Study on the Effect of Catalyst Loading on the DOC + SCR Coupled System of a Light-Duty Diesel Engine

Abstract

DOC coupled with SCR represents a key technological approach for reducing gaseous pollutant emissions from diesel engines. Based on engine bench testing using a light-duty diesel engine as a prototype, this study investigates the impact of DOC coupled with SCR at different catalyst loadings on diesel engine emission characteristics. Results indicate that higher DOC loadings lead to greater exhaust backpressure losses, with a maximum pressure difference reaching 4.3 kPa. The temperature difference across the DOC was minimally affected by catalyst loading. Higher DOC loading enhanced catalytic activity toward CO and THC. At medium-to-low loads, this effect was pronounced, while at high loads, the influence of catalyst loading diminished. Higher DOC loading enhances NO oxidation capacity. Under external characteristic conditions, elevated engine exhaust temperatures maximize post-DOC NO₂ formation, increasing post-DOC NO₂ production by over 100%. These findings provide useful guidance for optimizing diesel aftertreatment systems to achieve a better balance between pollutant reduction, energy consumption, and environmental sustainability, thereby supporting the sustainable development of cleaner diesel engine technologies.

1. Introduction

Diesel engines are widely used in commercial vehicles and off-road applications due to their high thermal efficiency, reliability, and power output [1]. On the other hand, diesel vehicle exhaust emissions pose significant pollution concerns. According to China’s Annual Report on Mobile Source Environmental Management [2], diesel vehicles emitted 961,000 tons of CO, 140,000 tons of HC, 4,040,000 tons of NO_X, and 45,000 tons of particulate matter (PM). These figures represent 14.4%, 8.1%, 78.3%, and over 90.0% of total automotive emissions, respectively. It is evident that diesel vehicles constitute the primary source of NO_X and PM emissions among atmospheric pollutants [3], posing significant risks to human health [4]. Consequently, China has established stringent emission regulations to control pollutant discharges from diesel vehicles [5,6]. Faced with increasingly rigorous emission standards, relying solely on in-engine purification technologies proves insufficient [7], making exhaust aftertreatment systems an essential technology for reducing diesel engine pollutant emissions [8].

Selective Catalytic Reduction (SCR) technology utilizes a catalyst to selectively react with NOx in an ammonia (NH₃) reduction atmosphere, producing non-toxic and pollution-free N₂ and H₂O, making it the most effective method for reducing diesel engine NO_X emissions [9]. Its primary reaction processes include standard reactions and rapid reactions [10]. Among these, the rapid reaction represents the most efficient pathway for SCR catalytic NO_X reduction. This reaction relies heavily on the NO₂ proportion within the NO_X mixture, achieving peak conversion efficiency when the NO:NO_X ratio reaches 1:1. Consequently, increasing the NO₂/NO_X ratio in diesel exhaust effectively enhances SCR catalytic efficiency.

