Research on the Impact of Typical SCR Faults on NOx Emission Deterioration of Heavy-Duty Vehicles

Abstract

Faults of the selective catalytic reduction (SCR) significantly exacerbate nitrogen oxide (NOx) emissions from heavy-duty vehicles, thereby posing a severe hazard to atmospheric environmental quality. Currently, the paucity of systematic studies on NOx emission degradation induced by typical SCR faults has severely hindered the advancement of precise emission regulation for heavy-duty vehicles in China. To address this critical gap, this study investigates the impact of typical SCR faults on NOx emission deterioration from heavy-duty vehicles. Initially, leveraging the China heavy-duty commercial vehicle test cycle as the benchmark, heavy-duty vehicle emission tests were designed and conducted under typical SCR faults. Emission datasets were acquired for three typical SCR faults—namely nozzle circuit disconnected fault, upstream temperature sensor inaccuracy fault, and urea-water replacement fault—as well as under normal operating conditions. Building upon these data, three representative scenarios were established by integrating vehicle operating condition, fuel consumption levels, and vehicle specific power states, enabling systematic quantification of the extent of NOx emission deterioration caused by each SCR fault. The findings reveal that the NOx emissions deterioration caused by urea-water replacement fault is the most severe, followed by nozzle circuit disconnected fault, and the impact of upstream temperature sensor inaccuracy fault is the least. This research provides crucial support for identifying key targets in emission control and enhancing the precision of heavy-duty vehicle emission regulation. Relevant authorities should prioritize cracking down on intentional non-compliant practices such as urea water substitution to safeguard a healthy and sustainable atmospheric environment.

1. Introduction

Air quality serves as a fundamental cornerstone for human survival and development. For a long time, automobile exhaust emissions have posed a severe threat to air quality [1,2]. Statistical data indicate that [3], heavy-duty diesel vehicles in China account for merely 10% of the total motor vehicle population, yet their nitrogen oxide (NOx) emissions constitute over 88% of the total NOx emissions from motor vehicles, making them a key contributor to atmospheric pollution. Consequently, strengthening the control of NOx emissions from heavy-duty diesel vehicles has become a critical measure to win the “Blue Sky Defense Campaign” and achieve the “dual carbon” strategic goals [4,5]. The selective catalytic reduction (SCR) system represents the most effective technical approach for controlling NOx emissions from heavy-duty vehicles [6,7] and serves as the core guarantee for vehicles to comply with the National VI b emission regulations. However, during actual operation, affected by multiple overlapping factors such as increasing vehicle service life, inadequate maintenance systems, and non-standard human operations [8,9], the SCR system is highly prone to various faults, leading to a significant decline in denitrification efficiency and excessive NOx emissions, thereby severely endangering air quality [10].

According to road inspection data from environmental protection departments in recent years, typical faults of SCR systems in heavy-duty vehicles can be categorized into three types [11,12,13]: (1) Nozzle circuit disconnected fault: aging or vibration-induced detachment of power supply lines causes the nozzle to fail to receive command signals from the electronic control unit (ECU), interfering with the normal injection of urea and resulting in partial or complete loss of reductant supply to the SCR system. (2) Upstream temperature sensor inaccuracy fault: due to prolonged vehicle use and insufficient maintenance, the sensor fails to accurately monitor the upstream temperature of the SCR system, leading the ECU to misjudge that the system has not reached the operating temperature threshold, thereby reducing or stopping urea injection. (3) Urea-water replacement fault: some vehicle owners substitute standard urea solution with tap water to reduce operating costs, which constitutes a typical emission cheating behavior and directly renders the SCR system unable to undergo NOx reduction reactions. The widespread existence of these faults has caused the actual emission levels of numerous heavy-duty vehicles to far exceed regulatory limits, becoming a prominent challenge in current emission supervision. Conducting systematic research on the relationship between typical SCR faults and the NOx emissions deterioration from heavy-duty vehicles is of great practical significance for accurately identifying key targets for emission control and formulating scientific supervision strategies.

