ammonia injection, solid SCR systems provide better NOx conversion efficiency than urea water solution systems, particularly at low exhaust gas temperatures [40]. Solid ammonium compounds include solid urea, ammonium carbamate, solid ammonium salts (such as ammonium carbonate, ammonium bicarbonate), and metal complexes of ammonia (such as ammonium magnesium chloride, ammonium calcium chloride) [41]. Liu et al. [42] examined the low-temperature performance of solid SCR on an engine test bench and found the solid SCR system could achieve a NOx conversion efficiency of 40% at a low exhaust temperature of 160 °C. Compared with urea SCR systems, the solid SCR system improves the WHSC NOx conversion efficiency from 78.9% to 82.2%, and the WHTC NOx conversion efficiency from 78.8% to 83.3%, with the same ammonia-to-nitrogen ratio. Diesel vehicles with urea SCR systems have NOx emissions that are 2.38 times and 1.73 times higher than those of solid SCR systems at 160 °C and 200 °C. However, the current storage, transportation, and refueling systems for urea aqueous solutions, as well as the related infrastructure, are relatively well-established. In contrast, the infrastructure for the replacement and supply of solid ammonium is still not fully developed, so large-scale application has not yet been realized [43].
In addition to ammonia, other reducing gases such as hydrocarbons (methane, isopropanol, octane, pentane, etc.) [44], hydrogen, and CO are also being researched as potential SCR reductants to remove NOx from diesel engine exhaust. In theory, these reductants effectively avoid crystallization issues and are not corrosive or insensitive to the NO/NO2 ratio. In addition, the unburned HC and CO already present in the exhaust can be used as SCR reductants [45]. However, a common issue is that, under the rich oxygen conditions in diesel engine exhaust, these reductants are more likely to react with O2 rather than NOx. Therefore, developing high-performance catalysts is a common technical challenge when using these reductants [46].

4. The Combination of SCR and Remote OBD

OBD systems are effective tools for monitoring vehicle emission statuses and play a crucial role in emission control and management [47]. The OBD system was initially promoted and implemented in California as an emission control technology, known as OBD I. Subsequently, the next generation of OBD systems—OBD II—was introduced in the U.S. and Europe (in the latter known as EOBD) [48]. OBD II expanded the monitoring scope of components and standardized both software and hardware protocols. Both OBD I and OBD II systems are wired transmission systems, with data used to alert vehicle owners about vehicle fault conditions. However, vehicle owners can still choose to continue driving their vehicles when facing high emission risks. OBD III systems, also known as remote OBD systems, combine OBD technology with wireless communication technology, sending operational data, emission statuses, and fault information from the vehicle to regulatory centers, enabling continuous and uninterrupted monitoring of in-use vehicles [49].
Currently, emissions regulations consistently emphasize the important role of OBD in future emissions monitoring and have begun mandating the installation of remote OBD systems in heavy-duty vehicles. As recommended by the International Council on Clean Transportation (ICCT) to address high emissions issues related to improper design and calibration of emission control systems, component failures, durability problems, and tampering with aftertreatment systems, the following specific suggestions are proposed: 1. Develop a standardized methodology for fleet screening to identify potentially non-compliant vehicles. 2. Establish remote-sensing standards and create a remote-sensing record database. 3. Strengthen anti-tampering provisions [5].
Since new heavy-duty vehicles’ SCR systems are all equipped with NOx emission sensors and can usually provide information on urea levels and injection quantities, collecting such data through remote OBD systems provides crucial information on the proper functioning of SCR systems [50]. Integrating the information provided by remote OBD systems with SCR systems has become a significant research area in recent years. Current research focuses mainly on two aspects: identifying high NOx-emission vehicles using OBD data and detecting SCR system tampering in diesel vehicles. These technologies effectively monitor the status of SCR systems throughout the entire lifecycle of heavy-duty diesel vehicles, ensuring low NOx emissions across the lifecycle of heavy-duty diesel vehicles.