The Diesel Oxidation Catalyst (DOC) typically consists of cordierite or a metal substrate coated with precious metal catalysts (such as Pt and Pd), primarily promoting the oxidation of CO, hydrocarbons, and NO. It should be noted that nitrogen oxides (NO_X) emitted by diesel engines are predominantly NO, accounting for approximately 85–95% of total NO_X under most operating conditions. Therefore, the Diesel Oxidation Catalyst (DOC) is essential upstream of the Selective Catalytic Reduction (SCR) system, as it oxidizes NO to NO₂, thereby increasing the NO₂/NO_X ratio and enabling the fast SCR reaction pathway. Simultaneously, the oxidation reaction releases heat, elevating exhaust temperature and further boosting SCR efficiency. The DOC primarily consists of four components: the housing, vibration-damping layer, carrier, and catalyst. The catalyst serves as the core element, with active components mainly comprising precious metals such as Pt and Pd. Research indicates that DOCs can elevate the NO₂/NO_X ratio in diesel exhaust to 60–70%. The oxidation performance of DOCs can be enhanced by optimizing catalyst formulations and structural parameters [11]. Research by Zhong [12] and Lou [13], among others, indicates that DOC geometric and structural parameters significantly affect oxidation performance. In general, longer DOCs exhibit superior oxidation capabilities for CO, THC, and NO, while carrier material, mesh size, and wall thickness also influence oxidation behavior to varying degrees. In addition, the oxidation performance of Pt-based catalysts is closely related to the catalyst activity state and Pt dispersion. Shi [14] reported that non-thermal plasma synergistic regeneration can reconstruct the performance of Pt catalysts, thereby improving their catalytic oxidation capability. Therefore, the oxidation performance of DOC is affected not only by structural design parameters but also by the physicochemical state of the active catalyst components. The study by Florcher P [15] shows that honeycomb ceramic carriers with high pore density and thin walls have the characteristics of low exhaust resistance and large reaction area, which helps to improve the activity of catalysts. Khosravi et al. [16] studied the reaction kinetics models of CO, NO, and C₃H₆ oxidation reactions in DOC using two catalyst formulations, Pt and Pt:Pd (4:1). The results showed that the catalyst with Pd added had higher catalytic activity and higher conversion efficiency for CO and THC. Beyond carrier material and structural parameters, the loading amount of DOC exerts a more pronounced effect on oxidation performance. Lou et al. [17] observed that increasing precious metal loading reduced the ignition temperatures of CO and THC. Ge et al. [18] found that higher precious metal loading improved the catalyst’s low-temperature activity.

Xiang et al. [19] found that coupling DOC with SCR can significantly reduce HC, CO, NO_X, NH₃, and N₂O emissions in ammonia/diesel dual fuel engines. The main reason for this is that DOC oxidizes HC and CO and converts NO to NO₂, thereby optimizing the SCR inlet exhaust composition and improving denitrification efficiency. Lyu et al. [20] investigated a typical Chinese Stage VI heavy-duty diesel engine equipped with DOC and SCR, revealing enhanced durability against N₂O emissions. Jung et al. [21] emphasized the importance of selecting appropriate catalysts for effective N₂O reduction, noting that vanadium dioxide catalysts demonstrated superior efficacy in suppressing N₂O formation compared to zeolite and copper-zeolite catalysts. Hu et al. [22] found that the DOC + SCR integrated post-treatment system can effectively reduce gaseous and particulate emissions under both steady-state and transient conditions. The emission reduction mechanism is mainly attributed to the oxidation of unburned components and soluble organic matter by DOC, as well as the synergistic regulation of the SCR reaction environment. Sun et al. [23] observed that the synergistic effect of DOC and SCR reduces the graphitization degree of particulate matter (PM), with higher exhaust temperatures yielding lower graphitization levels. Sun et al. [24] demonstrated that the “DOC + SCR” system exhibits strong robustness in controlling pollutant emission levels during purification. Particularly under high-speed and high-load conditions, CO concentrations approached 0 ppm for most of the time after catalytic treatment.

Reviewing the current state of domestic and international research, most existing studies focus on the DOC as a standalone component, investigating how parameter variations affect diesel engine emission characteristics. Research on the intercoupling relationships among exhaust aftertreatment components is scarce, failing to provide clear guidance on how aftertreatment systems influence diesel engine emission performance.

This study will investigate the effects of DOC coupled with SCR at different catalyst loadings on diesel engine emission characteristics based on engine bench testing. The anticipated findings are expected to provide guidance for optimizing diesel engine exhaust aftertreatment systems.

