Inner Selective Non-Catalytic Reduction Strategy for Nitrogen Oxides Abatement: Investigation of Ammonia Aqueous Solution Direct Injection with an SI Engine Model

: This study contributes to a method based on an aqueous solution of ammonia direct injection for NO x emissions control from internal combustion engines. Many previously published studies about deNO x technology are based on selective catalytic reduction (SCR), but only few deal with inner selective non-catalytic reduction (inner SNCR) technology, which is an intensive improvement of selective non-catalytic reduction (SNCR) applied in the in-cylinder puriﬁcation procedure. Before numerical calculations were carried out, the computational ﬂuid dynamic (CFD) simulation model was validated with steady-state experimental results. The main results revealed that with the increasing concentration of aqueous solution of ammonia, nitrogen oxides gradually decrease, and the largest decline of NO x is 65.1% with little loss of cylinder peak pressure. Unburned hydrocarbon (UHC) and carbon monoxide (CO) may increase using inner SNCR, and soot emissions show a decreased tendency. However, there is little change when ammonia content varies. Ulteriorly, reﬁning the direct injection phase is of great help to inner SNCR technology to enhance the reduction of NO x and reduce NH 3 oxidation and NH 3 slipping. oxidation (NH 3 + O 2 → NO + H 2 O), and slipping (not involved in any reactions). It is beneficial to analyze the proportion of different effects to promote the positive effects of ammonia and to inhibit the negative effects of ammonia. The contribution sequence for reduction effect is case 1 > case 2 > case 3. The contribution sequence for oxidation is case 2 > case 1 > case 3. The contribution sequence for slipping is case 3 > case 2 > case 1. The above results can be analyzed as follows: contribution for reduction and slipping are mainly related to the retention time of NH 3 in the combustion chamber and an early DI timing results in longer retention time and further increases the contribution of NH 3 for reduction and decreases the contribution of NH 3 for slipping. However, an early DI timing means that more NH 3 may be oxidized to NO under high cylinder temperature, which weakens the selectivity for deNO x . Additionally, the increase of ammonia composition is favorable to the enhancement of the deNO x effect, but the contribution of NH 3 for reduction is decreased. Thus, ammonia concentration is not likely to increase G.L., and D.L.; software, F.H.; validation, F.H. and Y.D.; formal analysis, F.H.; investigation, F.H.; resources, X.Y.; data curation, F.H.; writing—original draft preparation, F.H.; writing—review and editing, F.H.; visualization, F.H.; supervision, X.Y.; project administration, X.Y.; funding acquisition, X.Y.


Introduction
The Volkswagen emissions scandal once again attracts people's attention to automobile exhaust emissions, especially NO x emissions. With the emission regulations becoming more and more stringent, exhaust treatment technologies for engines need to be upgraded and adjusted. As common technical routes to reduce NO x emissions, two schemes are often mentioned: water injection or water fuel emulsion and selective catalytic reduction (SCR) [1][2][3][4][5]. In this section, these two schemes are briefly reviewed and an innovation scheme is put forward.
Much of the literature shows that both water injection and water emulsion will enhance fuel conversion efficiency and promote NO x emission reduction [6,7]. Parlak et al. [8] developed an electronic controlled steam injection system in the intake manifold for an spark ignition (SI) engine. In the experimental results, it is seen that the engine torque and the effective power increase up to 4.65% at 3200 rpm, specific fuel consumption reduces up to 6.44% at 2000 rpm, and NO emissions reduces 40% on average at 2800 rpm. Yang et al. [9] conducted a detailed experimental study to evaluate the effect of 10% water emulsion diesel on engine performance and emissions. The result shows that lower exhaust gas temperature and lower NO emissions are achieved at all load and engine speed condition for 10% On the basis of reverse engineering, a stereolithography (STL) file for a single cylinder of the commercial engine and its attachment (manifolds and valves) was obtained from 3D point clouds. Figure 1 shows the structure body (A), the surface mesh (B), and the two-dimensional section (C and D) of the structure mesh at 23 • CA after top dead centre (TDC).