In recent years, scholars at home and abroad have carried out a series of studies on the impact of SCR system faults on NOx emissions from heavy-duty vehicles. Hu et al. [12] investigated the drift fault of the SCR downstream temperature sensor, and the results showed that this fault increased NOx emissions from 1.90 g/kWh to 4.60 g/kWh. Huang et al. [14] studied vehicle emission conditions after interrupting urea supply, revealing that this operation increased the NO emission factor by 16 times. Su et al. [9] tested a diesel vehicle with a faulty SCR system using the portable emission measurement system (PEMS), and found that the NOx emission factors of the faulty vehicle under 0%, 50%, and 75% loads were 8.42 g/kWh, 6.15 g/kWh, and 6.26 g/kWh, respectively, which were 2.14, 2.10, and 2.47 times those of the vehicle with a normal SCR system. Tian et al. [15] tested 6 diesel vehicles with tampered SCR systems using PEMS, and analysis via driving cycle binning method showed that after SCR system tampering, the NOx emission factors of National V and National VI diesel vehicles increased by 2–4 times and 10–50 times, respectively.

Despite the fact that existing studies have preliminarily revealed the impact of some SCR faults on emission deterioration, there are still two significant shortcomings. (1) Limitation in driving cycle applicability: most studies are conducted based on foreign driving cycles such as those in Europe and the United States, while research on China heavy-duty commercial vehicle test cycle that align with China’s road characteristics, traffic flow distribution, and driving habits is relatively scarce, resulting in research conclusions that are difficult to directly support the practice of heavy-duty vehicle emission supervision in China. (2) Lack of comparative analysis of multiple faults: existing studies mostly focus on single fault types, and have not systematically quantified the differences in NOx emission deterioration caused by multiple typical SCR faults under a unified experimental platform and the same driving cycle. Thus, it is impossible to clarify the hazard levels and emission characteristics of various faults, which is unfavorable for environmental protection departments to formulate targeted fault identification, early warning, and control schemes.

In view of this, this study takes National VI heavy-duty diesel vehicles as the research object and uses the China heavy-duty commercial vehicle test cycle as the benchmark cycle to investigate the impact of typical SCR faults on the NOx emissions deterioration. By systematically analyzing the variation laws of NOx emissions under different vehicle operating conditions, fuel consumption levels, and vehicle specific power (VSP) states, the emission impact mechanisms and hazard degrees of various faults are clarified. The research results aim to provide theoretical basis and data support for environmental protection departments to formulate differentiated supervision strategies and optimize on-board diagnostic fault diagnosis algorithms, thereby helping to improve the precise supervision capability of NOx emissions from heavy-duty vehicles in China and providing technical guarantee for atmospheric pollution prevention and control.

2. Experiment and Methods

2.1. Heavy-Duty Vehicle Chassis Dynamometer Emission Test

In this study, a market-mainstream heavy-duty box truck was selected as the test subject, and emission tests were conducted on a chassis dynamometer platform [16]. During the test, the vehicle operated strictly in accordance with the China Heavy-Duty Commercial Vehicle Test Cycle (CHTC-HT). The CHTC-HT accurately reflects the actual road operation characteristics in China. Therefore, the test results can provide important references for guiding the emission control of heavy-duty vehicles in China. The detailed test procedures are as follows. The technical parameters of the test vehicle are presented in Table 1. To systematically compare the emission impacts of different SCR faults, this study designed and implemented four groups of parallel tests: no-fault group, nozzle circuit disconnected fault group, upstream temperature sensor inaccuracy fault group, and urea-water replacement fault group. The schematic diagram of the design for each test group is shown in Figure 1. The specific fault simulation schemes are as follows:

Table 1. Technical parameters of test vehicle.

Name of Technical Parameter	Technical Parameter Result
Curb weight	7975 kg
Maximum design gross mass	16,000 kg
Engine displacement	4.5 L
Emission standard	National VI b
Transmission type	ZF6AS1000TO
Final drive ratio	4.1
Aftertreatment type	DOC + DPF + SCR + ASC
Maximum net power	162 kW

Figure 1. Schematic diagram of heavy-duty vehicle emission test considering typical SCR faults.

(1) No-fault group: all components of the SCR system in the test vehicle functioned normally. It could efficiently denitrify the engine exhaust gas as designed, ensuring that the emission level met the requirements of the National VI b emission regulations.

(2) Nozzle circuit disconnected fault group: the power supply line controlling the urea nozzle in the SCR system of the test vehicle was artificially disconnected. This simulated the nozzle failure caused by line aging or vibration-induced detachment in actual operation, interfering with the normal injection of urea and blocking or partially blocking the reductant supply to the SCR system.

