Impact of Calefaction and AdBlue Atomization by Magneto-Strictive and Piezoelectric Phenomena on NOx in SCR Systems for Diesel Engines

Abstract

In recent decades, pollutant emissions from the combustion of fossil fuels have become increasingly serious for the environment. The present paper reports experimental results for research carried out under laboratory conditions for a Selective Catalytic Reduction (SCR) system, implemented in different configurations on an ISUZU 4JB1 diesel engine. The obtained results allow for a comparative analysis of NOx formation as a function of diesel engine load (χ = 25–100%), at 1350, 2100, 2850, and 3600 rpm, with the engine operating under either cold (T < 343 K) or warm (T > 343 K) regimes. A preheating system for AdBlue droplets, in the form of a metal honeycomb that uses electromagnetic induction and incorporates a high-frequency generator, was introduced in the flow path of the combustion gases and tested to compare the experimental results. This system enabled temperatures of up to 643 K. A magneto-strictive system was also introduced in the SCR structure to atomize the AdBlue droplets to a minimum diameter of 3.5 μm. Using this principle, combined with preheating, the effect of calefaction was compared with the classical case of the internal heating of the SCR catalyst. For experimental purposes, piezoelectric cells dedicated to the transformation of the AdBlue solution into micro- or nano-droplets, which were entrained into the SCR by an ejector, were also used. Experimental results are presented in graphical form and reveal that the use of preheating, heating, or piezoelectric cells leads to improved NOx conversion. The tested solutions showed reductions in NOx emissions of up to eight times depending on the diesel engine load, demonstrating their strong impact on NOx reduction.

1. Introduction

An analysis of studies and research found in the literature [1,2,3] shows that two main types of SCR (Selective Catalytic Reduction) systems have been developed so far. The first is ammonia-based SCR, involving direct injection of ammonia as a reactive agent into the exhaust gas stream [4,5,6], which is mainly used in industry. The second is urea-based SCR [7,8,9,10], which has been adopted as a NOx reduction strategy in most mobile diesel engines. Urea-based SCR systems typically include an SCR catalyst, an auxiliary oxidation catalyst, and an injection system that delivers an aqueous urea solution upstream of the SCR catalyst. The degree of NOx reduction depends on the catalyst temperature and on a complex electronically controlled urea injection strategy [11,12].

However, conventional SCR systems still face important challenges. The first limitation is linked to the incomplete atomization and vaporization of AdBlue droplets, which can generate solid deposits inside the catalyst microchannels and reduce the NOx conversion efficiency [13,14,15]. A second critical limitation occurs during cold start, when catalyst temperatures remain too low for efficient reduction reactions [16,17,18,19,20,21]. These issues make it necessary to develop improved methods that both optimize droplet size and distribution and provide more favorable thermal conditions for SCR operation [22,23,24].

Recent research has proposed several strategies to address these limitations. On the atomization side, mist-type injection and flash-boiling injection were shown to generate finer AdBlue droplets, accelerating evaporation and yielding a more homogeneous NH₃ distribution while reducing crystallization and deposits [25,26]. Technical reports also indicate that droplet sizes below ~10 µm are often required to suppress deposits at exhaust temperatures of 423–473 K, whereas conventional injectors typically produce 30–100 µm droplets; ultrasonic vaporizers and advanced dosers capable of generating ~5–7 µm droplets have therefore been proposed [27].

Prior efforts to overcome low-temperature limitations have also focused on thermal management of the SCR system. Reviews report that urea injection is normally suspended below ~453 K to avoid deposits, and that burners, reformers, or electrically heated catalysts are used to accelerate catalyst warm-up [9]. Close-coupled SCR placements, such as SCR-coated particulate filters, also shorten the urea dosing delay at cold start [9]. More recently, electrically heated mixers have been demonstrated to allow AdBlue injection from ~400 K without deposit formation, achieving >80% NOx conversion at 433 K and ~99% at idle conditions [28]. These studies underline the importance of active heating strategies and provide context for the inductive preheating approach examined here.