Geometric Model and Boundary Conditions
In this work, CONVERGE software [29] with kinds of sub-models was utilized to build and calibrate the engine research model. The virtual engine was modified from a four-cylinder SI engine and the main technical parameters are shown in Table 1. On the basis of reverse engineering, a stereolithography (STL) file for a single cylinder of the commercial engine and its attachment (manifolds and valves) was obtained from 3D point clouds. Figure 1 shows the structure body (A), the surface mesh (B), and the two-dimensional section (C and D) of the structure mesh at 23 °CA after top dead centre (TDC). SAGE, a detailed transient chemistry solver developed by Senecal et al. [30], was chosen as the combustion model for various reaction processes. This model calculated the chemical reaction rate of each step. The more detailed the chemical reaction mechanism used in the combustion model, the closer to the actual combustion is the simulation process in the cylinder. However, in fact, the calculating speed will slow down using the detailed mechanism. Taking account of saving computational time, it is necessary to simplify the reaction mechanism according to the recorded experiment results in the previous literature.
The mechanism used in this work is composed of three parts, involving gasoline surrogate mechanism, NOx generation mechanism, and ammonia deNOx mechanism. The primary reference fuel (PRF) mechanism [31] including 41 species and 124 reactions is appropriate for gasoline surrogate. This PRF mechanism has been validated in a series of experiments, such as shock tube, jet SAGE, a detailed transient chemistry solver developed by Senecal et al. [30], was chosen as the combustion model for various reaction processes. This model calculated the chemical reaction rate of each step. The more detailed the chemical reaction mechanism used in the combustion model, the closer to the actual combustion is the simulation process in the cylinder. However, in fact, the calculating speed will slow down using the detailed mechanism. Taking account of saving computational time, it is necessary to simplify the reaction mechanism according to the recorded experiment results in the previous literature.
The mechanism used in this work is composed of three parts, involving gasoline surrogate mechanism, NO x generation mechanism, and ammonia deNO x mechanism. The primary reference fuel (PRF) mechanism [31] including 41 species and 124 reactions is appropriate for gasoline surrogate. This PRF mechanism has been validated in a series of experiments, such as shock tube, jet stirred reactor, flow reactor, and homogeneous charge compression ignition (HCCI) engine, and it has also been widely  [32][33][34]. The NO x mechanism, a so-called extended Zeldovich mechanism, comes from Tao's work [35], which is widely referenced in published articles [36][37][38]. The ammonia deNO x mechanism is collated and verified by Golovitchev [39] and it has been tested and verified as well [40,41]. This work combines these three sub-mechanisms to simulate the coupled effect of water and ammonia for the engine deNO x process with inner SNCR strategy.
As a widely used empirical model, the Hiroyasu model was employed in the present soot prediction, which could make an all-round evaluation on the comprehensive applicability of the inner SNCR method [42]. Re-normalization group (RNG) k-ε model [43] is used for turbulence simulation. The wall heat transfer model, developed by Han and Reitz [44], is used for the wall heat loss. The Kelvin-Helmholtz and Rayleigh-Taylor (KH-RT) hybrid model [45] is used for fuel injection and spray breakup. In detail, the primary breakup is controlled by the KH model and the sub-droplets are produced in this process. The subsequent breakup is determined by the competition mechanism between KH model and RT model. It is necessary to consider turbulence in the continuous phase with the motion of oil droplets, so the O'Rourke model [46] is chosen for turbulence in this work. A spherical module with energy of 40 mJ simulates ignition by the spark plug at the side of the exhaust manifold. Figure 2 shows the relative locations of the port fuel injector, the direct injector, and the spark plug in the CFD model. stirred reactor, flow reactor, and homogeneous charge compression ignition (HCCI) engine, and it has also been widely applied or verified in other studies [32][33][34]. The NOx mechanism, a so-called extended Zeldovich mechanism, comes from Tao's work [35], which is widely referenced in published articles [36][37][38]. The ammonia deNOx mechanism is collated and verified by Golovitchev [39] and it has been tested and verified as well [40,41]. This work combines these three submechanisms to simulate the coupled effect of water and ammonia for the engine deNOx process with inner SNCR strategy. As a widely used empirical model, the Hiroyasu model was employed in the present soot prediction, which could make an all-round evaluation on the comprehensive applicability of the inner SNCR method [42]. Re-normalization group (RNG) k-ε model [43] is used for turbulence simulation. The wall heat transfer model, developed by Han and Reitz [44], is used for the wall heat loss. The Kelvin-Helmholtz and Rayleigh-Taylor (KH-RT) hybrid model [45] is used for fuel injection and spray breakup. In detail, the primary breakup is controlled by the KH model and the sub-droplets are produced in this process. The subsequent breakup is determined by the competition mechanism between KH model and RT model. It is necessary to consider turbulence in the continuous phase with the motion of oil droplets, so the O'Rourke model [46] is chosen for turbulence in this work. A spherical module with energy of 40 mJ simulates ignition by the spark plug at the side of the exhaust manifold. Figure 2 shows the relative locations of the port fuel injector, the direct injector, and the spark plug in the CFD model.