4.1. Identification of High NOx Emission Vehicles

Research has revealed that some in-use heavy-duty diesel vehicles suffer from issues such as aging SCR systems, untimely urea addition, or tampering with the emission after-treatment system’s hardware and software. These vehicles emit NOx levels that far exceed regulatory limits and are referred to as “high-emission vehicles”. Remote OBD systems enable real-time monitoring of NOx emissions throughout the lifecycle of vehicles, allowing for the identification of NOx exceedance and facilitating further analysis of SCR system issues in these high-emission vehicles.
The precision of NOx emission measurements provided by remote OBD systems relies on the integration of SCR systems and OBD technology. Cheng et al. [51] compared the NOx emission data per unit distance under typical diesel heavy-duty vehicle operation conditions between remote OBD systems and portable emission measurement systems (PEMSs). They found that the values were very similar (average difference less than 1%) with an average relative error ranging from −13% to +22%, and deviations only occurred in a few transient tests.
Jiang et al. [52] aimed to improve the accuracy of remote OBD technology in detecting high NOx emissions from heavy-duty diesel vehicles by testing vehicles meeting China V and China VI emission standards. The results showed that, compared to using NOx concentration alone, the ratio of NO to CO2 (carbon dioxide) was more effective in identifying high NOx emission vehicles. Specifically, high-emission vehicles could be identified when the NO/CO2 ratio exceeded 200 × 10−4 for China V vehicles and 25 × 10−4 for China VI vehicles.
He et al. [53] addressed issues such as missing or abnormal values in remote OBD data by proposing a systematic approach that includes data quality assessment, repair, and NOx exceedance determination. When all data variables are missing or abnormal, data repair algorithms can improve data quality. If only NOx data are problematic, data can be repaired using other normal variables or by predicting emissions exceedance with other variables. Among data repair algorithms, Probabilistic Principal Component Analysis was found to be more accurate than other methods such as Non-Negative Matrix Factorization and k-Nearest Neighbors.
Wang et al. [54] proposed an integrated regulatory system that relies on remote OBD data from in-use vehicles and incorporates functions such as vehicle emission factor calculation, high-emission vehicle identification, and emission after-treatment system tampering detection. To more accurately determine if a vehicle meets the PEMS test limits, they suggested extracting and stitching OBD data streams according to vehicle types and dividing them into PEMS-like data segments based on regulatory road conditions. These stitched PEMS-like data segments are then analyzed using a “power-based window” method. However, current remote OBD data lack the engine reference torque data and engine transient cycle cumulative power data required for power window analysis, and the authors did not provide a solution for this issue in their paper.
To address the above problems, the authors [55] propose a data analysis method that uses vehicle fuel consumption data from the remote OBD system instead of power-based data as a baseline for window division and emission calculation. By correlating the specific power of the heavy-duty vehicle’s Adapted World Transient Vehicle Cycle (C-WTVC) cycle and the 100 km fuel consumption provided by the Ministry of Industry and Information Technology (the network condition data), the correlation equation was established between the vehicle fuel consumption and work performed. Therefore, the transformation between fuel consumption and the work conducted by the vehicle engine was realized. The emission windows derived by the work-based window (WBW) method and the fuel-consumption-based window (FBW) method yield closely similar outcomes (Figure 3).