2. Test System and Methods

2.1. Test System

Figure 1 presents a schematic diagram of the engine dynamometer test system. The system mainly consists of a Kaimai electric dynamometer (Kaimai (Luoyang) Electromechanical Co., Ltd., Luoyang, China), a Tocell fuel consumption meter (Shanghai Tocell Engine Testing Equipment Co., Ltd., Shanghai, China), and auxiliary bench equipment, including a coolant temperature control system, an engine oil temperature control device, an engine data acquisition unit, and an air filter. The main emission measurement instruments include a MEXA-1600D gaseous analyzer (HORIBA, Ltd., Kyoto, Japan), an EEPS-3090 particle size analyzer (TSI Incorporated, Shoreview, MN, USA), and a Dekati DI-2000 ejector diluter (Dekati Ltd., Kangasala, Finland). The bench control console communicates with the dynamometer, fuel consumption meter, emission measurement system, and various sensors to coordinate test operation and record the corresponding data.

2.2. Engine Specifications

The test engine was a light-duty diesel engine with a displacement of 1.91 L, an in-line four-cylinder configuration, and turbocharging with intercooling. Its main technical specifications are listed in

Table 1. Technical specifications of the test engine.

Intake System	Forced Induction with Intercooler
Engine configuration	Dual overhead camshaft
Bore × stroke (mm)	80 × 92
Compression ratio	18.1
Displacement (L)	1.91
Maximum power output (kW)	75
Maximum torque speed (r/min)	2000
Maximum torque (N·m)	250
Rated speed (r/min)	3200

.

2.3. Test Exhaust Aftertreatment System

The test exhaust aftertreatment system consists of a DOC coupled with an SCR unit. The DOC employs three different catalyst loading levels, while the coupled SCR unit maintains fixed parameters. In this study, precious metal loadings of 25, 40, and 60 g/ft³ were selected to represent low, medium, and high loading levels, respectively, for investigating the variation in emission characteristics of the DOC + SCR coupled system over a typical loading range. DOC-specific technical parameters are detailed in

Table 2. Experimental DOC technical parameters.

Project	NO. 1	NO. 2	NO. 3
Pore density (cpsi)	300	300	300
Carrier diameter (mm)	144	144	144
Carrier length (mm)	118	118	118
Aspect ratio	0.82	0.82	0.82
Volume (L)	1.92	1.92	1.92
Wall thickness (mil)	5	5	5
Carrier material	Cordierite	Cordierite	Cordierite
Coating material	γ-Al2O3	γ-Al2O3	γ-Al2O3
Micro pore diameter (μm)	1–10	1–10	1–10
Median value of micropores (μm)	1–4	1–4	1–4
Specific surface area (m2·g−1)	120–180	120–180	120–180
Precious metal loading capacity (g·ft−3)	25	40	60
Precious metal components	Pt/Pd/Rh	Pt/Pd/Rh	Pt/Pd/Rh
Proportion of precious metals	5:1:0	5:1:0	5:1:0

. When the engine exhaust enters the exhaust pipe, a certain amount of urea solution is sprayed into the exhaust pipe by the urea injection device. The urea solution is atomized under high temperature, undergoes hydrolysis and pyrolysis reactions, and generates ammonia gas (NH₃) required for the reduction reaction [25]. Under the action of the catalyst, NO_X is selectively reduced to nitrogen gas. The urea-SCR system can be equipped with DOC at the front end of the SCR catalyst to achieve rapid SCR reaction, thereby improving the conversion rate of NO_X [26,27]. Due to the secondary pollution caused by NH₃ escape caused by the installation of NH₃-SCR, the technical solution of optimizing the urea injection strategy and installing ASC (Ammonia Slip Catalyst) catalyst to oxidize excess NH₃ to generate N₂ has attracted the attention of various host factories [28]. Specific technical parameters of SCR are shown in

Table 3. SCR system parameters.

Project	System Parameters
Pore density (cpsi)	300
Carrier diameter (mm)	144
Carrier length (mm)	150
Aspect ratio	1.04
Volume (L)	2.44
Wall thickness (mil)	5
Coating material	TiO2-based coating
Micro pore diameter (μm)	5–20
Median value of micropores (μm)	8–12
Carrier material	Honeycomb ceramics
Catalyst type	V2O5-WO3/TiO2
V2O5 content (wt%)	1.2
WO3 content (wt%)	6
Catalyst loading (g/L)	140
SCR active temperature window (°C)	200–500

.