Initial Parameters and Boundary Conditions
Initial parameters and boundary conditions of the engine model were mainly based on experimental measurement as well as calculation values from GT-SUITE code. For four-stroke engines, it is necessary to make it clear that 0 °CA represents the top dead centre (TDC) for firing in this investigation. Thus, the negative values mean the event happening before TDC and positive values mean the event happening after TDC. Table 2 shows the initial parameters and boundary conditions. The test fuel, gasoline with research octane number (RON) 90, was applied in the calibration based on combustion and emissions. The injected aqueous solution of ammonia was preconfigured with ammonia mass fraction (AMF) of 0%, 10%, and 20%, respectively. The direct injection (DI) timing of the aqueous solution of ammonia was set to −105 °CA, 0 °CA, and 60 °CA, respectively. All of the simulations began at −383 °CA and ended at 120 °CA. In order to facilitate and understand the comparison of the results in Section 3, the cases and the legend nomenclature are shown in Table 3.

Initial Parameters and Boundary Conditions
Initial parameters and boundary conditions of the engine model were mainly based on experimental measurement as well as calculation values from GT-SUITE code. For four-stroke engines, it is necessary to make it clear that 0 • CA represents the top dead centre (TDC) for firing in this investigation. Thus, the negative values mean the event happening before TDC and positive values mean the event happening after TDC. Table 2 shows the initial parameters and boundary conditions. The test fuel, gasoline with research octane number (RON) 90, was applied in the calibration based on combustion and emissions. The injected aqueous solution of ammonia was preconfigured with ammonia mass fraction (AMF) of 0%, 10%, and 20%, respectively. The direct injection (DI) timing of the aqueous solution of ammonia was set to −105 • CA, 0 • CA, and 60 • CA, respectively. All of the simulations began at −383 • CA and ended at 120 • CA. In order to facilitate and understand the comparison of the results in Section 3, the cases and the legend nomenclature are shown in Table 3.   Table 3. Setting of the simulation cases and the legend nomenclature.

Model Validation
The test experiment system for calibration is illustrated in Figure 3. Detailed information of the test equipment and its accuracy were presented in the work of [27]. In order to ensure the accuracy of the verification, the experiment and simulation were under the same working conditions, namely at the speed of 1500 r/min and at the torque of 40 N·m. Meanwhile, the overall equivalence ratio of 1 was used for the stoichiometric condition. PRF 90 (isooctane: 90% vol. and n-heptane: 10% vol.) was used to match the experimental gasoline with RON 90. Cylinder pressure, heat release rate (HRR), and global NO x emissions were used for calibration, as shown in Figure 4.  In Figure 4, the simulation results coincided with the experimental results at most coordinates. The simulation error was mainly the result of the accuracy error of heat transfer loss and the reduced PRF mechanism. Because the average error was less than 5%, the subsequent results and analysis got sufficient reliable assurance.  In Figure 4, the simulation results coincided with the experimental results at most coordinates. The simulation error was mainly the result of the accuracy error of heat transfer loss and the reduced PRF mechanism. Because the average error was less than 5%, the subsequent results and analysis got sufficient reliable assurance.

