COMPUTATIONAL AND EXPERIMENTAL INVESTIGATION ON AFTER-TREAMENT SYSTEMS TO MEET FUTURE EMISSION NORMS FOR TRUCK APPLICATIONS S. Karthikeyan, R. Hariganesh, M. Sathyanandan, S. Krishnan Engine Product Development Department (Sunrise), Ashokleyland Technical Center Chennai, Tamil Nadu, India P. Vadivel, D.Vamsidhar CFD Department, Defiance Technologies Chennai, Tamil Nadu, India Abstract: Future emission legislation for light commercial vehicles can be met through appropriate design of after – treatment devices like Diesel Oxidation Catalyst (DOC) for HC, CO and partial PM reduction and Cooled –Exhaust Gas Recirculation (C-EGR) systems for NOx reduction for low cost solution. Computational analysis offer new possibilities for optimal design of DOC and C-EGR systems for low cost truck applications. Further the reduction in NOx emissions within the new legislation, forces the increase of EGR rates, for this EGR cooler with higher thermal dissipation capacity is required. Corrugated tube design for the cooler pipe increases the thermal efficiency of EGR cooler without affecting the engine performance. This paper provides the overview on exhaust flow uniformity index, pressure drop across the DOC and hot spot prediction in cooler tubes, pressure distributions inside the EGR cooler using Computational Fluid Dynamics (CFD). With the optimized DOC/C-EGR design the emission results are investigated on chassis dynamometer to meet BS-IV diesel emission norms. Keywords: Diesel Oxidation Catalyst; EGR, Emissions. 1. Introduction The new policies for emissions reduction from diesel engines have required the development of after- treatment systems that have been added to engines in order to reduce the emissions of CO, Unburned Hydrocarbons (UHC), NOx and particulates. As emission regulations have tightened, the complexity of the catalytic converter system has increased. Ceramic based catalytic converters are commonly utilized as a solution in meeting legislated emission requirements. Meeting the demands of tightening legislation depends on emission conversion efficiency and light-off behavior of catalytic converter. The emission conversion efficiency and light-off of a catalytic converter depends on various factors. On one hand, there are the properties of the monolith such as cross section, length, cell density, wall thickness, washcoat formulation and loading. On the other hand, the flow characteristics of the exhaust gas such as flow velocity, temperature, composition of raw emissions and flow distribution play an important role for the conversion rate and light-off behavior of catalytic converter [Michael G. Campbell (1995), Chandler.G.R (2000)]. Diesel engines are attractive for smaller trucks and passenger cars due to their low fuel consumption. But the high NOx emissions from diesel engines remain a major pollution problem. Combustion processes and engines will have to undergo further development to comply with future emission limits. Recent studies have demonstrated how a cooled exhaust gas recirculation system (EGR) system can reduce NOx emissions [Susan C(2008), Julia Windmann (2003)]. By reducing the inlet exhaust recirculation gas temperature with the EGR S. Karthikeyan et al. / International Journal of Engineering Science and Technology (IJEST) ISSN : 0975-5462 Vol. 3 No. 4 Apr 2011 3314
Transcript
COMPUTATIONAL AND EXPERIMENTAL INVESTIGATION ON
AFTER-TREAMENT SYSTEMS TO MEET FUTURE EMISSION NORMS
FOR TRUCK APPLICATIONS
S. Karthikeyan, R. Hariganesh, M. Sathyanandan, S. Krishnan
Engine Product Development Department (Sunrise), Ashokleyland Technical Center Chennai, Tamil Nadu, India
P. Vadivel, D.Vamsidhar
CFD Department, Defiance Technologies Chennai, Tamil Nadu, India
Abstract: Future emission legislation for light commercial vehicles can be met through appropriate design of after –treatment devices like Diesel Oxidation Catalyst (DOC) for HC, CO and partial PM reduction and Cooled –Exhaust Gas Recirculation (C-EGR) systems for NOx reduction for low cost solution. Computational analysis offer new possibilities for optimal design of DOC and C-EGR systems for low cost truck applications. Further the reduction in NOx emissions within the new legislation, forces the increase of EGR rates, for this EGR cooler with higher thermal dissipation capacity is required. Corrugated tube design for the cooler pipe increases the thermal efficiency of EGR cooler without affecting the engine performance. This paper provides the overview on exhaust flow uniformity index, pressure drop across the DOC and hot spot prediction in cooler tubes, pressure distributions inside the EGR cooler using Computational Fluid Dynamics (CFD). With the optimized DOC/C-EGR design the emission results are investigated on chassis dynamometer to meet BS-IV diesel emission norms.