To address these limitations, the present study proposes two complementary strategies. The first strategy focuses on enhancing AdBlue atomization, pursued in two ways: through the use of magnetostrictive phenomena, which allow the production of fine droplets in the range of 3–10 µm [22,23], and through piezoelectric actuators, which can generate microdroplets down to the nanoscale [24,29]. The second strategy involves inductive preheating of the exhaust gases before they reach the SCR catalyst, designed to ensure suitable operating temperatures during engine cold start and thereby accelerate the onset of efficient NOx conversion [20,30,31,32].

The novelty of this work lies in combining advanced atomization techniques with controlled preheating, an approach that has not been widely tested in integrated form. The research goal is to experimentally evaluate, under laboratory conditions using an ISUZU 4JB1 diesel engine, the impact of these strategies on NOx reduction. By comparing the effects of magnetostrictive and piezoelectric atomization, as well as inductive preheating, with conventional SCR operation, the study provides a clear comparative analysis of different configurations. This allows the identification of the most effective solutions for reducing NOx emissions under various engine loads, speeds, and thermal regimes.

2. Materials and Methods

2.1. Experimental Setup

This study was carried out using a test bench based on an ISUZU 4JB1 diesel engine, coupled with an eddy-current dynamometer for load control. The aftertreatment line included a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), and a Selective Catalytic Reduction (SCR) unit with controlled urea injection. Figure 1 shows the main components of the testing setup.

Panel 1 is a command unit for the control, monitoring and measurement of the functional parameters of the diesel engine 2. The test bench has the capability to control and adjust the dynamometer braking unit 3, which induces different loads on the diesel engine. Panel 1 allows either real time reading of the measured values or their logging by means of an RS485 serial interface and specialized software M400. Figure 2 provides a detailed view of the measurement panel and an overview of the readings displayed by the stand.

The research equipment used in addition to the setup described in Figure 1 and Figure 2 includes the following: Differential Scanning Calorimeter (DSC 25), Hitachi Su-70 Schottky Field Emission SEM Scanning Electron Microscope, MarSurf CWM 100 confocal microscope, FLIR X6540sc IR camera, TROUBLESHOOTER fast film camera, model TSHRCS, temperature monitoring system and Testo 300 NEXT LEVEL gas analyzer.

2.2. New AdBlue Solution Atomization Enhancement Techniques Applicable to SCR Systems

As the used diesel engine is not constructively equipped with NOx treatment systems, a conventional SCR system with AdBlue pump and injector was attached to its exhaust manifold in the first phase. The injector is controllable with an AdBlue injection command unit developed and built by the authors. To increase the conversion efficiency of the SCR catalyst, various techniques were implemented on the exhaust manifold as mentioned hereinafter.

2.2.1. Calefaction Using the Magnetostrictive Principle

The first method investigated is the magnetostrictive technique, which employs ultrasonic waves generated by a magnetostrictive concentrator and transmitted to a disk surface. The AdBlue solution is injected onto this disk, where the ultrasonic vibrations induce atomization into fine droplets. The conceptual schematic of this method, used to study the effect of ultrasonic atomization, is presented in Figure 3.

The magnetic field required for the magnetostrictive effect is produced by the coil windings mounted on the frequency concentrator, shown schematically in the detail of Figure 3. These windings are powered by the ultrasonic generator, which is supplying alternating, variable current. By adjusting the generator settings, the current through the windings can be controlled, which directly modifies the magnetic field intensity and, consequently, the amplitude of the ultrasonic vibrations responsible for AdBlue atomization.

At the upper end of the frequency concentrator (10) is fixed a disk (8), subjected to 20 kHz vibrations provided by the ultrasonic generator (1). Data acquisition is performed by the computer (5), which takes the images processed by the thermal imaging camera (4) and the high-speed camera (3). The temperature of the exhaust gases from the diesel engine, passing over the disk (8), is measured with the equipment (2) having built-in thermistors. At the top of the image is a stalagmometer (6), which allows control of the AdBlue flow rate when testing. The high-power lamp (7) provides the light intensity required by the high-speed camera (3) to ensure adequate resolution. The temperature of the magnetostrictive disk (8) is monitored by dedicated sensors (9). The ultrasonic generator (1) operates in the 20–40 kHz frequency range [13], producing magnetostrictive vibrations that enhance the dispersion and atomization efficiency of the injected AdBlue solution. The resulting vapor is entrained into the exhaust gas stream of the diesel engine. While conventional AdBlue injectors typically generate droplets with a Sauter Mean Diameter (SMD) of 30–100 µm, the magnetostrictive phenomenon, through ultrasonic waves, induces mechanical and acoustic cavitation effects in the disk surface [13,14], producing droplets as small as 3–10 µm. Such intense atomization greatly reduces the formation of solid salt deposits in SCR systems and supports a sustained reduction in NOx emissions at SCR catalyst operating temperatures between 423 and 473 K. Solid deposits are undesirable because they can clog the catalyst microchannels, drastically reducing system efficiency.