Cylinder Pressure
In Figure 5a, the DI event occurs before combustion (DI timing = −105 °CA). With AMF increasing, the peak pressure decreases first and then increases when AMF = 20%. The peak pressure change rate is −0.22%-0.37%. This is because that part of NH3 is oxidized, compensating for the pressure drop caused by water. In Figure 5b, the DI event occurs during combustion (DI timing = 0 °CA). A significant delay (0.9 °CA) is observed. However, the pressure is without noticeable changes with the variation of AMF, which indicates no compensation for pressure by NH3 addition. These two cases, DI timing = −105 °CA or 0 °CA, will cause lag of the phase angle corresponding to the peak pressure. Most of the components in the aqueous solution of ammonia are water, the local adiabatic flame temperature decreases in the cylinder due to the phase change of water, which is an endothermic process. Further, the temperature dropping further affects the chemical reaction rate. This is why the crank angle phase is delayed. Water direct injection could promote the formation of a highly humid air/fuel mixture, which inhibits the advance and spread of the flame front. Especially when the injection timing is near the combustion phase (DI timing = 0 °CA), it is likely to cause peak pressure decrease, peak phase angle delay, incomplete combustion, and power decrease as a result of more interference. In Figure 5c, because the direct injection happens after the combustion phase In Figure 4, the simulation results coincided with the experimental results at most coordinates. The simulation error was mainly the result of the accuracy error of heat transfer loss and the reduced PRF mechanism. Because the average error was less than 5%, the subsequent results and analysis got sufficient reliable assurance.

Cylinder Pressure
In Figure 5a, the DI event occurs before combustion (DI timing = −105 • CA). With AMF increasing, the peak pressure decreases first and then increases when AMF = 20%. The peak pressure change rate is −0.22%-0.37%. This is because that part of NH 3 is oxidized, compensating for the pressure drop caused by water. In Figure 5b, the DI event occurs during combustion (DI timing = 0 • CA). A significant delay (0.9 • CA) is observed. However, the pressure is without noticeable changes with the variation of AMF, which indicates no compensation for pressure by NH 3 addition. These two cases, DI timing = −105 • CA or 0 • CA, will cause lag of the phase angle corresponding to the peak pressure. Most of the components in the aqueous solution of ammonia are water, the local adiabatic flame temperature decreases in the cylinder due to the phase change of water, which is an endothermic process. Further, the temperature dropping further affects the chemical reaction rate. This is why the crank angle phase is delayed. Water direct injection could promote the formation of a highly humid air/fuel mixture, which inhibits the advance and spread of the flame front. Especially when the injection timing is near the combustion phase (DI timing = 0 • CA), it is likely to cause peak pressure decrease, peak phase angle delay, incomplete combustion, and power decrease as a result of more interference. In Figure 5c, because the direct injection happens after the combustion phase (DI timing = 40 • CA), there is no change in peak pressure and a slight fluctuation (±1.69%) occurs in cylinder pressure after 40 • CA with the variation of AMF.

Major Intermediates Combustion Species
Variations in highly active radicals such as OH, H 2 O 2 , and HO 2 can be captured as a crucial indicator to reflect combustion rate and intensity [47]. Figures 6-8 show the effect of AMF on OH, H 2 O 2 , and HO 2 at different DI timings. When DI timing = −105 • CA, there is enough time to form a high humidity mixture before burning. This injection strategy is the same as the inlet port injection or fumigation situation. Water evaporates and absorbs heat, causing the cylinder temperature decrease. Meanwhile, there is a lag in the peak concentration of all species. The hysteresis effect of combustion is inhibited with the increase of ammonia concentration. When DI timing = 0 • CA, the hysteresis is reduced and combustion phase does not change further with variation of AMF. When DI timing = 40 • CA, water (AMF = 0) or aqueous solution of ammonia (AMF = 10% and 20%) injection occurs after combustion phase and most of the curves overlap in different AMF in Figures 6c, 7c and 8c, which indicates very little effect in combustion using inner SNCR. OH has been identified as a marker of combustion occurrence in many literature studies [48][49][50][51]. H 2 O 2 and HO 2 also play an important role in high temperature reaction process through #1~#6, which are highly susceptible to water addition, especially causing temperature drop and reaction delay.

Major NO x Emissions
Most NO x emissions exist in the form of NO in engine cylinders [52,53]. Thus, this fundamental research takes NO as the representative of NO x to evaluate the proposed inner SNCR method.

Major Intermediates Combustion Species
Variations in highly active radicals such as OH, H2O2, and HO2 can be captured as a crucial indicator to reflect combustion rate and intensity [47]. Figures 6-8 show the effect of AMF on OH, H2O2, and HO2 at different DI timings. When DI timing = −105 °CA, there is enough time to form a high humidity mixture before burning. This injection strategy is the same as the inlet port injection or fumigation situation. Water evaporates and absorbs heat, causing the cylinder temperature decrease. Meanwhile, there is a lag in the peak concentration of all species. The hysteresis effect of combustion is inhibited with the increase of ammonia concentration. When DI timing = 0 °CA, the hysteresis is reduced and combustion phase does not change further with variation of AMF. When DI timing = 40 °CA, water (AMF = 0) or aqueous solution of ammonia (AMF = 10% and 20%) injection occurs after combustion phase and most of the curves overlap in different AMF in Figures 6c, 7c, and 8c, which indic

Major NOx Emissions
Most NOx emissions exist in the form of NO in engine cylinders [52,53]. Thus, this fundamental research takes NO as the representative of NOx to evaluate the proposed inner SNCR method.

