1. Introduction
Diesel engines have the advantages of a high thermal efficiency and reliability compared to other internal combustion engines. Furthermore, application of the material properties of the fuel and the high-pressure injector technology, Common Rail Direct Injection (CRDI), resulted in the simultaneous increase in these engines’ performance and their market [
1]. However, despite the high thermal efficiency of compression ignition engines, these engines have the disadvantage of producing a large amount of exhaust gas compared to spark ignition engines. Recently, with increasing attention toward the environmental pollution of automobiles, strengthened emissions regulations were adopted, making the installation of additional aftertreatment equipment inevitable [
2,
3]. For diesel engines, three-way catalyst devices cannot be installed due to the low exhaust gas temperature and operating air–fuel ratio; numerous aftertreatment equipment installations are necessary [
4,
5].
Currently, rather than pretreatment, regulations are satisfied through the use of aftertreatment equipment for most of the automobile exhaust gas. Beginning with the adoption of the off-road exhaust gas regulation Euro 6 in 2014, the off-road engine regulation Tier 4 Final was implemented in 2015, requiring the development of exhaust gas aftertreatment technologies [
6]. The existing aftertreatment equipment satisfied the exhaust gas regulations by installing a diesel oxidation catalyst (DOC), diesel particulate filter (DPF), or lean NO
X trap (LNT), which operate without an additional injection system. Installation of selective catalytic reduction (SCR) devices became inevitable on all diesel vehicles with the enforcement of Euro 6 D [
7,
8]. Simultaneously, strengthening of the off-road emissions regulations made SCR installation necessary for diesel internal combustion engines regardless of whether they were on-road or off-road. The biggest reason for the delay in SCR applications is the need to additionally install a urea injection device and management system necessary for the reaction, unlike the conventional catalyst system. The SCR system is evaluated as an environmentally friendly exhaust gas aftertreatment device because it can reduce most of the carbon monoxide and NO
X emissions produced from combustion [
9].
SCR is largely composed of a storage tank, urea injection nozzle, control module, and catalyst. SCR requires a supply of urea for the catalyst reaction; an additional thawing module is necessary due to the risk of freezing in cold weather as the freezing point of urea is −11 °C. The NO
X exchange rate increases for exhaust gas temperatures of 250 °C and higher, facilitating the reaction of the SCR catalyst. The exchange efficiency of the catalyst is proportional to the urea injection amount and catalyst urea distribution. Generally, a mixer is added to the outlet of the injection nozzle to increase the catalyst efficiency and improve the urea distribution and evaporation amount. Double-layered catalyst structure SCR development is being actively carried out to collect residual ammonia from the reaction through AOC coating of the outlet of the catalyst [
10].
The SCR catalysts can be largely categorized into the primary catalyst that the urea and exhaust gas react to and the secondary catalyst with an AOC coating added to prevent the residual ammonia from the reaction with the primary catalyst from entering the atmosphere. Ammonia is an irritant substance that can affect biological tissue. Exposure to 10 ppm or more of ammonia can cause eye and respiratory problems. Thus, along with the adoption of the Euro 6 standard, additional catalyst (AOC coating) installation is mandatory to reduce slip ammonia in the SCR-installed vehicle. For the urea used in SCR, urea is diluted in distilled water (urea 32.5%, distilled water 67.5%) and this aqueous solution is injected for vaporization and interaction in the catalytic reaction. Ammonia, an alkaline substance, has the problem of negatively impacting the catalyst durability when the urea that did not vaporize at the inlet of the catalyst is transported. Accordingly, development companies are carrying out research in urea evaporation amount and distribution improvement by installing a device with a mixer shape between the catalyst and nozzle [
11,
12,
13].
The NO
X uniformity index (UI) results of the SCR catalyst outlet play an important role in the prediction of the catalyst exhaust gas reduction performance and slip ammonia predictions. Most SCR-related studies have conducted SCR part analyses or single tests rather than reflecting actual engine test results [
14,
15]. The aftertreatment device in an actual internal combustion engine has numerous design parameters depending on the engine room space and, in the case of exhaust gas flow testing, the results are significantly influenced by the inlet and outlet design of the catalyst. Additionally, systematical performance prediction and investigation of the aftertreatment system is required. The design layout of the exhaust gas and aftertreatment system should be clearly determined, and the same design has to be used for the numerical analysis to analyze the NO
X emission and ammonia slip under the same conditions.
In this study, SCR performance prediction and improvement were made and examined for the actual Tier 4 Final-applied off-road engine. The determination of the test conditions utilized the exhaust gas temperature and flowrate conditions of the most dominant operating range in the exhaust gas assessment method used in certification testing. The test results were applied to increase the reliability of the governing equation and boundary conditions, and a numerical analysis was additionally conducted under the determined conditions.
In this paper,
Section 2 explains the structure of this paper and
Section 3 details the engine test conditions and numerical analysis boundary conditions and design.
Section 4 discusses the numerical analysis performed for performance enhancement and the NH3 uniformity index result, followed by a summary of the results in
Section 5.
4. Numerical Analysis Results
The flow-path shape of the aftertreatment device is an important variable for the efficiency and durability of the catalyst. The catalyst shear rate uniformity index plays an important role in predicting the uniform reaction of the catalyst. In the case of the SCR, the injected urea and exhaust gas react unlike conventional catalysts. To predict the catalyst reaction and durability of the SCR, the velocity uniformity at the inlet of the catalyst was compared with the NH
3 uniformity index. Using the same aftertreatment device design, 25 cases were conducted for 8 urea injection directions and 3 angles with the same boundary conditions.
