A New Dynamic Injection System of Urea-Water Solution for a Vehicular Select Catalyst Reduction System

Since the Euro-III standard was adopted, the main methods to inhibit NOx production in diesel engines are exhaust gas recirculation (EGR) and select catalyst reduction (SCR). On these methods SCR offers great fuel economy, so it has received wide attention. However, there also exists a trade-off law between NOx conversion efficiency and NH3 slip under dynamic conditions. To inhibit NH3 slip with high NOx conversion efficiency, a dynamic control method for a urea water solution (UWS) injection was investigated. The variation phenomena of SCR conversion efficiency with respect to the cross-sensitivity characteristics of the NOx sensor to NH3 have been thoroughly analyzed. The methodology of “uncertain conversion efficiency curve tangent analysis” has been applied to estimate the concentration of the slipped NH3. The correction factor “φ” of UWS injection is obtained by a comparative calculation of the NOx conversion ability and subsequent NH3 slip. It also includes methods of flow compensation and flow reduction. The proposed control method has been authenticated under dynamic conditions. In low frequency dynamic experiments, this control method has accurately justified the NH3 slip process and inhibits the NH3 emission to a lower level thereby improving the conversion efficiency to a value closer to the target value. The results of European transient cycle (ETC) experiments indicate that NH3 emissions are reduced by 90.8% and the emission level of NOx is close to the Euro-V standard.


Introduction
Direct injection diesel engines are preferred for their superior economy, power and emissions.Due to the high combustion temperature of the diesel engine, the nitrogen in the air is easily oxidized by oxygen and produces a large amount of NO x which have a significant pollution impact on the environment.Therefore, NO x emissions should be controlled.High pressure fuel injection and turbocharging technology have been used to change the ratio of particulate matter (PM) and NO x by regulating the fuel injection strategy.From the Euro-II to the Euro-III phase, the high injection pressure (high common rail fuel injection) system has been used to optimize in-cylinder combustion and regulate the ratio of PM and NO x to reach the emission goals.From the Euro-III to the Euro-IV phase, the main problem is how to significantly reduce PM and NO x emissions.There are two methods at present: one is to use exhaust gas recirculation (EGR) to reduce in-cylinder NO x , and out of the cylinder, with diesel particulate filter(DPF) to filter PM; the other method is to use select catalyst reduction (SCR) to eliminate NO x and PM.In small diesel engines, the SCR system is limited by the exhaust gas temperature, so EGR + DPF technology is used as the main method to solve the emission problem.The exhaust temperature of medium and heavy diesel engines is high.At high exhaust Energies 2017, 10, 12 2 of 17 temperatures, diesel engines with SCR + high-pressure common rail (HCR) technology are more economical than diesel engines with EGR + DPF technology, so SCR is more widely used in medium and heavy duty diesel engines.From the Euro-IV to the Euro-V phase, how to further reduce NO x has become the key problem.The main method is to optimize the SCR system to enhance the NO x conversion efficiency and reduce the NH 3 leakage [1][2][3][4][5][6].
At present, almost 99% of diesel vehicles work under dynamic operating conditions and thus their NO x emissions are also a dynamic process.Based on this condition, excellent dynamic performance is an essential characteristic of the SCR system.SCR control strategies mainly focus on optimizing the urea water solution (UWS) injection rate algorithms and NH 3 slip inhibition.However, there is a trade-off law between NO x conversion efficiency and NH 3 slip under dynamic conditions.When the actual injection rate is lower than the theoretical one, the NO x can't be completely reduced.When the actual injection rate is higher than the theoretical value, NH 3 can't be completely oxidized and thus generates secondary pollution [7][8][9].Furthermore, NH 3 storage and catalyst release make the NH 3 slip inhibition more difficult.
Some researchers believe that an oxidation processor installed at the end of the exhaust pipe may inhibit NH 3 slip [10].Nova and Tronconi [11] added an Ammonia Slip Catalyst (ASC) to the exhaust pipe downstream of the SCR system and completed some investigations by experiment and simulation.The results showed that the studied ASC could efficiently clean up the slipped NH 3 .Shrestha et al. [12] did some experiments and simulation research on multi-functional wash coated monolith catalysts.They compared the catalysts for a range of temperatures, space velocities, and feed compositions in the presence of H 2 O and CO 2 .Based on the data acquired from the experiments, a dynamic model of the NH 3 oxidation process was established.
Some researchers supposed that it is necessary to investigate the processes of NH 3 storage, release and reduction reaction as well as the SCR catalyst [13,14].Rauch et al. [15] monitored the ammonia loading of a vanadia-based SCR catalyst by a microwave-based method.Their experimental results showed that the method can be applied to different temperatures.It was also possible to determine the storage of ammonia from the ammonia-to-NO x feed ratio.Zhang and Wang [16] focused on the simultaneous estimation of ammonia coverage ratios and input.They configured a three-state nonlinear model with the high-gain observer method by assuming the states of the SCR system are homogenous inside and the SCR cell was a continuous stirred tank reactor.The NO x sensor was cross-sensitive to the NH 3 concentration, and the NO x sensor reading was corrected by precise NH 3 sensor measurements.The simulation results showed that the designed observer worked well.
NO x sensors are used to measure the NO x concentration downstream from the SCR system and feed it back to the SCR controller.However, research results have showed that NO x sensors have an enhanced cross sensitivity to NH 3 [17][18][19].According to the structural characteristics of NO x sensors, the NH 3 inside the sensor is easily oxidized to NO x .There are mainly three chemical reactions in this process, as described in Equations ( 1)-(3) [20][21][22][23]: Wang [24] believed that the main factor is temperature, which may affect the three chemical reactions.The cross sensitivity factor of the NO x sensor is changed as the temperature changes.Experiments proved that this factor was between 0.5 and 2 for a range of diesel engine exhaust temperatures.
This paper focuses on the trade-off law between NO x conversion efficiency and NH 3 slip by using a presented method of "uncertain conversion efficiency curve tangent analysis" based on the NH 3 cross sensitivity characteristics of the NO x sensor.The degree of NH 3 slip will be obtained from the calculation of the parameters which may affect the shape and locations of this tangent.Subsequently, the UWS injection correction factor "ϕ" will be calculated with the dynamic flow compensation and flow reduction.Finally the accuracy of the UWS correction model and the effectiveness of NH 3 slip inhibition will be verified under low-frequency and high-frequency (ETC cycle) dynamic working conditions.

