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Article

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

School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
*
Author to whom correspondence should be addressed.
Energies 2017, 10(1), 12; https://doi.org/10.3390/en10010012
Submission received: 28 September 2016 / Revised: 4 December 2016 / Accepted: 9 December 2016 / Published: 23 December 2016
(This article belongs to the Special Issue Automotive Engines Emissions and Control)

Abstract

:
Since the Euro-ІІІ 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-Ѵ standard.

1. 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 NOx which have a significant pollution impact on the environment. Therefore, NOx emissions should be controlled. High pressure fuel injection and turbocharging technology have been used to change the ratio of particulate matter (PM) and NOx by regulating the fuel injection strategy. From the Euro-ІІ to the Euro-ІІІ 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 NOx to reach the emission goals. From the Euro-ІІІ to the Euro-ІѴ phase, the main problem is how to significantly reduce PM and NOx emissions. There are two methods at present: one is to use exhaust gas recirculation (EGR) to reduce in-cylinder NOx, 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 NOx 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 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-ІѴ to the Euro-Ѵ phase, how to further reduce NOx has become the key problem. The main method is to optimize the SCR system to enhance the NOx conversion efficiency and reduce the NH3 leakage [1,2,3,4,5,6].
At present, almost 99% of diesel vehicles work under dynamic operating conditions and thus their NOx 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 NH3 slip inhibition. However, there is a trade-off law between NOx conversion efficiency and NH3 slip under dynamic conditions. When the actual injection rate is lower than the theoretical one, the NOx can’t be completely reduced. When the actual injection rate is higher than the theoretical value, NH3 can’t be completely oxidized and thus generates secondary pollution [7,8,9]. Furthermore, NH3 storage and catalyst release make the NH3 slip inhibition more difficult.
Some researchers believe that an oxidation processor installed at the end of the exhaust pipe may inhibit NH3 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 NH3. 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 H2O and CO2. Based on the data acquired from the experiments, a dynamic model of the NH3 oxidation process was established.
Some researchers supposed that it is necessary to investigate the processes of NH3 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-NOx 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 NOx sensor was cross-sensitive to the NH3 concentration, and the NOx sensor reading was corrected by precise NH3 sensor measurements. The simulation results showed that the designed observer worked well.
NOx sensors are used to measure the NOx concentration downstream from the SCR system and feed it back to the SCR controller. However, research results have showed that NOx sensors have an enhanced cross sensitivity to NH3 [17,18,19]. According to the structural characteristics of NOx sensors, the NH3 inside the sensor is easily oxidized to NOx. There are mainly three chemical reactions in this process, as described in Equations (1)–(3) [20,21,22,23]:
2 NH 3 + 2 O 2 N 2 O + 3 H 2 O
2 NH 3 + 2.5 O 2 2 NO   +   3 H 2 O
2 NH 3 + 3.5 O 2 2 NO 2 + 3 H 2 O
Wang [24] believed that the main factor is temperature, which may affect the three chemical reactions. The cross sensitivity factor of the NOx 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 NOx conversion efficiency and NH3 slip by using a presented method of “uncertain conversion efficiency curve tangent analysis” based on the NH3 cross sensitivity characteristics of the NOx sensor. The degree of NH3 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 NH3 slip inhibition will be verified under low-frequency and high-frequency (ETC cycle) dynamic working conditions.

2. Correction Strategy Mathematical Analysis

For the SCR system control strategy, the corrected UWS injection rate is calculated by Equation (4):
q uws , Act = ( 1 + φ ) × q uws , Bas
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):
a UWS , Act = q uws , Act t = q uws , Bas φ t + ( 1 + φ ) q uws , Bas t
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 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):
C N , Act = C NO x , Act + K T C NH 3 , Act
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):
C NO x , Trg = C NO x , Ori P Con , Trg
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 Section 2.2 and Section 2.3 for more details).