(3) Upstream temperature sensor inaccuracy fault group: the mounting base of the upstream temperature sensor in the SCR system of the test vehicle was raised. This altered the temperature measurement environment of the sensor, simulating the temperature measurement misalignment problem caused by long-term use and improper maintenance. Consequently, it interfered with the accurate control of the urea injection amount by the ECU.

(4) Urea-water replacement fault group: the standard urea solution in the urea tank of the test vehicle was completely replaced with tap water. This simulated the emission cheating behavior carried out by some vehicle owners to avoid operating costs, directly rendering the SCR system unable to undergo NOx reduction reactions.

It should be specially noted that except for the artificially set target faults, the functions of the engine, transmission system, and other auxiliary systems of the test vehicle remained consistent and normal in all four test groups.

All test operations were strictly performed in accordance with the test protocols specified in the national standard [17]. The main test equipment included a CDM-72HDD-4WD four-wheel-drive chassis dynamometer manufactured by MAHA (Hoyen, Haldenwang, Germany) and a MEXA-7200DTR full-flow dilution constant volume sampling (CVS) system produced by HORIBA (Minami-ku, Kyoto, Japan). Prior to the tests, all equipment underwent zero drift calibration, airtightness leakage detection, and range calibration to ensure that the equipment accuracy met the test requirements.

To simulate the actual driving conditions on the road, the driving resistance coefficient was selected based on the 90% full-load state of the vehicle. The 90% full-load mass is approximately 14.4 tonnes. The driving resistance coefficient was derived from References [18,19]. These coefficients were input into the chassis dynamometer control system to reproduce the actual driving resistance of the vehicle under 90% full load. Meanwhile, the test vehicle was subjected to sufficient warm-up treatment before the formal test until the engine coolant temperature, oil temperature, and exhaust temperature all reached a stable operating range.

During the tests, the CVS system was used to continuously collect the volume concentrations of CO₂ and NOx in the exhaust gas at a sampling frequency of 1 Hz, and the carbon balance method was simultaneously applied to calculate the second-by-second fuel consumption. The chassis dynamometer’s built-in sensors were used to real-time record operating parameters such as vehicle speed, acceleration, and wheel-side power. In each test group, the test vehicle was operated continuously for 3 cycles in accordance with the CHTC-HT. After the tests, the raw data were downloaded and the coefficient of variation method was used to verify data consistency. Finally, data groups with a coefficient of variation less than 5% were selected as valid research data to ensure the reliability and repeatability of the test results.

2.2. Analytical Method of This Paper

Based on the emission test data, this study investigates the impact of typical SCR faults on the NOx emission deterioration from heavy-duty vehicles. The main analysis method is illustrated in Figure 2.

Figure 2. Flowchart of the analysis method in this study.

Firstly, data preprocessing was performed on the input data. The preprocessed test data were divided into four sub-scenarios according to vehicle operating condition, namely the total scenario, brake scenario, idle speed scenario, and drive scenario. Based on this, the typical scenario 1 was established to study the impact of typical SCR faults on NOx emission deterioration under different operating condition. The definitions of the sub-scenarios are as follows: (1) The total scenario is defined as the complete operating data of the China heavy-duty commercial vehicle test cycle. (2) The brake scenario is defined as the operating data where acceleration ≤−0.15 m/s². (3) The idle speed scenario is defined as the operating data where the absolute value of acceleration <0.15 m/s² and vehicle speed <1 km/h. (4) The drive scenariois defined as the operating data excluding the brake scenario and idle speed scenario. Equations (1) and (2) were used to calculate the second-by-second NOx emission factor and the average NOx emission factor, respectively, thereby analyzing the impact of typical SCR faults on NOx emission deterioration under different operating conditions:

$$EF=E/(v/3600),$$

(1)

$$SEF=\left(\sum _{i=1}^{n}E{F}_i\right)/n,$$

(2)

where EF represents the NOx emission factor per second, with the unit of g/km; E represents the NOx emission per second, with the unit of g/s; v represents the vehicle speed per second, with the unit of km/h; SEF represents the average NOx emission factor of a certain scenario, with the unit of g/km; n represents the duration of a certain scenario.