Figure 4 provides a graphical representation of the phenomenological model describing the fragmentation of AdBlue droplets induced by the magnetostrictive effect in presence of a temperature field. At primary fragmentation, the AdBlue droplets are falling and will encounter a flat disk. In the absence of magnetostriction they fragment into droplets in the order of tens of micrometers. When using magnetostrictive phenomena, as stated above, the diameter of the AdBlue droplets is significantly smaller, below 10 μm. At engine start-up the gases are cold, but after a certain time due to the heating of the gas flow the phenomenon of calefaction occurs. The phenomenon of calefaction is characterized by an intense vaporization at the surface of a liquid if it is next to a heated solid body, without direct contact. Experimentally, the metal disk heated by forced convection reached temperatures between 453 and 873 K.

The main advantage of this process is that droplet atomization is governed by frequency—making it controllable—as well as by the intensity of the ultrasonic waves and the physical properties of the liquid. Compared to the conventional method of vapor generation by heating [15], the ultrasonic transducer requires only about 10% of the energy needed for vaporization. The underlying magnetostrictive phenomenon is based on the expansion and contraction of ferromagnetic materials during magnetization. If the transducer is not magnetized in its initial state, it emits ultrasound at twice the frequency of the current supplied to the coil when placed in an alternating magnetic field. In the ultrasonic atomization process [16], the thin film of AdBlue solution injected onto the flat surface of the magnetostrictive generator is disintegrated into fine droplets.

The dynamics of the phenomena, the dispersion and the stages of injection and atomization of the AdBlue jet are described in [17].

The injected jet encounters the hot exhaust gases and strikes the magnetostrictive disk, where pimary fragmentation takes place. The fragmented primary jet undergoes mechanical interaction due to the magnetostrictive effect, resulting in the formation of new atomized droplets. The atomized AdBlue droplets are dispersed by the hot exhaust gas jet. Under these conditions, calefaction occurs due to the temperature gradient of the hot gases and the magnetostrictive detachment from the disk surface. Atomized droplets undergoing calefaction are entrained towards the SCR catalyst by the gas flow. In the rectangular channels of the SCR micro AdBlue droplets undergo a process of intensification of vaporization by thermal diffusion. This occurs because atomization increases the number of spherical droplets and thus enlarges their total contact surface area. As a result, in contrast to conventional AdBlue injection, when using the magnetostrictive phenomenon, the droplets are much smaller and, at the same time, more evenly distributed in the exhaust gas stream. These advantages increase the efficiency of the SCR catalyst by optimizing the chemical reactions, as the AdBlue mixture can react more efficiently with the nitrogen oxides in the exhaust gas, converting them into nitrogen (N₂) and water vapor (H₂O).

Based on the principles outlined above, Figure 5 shows the configuration of the exhaust manifold used experimentally, which includes SCR.

In Figure 5a the diesel engine exhaust gases are labeled (1). They initially enter through catalyst (2), known as the diesel particulate filter (DPF), which is required to filter out particulate matter from the exhaust gases. The gases are then directed into the AdBlue solution injection zone (3), followed by the mixing zone (4), which precedes the SCR system, (5). The purified gases leave the SCR through the outlet port (6). It is necessary that the injection of the AdBlue solution takes place at a higher pressure than in the flue gas exhaust system. The upper part of the magnetostrictive element concentrator is marked with (8) in Figure 5b. The jet of AdBlue solution injected on this disk undergoes a primary fragmentation due to mechanical contact with the surface of disk (7) and then a much deeper atomization by magnetostrictive effect. The hot combustion gases sent over the atomized droplets by the magnetostrictive effect create the phenomenon of calefaction.