2.1. 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):
P Con , Fuz = C NO x , Ori C N , Act C NO x , Ori = 1 C N , Act C NO x , Ori
P Con , Abs = C NO x , Ori C NO x , Act C NO x , Ori = [ C NO x , Ori ( C N , Act K T C NH 3 , Act ) ] C NO x , Ori = P Con , Fuz + K T C NH 3 , Act C NO x , Ori
According to the results of Equations (8) and (9), the relative conversion efficiency can be calculated with Equation (10):
P Con , Rel = P Con , Abs P Con , Trg = P Con , Fuz + K T C NH 3 , Act C NO x , Ori P Con , Trg
The change rules of PCon,Fuz, PCon,Abs, and PCon,Rel can be obtained from Equations (11)–(13):
v Con , Fuz = P Con , Fuz = ( C N , Act C NO x , Ori ) t
v Con , Abs = P Con , Abs = P Con , Fuz + ( K T C NH 3 , Act C NO x , Ori ) t
v Con , Rel = v Con , Abs v Con , Trg = P Con , Fuz + ( K T C NH 3 , Act C NO x , Ori ) P Con , Trg t
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,FuzPCon,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 PCon,Fuz > PCon,Trg). For such an instance the NH3 slip is assumed to be zero, thus the UWS need not be corrected.
(2)
0 ≤ PCon,FuzPCon,Trg, and tanθ < 0 (vCon,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), PCon,Fuz becomes smaller due to the increase of the CN,Act. The enlargement of the CN,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 NH3 slip due unreacted ammonia.
  • The second case is that insufficient UWS may cause a slowing the process and lead to a growing amount of NOx remaining unreduced due to unavailability of reactant.
Therefore, it may be concluded that with the condition aUWS,Act ≤ 0 and PCon,AbsPCon,Trg, there is no NH3 slip, thus UWS compensation could be continued. When aUWS,Act > 0 and PCon,Abs = PCon,Trg, NH3 slip is severely increased, thus UWS injection should be reduced.
(3)
0 ≤ PCon,FuzPCon,Trg, and tanθ ≥ 0 (vCon,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 aUWS,Act > 0 and PCon,AbsPCon,Trg, there is no NH3 slip like the previous cases, thus UWS compensation should be continued. With the condition aUWS,Act ≤ 0 and PCon,AbsPCon,Trg, UWS injection should be reduced as NH3 slip is going to increase.
(4)
PCon,Fuz < 0
This particular case emerges on ruling out the test error and the engine calibration map error, thus under these circumstances CNOx,ActCNOx,Ori (theoretically), whereas, CN,Act > CNOx,Ori (PCon,Fuz < 0), CNH3,Act > 0 as shown in Equation (7). This case indicates a seriously high level of NH3 slip therefore UWS injection must be stopped immediately.

2.2. Urea Water Solution Flow Dynamic Compensation

There was no NH3 slip during the process of the UWS dynamic compensation. Therefore:
{ C NH 3 , Act 0 C NO x , Act C N , Act
It is indicated that the actual amount of injected UWS (including the NH3 released from the catalyst) was less than the demand of SCR reaction. The condition is 0 ≤ PCon,AbsPCon,Trg and zero NH3 slip. Therefore, the correction factor φ can be calculated using Equation (15):
φ Δ Q NO x , Red Q NO x , ActRed
where QNOx,Red (NOx conversion potential) is the difference between the target value and actual value of the total reduced NOx in a period as shown by Equation (16) and QNOx,AcRed is the actual value of the total reduced NOx in a specific period given by Equation (17):
Δ Q NO x , Re = C NO x , Ori P Con , Trg q Exh d t ( C NO x , Ori C NO x , Act ) q Exh d t = [ C N , Act C NO x , Ori ( 1 P Con , Trg ) ] q Exh d t
Q NO x , ActRe = ( C NO x , Ori C NO x , Act ) q Exh d t = ( C NO x , Ori C NO x , N ) q Exh d t
φ = Δ Q N O x , Red / t Q N O x , A c t Red / t = [ C N , Act C NO x , Ori ( 1 P Con , Trg ) ] q Exh ( C NO x , Ori C N , Act ) q Exh = C NO x , Ori P Con , Trg ( C NO x , Ori C N , Act ) 1
C N , Act C NO x , Ori
For the control method, the calculation of UWS injection rate and its acceleration may be accomplished with Equation (20) or Equation (21):
{ q uws , Act = C NO x , Ori P Con , Trg ( C NO x , Ori C NO x , N ) q uws , Bas a uws , Act = q uws , Bas ( C NO x , Ori P Con , Trg C NO x , Ori C N , Act ) t + ( C NO x , Ori P Con , Trg C NO x , Ori C N , Act ) q uws , Bas t C N , Act C NO x , Ori
{ q uws , Act = 0 a uws , Act = 0 C N , Act C NO x , Ori
where NH3 slip is assumed to be zero.
The value of NOx emission in this process may be obtained with:
{ Q N O x = C N , Act q Exh d t Q NH 3 0