Secondly, within the drive scenario, the concept of scenario subdivision based on a single variable was introduced, and fuel consumption level and VSP state were considered to further study the impact of typical SCR faults on NOx emission deterioration. The calculation method of VSP is shown in Equation (3). The drive scenario was subdivided into three levels (low, medium, and high) using fuel consumption and VSP as independent variables, respectively. The subdivision method is illustrated in Figure 3: all second-by-second data of the drive scenario were sorted in ascending order by the value of the independent variable, and the 20th and 80th percentiles were used as the boundaries of the subdivided scenarios. Based on this, the drive scenario was divided into the low fuel consumption (LFC), medium fuel consumption (MFC), and high fuel consumption (HFC) scenarios, as well as the low vehicle specific power (LVSP), medium vehicle specific power (MVSP), and high vehicle specific power (HVSP) scenarios. Figure 3 shows the distribution of fuel consumption scenarios and VSP scenarios:

$$VSP=av+{\displaystyle \frac{A}{m}}v+{\displaystyle \frac{B}{m}}{v}^2+{\displaystyle \frac{C}{m}}{v}^3,$$

(3)

where VSP represents the vehicle specific power, with the unit of kW/ton; v represents the instantaneous speed, with the unit of m/s; a represents the acceleration, with the unit of m/s²; A represents the rolling resistance coefficient, with the unit of kW s/m; B represents the rotational resistance coefficient, with the unit of kW s²/m²; C represents the aerodynamic resistance coefficient, with the unit of kW s³/m³; m represents the vehicle mass, with the unit of t. For the heavy-duty diesel vehicles in this paper, the values of A/m, B/m and C/m are 0.0875, 0 and 0.000331 [19,20].

Figure 3. Distribution of fuel consumption scenarios and VSP scenarios.

Finally, the analysis method in this study takes heavy-duty vehicle chassis dynamometer emission test data as input, establishes three typical scenarios by considering vehicle operating condition, fuel consumption level, and VSP state, and based on these, conducts a more detailed study on the impact of typical SCR faults on NOx emission deterioration.

In addition, to ensure the reliability of the NOx emission data analysis results, this study analyzed and estimated the measurement uncertainty based on the Guide to the Expression of Uncertainty in Measurement (GUM) [20]. The calculation results in this paper may have errors from several aspects, including but not limited to (1) the test error of the equipment itself; (2) the speed-following error during the test process. Given these errors, in the analysis process of this paper, approximate values of the real data after calculation will be provided.

3. Result and Analysis

3.1. The NOx Emissions Deterioration Caused by Nozzle Circuit Disconnected Fault

Based on the emission data from the nozzle circuit disconnected fault group and no-fault group, this study quantitatively analyzed the NOx emission deterioration caused by the nozzle circuit disconnected fault, as shown in Figure 4. In Figure 4a, the NOx emission units for the brake and idle scenarios are g/s, while those for the drive and total scenarios are g/km. The unit definitions in Figure 5a and Figure 6a are consistent with this. In the Figure 4, Figure 5 and Figure 6, the red five-pointed stars represent the mean values. The gray dots in the figure represent the outliers of the box plot. Here, the NOx emissions in the braking scenario and idle scenario have a relatively weak correlation with the mileage. Using the unit of g/km is not sufficient to describe the actual nitrogen oxide emission situation. Therefore, the unit of g/s is adopted. During the analysis process, this paper calculated the average value of the effective results for each scenario, which can, to a certain extent, minimize the interference of other factors on the analysis results and improve the credibility. Hereafter, the nozzle circuit disconnected fault is abbreviated as Fault A.

Figure 4. Results of NOx emission deterioration caused by Fault A (Nozzle circuit disconnected fault).

Figure 5. Results of NOx emission deterioration caused by Fault B (Upstream temperature sensor inaccuracy fault).

Figure 6. Results of NOx emission deterioration caused by Fault C (Urea-water replacement fault).