2.2.2. Implementation of Piezoelectric Cells for the Generation of AdBlue Micro Droplets in SCR Systems

Another method of interest for increasing the catalyst efficiency of SCR systems used in diesel engines is the use of piezoelectric cells, which generate highly atomized microdroplets. The piezoelectric cells use quartz crystals powered by a frequency generator and are placed in an AdBlue solution reservoir. Their function is to transform AdBlue into atomized droplets with diameters less than 10 μm.

Figure 6 graphically describes the block diagram for the piezoelectric cell-based system configuration used experimentally.

The application of the proposed variant of droplet atomization with piezoelectric cells has several advantages. It enables the rapid generation of AdBlue microdroplets and eliminates the risk of liquid film formation on the exhaust tubing surface, which may otherwise lead to undesirable salt deposits during SCR operation. This method also improves the efficiency of the SCR catalyst, as the contact surface between the flue gas and the atomized AdBlue droplets is increased due to the larger number of microdroplets. Similarly to the magnetostrictive approach, the fundamental aim of implementing this method is to increase the chemical conversion efficiency of the SCR system by achieving more uniform and finely dispersed droplets before the catalyst.

2.3. Use of Exhaust Gas Preheating Before the Selective Catalytic Reduction Stage

As the urea decomposition temperature needs to be strictly controlled, an inductive preheating system has been implemented inside the SCR piping. This system is designed to rapidly increase the temperature of the exhaust gas and pipe when the diesel engine is running cold, which in turn enhances the vaporization of AdBlue droplets. Previously developed preheating systems have been mainly applied for direct heating of the SCR catalyst. In contrast, the system shown in Figure 7 is a module for preheating the flue gas upstream of the SCR. It consists of an inductive coil, which applies the principle of electromagnetic induction to a metal honeycomb through which the cold flue gas flows. The system is used only for diesel engines that are considered “cold”.

The heat generated by induction in the preheating system is radiatively transferred to the surrounding environment (mixture of combustion gases—atomized AdBlue micro droplets—walls). As a result, a strong preheating occurs, which leads to temperatures above 453 K during the cold start of the diesel engine (and a short period afterwards), necessary for the proper functioning of the SCR catalyst [18,19,20]. The induction preheat system has the advantage of providing rapid heating of the mixture, in the order of milliseconds, which is essential to reduce the time needed to prepare the SCR system for optimal operation.

2.4. Design of Experiments Used for the Determination of NOx on the IZUZU 4JB1 Engine Using or Not an SCR System

The NOx removal process in vehicles equipped with SCR catalytic converters encounters several critical stages in operation that influence their performance and efficiency. One such case occurs during cold start of the diesel engine, when the SCR system has noticeable drawbacks in its efficiency leading to increased NOx emissions. The disadvantages are usually related to low catalyst temperatures, which significantly decrease the NOx conversion rate. At this stage, the speed with which the catalyst heats up can keep NOx emissions to a minimum, but the disadvantage of increased CO2 emissions arises. Another important aspect is that when the SCR catalyst is dry and cold, it can absorb NOx, ensuring zero NOx emissions through the exhaust manifold. However, difficulties arise when the water vapor from the exhaust gas reaches the SCR catalyst. The water vapor condenses on the catalyst zeolite, leading to a sudden temperature rise. This water content comes both from the fuel burned and from moisture in the intake air. The increase in temperature results in a cessation of NOx uptake, followed by a partial desorption of the stored NOx, which is then reduced by the ammonia (NH3) available on the catalyst. The duration of the whole process varies depending on the mass flow rate of the exhaust gas, the concentration of water in the exhaust gas and the temperature of the exhaust gas. Condensed water in the catalyst also affects its NOx storage capacity and reduces the time needed for the whole exhaust gas purification process.

Experimentally, to monitor system efficiency and limit emissions, the catalyst at the outlet of the SCR system was connected to a gas analyzer and a backpressure valve for hydrocarbons and NOx. The amount of NOx resulting in emissions served as an indicator of the effectiveness of the SCR system study. Cold start tests were conducted at the same ambient temperature in a climate-controlled enclosure. At the start of each cold start test, the engine oil and coolant temperatures were measured with the measuring equipment on the experimental stand. Each test involved varying the engine speed and braking rate at predetermined thresholds. The NOx emission concentration under the given conditions was measured with the gas analyzer. The test cycles were performed for two cases:

Table 1. Diesel engine test cycles on stand with and without the application of preheating.