2.4. Test Conditions

Two typical operating conditions were selected: external characteristic and load characteristic. The specific parameters are shown in

Table 4. Operating conditions for external and load characteristic tests.

Speed (r/min)	Loading (%)
1000	100
1200	100
1400	100
1600	100
1800	100
2000	100
2200	100
2400	100
2600	100
2800	100
3000	100
3200	100
2000	10
2000	25
2000	50
2000	75
2000	100
3200	10
3200	25
3200	50
3200	75
3200	100

.

(1)

External characteristic condition: The test engine operates at full load across a speed range from 1000 r/min to 3200 r/min. Test speeds are selected at 200 r/min intervals, resulting in 12 external characteristic test points.

(2)

Load Characteristic Test Conditions: Based on the engine’s external characteristic test data, the maximum torque speed is 2000 r/min and the rated speed is 3200 r/min. Load levels corresponding to 10%, 25%, 50%, 75%, and 100% of full load at each respective speed were selected as load characteristic test points.

3. Results and Discussion

3.1. Exhaust Backpressure and Exhaust

The exhaust back pressure measured in the experiment is mainly determined by the physical properties of the catalyst structure, including coating thickness and pore characteristics, which are dominated by the total load. In this experimental batch, the total load is proportional to PGM, and the two change synchronously. Figure 2 shows the effect of DOC-coupled SCR with different catalyst loadings on exhaust back pressure. According to Figure 2a,b, under external characteristic conditions, the exhaust pressure difference before and after DOC shows a fluctuating trend of first increasing, then decreasing, and then increasing again with the increase in rotational speed. This change is mainly influenced by the exhaust flow velocity and device flow resistance. As the rotational speed increases, the exhaust flow rate increases, and the friction loss in the DOC + SCR system increases, resulting in an overall increase in back pressure level [29]. At the same time, under the same carrier structure conditions, the loading of precious metals increased from 25 g/ft³ to 60 g/ft³, resulting in an increase in catalyst coating amount and a decrease in effective flow cross-sectional area. Therefore, the exhaust pressure difference in DOC + SCR No. 3 at all speeds was higher than that of DOC + SCR Nos. 1 and 2.

Under load characteristic conditions (Figure 2c,d), the exhaust pressure difference before and after DOC increases monotonically with increasing load at 2000 r/min and 3200 r/min and reaches its maximum value at 100% load. At this time, the maximum pressure difference in DOC + SCR with noble metal loadings of 25, 40, and 60 g/ft³ was 3.3, 3.6, and 4.3 KPa, respectively, indicating that the influence of noble metal loadings on system flow resistance is more significant under high exhaust flow conditions.

Overall, the variation in exhaust back pressure is mainly determined by the engine operating conditions, and the loading of precious metals has a secondary but stable effect on the back pressure level by changing the characteristics of the catalyst coating. The increase in back pressure may have adverse effects on the engine ventilation process, thereby forming certain constraints on the combustion process under high load conditions. Although the present study mainly focused on the effects of DOC loading on exhaust backpressure and emission characteristics, the increase in exhaust flow resistance may also influence the turbocharger operating condition, intake air flow, exhaust gas temperature, and fuel consumption. Therefore, the results reported in this study are primarily intended to reflect the relative differences in outlet pollutant concentrations and NO₂/NOx composition among different DOC loading levels under the tested operating conditions. These coupled effects should be further considered in future studies when comprehensively evaluating the effectiveness of different catalyst loading levels.