Figure 4a shows the NOx emission deterioration caused by Fault A under different operating conditions. Compared with the no-fault group (average NOx emission is 0.11 g/km), the occurrence of Fault A resulted in an approximately 3400% increase in NOx emissions from the heavy-duty vehicle (average NOx emission under Fault A is 3.61 g/km). Specifically, the NOx emissions in braking, idle speed, and driving scenarios increased by approximately 1900%, 700%, and 2600%, respectively. Notably, although NOx emissions in the brake and idle speed scenarios increased significantly after Fault A occurred, they still met the regulatory limit requirements. Among the operating condition scenarios, the drive scenario exhibited the largest increase in NOx emissions, reaching 4.33 g/km under the fault condition. The primary reason for this phenomenon is that the drive scenario corresponds to the typical operating condition of the engine with medium-high load and medium-high speed, where the original NOx generation is extremely high. The SCR system is the core technical means to achieve compliant NOx emissions. Thus, the interruption of reductant supply caused by Fault A inevitably leads to a sharp increase in NOx emissions. In contrast, in the brake and idle speed scenarios, the engine operates under low load and low combustion temperature, resulting in inherently low original NOx generation. Even if the SCR system fails, the actual emissions can still comply with regulatory requirements.

Figure 4b shows the NOx emission deterioration caused by Fault A under different fuel consumption levels. When Fault A occurred, NOx emissions generally showed a gradual upward trend with increasing fuel consumption, and the dispersion of emission data expanded as fuel consumption increased, with the HFC scenario exhibiting the widest range of emission values. In terms of emission increase rates, the NOx emissions under the LFC, MFC and HFC scenarios increased by approximately 1300%, 2900% and 1200%, respectively, with the MFC scenario showing a significantly higher increase than the LFC and HFC scenarios.

Figure 4c shows the NOx emission deterioration caused by Fault A under different VSP states. When Fault A occurred, NOx emissions showed a gradual upward trend with increasing VSP, while the dispersion of emission data first increased and then decreased, with the MVSP scenario having the widest range of NOx emission values. Regarding the emission growth rate, the NOx emissions under the LVSP, MVSP and HVSP scenarios increased by approximately 1400%, 2600% and 3400%, respectively, with the HVSP scenario showing the largest growth rate.

When Fault A occurs in the SCR system, the variation patterns of NOx emissions with fuel consumption and VSP are significantly different. This is because VSP has a strong linear correlation with engine load and directly reflects the engine power demand, whereas fuel consumption is not only affected by engine load but also constrained by the operating efficiency of the engine and the entire powertrain. Since the original NOx emission of the engine is directly related to load, the HVSP scenario (corresponding to high load) exhibits the largest increase in NOx emissions, while the HFC scenario does not show the highest increase due to efficiency and other factors.

3.2. The NOx Emissions Deterioration Caused by Upstream Temperature Sensor Inaccuracy Fault Group

Based on the emission data from the upstream temperature sensor inaccuracy fault group and the no-fault group, this study quantitatively analyzed the NOx emission deterioration caused by the upstream temperature sensor inaccuracy fault, as shown in Figure 5. Hereafter, the upstream temperature sensor inaccuracy fault is abbreviated as Fault B.

Figure 5a shows the NOx emission deterioration caused by Fault B under different operating conditions. Compared with the no-fault group (average NOx emission is 0.11 g/km), the occurrence of fault B led to an approximately 700% increase in nitrogen oxide emissions from heavy vehicles (average NOx emission under Fault B is 0.8 g/km). Specifically, the NOx emissions increased by approximately 400%, 80%, and 600%, respectively, in the braking, idle speed, and driving scenarios. In terms of the pattern of emission increase, Fault B showed similarity to Fault A (nozzle circuit disconnected fault), with both exhibiting a significant increase in emissions in the drive scenario. However, in terms of the magnitude of the increase, the NOx emission increase caused by Fault B was significantly lower than that caused by Fault A.

Figure 5b shows the NOx emission deterioration caused by Fault B under different fuel consumption levels. When Fault B occurred, the NOx emission values in the LFC and MFC scenarios were roughly equivalent, while the HFC scenario exhibited the highest NOx emission values. Regarding emission increase rates, the NOx emission increases in the LFC MFC and HFC scenarios were approximately 400%, 300%, and 500%, respectively, which differed significantly from the NOx emission pattern observed when Fault A occurred in the SCR system.

Figure 5c shows the NOx emission deterioration caused by Fault B under different VSP states. When Fault B occurred, NOx emissions showed a slow upward trend with increasing VSP, and the distribution of NOx emission values was roughly consistent across all VSP scenarios. In terms of emission increase rates, the NOx emission increases in the LVSP, MVSP and HVSP scenarios were approximately 600%, 500%, and 600%, respectively, which also differed distinctly from the NOx emission variation pattern when Fault A occurred in the SCR system.