4.2. Detection of SCR Tampering Vehicles

A diesel vehicle’s aftertreatment system can perform poorly due to deterioration, poor maintenance, and tampering with the exhaust aftertreatment system [56]. Vehicle owners tamper with their vehicles to avoid costs related to consumables, maintenance, and/or repair of the emissions control systems. Other reasons include reducing downtime costs, improving fuel economy, performance tuning, and adjusting exhaust sound levels [57]. A subproject of the EU H2020 program, diagnostic anti-tempering systems (DIASs), conducted a detailed investigation into tampering phenomena in vehicle aftertreatment systems. The results showed that most tampering behaviors in heavy-duty diesel vehicles were concentrated in the SCR system [58], with the primary aim of reducing urea consumption and SCR maintenance costs, thereby significantly saving operational expenses. Currently, Electric Control Unit (ECU) reprogramming and signal simulators are the two main tampering methods used in SCR systems. Through these means, even if actual NOx emissions exceed standards, data streams showing low NOx emissions can be generated without triggering any fault codes. This allows vehicles with excess emissions to operate normally, avoiding detection by environmental protection agencies.
Tampering with SCR systems in heavy-duty diesel vehicles significantly increases NOx emissions and pollutes the atmospheric environment. Research by He et al. [59] found that NOx emissions from Euro 5 and Euro 6 heavy-duty diesel vehicles with SCR tampering can reach or even exceed those of Euro 3 heavy-duty diesel vehicles. Chen et al. [51] found that simulated tampered diesel vehicles that do not inject urea often have NOx emissions per unit of fuel consumption that are more than twice that of normal diesel vehicles. Therefore, detecting tampering with heavy-duty vehicle SCR systems is key to improving the actual conversion efficiency of SCR systems in use.
Currently, annual inspection programs are the main means for assessing the operational status of in-use vehicle aftertreatment systems, and annual inspection programs for in-use vehicles are mandatory to ensure the proper functioning of the emissions control system. The annual inspection system can effectively detect vehicles with aftertreatment system failures caused by non-human factors. However, tampering can be difficult to detect because owners may restore the aftertreatment system to its original state before the inspection. Remote OBD systems are emerging as a potentially effective tool for identifying these tampering issues [60]. With the implementation of remote OBD systems, vehicle data are transmitted in real-time to a central maintained by environmental agencies, allowing for timely detection of tampering behaviors through data stream integration and relevant vehicle control. This will significantly reduce SCR system tampering. On the other hand, whether it is ECU reprogramming or signal simulator installation, both involve meticulously designed data streams, making it difficult for even experienced engineers to quickly identify tampering through remote OBD data. Therefore, the core issue of research has become designing algorithms to detect tampering behaviors in remote OBD data.
In the remote OBD data flow comprehensive supervision system constructed by Wang et al. [54], an algorithm framework for detecting SCR system tampering was proposed: calculating the theoretical urea consumption of the SCR system and the actual urea consumption in the urea tank separately, and, if there is a significant difference between the two, it indicates the possibility of tampering. The tampering detection method proposed by Wang et al. has the advantages of simplicity and low computational requirements. However, it heavily relies on the accuracy of urea consumption measurements, which is a challenge under the current limitations of remote OBD system data accuracy. Thus, the reliability of this algorithm is affected.
Roland et al. [61,62] introduced a complex data-driven approach to determine tampering behaviors in remote OBD data. As shown in Figure 4, the algorithm of the approach mainly consists of three parts, predictors, detectors, and an adaptive majority voting scheme. Before the real application, predictors for different emission parameters (NOx concentrations upstream and downstream of the SCR, oxygen concentration recorded by the oxygen sensor, and the amount of urea, etc.) are trained offline with tamper-free data. Long Short-Term Memory (LSTM) predictive networks are selected as the predictive model. Then, during the detection phase, each detector compares and analyzes the deviations between the calculated value and the real signal value. At last, an adaptive weighted voting scheme is then used to integrate the comparison results from the detectors to indicate whether tampering has occurred. Additionally, Pisoska et al. [63] innovated a comprehensive system called VetaDetect for detecting vehicle aftertreatment tampering. VetaDetect consists of multiple input single output autoregressive moving average models and is combined with Dempster–Shafer evidence theory for tampering determination in closed-loop scenarios. Although these methods have demonstrated high determinative accuracy across multiple datasets, obstacles such as insufficient remote OBD data variables and high computational complexity still hinder their practical application.
Zhang et al. [64] collected data on both cheating and normal OBD from different engines. They developed three types of vehicle SCR tampering models based on BP neural networks, logistic regression, and support vector machines (SVMs), and validated their accuracy. The study found that the support vector machine model, due to its high accuracy, strong generalization ability, and simple and fast computational process, is well-suited as a testing model for detecting SCR tampering.