2.3. Urea Water Solution Flow Dynamic Reduction

During the UWS dynamic process, reduction is the response of increasing NH3 slip. Therefore:
{ C NH 3 , Act 0 C NO x , Act + K T C NH 3 , Act = C N , Act
The case of 0 ≤ PCon,AbsPCon,Trg and presence of evident NH3 slip shows that the actual amount of injected UWS (including the NH3 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):
C NO x , Act = C NO x , Ori ( 1 P Con , Trg )
According to Equation (7):
C NH 3 , Act = C N , Act C NO x , Ori ( 1 P Con , Trg ) K T
The correction factor φ can be calculated as Equation (26):
φ = Q N H 3 R AN ( Q N O x , TrgRed + Q N H 3 R AN ) = Q N H 3 ( R AN Q N O x , TrgRed + Q N H 3 )
where QNH3 is the total NH3 emission amount in a specific period of time given by Equation (27), QNOx,TrgRed is the total reduced NOx with target conversion efficiency of Equation (28) and RAN is the ammonia nitrogen ratio constant set in the SCR system control strategy:
Q NH 3 = C NH 3 , Act q Exh d t = [ C N , Act C NO x , Ori ( 1 P Con , Trg ) K T ] q Exh d t
φ = Q N H 3 / t ( R AN Q N O x , TrgRed + Q N H 3 ) / t = [ C N O x , Ori ( 1 P Con , Trg ) C N , Act ] q Exh K T ( R AN P Con , Trg C NO x , Ori C NO x , Ori ( 1 P Con , Trg ) C N , Act K T ) q Exh = C NO x , Ori ( 1 P Con , Trg ) C N , Act K T R AN P Con , Trg C NO x , Ori C NO x , Ori ( 1 P Con , Trg ) + C N , Act
Due to qUWS,Act ≥ 0 and 1 + φ ≥ 0:
C N , Act C NO x , Ori ( 1 + K T R AN ) P Con , Trg C NO x , Ori
In the presence of NH3 Slip, the control method may be applied to calculate the UWS injection rate and its acceleration is given by Equation (30) or Equation (31):
{ q uws , Act = ( 1 1 K T R AN + C NO x , Ori C N , Act K T R AN P Con , Trg C NO x , Ori ) q uws , Bas a uws , Act = q uws , Bas ( C NO x , Ori C N , Act R AN P Con , Trg C NO x , Ori 1 K T R AN ) t + ( 1 1 K T R AN + C NO x , Ori C N , Act K T R AN P Con , Trg C NO x , Ori ) q uws , Bas t C N , Act C NO x , Ori ( 1 + K T R AN ) P Con , Trg C NO x , Ori
{ q uws , Act = 0 a uws , Act = 0 C N , Act C NO x , Ori ( 1 + K T R AN ) P Con , Trg C NO x , Ori
The amount of NOx emission and NH3 emission in this process could be determined by:
{ Q N O x = C N , Act q Exh d t = C NO x , Ori ( 1 P Con , Trg ) q Exh d t Q NH 3 = [ C N , Act C NO x , Ori ( 1 P Con , Trg ) ] q Exh d t

2.4. Special Case

For the special case of PCon,Abs > PCon,Trg, the actual value is more than the target value of the system conversion efficiency and NH3 slip is assumed to be zero, as can be seen Equation (33):
{ C NH 3 , Act 0 C NO x , Act C N , Act
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
The amount of NOx emission and NH3 emission in this process are:
{ Q N O x = C N , Act q Exh d t Q NH 3 0
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:
{ q uws , Act = 0 a uws , Act = 0
The amount of NOx emission and NH3 emission in this case can be described as follows:
{ Q N O x = C N , Act q Exh d t = C NO x , Ori ( 1 P Con , Trg ) q Exh d t Q NH 3 = [ C N , Act C NO x , Ori ( 1 P Con , Trg ) ] q Exh d t

3. 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.
The heavy duty diesel engine parameters used in the experiment are given in Table 2.

3.1. 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.
(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.
(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.