The core reason for the difference in the NOx emission deterioration patterns between Fault B and Fault A lies in the fact that upstream temperature sensor misalignment does not cause complete failure of the SCR system. As the engine exhaust temperature increases, it creates a relatively optimal measurement condition for the misaligned upstream temperature sensor. Consequently, in the MFC and HFC scenarios as well as the MVSP and HVSP scenarios, the SCR system can still maintain a certain level of working efficiency, exerting a certain reduction effect on the original NOx emissions of the engine.

3.3. The NOx Emissions Deterioration Caused by Urea-Water Replacement Fault

Based on the emission data from the urea-water replacement fault group and the no-fault group, this study quantitatively analyzed the NOx emission deterioration caused by the urea-water replacement fault, as shown in Figure 6. Hereafter, the urea-water replacement fault is abbreviated as Fault C.

Figure 6a shows the NOx emission deterioration caused by Fault C under different operating conditions. Compared with the no-fault group (average NOx emission is 0.11 g/km), the occurrence of Fault C resulted in a approximately 6700% increase in NOx emissions from the heavy-duty vehicle (average NOx emission under Fault C is 0.8 g/km). Specifically, the NOx emission increases in the brake, idle speed, and drive scenarios were approximately 3200%, 2600%, and 4500%, respectively. Compared with Fault A and Fault B, Fault C showed no significant difference in the pattern of emission increase across operating characteristic scenarios, but the magnitude of the increase was significantly more prominent.

Figure 6b shows the NOx emission deterioration caused by Fault C under different fuel consumption levels. When Fault C occurred, NOx emissions generally exhibited a gradual upward trend with increasing fuel consumption, and the dispersion of emission data expanded as fuel consumption increased, with the HFC scenario having the widest range of emission values. In terms of emission increase rates, the NOx emission increases in the LFC, MFC and HFC scenarios were approximately 4500%, 4700%, and 2500%, respectively, which was basically consistent with the NOx emission deterioration pattern caused by Fault A (nozzle circuit disconnected fault).

Figure 6c shows the NOx emission deterioration caused by Fault C under different VSP states. When Fault C occurred, NOx emission values remained basically stable with increasing VSP, and the dispersion of emission data gradually decreased, with the emission values in the HVSP scenario being more concentrated. Regarding emission increase rates, the NOx emission increases in the LVSP, MVSP and HVSP scenarios were approximately 3800%, 4800%, and 4200%, respectively, which differed greatly from the NOx emission deterioration patterns caused by Fault A and Fault B.

The core reason for the difference in the NOx emission deterioration patterns between Fault C and Fault A/Fault B is as follows: in the dimension of fuel consumption level, the NOx emission pattern of Fault C was basically consistent with that of Fault A, indicating that the two faults share a similar mechanism for causing SCR system failure. However, in the dimension of VSP state, the emission pattern of Fault C was significantly different from the other two faults. This is because urea-water replacement causes the SCR system to completely lose its denitrification function, making it unable to reduce the original NOx emissions of the engine under high VSP scenarios, thus fully revealing the characteristics of high and concentrated original NOx emission values of the engine under this fault. In contrast, after the SCR nozzle is disconnected, the SCR system still has a small amount of urea injection, which can play a certain role in reducing the original NOₓ emissions of the engine.

3.4. Discussion on Typical Faults

3.4.1. Discussion of the Results of This Paper

Figure 7 presents the comparison results of NOx emissions from the heavy-duty vehicle under four states: no-fault, Fault A (nozzle circuit disconnected fault), Fault B (upstream temperature sensor inaccuracy fault), and Fault C (urea-water replacement fault). To facilitate the comparison of NOx emission increase rates on the same scale, the NOx emission increase rates under different fuel consumption and VSP scenarios were normalized. The maximum increase rate in each group was set as 1, and other increase rates were converted into ratios relative to the maximum increase rate (i.e., the proportion of NOx increase), resulting in Figure 7b,c.

Figure 7. Comparison of NOx emissions from heavy-duty vehicles under typical SCR faults.

Figure 7a shows the total NOx emission results of the heavy-duty vehicle. Among the three typical SCR faults, the NOx emission level was the highest under Fault C, the lowest under Fault B, and intermediate under Fault A. This result indicates that urea-water replacement causes the SCR system to completely lose its denitrification function, making it unable to reduce the original NOx emissions of the engine, thus resulting in the most severe NOx emissions from the vehicle. After the SCR nozzle is disconnected, although NOx emissions increase, the fault does not completely block urea injection, and the SCR system can still inject a small amount of urea to achieve partial NOx reduction. When the upstream temperature sensor is misaligned, although the sensor cannot accurately obtain the SCR operating temperature, it can still ensure urea injection, thereby playing a certain role in reducing NOx emissions.