Figure 7 shows the urea injection conditions of the numerical analysis. The NH
3 uniformity index results for the SCR mixer outlet and SCR catalyst inlet were investigated according to the urea injection conditions with the same flow-path design.
4.1. Device and Mixer Flow Characteristics
The catalyst inlet flow path and cross section velocity distributions were examined to predict the durability and performance of the aftertreatment device catalyst.
Figure 8 shows the velocity distribution of the inlet of each catalyst. The flow was concentrated at the center in the DOC inflow catalyst inlet; the velocity distribution and flow concentration in a lattice shape were observed at the center due to the effect of the perforated plate. For the DPF, it was observed that the velocity distribution stabilized after passing through the DOC catalyst. The velocity distribution at the SCR inlet stabilized overall after passing through the DOC and DPF catalysts. However, the uniformity, at 0.733, did not significantly improve compared to the case of DPF. This result was thought to be due to the effect of the flow rotation caused by the shape of the flow between the mixer outlet and the SCR inlet. Additional space for stabilization of the SCR catalyst inlet flow or installation of an additional plate are thought to be options that would provide increased flow stability. In this research, additional design changes were not carried out as the velocity distribution and uniformity index were determined to be stable.
While the uniformity index of the exhaust gas inflow to the catalyst is important in enhancing the SCR catalyst reaction, the uniformity index of the urea is also important. The mixer performance for improved urea uniformity was examined. To observe the effect of the mixer, the turbulent kinetic energy distribution is shown for eight cross sections from the injection point up to 400 mm at 50 mm intervals, along with the uniformity index and a streamline, shown in
Figure 9. For the same cross sections, the NH
3 distribution and uniformity index are shown in
Figure 10. After passing through the 120 mm point where the mixer is located, NH
3 developed towards the cross-section center at the 150 mm cross section and the uniformity index began to rapidly increase. As shown for the 200 mm cross section, the turbulent kinetic energy sharply increased at the center and the NH
3 distribution increased in a similar way. It was concluded that the turbulent kinetic energy has an effect on the distribution development, along with the evaporation of NH
3. It was observed that the NH
3 distribution increased up to 0.902 at the mixer outlet. These results are plotted in
Figure 11 to allow trends to be observed.
4.2. Mixer Outlet NH3 Uniformity Index Distribution According to the Urea Injection Angle
Numerical analysis was performed for the 25 cases to observe the NH
3 uniformity index at the mixer outlet according to the urea injection angle for the same mixer and flow path shape. The case of injection in the center direction, used in the reliability evaluation, was set as the reference. The resulting graph is plotted in
Figure 12 to allow comparison of the NH
3 uniformity index at 400 mm of the injection outlet; the resulting values are shown in
Table 3.
As the diameter decreased at the DPF outlet, the main stream was formed diagonally at the injector nozzle inlet. The main stream direction is marked on the graph when looking in the injection direction from the injector. For the condition of injection in the main stream direction, the NH3 uniformity index showed a decreasing trend. This result is thought to be due to the injector energy and main stream energy acting on the injected urea evaporation and flow. When the injected urea main stream direction and injection direction are the same, the NH3 uniformity index appears to decrease as the time to reach the mixer along the flow decreases. When the injection direction is perpendicular to the main stream, the NH3 uniformity index improved for the E and SE directions.
The mixer outlet uniformity index showed a minimum value of 0.828 in the SW direction and maximum value of 0.966 in the E direction. The results for each angle for all directions showed the same trend. Regardless of the injection direction, the uniformity index tended to be high for small injection angle conditions. As energy transfer occurred due to the main stream for 15° in all directions, on average a high NH3 uniformity index was established in comparison to the other angles. The 45° condition was thought to be less affected by the turbulent flow due to the high injection angle compared to the injector nozzle shape.
4.3. SCR Inlet NH3 Uniformity Index Distribution According to Urea Injection Angle
The imbalance of the SCR catalyst inlet uniformity index can increase the probability of NH3 slip occurring and cause a durability imbalance in the catalyst. Numerical analysis for 25 cases was carried out and compared to investigate the NH3 uniformity index at the SCR inlet according to the urea injection angle for the same mixer and flow path shapes. The case with the injection in the center direction, used in the comparison with the experimentation and reliability evaluation, was set as the reference.
Figure 13 and
Table 4 shows the NH3 uniformity index at the SCR catalyst inlet.
The SCR catalyst inlet uniformity index showed results very similar to those of the mixer. It was thought that the initial injection evaporation maintained a high uniformity index until the SCR inlet. When the main stream direction and injection direction were similar, a low NH3 uniformity index was obtained at the SCR catalyst inlet regardless of the injection angle, while a high NH3 uniformity index was obtained when the main stream direction and injection direction were perpendicular to each other.
The maximum NH3 uniformity index at the SCR inlet was 0.981 for the SE direction 15° condition and the minimum uniformity index was 0.954 for the N direction 45° condition. The difference between the maximum and minimum was very small at 0.027. Although the difference appears to be large in the graph plot, it can be observed that the average difference for the injection angles is very small. This result was due to the sufficient space necessary for flow stabilization as the flow slowed down with the expansion of the shape from the mixer outlet to the SCR catalyst inlet.