Figure 3 shows the effect of DOC-coupled SCR with different noble metal loadings on exhaust temperature characteristics. Under the external characteristic condition, the temperature difference between the front and rear exhaust of the DOC shows a fluctuating trend of first increasing, then decreasing, and then increasing again with the increase in speed. At 1200 r/min, the maximum temperature difference in DOC + SCR with precious metal loads of 25, 40, and 60 g/ft³ is 55 °C, 51 °C, and 56 °C, respectively. This fluctuation mainly reflects the comprehensive effect between the change in exhaust temperature and the heat release intensity of catalytic oxidation. When the loading of precious metals increases, the temperature difference actually decreases. This is because the transport rate of reactants limits the reaction rate. Even if more precious metals are added to the catalyst, the pollutants that can participate in the reaction will not significantly increase. Therefore, the heat release tends to be saturated, and the change in ΔT is not significant. A higher total load will increase the thickness of the washcoat and increase the thermal capacity of the catalyst. This means that the catalyst absorbs more heat, while the temperature peak transferred from the exhaust to the SCR is slightly weakened, which may result in a slight decrease in ΔT with increasing PGM.

Under load characteristic conditions, the temperature difference before and after DOC monotonically increases with increasing load at 2000 r/min and 3200 r/min and reaches its maximum value at 100% load. Correspondingly, the maximum temperature differences at 2000 r/min are 25 °C, 45 °C, and 45 °C, respectively, while at 3200 r/min they are 29 °C, 43 °C, and 52 °C, respectively. This indicates that as the load increases, the oxidation reactions of CO, THC, and NO intensify, and the exothermic effect gradually becomes dominant.

Overall, under the same carrier structure conditions, the increase in temperature difference on DOC is related to the enhanced oxidation activity of the catalyst. A higher PGM load promotes the oxidation of NO to NO₂, and the generated NO₂ can further participate in the oxidation of carbonaceous substances, including particulate matter, which is an exothermic process. This additional oxidation heat helps to observe an increase in DOC exhaust temperature. Increasing the loading of precious metals can enhance the oxidation activity of DOC, resulting in higher temperature rise levels under various load conditions. At the same time, the overall temperature difference at rated speed is higher than the maximum torque speed, indicating that under high-speed conditions, the influence of exhaust thermal state and reaction intensity on temperature rise is stronger than the heat loss effect inside the device [30].

3.2. Gaseous Emission Characteristics

Figure 4 shows the temperature-dependent characteristics of CO conversion rates for different DOC samples. It can be seen that as the temperature increases, the conversion rates of CO by the three types of DOC gradually increase, but due to the different loading amounts of precious metals, their temperature rise curves show significant differences. The conversion curve of DOC No. 1 is the smoothest, reaching only 10% conversion rate at 150 °C, and then slowly increasing with temperature. The conversion rate of DOC 2 reached 10% at 143 °C, and its heating response was slightly faster than that of DOC 1. In contrast, DOC No. 3 exhibits significantly low-temperature ignition characteristics, achieving a conversion rate of 10% at 114 °C and complete conversion of CO at 157 °C. Correspondingly, the ignition temperatures T₅₀ of DOC 1, 2, and 3 are 168, 161, and 128 °C, respectively, and the complete conversion temperatures T₉₀ are 185, 169, and 136 °C, respectively, indicating that DOC 3 has the highest reaction rate in the low, medium, and high temperature ranges.

The results shown in Figure 5 indicate that the oxidation performance of CO by DOC with different catalyst loadings is mainly influenced by the combined effects of exhaust temperature and airspeed characteristics. Under external characteristic conditions, as the speed increases, the CO emissions before DOC decrease, mainly due to the increase in exhaust temperature, which makes combustion more complete. Under these conditions, the inlet temperature of the DOC has reached the CO ignition range, indicating that all three sets of DOC exhibit high CO oxidation efficiency. The average conversion rates of DOC with noble metal loadings of 25, 40, and 60 g/ft³ at various speeds reach 62.7%, 72.0%, and 92.6%, respectively, indicating that increasing the noble metal loading can significantly enhance CO oxidation activity.