Major NOx Emissions
Most NOx emissions exist in the form of NO in engine cylinders [52,53]. Thus, this fundamental research takes NO as the representative of NOx to evaluate the proposed inner SNCR method.  In order to fully compare the evolution of NO emissions with different DI timings, the image post-processing of the calculated results starts from 50 • CA and ends at 110 • CA. Again, the original baseline represents the NO emissions of the prototype engine without inner SNCR and the AMF value represents the different ammonia concentration in the aqueous solution of ammonia. When AMF = 0, namely pure water injection, the NO emissions show a clear decreasing trend compared with the original baseline in Figures 9-11. This can be easily explained that the water injection reduces the in-cylinder temperature and the lower flame temperature inhibits the formation of NO x . On the basis of the theory from Zeldovich [54], thermal NO x production doubles for every 90 K temperature increase when the flame temperature is above 2000 K. Therefore, it can be seen that these results conform to Zeldovich mechanism with different DI timings using water injection method, and similar conclusions appear in the literature [55][56][57]. With the increase of ammonia concentration, the regional concentration of NO obviously shrinks in these three figures, which is because high concentration aqueous solution of ammonia contributes to perfect NO reduction. For DI timing, the retention time of aqueous solution of ammonia in the combustion chamber is longer when DI timing is earlier, so NO emissions performance of DI timing = −105 • CA is better than that of DI timing = 40 • CA. Additionally, when DI timing = 40 • CA, poor diffusion of ammonia slows down NO reduction, so the final NO concentration is still at a high level. Particularly, DI timing = 0 • CA achieved better NO emissions effect than DI timing = −105 • CA and DI timing = 40 • CA. This is mainly because the DI timing (0 • CA) is close to the ignition timing (−13 • CA), which causes greater disturbance and temperature drop at the front of the flame. The consequence is that the peak cylinder pressure and active ingredients are sacrificed, as shown in Figures 5b and 6b. In order to demonstrate the effect of deNO x with the inner SNCR method, compared with the original baseline, Table 4 is calculated and recorded based on the data at the end of the simulation (120 • CA).