Figure 7b shows the variation results of NOx emission increase rates under different fuel consumption levels. The pattern of NOx emission increase caused by Fault B was completely opposite to that caused by Fault A and Fault C. In the MFC scenario, Fault B exhibited the smallest NOx emission increase, which may be due to the difference in the action mechanism of Fault B compared with the other two faults. Despite the misalignment of the upstream temperature sensor, the large amount of stable high-temperature exhaust gas generated by the engine in the MFC scenario still enabled the elevated temperature sensor to identify an effective operating temperature range, thereby controlling the urea injection system to spray a certain amount of urea and achieve NOx reduction.

Figure 7c shows the variation results of NOx emission increase rates under different VSP states. The pattern of NOx emission increase caused by Fault B also showed a completely opposite trend to that caused by Fault A and Fault C. In addition, Fault C exhibited high NOx emission increase rates in LVSP, MVSP and HVSP scenarios, while Fault A and Fault B did not show this phenomenon. In LVSP and MVSP scenarios, Fault A and Fault B showed lower NOx increase rates, mainly because these two faults did not cause the SCR system to completely lose its function and could still work normally under some operating conditions to reduce NOx emissions.

3.4.2. Comparative Discussion with Existing Literature

Building upon the preceding analysis, this paper also surveyed a number of studies focusing on vehicle NOx emission increments induced by malfunctions. A comparative discussion was conducted between these findings and the results of this study, with detailed information presented in Table 2.

Table 2. Comparative Discussion with the Existing Literature.

Fault Name	The Increase in NOx Emissions Caused
Upstream NOx sensor malfunctions [9]	214% (No load) 210% (Half load) 247% (75% Full load)
SCR system tampering [16]	200%~400% (National V) 1000%~5000% (National VI)
Conventional tampering operations [21]	140%
Nozzle circuit disconnected fault (Fault A)	3400%
Upstream temperature sensor inaccuracy fault group (Fault B)	700%
Urea-water replacement fault (Fault C)	6700%

Reference [9] investigates the impact of upstream NOx sensor malfunctions on the NOx emissions of National IV heavy-duty diesel vehicles. It is indicated that such malfunctions lead to an underestimation of the measured upstream NOx concentration, which in turn affects the urea injection volume and ultimately results in increased NOx emissions. The study reveals that under no-load, half-load, and 75% full-load conditions, the upstream NOx sensor malfunctions cause the vehicle emissions to increase by 214%, 210%, and 247%, respectively. From a theoretical perspective, this type of upstream NOx sensor malfunction is analogous to illegal OBD program rewriting by vehicle owners, where the actual readings of the NOx sensor are adjusted to reduce urea consumption. Both practices induce a comparable degree of NOx emission deterioration.

Reference [16] explores the influence of SCR system tampering on diesel vehicle NOx emissions. It points out that SCR system tampering leads to the deactivation or failure of the SCR system, thereby severely impairing NOx emission control. The research finds that after SCR system tampering, the NOx emission factor of National V diesel trucks increases by 200% to 400%, while that of National VI diesel trucks surges by 1000% to 5000%. In terms of mechanism, SCR system tampering or failure is similar to Fault C (urea-water replacement fault) in this paper, and the resulting magnitudes of NOx emission increments are basically consistent.

Reference [21] analyzes the existing conventional tampering operations and explores their impacts on the NOx emissions of National VI heavy-duty vehicles using remote monitoring data. It notes that the current tampering practices mainly include denitrification system tampering, OBD monitor disabling, torque limit function disabling, and calibration data modification. All these tampering operations cause varying degrees of deterioration in vehicle NOx emissions. The study demonstrates that the NOx emissions of tampered vehicles increase by approximately 140%.

In summary, both vehicle malfunctions and artificial tampering result in varying degrees of NOx emission deterioration. Notably, the three typical faults investigated in this paper (Nozzle circuit disconnected fault, Upstream temperature sensor inaccuracy fault group, and Urea-water replacement fault) induce the most severe NOx emission degradation, which requires special attention in practical applications.