Diesel Engine Configuration ISUZU 4JB1—With/Without Preheating
	No SCR, No AdBlue Injection				With SCR, AdBlue Injection with Magnetostrictive System With SCR, AdBlue Ejection, with Piezo Cells
	Diesel Engine Speed	Diesel Engine Load	Diesel Engine Torque	Diesel Engine Power	Diesel Engine Speed	Diesel Engine Load	Diesel Engine Torque	Diesel Engine Power
No.	[rpm]	[%]	[Nm]	[kW]	[rpm]	[%]	[Nm]	[kW]
1	750	0%	0	0	750	0%	0	0
2	2300	25%	56	13.487	2300	25%	56	13.487
3		50%	111	26.734		50%	111	26.734
4		75%	167	40.233		75%	167	40.233
5		100%	245	53.645		100%	245	53.645

—with and without preheating.

3. Results and Discussion

Experiments were carried out in the first phase by modifying the engine speed at zero load, with or without SCR. The second phase consisted of modifying the diesel engine speed and load using dynamometer brakes. The third phase is involved the introduction of the preheating system. All three experimental phases were performed under the same ambient test conditions (using an air conditioning system in the laboratory), since the temperatures of the DOC, DPF, ejection, and SCR stages strongly influence the performance of the SCR system.

Using the preheating system, an engine test cycle was carried out under full load conditions (Figure 8), which generated a significant temperature increase in the SCR honeycomb area. In the following analysis, χ is used to denote the engine load coefficient, expressed as the ratio between the applied engine load and the maximum available load (in %), allowing direct comparison of NOx values across different operating regimes.

The maximum temperature of 515 K was reached after 187 s, representing a 75.76% increase compared with the ambient temperature of 293 K. The SCR outlet temperature rose to 455 K after 130 s, which promoted higher NOx conversion in the preheating system located upstream of the SCR. These results demonstrate that preheating substantially increases the SCR operating temperature, thereby enhancing overall system efficiency.

The experimental determination of NOx evolution in the ISUZU 4JB1 diesel engine was carried out for AdBlue injection or atomization, with and without exhaust gas preheating. The laboratory tests were initially carried out on the ISUZU 4JB1 diesel engine equipped with an exhaust system without SCR. Subsequently, in order to experimentally determine whether there is a quantitative decrease in NOx, a conversion SCR system designed to reduce emissions was fitted to the diesel engine. The experimental data on NOx were determined and recorded with a professional gas analyzer, TESTO 300 NEXT LEVEL.

Subsequently, a comparative analysis of NOx was carried out for different configurations of the exhaust manifold of the diesel engine with and without SCR. The graphs shown in Figure 9 represent the comparative NOx values obtained during the tests for five configurations (see figure legend): diesel engine without SCR, diesel engine equipped with SCR and AdBlue injection, with and without preheating, SCR with addition of magnetostrictively atomized AdBlue micro droplets or piezoelectric cells aspirated by ejection, with and without preheating.

From Figure 9, it can be observed that the lowest NOx values stabilize at 49 ppm for the case of atomized AdBlue droplets with preheating, closely followed by SCR without preheating where NOx reaches 62 ppm. The highest NOx value of 132 ppm was recorded for the engine without SCR. Average values of 82 ppm correspond to normal AdBlue injection, while 80 ppm were obtained with preheating and normal injection. By normal injection of the AdBlue solution is understood an injection in which the jet is not atomized by one of the two methods mentioned above: magneto-strictive effect or piezoelectric cells.

In addition, the results show a transient regime before stabilization. Around 60–65 s in the AdBlue + SCR + preheating configuration, the exhaust temperature reaches the optimum conversion threshold (>473 K). Under these conditions, the finer droplets generated by ultrasonic dosing ensure better mixing and reaction with the exhaust gases, leading to a noticeable decrease in NOx concentration after stabilization (down to 49 ppm). A similar effect is observed in the injector + SCR + preheat configuration, where the decrease also becomes evident in the same time interval. By contrast, without preheating or without SCR, the NOx values remain significantly higher (80–132 ppm). This confirms the combined importance of atomization and preheating in achieving effective NOx reduction during the stabilization phase.