5. Conclusions

This work reviewed the recent advances in heavy-duty diesel vehicle SCR technology regarding low-temperature NOx reduction technologies and the combination of SCR and remote OBD. The conclusions are as follows:
  • The copper-based molecular sieve catalysts have significant advantages in low-temperature NOx conversion efficiency, but relatively low efficiency in high-temperature conditions, and may lead to high N2O formation in the SCR system. The combination of multiple catalytic materials shows potential for achieving high conversion efficiency across a wide temperature range, and reduced N2O formation. The durability of SCR copper molecular sieve catalysts is crucial to ensuring low NOx emissions from diesel vehicles throughout their lifecycle.
  • Accelerating the exhaust temperature rise during low-temperature engine operation to meet the urea injection requirements has become a crucial research focus. Both cc-SCR and CDPF technologies can accelerate the increase in exhaust temperature within the SCR system, effectively improving NOx conversion efficiency during cold starts. However, for aftertreatment systems that include cc-SCR, the control strategy is a challenge; and, for CDFP, durability constraints limit its widespread use.
  • Solid ammonium and gaseous reductants can effectively address the issue of urea not being able to be injected under low-temperature exhaust conditions and have significant potential to greatly improve the low-temperature NOx conversion efficiency of SCR systems. However, the refueling infrastructure for solid ammonium is still inadequate, which limits its large-scale use. For gaseous reductants, such as hydrocarbons, the main future technical challenge is to enhance the selectivity of the catalyst for the reaction between gaseous reductants and NOx.
  • Remote OBD is an important technological measure used in recent emission regulations to monitor the NOx conversion efficiency of diesel vehicle SCR systems throughout the entire lifecycle. ICCT proposed the future function of remote OBD: 1. Develop a standardized methodology for fleet screening to identify potentially non-compliant vehicles. 2. Establish remote sensing standards and create a remote sensing record database.3. Strengthen anti-tampering provisions.
  • The accuracy of NOx emission data provided by remote OBD can support accurate assessment of NOx emissions from heavy-duty vehicles, but it requires the use of necessary data correction algorithms. Additionally, it is important to utilize the limited data items available from remote OBD to as closely as possible replicate the emission testing process required by the specific emission regulations.
  • Tampering with the SCR systems of heavy-duty diesel vehicles significantly increases NOx emissions and pollutes the atmospheric environment. Remote OBD systems are an important technology for identifying tampering in SCR systems of heavy-duty vehicles. The reliability of SCR tampering detection based on remote OBD systems depends on the accuracy of the data. Additionally, due to the immense computational load required for identifying tampered vehicles, there is a need to develop SCR tampering detection algorithms that balance both accuracy and computational efficiency.

Author Contributions

Methodology, H.L.; validation, H.L.; resources, Q.L.; writing—original draft preparation, Z.C.; writing—review and editing, H.L.; supervision, T.W.; funding acquisition, H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Excellent Young Scientists Fund Program (Overseas) (Admission No. 21FAA01116).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Working principle of urea SCR technology.
Figure 1. Working principle of urea SCR technology.
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Figure 2. A diesel cc-SCR engine aftertreatment system designed by Southwest Researchers, United [29].
Figure 2. A diesel cc-SCR engine aftertreatment system designed by Southwest Researchers, United [29].
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Figure 3. Comparison of NOx and window average power rate (APR) distribution of PEMS tests using work-based window and fuel-based window methods ((A–D) stand for different vehicle PEMS test cases).
Figure 3. Comparison of NOx and window average power rate (APR) distribution of PEMS tests using work-based window and fuel-based window methods ((A–D) stand for different vehicle PEMS test cases).
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Figure 4. Schematic diagram of tampering detection of aftertreatment system proposed by Roland et al. [61].
Figure 4. Schematic diagram of tampering detection of aftertreatment system proposed by Roland et al. [61].
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Chen, Z.; Liu, Q.; Liu, H.; Wang, T. Recent Advances in SCR Systems of Heavy-Duty Diesel Vehicles—Low-Temperature NOx Reduction Technology and Combination of SCR with Remote OBD. Atmosphere 2024, 15, 997. https://doi.org/10.3390/atmos15080997

AMA Style

Chen Z, Liu Q, Liu H, Wang T. Recent Advances in SCR Systems of Heavy-Duty Diesel Vehicles—Low-Temperature NOx Reduction Technology and Combination of SCR with Remote OBD. Atmosphere. 2024; 15(8):997. https://doi.org/10.3390/atmos15080997

Chicago/Turabian Style

Chen, Zhengguo, Qingyang Liu, Haoye Liu, and Tianyou Wang. 2024. "Recent Advances in SCR Systems of Heavy-Duty Diesel Vehicles—Low-Temperature NOx Reduction Technology and Combination of SCR with Remote OBD" Atmosphere 15, no. 8: 997. https://doi.org/10.3390/atmos15080997

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