Other Emissions
Figures 12-14 show the recorded data at the end of the CFD simulation (120 • CA). As shown in Figure 12, compared with the original values, SNCR technology tends to increase UHC, and the maximum increment is 27.6%; whereas with the increase of ammonia concentration, slightly UHC is suppressed. For most ICE, the wall quenching and the slit effect may deteriorate UHC emissions. In this work, however, the result above is mainly because the mass of the aqueous solution injected in the combustion room is constant. Therefore, the increase of ammonia will inevitably lead to the decrease of water, which is the main deteriorator of UHC. When DI timing = 0 • CA, severe flame interference and heat transfer barriers promote larger generation of UHC than that when DI timing = −105 • CA and 40 • CA. As an additional confirmation for the effect of aqueous solution, Lanzanova pointed that increased air dilution reduces the combustion temperature and lowers post-flame UHC oxidation [58].
In Figure 13, the change of CO concentration is similar to UHC with the variation of DI timing, although their formation mechanism is essentially different. The biggest change took place in DI = 0 and AMF = 20%, with a maximum increase of 15.3%. For incomplete combustion products, inert substances direct injection hinders flame development. On the one hand, the reaction formula, CO + OH = CO 2 + H, is the key reaction pathway for CO oxidation to CO 2 [59]. On the basis of Figure 6b, an obvious OH decline occurred as aqueous solution was injected, and this inhibits the above-mentioned positive reaction process. On the other hand, a part of ammonia may react with oxygen to produce NO x under high temperature, resulting in local oxygen deficiency, which increases incomplete oxidation of the hydrocarbon fuel. Generally, increase in ammonia concentration leads to a slight increase in CO in these three DI timings.
In Figure 14, there is no obvious soot fluctuation compared with the original baseline (calibrated value), which means that the inner SNCR method maintained good soot tolerance for the addition of ammonia aqueous solution at different DI strategies. When DI timing = −105 • CA and 0 • CA. The earlier DI timing inhibits the formation of soot, mainly as a result of the inhibition mechanism of aqueous solution on high temperature. The maximum reduction is 16.9%. When DI timing = 40 • CA, the DI method of aqueous solution of ammonia plays little effect on soot formation or consumption.  Figures 12-14 show the recorded data at the end of the CFD simulation (120 °CA). As shown in Figure 12, compared with the original values, SNCR technology tends to increase UHC, and the maximum increment is 27.6%; whereas with the increase of ammonia concentration, slightly UHC is suppressed. For most ICE, the wall quenching and the slit effect may deteriorate UHC emissions. In this work, however, the result above is mainly because the mass of the aqueous solution injected in the combustion room is constant. Therefore, the increase of ammonia will inevitably lead to the decrease of water, which is the main deteriorator of UHC. When DI timing = 0 °CA, severe flame interference and heat transfer barriers promote larger generation of UHC than that when DI timing = −105 °CA and 40 °CA. As an additional confirmation for the effect of aqueous solution, Lanzanova pointed that increased air dilution reduces the combustion temperature and lowers post-flame UHC oxidation [58].     Figures 12-14 show the recorded data at the end of the CFD simulation (120 °CA). As shown in Figure 12, compared with the original values, SNCR technology tends to increase UHC, and the maximum increment is 27.6%; whereas with the increase of ammonia concentration, slightly UHC is suppressed. For most ICE, the wall quenching and the slit effect may deteriorate UHC emissions. In this work, however, the result above is mainly because the mass of the aqueous solution injected in the combustion room is constant. Therefore, the increase of ammonia will inevitably lead to the decrease of water, which is the main deteriorator of UHC. When DI timing = 0 °CA, severe flame interference and heat transfer barriers promote larger generation of UHC than that when DI timing = −105 °CA and 40 °CA. As an additional confirmation for the effect of aqueous solution, Lanzanova pointed that increased air dilution reduces the combustion temperature and lowers post-flame UHC oxidation [58].    In Figure 13, the change of CO concentration is similar to UHC with the variation of DI timing, although their formation mechanism is essentially different. The biggest change took place in DI = 0 and AMF = 20%, with a maximum increase of 15.3%. For incomplete combustion products, inert substances direct injection hinders flame development. On the one hand, the reaction formula, CO + OH = CO2 + H, is the key reaction pathway for CO oxidation to CO2 [59]. On the basis of Figure 6b, an obvious OH decline occurred as aqueous solution was injected, and this inhibits the abovementioned positive reaction process. On the other hand, a part of ammonia may react with oxygen to produce NOx under high temperature, resulting in local oxygen deficiency, which increases incomplete oxidation of the hydrocarbon fuel. Generally, increase in ammonia concentration leads to a slight increase in CO in these three DI timings.

Other Emissions
In Figure 14, there is no obvious soot fluctuation compared with the original baseline (calibrated value), which means that the inner SNCR method maintained good soot tolerance for the addition of ammonia aqueous solution at different DI strategies. When DI timing = −105 °CA and 0 °CA. The earlier DI timing inhibits the formation of soot, mainly as a result of the inhibition mechanism of aqueous solution on high temperature. The maximum reduction is 16.9%. When DI timing = 40 °CA, the DI method of aqueous solution of ammonia plays little effect on soot formation or consumption.

Contribution of Ammonia
In this work, ammonia was introduced to promote the reduction of NOx on the basis of incylinder water direct injection. By counting the types of ammonia reactions in each cell, contribution of NH3 were obtained. In Figure 15, two concentrations (AMF = 10% and 20%) and three DI timings (DI timing = −105 °CA, 0 °CA, and 40 °CA) are analyzed. Additionally, the contribution of ammonia is divided into three parts, for reduction (NH3 + NO → N2 + H2O), oxidation (NH3 + O2 → NO + H2O), and slipping (not involved in any reactions). It is beneficial to analyze the proportion of different effects to promote the positive effects of ammonia and to inhibit the negative effects of ammonia. The contribution sequence for reduction effect is case 1 > case 2 > case 3. The contribution sequence for oxidation is case 2 > case 1 > case 3. The contribution sequence for slipping is case 3 > case 2 > case 1. The above results can be analyzed as follows: contribution for reduction and slipping are mainly related to the retention time of NH3 in the combustion chamber and an early DI timing results in longer retention time and further increases the contribution of NH3 for reduction and decreases the contribution of NH3 for slipping. However, an early DI timing means that more NH3 may be oxidized to NO under high cylinder temperature, which weakens the selectivity for deNOx. Additionally, the increase of ammonia composition is favorable to the enhancement of the deNOx effect, but the contribution of NH3 for reduction is decreased. Thus, ammonia concentration is not likely to increase