3.2. Low-Frequency Dynamic Process Correction Result

The low-frequency process has been repeated with the dynamic correction of the UWS injection. The NOx and NH3 emissions after the correction are shown in Figure 8.
(1)
From the UWS injection starting position to 60th second there was no NH3 slip. Between the 60th and 120th second the second two humps appeared in the NH3 concentration curve at less than 80 ppm. That’s because of the fractionally unreacted NH3 slip downstream of the SCR system.
(2)
In the low-frequency process, the actual tested value of the NOx emission was almost the same as the target value. The actual tested value was slightly lower than the target value in the hump region of the NH3 slip. That’s because of the undue UWS injection and more NOx was restored by the excessive NH3.
(3)
In the hump region of the NH3 slip, the measured value is slightly higher than the target value of the NOx concentration. That’s because the NOx sensor was influenced by NH3 and NOx at the same time as shown in Equation (7).
Comparing the engine emissions before and after the UWS dynamic correction:
(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 low-frequency 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.
(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.
(1)
In the corrected low-frequency process the trend of the calculated NH3 concentration was the same with that of the experimental value. The deviation of the calculation was more obvious in the region of high NH3 slip as compared to the results shown in Figure 7. The deviation oscillated between −10 to 0 ppm and −5 to 0 ppm with NH3 slip and between −40 to 100 ppm and −20 to 15 ppm without NH3 slip.
(2)
Moreover, during the corrected low−frequency process trend of the calculated NOx concentration was the same as the actual tested value. The calculation deviation was uniformly distributed and oscillated between −70 to 70 ppm and −20 to 20 ppm as compared with Figure 7.
(3)
NOx concentration calculation and NH3 concentration downstream of the SCR system would become more accurate with application of the UWS dynamic correction.
It is compared by the correction factor φ before and after applying the UWS dynamic control method as illustrated in Figure 11A 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.
(4)
φA and φB were greater than 0 and declined gradually from the starting position of UWS injection till the 20th second. However, the decrease of φA was faster than that of φB.
(5)
φA and φB remained close to 0 from 20th till the 60th second.
(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.
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.

3.3. 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 result indicated a great high efficiency of NOx conversion in the two tests. The NOx emission was improved by 53.9% and the NH3 slip was reduced by 90.8% with the application of the control method. The NH3 slip was also inhibited significantly. The levels of engine NOx emissions and NH3 slip were improved and conform to the Euro-V standard.

4. Conclusions

(1)
It can be generalized that the “uncertain conversion efficiency curve tangent analysis” method can accurately justify the different NH3 slip situation.
(2)
The calculation deviation can be controlled with NOx between −20 ppm and 20 ppm and NH3 between −20 ppm to 15 ppm by application of UWS dynamic correction in a low-frequency process. The NOx emission was improved by 5.58% and NH3 slip was reduced by 92.68%.
(3)
In the application of UWS dynamic correction in high-frequency process (i.e., during of ETC test), in spite the fact the NOx emission has been improved by 53.9%, the NH3 slip was reduced by 90.8%. The level of engine NOx emissions and NH3 slip has been improved up to Euro-IV and closer to the Euro-V standard. The newly developed method presents a significant NH3 slip inhibition.

Author Contributions

Long Li and Wei Lin made the method. Long Li and Wei Lin designed the experiment and organized the entire experiment process. Youtong Zhang made many experimental suggestions and collated the experiment data.

Conflicts of Interest

The authors declare no conflict of interests.

Abbreviations

ASCAmmonia slip catalysts
ETCEuropean transient cycle
HCRHigh-pressure common rail
PMParticulate matter
SCRSelect catalyst reduction
UWSUrea water solution

Symbols

aUWS,ActAcceleration of qUWS,Act
CN,ActNOx concentration measured by the NOx sensor
CNOx,ActActual NOx concentration at the testing position
CNOx,OriOriginal NOx concentration of the engine before aftertreatment
CNOx,TrgThe target of NOx concentration downstream SCR system
CNH3,ActActual NH3 concentration at the testing position
CDNH3,ActTest error of actual NH3 concentration
CDNOx,ActTest error of actual NOx concentration
KTCross sensitive factor
PCon,FuzUncertain conversion efficiency
PCon,AbsAbsolute conversion efficiency
PCon,RelRelative conversion efficiency
PCon,TrgTargeted conversion efficiency
qUWS,ActReal-time UWS injection rate after correction
qUWS,BasBasic UWS injection rate before correction
QNOx,RedNOx conversion potential
QNOx,AcRedActual value of the total reduced NOx
QNOx,TrgRedTotal reduced NOx under target conversion efficiency
RANAmmonia nitrogen ratio set in the SCR control strategy
φCorrection factor of the UWS injection rate
φACorrect factor after applying the control method
φBCorrect factor before applying the control method Reference