At the same time, as the rotational speed increases, the exhaust flow rate and carrier air velocity increase, and the residence time of gas in the catalyst shortens, gradually shifting the CO oxidation reaction from temperature control to kinetic limitation, resulting in a decrease in the apparent oxidation efficiency of DOC with increasing rotational speed. In contrast, higher loading of precious metals can maintain higher CO conversion capacity under high altitude conditions by providing more active sites.

Under load characteristic conditions, at 2000 r/min and 3200 r/min, as the load increases, the CO emissions before DOC show a decreasing trend, reflecting the enhancement of combustion temperature and oxidation environment. Correspondingly, the average oxidation efficiency of CO by DOC with precious metal loadings of 25, 40, and 60 g/ft³ under different loads was 52.5%, 60.5%, and 80.7%, and 52.5%, 59.9%, and 68.8%, respectively. Among them, in the low load range, due to the lower exhaust temperature, CO oxidation relies more on the intrinsic activity of the catalyst, and the noble metal loading DOC shows a more significant conversion advantage; In the high load range, temperature factors dominate, and the differences between different load levels are relatively reduced.

Figure 6 shows the effect of DOC-coupled SCR with different noble metal loadings on THC emission characteristics. Under the external characteristic conditions, there is no obvious pattern of THC emissions before DOC changing with speed, while all three sets of DOC show high THC oxidation efficiency. Among them, the average conversion rates of DOC with noble metal loading of 25, 40, and 60 g/ft³ at each speed reach 78.6%, 83.4%, and 88.9%, respectively, indicating that increasing the noble metal loading can enhance the oxidation activity of DOC towards THC.

As the rotational speed increases, the apparent oxidation efficiency of DOC towards THC gradually decreases, mainly due to the increase in exhaust flow rate and carrier air velocity, which shortens the residence time of gas in the catalyst and limits the reaction process kinetically. Under load characteristic conditions, the pre-DOC THC emissions of the engine decrease with increasing load at 2000 r/min and 3200 r/min, reflecting the enhancement of combustion temperature and oxidation environment. Correspondingly, under the condition of 2000 r/min, the average oxidation efficiency of THC by DOC with noble metal loadings of 25, 40, and 60 g/ft³ was 62.3%, 66.2%, and 74.2%, respectively; At 3200 r/min, the average oxidation efficiencies of DOC 1, 2, and 3 were 43.6%, 56.4%, and 61.0%, respectively.

But as the engine speed increases, the exhaust gas temperature rises due to the increase in combustion frequency and the accumulation of heat release per unit time. The increase in cylinder and exhaust temperatures enhances the integrity of fuel oxidation, thereby reducing the formation of incomplete combustion products such as CO and THC at the engine output position. In addition, higher exhaust temperatures promote the oxidation of CO and unburned hydrocarbons in the cylinder and exhaust manifold before reaching the DOC. This thermally driven oxidation process becomes more effective at higher engine speeds, further contributing to the observed reduction in CO and THC emissions before DOC.

Overall, the catalytic oxidation performance of DOC on THC is determined by the noble metal loading, exhaust temperature, and airspeed characteristics. Increasing the loading of precious metals can significantly improve the THC conversion capacity under low load and high airspeed conditions, while under high load conditions, temperature factors dominate and the efficiency differences between different loading levels are relatively reduced.

Figure 7 shows the NO_X emission characteristics under external characteristic conditions. From Figure 7a, it can be seen that the pre-DOC NO_X emissions of the engine show a trend of first increasing and then decreasing with increasing engine speed. Although the measured NO_X concentration after the DOC + SCR system decreases by 10.3%, 15.7%, and 17.6% at various engine speeds, this result should be interpreted as the overall response of the coupled aftertreatment system rather than a direct indication that the DOC itself removes total NO_X efficiently. From Figure 7b, it can be seen that the engine NO_X is mainly composed of NO, accounting for over 90%. Its trend with the change in engine speed is consistent with the total amount of NO_X. After DOC, NO decreases by 15.6%, 33.0%, and 28.6%, respectively. This suggests that the main role of the DOC is to promote the oxidation of NO to NO₂, thereby changing the NO/NO₂ distribution at the SCR inlet and further influencing the overall NO_X conversion behavior of the coupled system.