3.4.3. Analysis of the Applicability of Research Results

In addition, this paper discusses the applicable scope of the research results. Given that the experimental data of this study were acquired under the CHTC-HT, which closely aligns with the actual road characteristics, traffic flow distribution, and driving habits in China, the findings can be extended to other vehicles or fleets under the following conditions: (1) The findings are applicable to heavy-duty vehicles sharing similar aftertreatment technologies with the test vehicle, specifically China National VIb -compliant heavy-duty diesel vehicles equipped with a DOC (Diesel Oxidation Catalyst) + DPF (Diesel Particulate Filter) + SCR (Selective Catalytic Reduction) + ASC (Ammonia Slip Catalyst) aftertreatment system. This includes, but is not limited to, box trucks, flatbed trucks, and tractor units. (2) The findings are also relevant to medium-duty commercial vehicles where engine displacement and maximum net power are comparable to those of the test vehicle. For such vehicles, the design load of the SCR system and the urea demand characteristics exhibit high similarity to the test vehicle, ensuring consistent responses to the studied SCR faults. (3) The findings are suitable for commercial vehicle fleets operating in China, such as those engaged in urban distribution and short-haul trunk transportation. The operating conditions of vehicles in these fleets are highly consistent with the CHTC-HT, leading to uniform patterns of NOx emission deterioration induced by the targeted SCR faults. (4) The findings apply to fleets with moderate operation and maintenance (O&M) management standards, as well as fleets with vehicles of extended service life. In such fleets, SCR systems are more prone to common issues including sensor aging and wiring failures—fault types that are highly consistent with the three typical SCR faults investigated in this study. (5) The findings are valuable for fleets in need of strengthened precise supervision of NOx emissions. The study’s conclusions can directly serve as a basis for guiding fault diagnosis and troubleshooting in fleets, as well as for defining key priorities in emission supervision strategies.

4. Conclusions

In response to the lack of systematic research on the NOx emissions deterioration from heavy-duty vehicles caused by typical SCR faults, this study focused on three types of typical SCR faults and conducted a quantitative study on the degree of NOx emission deterioration from heavy-duty vehicles based on the CHTC. The aim is to provide a scientific basis and technical support for identifying key targets for emission control and improving the precise emission supervision capability of heavy-duty vehicles. The main research conclusions are as follows:

(1) A self-designed chassis dynamometer emission test for heavy-duty vehicles under typical SCR faults was carried out. Taking the CHTC as the benchmark cycle, emission data of heavy-duty vehicles under three types of typical SCR faults (nozzle circuit disconnected fault, upstream temperature sensor inaccuracy fault, and urea-water replacement fault) and normal state were collected, laying a reliable data foundation for subsequent quantitative analysis.

(2) Combined with vehicle operating conditions, fuel consumption levels, and VSP states, three types of typical analysis scenarios were established to systematically quantify and analyze the degree of NOx emission deterioration under different SCR faults. The results show that the urea-water replacement fault causes the most severe NOx emission deterioration, followed by the nozzle circuit disconnected fault, and the upstream temperature sensor inaccuracy fault has the least impact.

(3) Analysis from the dimension of operating conditions revealed that under various SCR faults, the NOx emission values and increase rates in the drive scenario were significantly higher than those in the brake and idle speed scenarios. Although NOx emissions in the brake and idle speed scenarios increased after the occurrence of faults, their emission levels still met regulatory limits due to the low original NOx generation of the engine in these two scenarios.

(4) Analysis from the dimension of fuel consumption levels revealed that the pattern of NOx emission increase caused by the upstream temperature sensor inaccuracy fault was completely opposite to that caused by the nozzle circuit disconnected fault and urea-water replacement fault, and the total deterioration degree was the lightest. This is mainly due to the sufficient exhaust temperature in MFC and HFC scenarios, which can partially compensate for the impact of sensor misalignment and maintain the partial denitrification function of the SCR system.

(5) Analysis from the dimension of VSP states revealed that the NOx emission increase rates of the three types of faults all increased with the rise in VSP, but there were differences in the change rates. Among them, the urea-water replacement fault exhibited extremely high NOx emission increase rates in LVSP, MVSP and HVSP scenarios and caused complete failure of the SCR system, thus should be regarded as the primary key target for heavy-duty vehicle emission control.