Correction Strategy Mathematical Analysis
For the SCR system control strategy, the corrected UWS injection rate is calculated by Equation (4): where q UWS,Act is the real-time UWS injection rate after correction, q UWS,Bas is the basic UWS injection rate before correction and ϕ is the correction factor of the UWS injection rate.The Simulink model of the correction strategy is shown in Figure 1.
Energies 2017, 10, 12 3 of 17 effectiveness of NH3 slip inhibition will be verified under low-frequency and high-frequency (ETC cycle) dynamic working conditions.

Correction Strategy Mathematical Analysis
For the SCR system control strategy, the corrected UWS injection rate is calculated by Equation ( 4): where qUWS,Act is the real-time UWS injection rate after correction, qUWS,Bas is the basic UWS injection rate before correction and φ is the correction factor of the UWS injection rate.The Simulink model of the correction strategy is shown in Figure 1.The model consists of two sub-models.The "Basic UWS Injection Rate Model" sub-model collects three pieces of data: (1) engine operating data EngineMsg, which include speed, torque, original emissions, etc.; (2) exhaust gas processor data EGPMsg, which include exhaust temperature before and after the catalyst, gas flow, etc.; (3) injection system data UDSMsg.These data are combined to calculate qUWS,Bas.
The "UWS Correction Model" sub-model collects four sets of data: (1) engine operating data; (2) waste gas treatment data; (3) urea injection system data; and (4) NOx sensor data.These data are processed in the module to obtain the UWS injection rate correction factor φ. The qUWS,Act is calculated using the factor φ and qUWS,Bas.
The change rule of qUWS,Act can be obtained from Equation ( 5): ( ) where aUWS,Act is the acceleration of qUWS,Act, and t is the time.
The "φ" (the initial value is "0") is the key factor to correct the UWS injection and keep the SCR system at a low NH3 slip level with a high conversion efficiency.The UWS correction model is shown in Figure 2. The model consists of two sub-models.The "Basic UWS Injection Rate Model" sub-model collects three pieces of data: (1) engine operating data EngineMsg, which include speed, torque, original emissions, etc.; (2) exhaust gas processor data EGPMsg, which include exhaust temperature before and after the catalyst, gas flow, etc.; (3) injection system data UDSMsg.These data are combined to calculate q UWS,Bas .
The "UWS Correction Model" sub-model collects four sets of data: (1) engine operating data; (2) waste gas treatment data; (3) urea injection system data; and (4) NO x sensor data.These data are processed in the module to obtain the UWS injection rate correction factor ϕ. The q UWS,Act is calculated using the factor ϕ and q UWS,Bas .
The change rule of q UWS,Act can be obtained from Equation (5): where a UWS,Act is the acceleration of q UWS,Act , and t is the time.
The "ϕ" (the initial value is "0") is the key factor to correct the UWS injection and keep the SCR system at a low NH 3 slip level with a high conversion efficiency.The UWS correction model is shown in Figure 2. The UWS correction model consists of five sub-models: The "KT Model" sub-model calculates the real-time NH3 sensitivity factor of the sensor based on the exhaust gas temperature near the NOx sensor.The NOx emission was measured by a NOx sensor under dynamic conditions.The surrounding NH3 may be converted into NOx easily in the NOx sensor.Therefore, the values of NH3 and NOx at the same time will be influenced by the data which is measured by the NOx sensor, as given by Equation (6): where CN,Act is the NOx concentration measured by the NOx sensor.CNOx,Act is the actual NOx concentration at the testing position.KT is the NH3 cross sensitivity factor of the NOx sensor which could be obtained from the KT map and CNH3,Act is the actual NH3 concentration at the testing position.The "Engine Emission Model" sub-model is used to calculate the original engine NOx emissions, the target conversion efficiency, the ammonia-nitrogen ratio and the target NOx emission concentration which would support service for the other models as shown in Figure 3.For example the targeted conversion efficiency could be calculated using Equation ( 7): where PCon,Trg is the target conversion efficiency (the highest value without NH3 slip) which can be obtained from the engine emission map, CNOx,Ori is the original NOx concentration of the engine before after treatment and can be obtained by inserting the value calculation of the steady map and CNOx,Trg is the target NOx concentration.