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Figure 1. The correction strategy model. UWS: urea water solution.
Figure 1. The correction strategy model. UWS: urea water solution.
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Figure 2. The UWS correction model.
Figure 2. The UWS correction model.
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Figure 3. Low frequency dynamic process.
Figure 3. Low frequency dynamic process.
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Figure 4. Engine experiment platform block diagram. 1: Fuel consumption; 2: Air flowmeter; 3: Dynamometer; 4: Electronic control unit (ECU); 5: Unified diagnostic services (UDS); 6: SCR control 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 4. Engine experiment platform block diagram. 1: Fuel consumption; 2: Air flowmeter; 3: Dynamometer; 4: Electronic control unit (ECU); 5: Unified diagnostic services (UDS); 6: SCR control 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.
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Figure 5. NOx emissions in dynamic conditions before the correction.
Figure 5. NOx emissions in dynamic conditions before the correction.
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Figure 6. The conversion efficiency tangents.
Figure 6. The conversion efficiency tangents.
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Figure 7. The values and deviations of the NOx and NH3 before the correction.
Figure 7. The values and deviations of the NOx and NH3 before the correction.
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Figure 8. The NOx and NH3 emissions before and after the correction.
Figure 8. The NOx and NH3 emissions before and after the correction.
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Figure 9. The four kinds of conversion efficiency after the correction.
Figure 9. The four kinds of conversion efficiency after the correction.
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Figure 10. The values and deviations of the NOx and NH3 after the correction.
Figure 10. The values and deviations of the NOx and NH3 after the correction.
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Figure 11. The dynamic correction fact before and after the correction.
Figure 11. The dynamic correction fact before and after the correction.
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Figure 12. Conversion efficiency and its velocity before and after correction.
Figure 12. Conversion efficiency and its velocity before and after correction.
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Figure 13. Emissions in the engine ETC cycle. (a) Before UWS injection dynamic correction; and (b) after UWS injection dynamic correction.
Figure 13. Emissions in the engine ETC cycle. (a) Before UWS injection dynamic correction; and (b) after UWS injection dynamic correction.
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Table 1. Specifications of main equipment.
Table 1. Specifications of main equipment.
NameTypeManufacturerLocationRemark
Diesel engineISDe270 40DCECXiangfan, ChinaL6
DynamometerGWD300POWERLINKChangsha, ChinaEddy current style
Fuel consumptionFC2210POWERLINKChangsha, ChinaQuality style
Air flow meterToCeilShanghai ToCeil Engine Testing EquipmentShanghai, ChinaHot film style
Emissions analyzerSESAM4.0AVLGraz, AustriaFourier transform infrared spectroscopy
Table 2. Parameters of ISDe270 40 diesel engine.
Table 2. Parameters of ISDe270 40 diesel engine.
Cylinder NumberBore/StrokeCapacityCompression RatioRated Power/SpeedMax TorqueFuel Supply Type
-mm/mmL-kW/rpmNm/rpm-
L6107/1246.717.3:1198/2500970/1400high pressure common rail

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MDPI and ACS Style

Li, L.; Lin, W.; Zhang, Y. A New Dynamic Injection System of Urea-Water Solution for a Vehicular Select Catalyst Reduction System. Energies 2017, 10, 12. https://doi.org/10.3390/en10010012

AMA Style

Li L, Lin W, Zhang Y. A New Dynamic Injection System of Urea-Water Solution for a Vehicular Select Catalyst Reduction System. Energies. 2017; 10(1):12. https://doi.org/10.3390/en10010012

Chicago/Turabian Style

Li, Long, Wei Lin, and Youtong Zhang. 2017. "A New Dynamic Injection System of Urea-Water Solution for a Vehicular Select Catalyst Reduction System" Energies 10, no. 1: 12. https://doi.org/10.3390/en10010012

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