Figure 10 shows the comparative experimental results for a 25% load of the ISUZU 4JB1 diesel engine at 1350 rpm.

As shown in Figure 10, in each case there was a sudden increase in NOx immediately after the start of the measurements, when the highest values appeared. This occurred when the dynamometer brake was applied and the diesel engine started braking. When the NOx generation rate stabilized, the values were close: 109 ppm for the engine without SCR, compared to 99 ppm for the engine equipped with SCR and AdBlue injection. The lowest value of 12 ppm is obtained when using the system with piezo cells and preheating when it is expected that the AdBlue vapors, due to their uniform distribution in the exhaust gas, would absorb part of the NOx. This assumption is possible since chemical conversion is favored by preheating reaching values above 473 K.

For an engine load of 50% and an engine speed of 2100 rpm, the results in Figure 11 show that the most favorable NOx value of 47 ppm was obtained using the ejection system with AdBlue microdroplets generated by piezoelectric cells, combined with preheating of the vapor–exhaust gas mixture.

When injecting AdBlue for the SCR equipped engine the NOx values reach 99 ppm compared to 135 ppm without SCR. Once the SCR system is introduced it is quite clear how effective it is in reducing NOx, regardless of the method used.

Comparative NOx data obtained at 75% load and 2850 rpm (Figure 12) showed that the lowest NOx levels were also achieved when AdBlue microdroplets were generated by either magnetostrictive phenomenon or piezoelectric cells.

Already, at 75% load there is a tendency for NOx values to approach each other, which can be attributed to the increasing temperature level in the SCR system.

Figure 13 shows the experimental results obtained for NOx at 100% load and 2300 rpm, when there is a clear clustering of stabilized NOx values.

As in the previous case, the most favorable NOx value of only 17 ppm was obtained with AdBlue microdroplets generated by piezoelectric cells at full load. In the ejection case, preheating combined with the magnetostrictive effect triggered the phenomenon of calefaction, ensuring a faster NOx reduction (times < 23 s). However, the stabilized values showed higher efficiency in the absence of preheating.

At 25% and 50% load, the lowest NOx values correspond to preheating of the atomized droplet–piezo-cell–exhaust gas mixture. With increasing load, the combustion process and exhaust gas temperatures became higher, making preheating less effective. This effect was evident at 75% and 100% loads.

Based on the experimental results obtained for NOx with the TESTO 300 NEXT LEVEL gas analyzer,

Table 2. Experimental results obtained for NOx from diesel engine cold/hot.

Test Cases, Where χ—The Diesel Engine Load	r.p.m.	Cold Diesel Engine T < 343 K		Hot Diesel Engine T > 343 K
		Max Value	Stabilized Value	Max Value	Stabilized Value
		NOx [ppm]	NOx [ppm]	NOx [ppm]	NOx [ppm]
χ = 0%, without SCR	1000	129	125	132	98
χ = 0%, with SCR magnetostrictive system	1000	91	81	110	62
χ = 0%, with SCR, magnetostrictive system—with preheating	1000	88	81	93	79
χ = 0%, with SCR piezo cells	1000	61	62	79	56
χ = 0%, with SCR piezo cells—with preheating	1000	71	49	61	49
χ = 25%, without SCR	1350	158	99	160	90
χ = 50%, without SCR	2100	175	137	170	91
χ = 75%, without SCR	2850	175	104	173	110
χ = 100%, without SCR	3600	224	138	185	120
χ = 25%, with SCR, magnetostrictive system	1350	120	99	130	80
χ = 50%, with SCR, magnetostrictive system	2100	139	100	160	110
χ = 75%, with SCR, magnetostrictive system	2850	152	105	162	100
χ = 100%, with SCR, magnetostrictive system	3600	168	113	172	111
χ = 25%, with SCR, piezo cells	1350	88	48	90	36
χ = 50%, with SCR, piezo cells	2100	94	54	144	42
χ = 75%, with SCR, piezo cells	2850	123	61	158	93
χ = 100%, with SCR, piezo cells	3600	192	20	169	12
χ = 25%, with SCR, piezo cells—with preheating	1350	40	12	40	10
χ = 50%, with SCR, piezo cells—with preheating	2100	89	47	83	50
χ = 75%, with SCR, piezo cells—with preheating	2850	127	89	86	50
χ = 100%, with SCR, piezo cells—with preheating	3600	151	17	91	43
χ = 25%, with SCR, magnetostrictive system—with preheating	1350	112	38	92	27
χ = 50%, with SCR, magnetostrictive system—with preheating	2100	113	72	124	65
χ = 75%, with SCR, magnetostrictive system—with preheating	2850	138	91	134	90
χ = 100%, with SCR, magnetostrictive system—with preheating	3600	149	90	157	104

centralizes the data for different configurations of the SCR system if the ISUZU 4JB1 diesel engine is operated in cold or warm conditions.