The "NH3 Slip Situation Model" sub-model is based on the output of the first two models to determine the current NH3 leak situation; more details can be seen in Section 2.1.The "UWS Flow Dynamic Reduction Model" sub-model is triggered when an NH3 leak occurs.When there is no NH3 leakage, it is necessary to consider whether there is little UWS injection and trigger the "UWS Flow Dynamic Compensation Model".
The "UWS Flow Dynamic Compensation Model" and "UWS Flow Dynamic Reduction Model" sub-models are used to calculate the correction factor and compensation factor of the UWS injection rate, respectively (see Sections 2.2 and 2.3 for more details).The UWS correction model consists of five sub-models: The "K T Model" sub-model calculates the real-time NH 3 sensitivity factor of the sensor based on the exhaust gas temperature near the NO x sensor.The NO x emission was measured by a NO x sensor under dynamic conditions.The surrounding NH 3 may be converted into NO x easily in the NO x sensor.Therefore, the values of NH 3 and NO x at the same time will be influenced by the data which is measured by the NO x sensor, as given by Equation ( 6): where C N,Act is the NO x concentration measured by the NO x sensor.C NO x ,Act is the actual NO x concentration at the testing position.K T is the NH 3 cross sensitivity factor of the NO x sensor which could be obtained from the K T map and C NH 3 ,Act is the actual NH 3 concentration at the testing position.
The "Engine Emission Model" sub-model is used to calculate the original engine NO x emissions, the target conversion efficiency, the ammonia-nitrogen ratio and the target NO x emission concentration which would support service for the other models as shown in Figure 3.For example the targeted conversion efficiency could be calculated using Equation ( 7): where P Con,Trg is the target conversion efficiency (the highest value without NH 3 slip) which can be obtained from the engine emission map, C NO x ,Ori is the original NO x concentration of the engine before after treatment and can be obtained by inserting the value calculation of the steady map and C NO x ,Trg is the target NO x concentration.
The "NH 3 Slip Situation Model" sub-model is based on the output of the first two models to determine the current NH 3 leak situation; more details can be seen in Section 2.1.The "UWS Flow Dynamic Reduction Model" sub-model is triggered when an NH 3 leak occurs.When there is no NH 3 leakage, it is necessary to consider whether there is little UWS injection and trigger the "UWS Flow Dynamic Compensation Model".
The "UWS Flow Dynamic Compensation Model" and "UWS Flow Dynamic Reduction Model" sub-models are used to calculate the correction factor and compensation factor of the UWS injection rate, respectively (see Sections 2.2 and 2.3 for more details).

NH3 Slip Situation Analysis
In order to justify and analyze the real-time NH3 slip situation of the engine, a method called "uncertain conversion efficiency curve tangent analysis" is presented.From the real-time measured value CN,Act, the uncertain conversion efficiency PCon,Fuz can be calculated using Equation (8).The absolute conversion efficiency PCon,Abs can be obtained from the calculated valve CNOx,Act by Equation ( 9): Con,Abs Con,Fuz NO ,Ori NO ,Ori NO ,Ori According to the results of Equations ( 8) and ( 9), the relative conversion efficiency can be calculated with Equation (10): Con,Rel Con,Abs Con,Trg Con,Fuz Con,Trg NO ,Ori The change rules of PCon,Fuz, PCon,Abs, and PCon,Rel can be obtained from Equations ( 11)-( 13): where vCon,Abs, vCon,Rel, and vCon,Fuz are their velocities.There are several kinds of NH3 slip situations, as follows: (1) PCon,Fuz > PCon,Trg For the original map of PCon,Trg obtained from the engine calibration experiments, from the theoretically point of view, with the PCon,Fuz ≤ PCon,Trg under any circumstances.In the actual