Contribution of Ammonia
In this work, ammonia was introduced to promote the reduction of NO x on the basis of in-cylinder water direct injection. By counting the types of ammonia reactions in each cell, contribution of NH 3 were obtained. In Figure 15, two concentrations (AMF = 10% and 20%) and three DI timings (DI timing = −105 • CA, 0 • CA, and 40 • CA) are analyzed. Additionally, the contribution of ammonia is divided into three parts, for reduction (NH 3 + NO → N 2 + H 2 O), oxidation (NH 3 + O 2 → NO + H 2 O), and slipping (not involved in any reactions). It is beneficial to analyze the proportion of different effects to promote the positive effects of ammonia and to inhibit the negative effects of ammonia. The contribution sequence for reduction effect is case 1 > case 2 > case 3. The contribution sequence for oxidation is case 2 > case 1 > case 3. The contribution sequence for slipping is case 3 > case 2 > case 1. The above results can be analyzed as follows: contribution for reduction and slipping are mainly related to the retention time of NH 3 in the combustion chamber and an early DI timing results in longer retention time and further increases the contribution of NH 3 for reduction and decreases the contribution of NH 3 for slipping. However, an early DI timing means that more NH 3 may be oxidized to NO under high cylinder temperature, which weakens the selectivity for deNO x . Additionally, the increase of ammonia composition is favorable to the enhancement of the deNO x effect, but the contribution of NH 3 for reduction is decreased. Thus, ammonia concentration is not likely to increase limitlessly, and DI timing matching the appropriate ignition phase is the best choice to play the maximum positive role of ammonia for inner SNCR. limitlessly, and DI timing matching the appropriate ignition phase is the best choice to play the maximum positive role of ammonia for inner SNCR.

Conclusions
Combining the respective characteristics of water DI technology and SNCR technology, this paper contributed to an innovation method, inner SNCR, which was verified based on CFD scheme in an SI engine model with port fuel injection and ammonia aqueous solution direct injection. A series of results with three AMF values and three DI timings were fully presented under stoichiometric

Conclusions
Combining the respective characteristics of water DI technology and SNCR technology, this paper contributed to an innovation method, inner SNCR, which was verified based on CFD scheme in an SI engine model with port fuel injection and ammonia aqueous solution direct injection. A series of results with three AMF values and three DI timings were fully presented under stoichiometric condition. Some main conclusions are summarized as follows:

1.
Inner SNCR, an innovate method put forward for ICE, can effectively inhibit NO x emissions with less negative effects. More NO reduction can be achieved by NH 3 addition rather than by increasing water injection. With the increase of ammonia composition, the largest decline of NO x is 65.1%, when DI timing = 0 • CA for stoichiometric condition.

2.
By using ammonia aqueous solution direct injection, the in-cylinder pressure will decrease slightly, and the corresponding phase of peak pressure will lag behind. However, owing to the small amount of injected aqueous solution (about 20% water fuel ratio), the impact on dynamic performance is very small.

3.
As a relatively inert component, aqueous solution of ammonia inhibits the rapid accumulation of active radicals in the combustion chamber, which directly leads to retard of combustion reaction rate and delay of peak pressure.

4.
Improper ammonia solution injection phase may cause incomplete combustion and an increase in UHC and CO emissions. The maximum increase of UHC is 27.6% and the maximum increase of CO is 15.3%. However, for soot, there is a decline when DI timing = −105 • CA and 0 • CA, and the maximum reduction is 16.9%.

5.
The contribution of the ammonia addition is divided into three parts: reduction, oxidation, and slipping. The reduction effect and slipping effect are mainly related to the retention time, so an earlier DI timing helps to strengthen reduction of NO by NH 3 and reduce NH 3 slipping. Nevertheless, an early DI timing means that more NH 3 may be oxidized to NO under high cylinder temperature simultaneously.