From Figure 7c,d, it can be seen that DOC significantly promotes the conversion of NO to NO₂, and with the increase in precious metal loading, the production of NO₂ and its proportion in NO_X significantly increase. Among them, the production of NO₂ increases by 105.6%, 210.8%, and 113.8% respectively, and the proportion of NO₂ increases by 5.1%, 18.3%, and 12.0% respectively. In the range of 1000–2000 r/min, the generation rate of NO₂ accelerates, mainly due to the decrease in CO and THC emissions, which weakens its competition with NO on the active sites of the catalyst and makes NO oxidation reaction more likely to occur [31].

When the speed is further increased to 2000–3200 r/min, although the exhaust temperature increases, the rate of NO₂ generation actually decreases, even lower than the pre-DOC level. This phenomenon may be related to the shortened gas residence time in the catalyst at high engine speeds, together with the combined effects of NO oxidation and NO₂ consumption/decomposition related reactions, which limit the increase in NO₂ formation under these conditions.

Figure 8 shows the NO_X emission characteristics under different load conditions at 2000 r/min. From Figure 8a, it can be seen that the NO_X emissions before DOC of the engine increase with an increase in load. The change in NO_X emissions before and after DOC is not significant under medium and low loads. However, under high load conditions, with the increase in precious metal load, the NO_X emissions after DOC show a more obvious decreasing trend.

As shown in Figure 8b, NO emissions increase with increasing load. Under low to medium load conditions, the emission reduction effect of DOC on NO is limited, and even slightly increased. Under high load conditions, the reduction in NO emissions is significantly enhanced. At 100% load, DOC with precious metal loadings of 25, 40, and 60 g/ft ³ showed a decrease of 33.3%, 40.1%, and 39.4% in NO emissions, respectively. This is mainly related to the lower exhaust temperature under low load, which is not conducive to the NO oxidation reaction.

As shown in Figure 8c,d, NO₂ emissions increase with increasing load, and their proportion in total NO_X shows a trend of decreasing first and then increasing with load. At 100% load, the production of NO₂ after DOC with noble metal loadings of 25, 40, and 60 g/ft³ increased by 86.0%, 157.6%, and 111.8%, respectively, indicating that the NO oxidation process becomes more favorable under relatively high-load and high-temperature conditions. At the same time, the non-monotonic variation with load suggests that the NO₂ response is also affected by the combined influence of catalytic activity, reactant competition, and residence time, rather than by temperature alone.

Figure 9 shows the NO_X emission characteristics under different load conditions at 3200 r/min. As shown in Figure 9a, the NO_X emissions before DOC of the engine increase with the increase in load. Under high load conditions, with the increase in precious metal load, the NO_X emissions after DOC show a relatively decreasing trend. Under some low load conditions, there is a slight increase in NO_X after DOC, which is due to the partial oxidation of NO to NO₂ under lower exhaust temperature conditions. In this operating range, the SCR catalyst has not yet entered the high-efficiency operating temperature window, and DOC mainly changes the composition of NO_X rather than reducing its total amount, resulting in a slight increase in the measured NO_X concentration. With the increase in load and exhaust temperature, the NO₂ generated by DOC effectively promotes the rapid SCR reaction pathway, significantly improving the NO_X conversion efficiency. Under these conditions, NO₂ is rapidly consumed by the SCR catalyst, and the positive effect of DOC promoting NO oxidation on the overall denitrification performance dominates.