NH 3 Slip Situation Analysis
In order to justify and analyze the real-time NH 3 slip situation of the engine, a method called "uncertain conversion efficiency curve tangent analysis" is presented.From the real-time measured value C N,Act , the uncertain conversion efficiency P Con,Fuz can be calculated using Equation (8).The absolute conversion efficiency P Con,Abs can be obtained from the calculated valve C NO x ,Act by Equation ( 9): According to the results of Equations ( 8) and ( 9), the relative conversion efficiency can be calculated with Equation (10): The change rules of P Con,Fuz , P Con,Abs , and P Con,Rel can be obtained from Equations ( 11)-( 13): where v Con,Abs , v Con,Rel , and v Con,Fuz are their velocities.There are several kinds of NH 3 slip situations, as follows: Energies 2017, 10, 12 6 of 17 (1) P Con,Fuz > P Con,Trg For the original map of P Con,Trg obtained from the engine calibration experiments, from the theoretically point of view, with the P Con,Fuz ≤ P Con,Trg under any circumstances.In the actual conditions when the engine calibration points are not enough, engine working instability or sensor testing errors might occur and lead to an abnormal circumstance (like P Con,Fuz > P Con,Trg ).
For such an instance the NH 3 slip is assumed to be zero, thus the UWS need not be corrected.
(2) 0 ≤ P Con,Fuz ≤ P Con,Trg , and tanθ < 0 (v Con,Fuz < 0) In this case, the uncertain conversion efficiency is lower than its target and stays away from the target value gradually.According to Equation ( 8), P Con,Fuz becomes smaller due to the increase of the C N,Act .The enlargement of the C N,Act may be caused by the following two cases: • The first case is that the excessively injected UWS caused an acceleration of the process and subsequently an increasing NH 3 slip due unreacted ammonia.

•
The second case is that insufficient UWS may cause a slowing the process and lead to a growing amount of NO x remaining unreduced due to unavailability of reactant.
Therefore, it may be concluded that with the condition a UWS,Act ≤ 0 and P Con,Abs ≤ P Con,Trg , there is no NH 3 slip, thus UWS compensation could be continued.When a UWS,Act > 0 and P Con,Abs = P Con,Trg , NH 3 slip is severely increased, thus UWS injection should be reduced.
(3) 0 ≤ P Con,Fuz ≤ P Con,Trg , and tanθ ≥ 0 (v Con,Fuz ≥ 0) In this case, the uncertain conversion efficiency is lower than its target and becomes close to the target value gradually.In this case it can be concluded that when a UWS,Act > 0 and P Con,Abs ≤ P Con,Trg , there is no NH 3 slip like the previous cases, thus UWS compensation should be continued.With the condition a UWS,Act ≤ 0 and P Con,Abs ≥ P Con,Trg , UWS injection should be reduced as NH 3 slip is going to increase.
(4) P Con,Fuz < 0 This particular case emerges on ruling out the test error and the engine calibration map error, thus under these circumstances C NO x ,Act ≤ C NO x ,Ori (theoretically), whereas, C N,Act > C NO x ,Ori (P Con,Fuz < 0), C NH 3 ,Act > 0 as shown in Equation (7).This case indicates a seriously high level of NH 3 slip therefore UWS injection must be stopped immediately.

Urea Water Solution Flow Dynamic Compensation
There was no NH 3 slip during the process of the UWS dynamic compensation.Therefore: It is indicated that the actual amount of injected UWS (including the NH 3 released from the catalyst) was less than the demand of SCR reaction.The condition is 0 ≤ P Con,Abs ≤ P Con,Trg and zero NH 3 slip.Therefore, the correction factor ϕ can be calculated using Equation (15): Energies 2017, 10, 12 7 of 17 where Q NO x ,Red (NO x conversion potential) is the difference between the target value and actual value of the total reduced NO x in a period as shown by Equation ( 16) and Q NO x ,AcRed is the actual value of the total reduced NO x in a specific period given by Equation ( 17): For the control method, the calculation of UWS injection rate and its acceleration may be accomplished with Equation (20) or Equation ( 21): where NH 3 slip is assumed to be zero.
The value of NO x emission in this process may be obtained with:

Urea Water Solution Flow Dynamic Reduction
During the UWS dynamic process, reduction is the response of increasing NH 3 slip.Therefore: The case of 0 ≤ P Con,Abs ≤ P Con,Trg and presence of evident NH 3 slip shows that the actual amount of injected UWS (including the NH 3 released from the catalyst) is much more than the demand of the SCR reaction.This current situation indicates that the SCR reaction is saturated as shown by Equation ( 24): According to Equation (7): Energies 2017, 10, 12 8 of 17 The correction factor ϕ can be calculated as Equation ( 26): where Q NH3 is the total NH 3 emission amount in a specific period of time given by Equation ( 27), Q NOx,TrgRed is the total reduced NO x with target conversion efficiency of Equation ( 28) and R AN is the ammonia nitrogen ratio constant set in the SCR system control strategy: Due to q UWS,Act ≥ 0 and 1 + ϕ ≥ 0: In the presence of NH 3 Slip, the control method may be applied to calculate the UWS injection rate and its acceleration is given by Equation (30) or Equation (31): The amount of NO x emission and NH 3 emission in this process could be determined by:

Special Case
For the special case of P Con,Abs > P Con,Trg , the actual value is more than the target value of the system conversion efficiency and NH 3 slip is assumed to be zero, as can be seen Equation ( 33): Energies 2017, 10, 12 9 of 17 The UWS injection rate doesn't require any dynamic adjustment thus ϕ ≡ 0. Therefore, it can be described with Equation (34) as follows: q uws,Act = q uws,Bas a uws,Act = ∂q uws,Bas ∂t (34) The amount of NO x emission and NH 3 emission in this process are: While in another special case where P Con,Fuz < 0 indicates that NH 3 slip is very high.Thus UWS injection must be stopped immediately.Now ϕ ≡ 0, and: The amount of NO x emission and NH 3 emission in this case can be described as follows:

Experiments and Result Analysis
The low-frequency dynamic working conditions of the engine are reproduced as shown in Figure 1.The detailed dynamic process is shown in Figure 3.The related parameters of the SCR system are changed in a slower manner for this process by an explicit analysis of their relationships and interaction factors.Changes of NO x emission and NH 3 slip are compared before and after the UWS dynamic correction.The ETC cycle was adopted for the high-frequency dynamic process for further verification of the UWS control method performance.The engine experiment platform is shown in Figure 4.
The UWS injection rate doesn't require any dynamic adjustment thus ϕ ≡ 0. Therefore, it can be described with Equation (34) as follows: uws,Bas uws,Bas uws,Act q q q a t (34) The amount of NOx emission and NH3 emission in this process are: While in another special case where PCon,Fuz < 0 indicates that NH3 slip is very high.Thus UWS injection must be stopped immediately.Now ϕ ≡ 0, and: The amount of NOx emission and NH3 emission in this case can be described as follows: ( ) ( )

Experiments and Result Analysis
The low-frequency dynamic working conditions of the engine are reproduced as shown in Figure 1.The detailed dynamic process is shown in Figure 3.The related parameters of the SCR system are changed in a slower manner for this process by an explicit analysis of their relationships and interaction factors.Changes of NOx emission and NH3 slip are compared before and after the UWS dynamic correction.The ETC cycle was adopted for the high-frequency dynamic process for further verification of the UWS control method performance.The engine experiment platform is shown in Figure 4.The specifications of main equipment in the engine experiment platform are listed in Table 1.
Energies 2017, 10, 12 10 of 17 The heavy duty diesel engine parameters used in the experiment are given in Table 2.The specifications of main equipment in the engine experiment platform are listed in Table 1.The heavy duty diesel engine parameters used in the experiment are given in Table 2.