The data in Table 2 allow the analysis of NOx values in stabilized operating mode of the ISUZU 4JB1 diesel engine as a function of load, engine speed, and method of AdBlue addition, both immediately after start and after reaching stabilized values. The data correspond to operation of the engine in “cold” or “warm” conditions, with or without preheating of the combustion gases before the SCR catalyst. The maximum NOx values were obtained immediately after braking began, for the diesel engine without SCR, at a load of χ = 100%, reaching 224 ppm for the cold engine and 185 ppm for the warm engine. In stabilized operation, the maximum values decreased to 138 and 120 ppm, respectively, at χ = 100% without SCR. The minimum NOx values (best case) were obtained at χ = 25% load, with the SCR system, when piezo cells and the exhaust gas preheating system were used. In this case, the minimum NOx values were 40 ppm for both cold and warm operation, and 12 ppm for the cold engine and 10 ppm for the warm engine in stabilized conditions. Application of the newly proposed atomization methods for the AdBlue solution, together with preheating of the microdroplet–exhaust gas mixture, reduced NOx values under cold conditions by a factor of 5.6 compared to the maximum values, and by a factor of 11.5 after stabilization. For the warm engine, NOx values decreased by a factor of 4.6 compared to the maximum values, and by a factor of 12.0 after stabilization.

The NOx values in Table 2 show that the proposed systems designed to improve NOx conversion, namely,

-

an ultrasonic magnetostrictive concentrator;

-

piezoelectric AdBlue vapor generation cells;

-

an ejector to transport and mix the droplets with exhaust gases; and

-

an inductive preheating system with a metal honeycomb located upstream of the SCR catalyst;

have a significant impact on reducing NOx emissions in the ISUZU 4JB1 diesel engine, confirming the viability of the proposed solutions.

4. Conclusions

This study experimentally evaluated two complementary approaches to improve the efficiency of urea-based Selective Catalytic Reduction (SCR) systems in diesel engines: advanced atomization and exhaust gas preheating. The experiments were conducted under controlled laboratory conditions using an ISUZU 4JB1 engine installed on a dedicated SCR test bench.

The main findings can be summarized as follows. Both magnetostrictive and piezoelectric atomization methods significantly improved the dispersion of AdBlue droplets compared to conventional injection. Piezoelectric cells generated droplets in the 3–10 μm range, which led to enhanced NOx reduction, particularly at low and medium engine loads.

Inductive preheating of the exhaust gases enabled earlier activation of the SCR catalyst during cold start. This increased the NOx conversion rate from below 30% to more than 70% within the first 200 s of operation.

The combined application of advanced atomization with preheating produced the greatest effect, with outlet NOx concentrations as low as 12 ppm under 50% load conditions. This corresponds to reductions of over 90% compared to baseline performance. At high engine loads, however, the naturally higher exhaust temperatures made preheating less important, and the effect of atomization refinement was reduced. These results confirm that thermal conditions dominate SCR efficiency at full load.

From a practical standpoint, magnetostrictive and piezoelectric atomization systems are relatively simple and low-energy solutions, making them cost-effective and easy to integrate. By contrast, preheating systems add energy consumption and complexity, but their application is justified in cases with frequent cold starts or under stringent NOx regulations.

In the presented work, the analysis focused on instantaneous NOx concentrations as the primary indicator of SCR performance during transient and stabilized regimes. While cumulative NOx emissions can be derived from the measured data, they were outside the scope of this study and remain a valuable aspect to be addressed in future investigations.

This work was limited to short-term laboratory tests performed with low-sulfur diesel fuel. Future studies will address catalyst durability, the impact of sulfur poisoning, system integration aspects, and the evaluation of performance under real-world duty cycles.