As shown in Figure 9b, NO emissions increase with increasing load. Under low to medium load conditions, the emission reduction effect of DOC on NO is limited and even slightly increases. Under high load conditions, the reduction in NO emissions shows a trend of first increasing and then decreasing. At 75% load, the reduction in NO after DOC with precious metal loadings of 25, 40, and 60 g/ft³ is 20.1%, 36.4%, and 38.4%, respectively. This is mainly related to the lower exhaust temperature and limited NO oxidation reaction under low load.

As shown in Figure 9c,d, the emissions of NO₂ and its proportion in NO_X both show a trend of first increasing and then decreasing with increasing load. At 75% load, the production of NO₂ after DOC with noble metal loadings of 25, 40, and 60 g/ft³ increased by 46.6%, 56.4%, and 88.0%, respectively, indicating that under medium to high load conditions, exhaust temperature and catalytic activity jointly promote the conversion of NO to NO₂, while at higher loads, this process gradually becomes limited by reaction equilibrium and shortened residence time.

4. Conclusions

This study employed an engine test bench using a 1.91 L light-duty diesel engine to investigate the effects of DOC coupled with SCR at different catalyst loading levels on diesel engine emission characteristics. The main conclusions are as follows:

(1)

Under the same substrate geometry and coating structure, increasing the DOC loading generally leads to higher exhaust backpressure under most operating conditions, indicating that higher catalyst loading increases system flow resistance. Under load characteristic conditions, the maximum pressure differences in the DOC + SCR system with catalyst loadings of 25, 40, and 60 g/ft³ reached 3.3, 3.6, and 4.3 kPa, respectively, at 100% load. In contrast, no obvious monotonic relationship was observed between the DOC temperature rise and catalyst loading, suggesting that the DOC temperature difference is mainly governed by engine operating conditions and exhaust thermal state.

(2)

Higher DOC loading enhances the oxidation activity toward CO and THC. The 60 g/ft³ DOC showed the highest CO and THC conversion capability over the tested conditions, while the influence of catalyst loading was more significant at medium-to-low loads and became less pronounced at high loads.

(3)

Higher DOC loading enhances NO oxidation capacity. Under external characteristic conditions, elevated engine exhaust temperatures maximize NO₂ formation after the DOC. At 2000 r/min, the amount of NO₂ produced after the DOC increased by 86.0%, 157.6%, and 111.8% for precious metal loadings of 25, 40, and 60 g/ft³, respectively. At the rated speed of 3200 r/min, the amount of NO₂ produced after the DOC increased by 46.6%, 56.4%, and 88.0% for precious metal loadings of 25, 40, and 60 g/ft³, respectively.

(4)

From a practical application perspective, the selection of DOC loading should consider not only oxidation performance and exhaust flow resistance, but also the potential catalyst cost and the possible influence on engine fuel consumption. Although 60 g/ft³ provides the strongest oxidation activity for CO and THC, it also results in the highest backpressure penalty and requires a higher catalyst coating amount. Considering emission improvement and system penalty together, the 40 g/ft³ DOC exhibits the best overall balance under the tested operating conditions and can be regarded as a more practically favorable catalyst loading in this study. In applications where oxidation activity is prioritized over cost and flow resistance, a higher loading, such as 60 g/ft³, may still be preferred.

Overall, increasing the DOC precious metal loading improves the oxidation of CO, THC, and NO, although the effect depends strongly on engine operating conditions. More importantly, the results suggest that appropriate catalyst loading optimization can help achieve a better balance among pollutant reduction, exhaust backpressure, catalyst utilization, and fuel economy. Therefore, optimizing DOC loading in DOC-SCR systems is beneficial not only for improving diesel emission control performance but also for supporting the sustainable development of cleaner and more efficient diesel engine technologies. Since this study was conducted only on a single light-duty diesel engine under typical steady-state conditions, further work is still needed on transient validation, catalyst characterization, long-term durability, and DOC-SCR synergistic optimization.