Mathematical Model Validation
As shown in Figure 5 (in this paper, A indicates the values after correction, B indicates the values before correction, T indicates the values obtained from equipment testing, C indicates the values obtained from calculation and M indicates the values obtained from the maps).The UWS injection was started at the 6th second.It can be seen in Figure 5 that the CN,Act curve declined gradually in the first 30 s, became stable at a very low level in the second 30 s, and two noticeable humps can be observed in the last 60 s.Considering the NH3 cross sensitivity of the NOx sensor, it could be initially assumed that the conversion efficiency was low in the first 30 s as the UWS injection was not sufficient.The NH3 slip increased significantly in the position of the two humps with the severely overloaded UWS injection.The UWS correction under dynamic conditions is critical for improving SCR conversion efficiency and NH3 slip inhibition.The change rules of the four kinds of conversion efficiency which have been discussed in Section 2.1 are shown in Figure 6.
(1) In the beginning, PCon,Abs was less than the target valve PCon,Trg.However, these two values are the same as that after the 30th second.The change rules of the four kinds of conversion efficiency which have been discussed in Section 2.1 are shown in Figure 6.
(1) In the beginning, P Con,Abs was less than the target valve P Con,Trg .However, these two values are the same as that after the 30th second.(2) P Con,Fuz and P Con,Abs remain the same as that before the 60th second.Then, two serious sinks appeared in the P Con,Fuz curve.(3) After the beginning of UWS injection, P Con,Rel was stable near a 0 value between the 20th and 60th second and fluctuated in a range of ±30% between the 60th and 120th second.
(2) PCon,Fuz and PCon,Abs remain the same as that before the 60th second.Then, two serious sinks appeared in the PCon,Fuz curve.(3) After the beginning of UWS injection, PCon,Rel was stable near a 0 value between the 20th and 60th second and fluctuated in a range of ±30% between the 60th and 120th second.As a result, between the 30th and 60th second, the NH3 slip is assumed to be zero thus the UWS needs no correction.Between 60th and 90th or 100th and 120th second, NH3 slip is severely increased, thus UWS injection should be reduced.Between the 0th and 30th second there is no NH3 slip, thus UWS compensation be continued.The calculated value and actual experimental value of the NOx and NH3 are compared in the low-frequency process.The results are shown in Figure 7.As a result, between the 30th and 60th second, the NH 3 slip is assumed to be zero thus the UWS needs no correction.Between 60th and 90th or 100th and 120th second, NH 3 slip is severely increased, thus UWS injection should be reduced.Between the 0th and 30th second there is no NH 3 slip, thus UWS compensation be continued.The calculated value and actual experimental value of the NO x and NH 3 are compared in the low-frequency process.The results are shown in Figure 7.
(1) From the 0 to the 60th second and the 90th to 100th second, the experimental value of the NH 3 concentration downstream from the SCR system is almost 0 ppm.The NO (2) PCon,Fuz and PCon,Abs remain the same as that before the 60th second.Then, two serious sinks appeared in the PCon,Fuz curve.(3) After the beginning of UWS injection, PCon,Rel was stable near a 0 value between the 20th and 60th second and fluctuated in a range of ±30% between the 60th and 120th second.As a result, between the 30th and 60th second, the NH3 slip is assumed to be zero thus the UWS needs no correction.Between 60th and 90th or 100th and 120th second, NH3 slip is severely increased, thus UWS injection should be reduced.Between the 0th and 30th second there is no NH3 slip, thus UWS compensation be continued.The calculated value and actual experimental value of the NOx and NH3 are compared in the low-frequency process.The results are shown in Figure 7.   (4) P Con,Abs and P Con,Trg were achieved in a shorter period as compared to Figure 4.The sinking amplitude of the P Con,Fuz curve has significantly decreased after the 60th second and became stable in the last 60 s.
(1) The CNOx,Act was slightly reduced in the last 60 s on application of the UWS dynamic correction.However, the control method has no significant effect on the value of CNOx,Act in the lowfrequency process.
(2) The CNH3,Act was also significantly reduced in the last 60 s after the UWS dynamic correction application.The values of CNH3,Act in the low-frequency process are also greatly influenced by the application of the correction.(3) Overall, the application of UWS dynamic control method has reduced ∫CNH3,Act dt by 92.68%, respectively.Moreover it has also improved ∫CNOx,Act dt by 5.58%.
The change trends of the all four kinds of conversion efficiencies are compared after applying the control method as shown in Figure 9.The calculated values and experimental values of the NO x and NH 3 were further compared to ensure that the revised data can be well trusted as shown in Figure 10.
Energies 2017, 10, 12 13 of 17 (1) In the beginning of the low-frequency process, PCon,Abs was smaller than PCon,Trg.However, the two curves overlapped after the 15th second.(2) From 0 to 60th second, PCon,Fuz was the same as PCon,Abs, whereas, after the 60th second PCon,Fuz started to deviate with a slightly sinking trend.(3) Initially PCon,Abs was less than 0 with a rising trend, whereas, it became stable when close to 0 and 15th to 60th second, while from 60th to 120th second it fluctuated many times with an amplitude between −15% and 15%.(4) PCon,Abs and PCon,Trg were achieved in a shorter period as compared to Figure 4.The sinking amplitude of the PCon,Fuz curve has significantly decreased after the 60th second and became stable in the last 60 s.
The calculated values and experimental values of the NOx and NH3 were further compared to ensure that the revised data can be well trusted as shown in Figure 10.It is compared by the correction factor φ before and after applying the UWS dynamic control method as illustrated in Figure 11 (φA and φB indicate the correction factors after and before applying the control method, respectively).
(1) From the UWS injection starting to 20th second, φB remained at more than 0 with a gradually declining trend.That is for the catalyst NH3 storage characteristic therefore UWS injection should be compensated.(2) From 20th to 60th second, φB remained constant and close to 0. Now catalyst NH3 storage has been saturated without NH3 slip so UWS injection may not be corrected.(3) From 60th to 120th second, φB was less than 0. As compared to Figure 7 it is observed that the change trend of φB was contrary to the change trend of NH3 concentration.It is because of the severely increasing NH3 slip and UWS injection must be reduced.It is compared by the correction factor ϕ before and after applying the UWS dynamic control method as illustrated in Figure 11 (ϕ A and ϕ B indicate the correction factors after and before applying the control method, respectively).
(1) From the UWS injection starting to 20th second, ϕ B remained at more than 0 with a gradually declining trend.That is for the catalyst NH 3 storage characteristic therefore UWS injection should be compensated.
(2) From 20th to 60th second, ϕ B remained constant and close to 0. Now catalyst NH 3 storage has been saturated without NH 3 slip so UWS injection may not be corrected.(3) From 60th to 120th second, ϕ B was less than 0. As compared to Figure 7   The uncertain conversion efficiency PCon,Fuz and its velocity vCon,Fuz before and after applying the UWS dynamic correction are compared as shown in Figure 12: (1) PCon,Fuz and vCon,Fuz were changed in the last 60 s on applying UWS injection dynamic correction during a high level of NH3 slip.(2) The PCon,Fuz after correction appeared more close to PCon,Trg and its range was reduced from −250%-95% to 40%-95% compared to the values before correction.(3) The range of the vCon,Fuz was reduced from −70%-30% to −10%-10% compared to the value before correction.
Figure 12.Conversion efficiency and its velocity before and after correction.

High-Frequency Dynamic Process Correction Result
The emission data comparison in ETC cycle before and after the UWS dynamic control method application is shown in Figure 13.The uncertain conversion efficiency P Con,Fuz and its velocity v Con,Fuz before and after applying the UWS dynamic correction are compared as shown in Figure 12: (1) P Con,Fuz and v Con,Fuz were changed in the last 60 s on applying UWS injection dynamic correction during a high level of NH 3 slip.(2) The P Con,Fuz after correction appeared more close to P Con,Trg and its range was reduced from −250%-95% to 40%-95% compared to the values before correction.(3) The range of the v Con,Fuz was reduced from −70%-30% to −10%-10% compared to the value before correction.The uncertain conversion efficiency PCon,Fuz and its velocity vCon,Fuz before and after applying the UWS dynamic correction are compared as shown in Figure 12: (1) PCon,Fuz and vCon,Fuz were changed in the last 60 s on applying UWS injection dynamic correction during a high level of NH3 slip.(2) The PCon,Fuz after correction appeared more close to PCon,Trg and its range was reduced from −250%-95% to 40%-95% compared to the values before correction.(3) The range of the vCon,Fuz was reduced from −70%-30% to −10%-10% compared to the value before correction.
Figure 12.Conversion efficiency and its velocity before and after correction.

High-Frequency Dynamic Process Correction Result
The emission data comparison in ETC cycle before and after the UWS dynamic control method application is shown in Figure 13.

3. 1 .
Mathematical Model ValidationAs shown in Figure5(in this paper, A indicates the values after correction, B indicates the values before correction, T indicates the values obtained from equipment testing, C indicates the values obtained from calculation and M indicates the values obtained from the maps).The UWS injection was started at the 6th second.It can be seen in Figure5that the C N,Act curve declined gradually in the first 30 s, became stable at a very low level in the second 30 s, and two noticeable humps can be observed in the last 60 s.Considering the NH 3 cross sensitivity of the NO x sensor, it could be initially assumed that the conversion efficiency was low in the first 30 s as the UWS injection was not sufficient.The NH 3 slip increased significantly in the position of the two humps with the severely overloaded UWS injection.The UWS correction under dynamic conditions is critical for improving SCR conversion efficiency and NH 3 slip inhibition.unit (SCU); 7: UWS tank; 8: Air pump; 9: Monitor; 10: Exterior gateway protocol (EGP); 11: Temperature sensor; 12: NOx sensor; 13: Emissions analyzer; and 14: Diesel engine.

Figure 5 .
Figure 5. NOx emissions in dynamic conditions before the correction.

Figure 5 .
Figure 5. NO x emissions in dynamic conditions before the correction.

( 1 )
From the 0 to the 60th second and the 90th to 100th second, the experimental value of the NH3 concentration downstream from the SCR system is almost 0 ppm.The NOx concentration and NH3 concentration calculated by the correction model completely overlap with the experimental values.(2) From the 60th to 90th second and the 100th to 120th second, there are slight deviations in the hump position between the calculated value and the experimental value of the NH3 concentration.The two compared values of NOx concentration no longer completely overlap, but the range and the change rate are apparently the same.(3) At zero NH3 slip condition, the calculation deviation of NH3 concentration was between −10 and 0 ppm and that of the NOx concentration was between −20 and 20 ppm.(4) Under high NH3 slip conditions, the calculation deviation of NH3 concentration was between −40 and 100 ppm and that of NOx concentration was between −70 and 70 ppm.

Figure 7 .
Figure 7.The values and deviations of the NOx and NH3 before the correction.
x concentration and NH 3 concentration calculated by the correction model completely overlap with the experimental values.(2) From the 60th to 90th second and the 100th to 120th second, there are slight deviations in the hump position between the calculated value and the experimental value of the NH 3 concentration.The two compared values of NO x concentration no longer completely overlap, but the range and the change rate are apparently the same.(3) At zero NH 3 slip condition, the calculation deviation of NH 3 concentration was between −10 and 0 ppm and that of the NO x concentration was between −20 and 20 ppm.(4) Under high NH 3 slip conditions, the calculation deviation of NH 3 concentration was between −40 and 100 ppm and that of NO x concentration was between −70 and 70 ppm.Energies 2017, 10, 12 11 of 17

( 1 )
From the 0 to the 60th second and the 90th to 100th second, the experimental value of the NH3 concentration downstream from the SCR system is almost 0 ppm.The NOx concentration and NH3 concentration calculated by the correction model completely overlap with the experimental values.(2) From the 60th to 90th second and the 100th to 120th second, there are slight deviations in the hump position between the calculated value and the experimental value of the NH3 concentration.The two compared values of NOx concentration no longer completely overlap, but the range and the change rate are apparently the same.(3) At zero NH3 slip condition, the calculation deviation of NH3 concentration was between −10 and 0 ppm and that of the NOx concentration was between −20 and 20 ppm.(4) Under high NH3 slip conditions, the calculation deviation of NH3 concentration was between −40 and 100 ppm and that of NOx concentration was between −70 and 70 ppm.

Figure 7 .
Figure 7.The values and deviations of the NOx and NH3 before the correction.Figure 7. The values and deviations of the NO x and NH 3 before the correction.

Figure 7 .
Figure 7.The values and deviations of the NOx and NH3 before the correction.Figure 7. The values and deviations of the NO x and NH 3 before the correction.

Figure 9 .
Figure 9.The four kinds of conversion efficiency after the correction.Figure 9.The four kinds of conversion efficiency after the correction.

Figure 9 .
Figure 9.The four kinds of conversion efficiency after the correction.Figure 9.The four kinds of conversion efficiency after the correction.

Figure 10 .
Figure 10.The values and deviations of the NOx and NH3 after the correction.

Figure 10 .
Figure 10.The values and deviations of the NO x and NH 3 after the correction.

17 ( 6 )
it is observed that the change trend of ϕ B was contrary to the change trend of NH 3 concentration.It is because of the severely increasing NH 3 slip and UWS injection must be reduced.(4) ϕ A and ϕ B were greater than 0 and declined gradually from the starting position of UWS injection till the 20th second.However, the decrease of ϕ A was faster than that of ϕ B .(5) ϕ A and ϕ B remained close to 0 from 20th till the 60th second.(6) Values of both the factors (ϕ A and ϕ B ) became less than 0 during the last 60 s.Both factors shared two troughs.The trough values of ϕ B ranged from −5 to −8 and that of the ϕ A was −1 to 0 in curve.Energies 2017, 10, 12 14 of Values of both the factors (φA and φB) became less than 0 during the last 60 s.Both factors shared two troughs.The trough values of φB ranged from −5 to −8 and that of the φA was −1 to 0 in curve.

Figure 11 .
Figure 11.The dynamic correction fact before and after the correction.

Figure 11 .
Figure 11.The dynamic correction fact before and after the correction.

Energies 2017, 10 , 12 14 of 17 ( 6 )
Values of both the factors (φA and φB) became less than 0 during the last 60 s.Both factors shared two troughs.The trough values of φB ranged from −5 to −8 and that of the φA was −1 to 0 in curve.

Figure 11 .
Figure 11.The dynamic correction fact before and after the correction.

Figure 12 .
Figure 12.Conversion efficiency and its velocity before and after correction.

Table 1 .
Specifications of main equipment.

Table 1 .
Specifications of main equipment.
Symbols a UWS,ActAcceleration of q UWS,Act C N,Act NO x concentration measured by the NO x sensor C NO x ,ActActual NO x concentration at the testing positionC NO x ,OriOriginal NO x concentration of the engine before aftertreatmentC NO x ,TrgThe target of NO x concentration downstream SCR systemC NH 3 ,Act Actual NH 3 concentration at the testing position C DNH 3 ,Act Test error of actual NH 3 concentration C DNO x ,Act Test error of actual NO x concentration K T Cross sensitive factor P Con,Fuz Uncertain conversion efficiency P Con,Abs Absolute conversion efficiency P Con,Rel Relative conversion efficiency P Con,Trg Targeted conversion efficiency q UWS,Act Real-time UWS injection rate after correction q UWS,Bas Basic UWS injection rate before correction Q NO x ,Red NO x conversion potential Q NO x ,AcRed Actual value of the total reduced NO x Q NO x ,TrgRed Total reduced NO x under target conversion efficiency R AN Ammonia nitrogen ratio set in the SCR control strategy ϕ Correction factor of the UWS injection rate ϕ A Correct factor after applying the control method B Correct factor before applying the control method Reference