The new policies for emissions reduction from diesel engines have required the development of after-treatment systems that have been added to engines in order to reduce the emissions of CO, Unburned Hydrocarbons (UHC), NOx and particulates. As emission regulations have tightened, the complexity of the catalytic converter system has increased. Ceramic based catalytic converters are commonly utilized as a solution in meeting legislated emission requirements. Meeting the demands of tightening legislation depends on emission conversion efficiency and light-off behavior of catalytic converter. The emission conversion efficiency and light-off of a catalytic converter depends on various factors. On one hand, there are the properties of the monolith such as cross section, length, cell density, wall thickness, washcoat formulation and loading. On the other hand, the flow characteristics of the exhaust gas such as flow velocity, temperature, composition of raw emissions and flow distribution play an important role for the conversion rate and light-off behavior of catalytic converter [Michael G. Campbell (1995), Chandler.G.R (2000)]. Diesel engines are attractive for smaller trucks and passenger cars due to their low fuel consumption. But the high NOx emissions from diesel engines remain a major pollution problem. Combustion processes and engines will have to undergo further development to comply with future emission limits. Recent studies have demonstrated how a cooled exhaust gas recirculation system (EGR) system can reduce NOx emissions [Susan C(2008), Julia Windmann (2003)]. By reducing the inlet exhaust recirculation gas temperature with the EGR
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cooler, the NOx emissions reduction is achieved. Most EGR –coolers are multi-tubular, single or multiple-pass, gas -to- liquid heat exchangers made of stainless steel. The exhaust gas composed of engine combustion products, at high temperature (often higher than 500 oC) passes inside the internal tubes and its heat is transferred to the coolant that flows between the external side of the tubes and the shell. In the design of EGR coolers it is necessary to take into account several aspects such as, heat transfer, pressure drop in the gas and coolant side, temperature control at the outlet and fouling efficiency [Terry Alger (2009)].
2. Combination of Catalyst for Diesel Oxidation system
Palladium when compared to platinum is less active catalytic material for oxidation reactions. On its own in a diesel catalyst, it will often exist in its less active oxide form. When palladium is used in addition to platinum, it stabilize the support catalyst, when heated it forms bimetallic particles. In Diesel oxidation catalyst (DOC) at the highest temperatures platinum often sinters as in a three-way catalyst. This means that the nanoscale platinum particles start to move around the support and merge to form large particle aggregates with an associated loss of efficiency in the catalyst. The addition of palladium can stabilize the size of the precious metal particles when a
catalyst is heated and thus ensure that the performance of the diesel catalyst is maintained at a high level for much longer. So platinum and palladium catalyst in 2:1 for close couple and 1:1 for under floor combination is used for this BS-IV application. Fig 1. Shows the cross section of diesel oxidation catalyst. 3. Long route/short route EGR location Long route EGR is favorable for low loads and short route EGR is favorable for high loads engine operation. Long route EGR has advantages of having more homogeneously EGR distribution per cylinder and needs much smaller cooler. In this BS-IV application the EGR valve is located on the long route. 3.1. Hot side/cold side EGR location If the exhaust gas is recycled to the intake directly, the operation is called hot EGR. If the re-circulated exhaust gas passes through the cooler, it is called C-EGR. In this BS-IV application, EGR valve is placed on the HOT side before the cooler.
Fig. 1. Cross section of Diesel Oxidation Catalyst
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3.2. High pressure/ low pressure EGR concepts Light commercial vehicles can be equipped with a relatively simple EGR system. EGR gas taken from the exhaust side upstream of the turbine and fed to the intake system over an EGR cooler. To increase the EGR mass flow rate, a small pressure difference is created locally at EGR feed point in the intake system. Normally location of EGR valve on high or low pressure side plays a major role for creating this pressure difference. A low pressure loop EGR is achievable because a positive differential pressure between the turbine outlet and compressor inlet is generally than zero. However, conventional compressors and inter-coolers are not designed to endure the temperature and contamination of Diesel exhausts. In general, the low pressure loop approach of EGR is not applicable except for exhaust gas designated compressors. Although options are available, the preferred practice is to re-circulate the exhaust gas from upstream of the turbine to downstream of the compressor (or downstream of the inter-cooler if applicable), i.e. a high pressure side EGR. The compressor and inter-cooler is therefore, not exposed to the exhaust. However, such high pressure side EGR is only applicable when the turbine upstream pressure is sufficiently higher than the boost pressure. In this study the valve is placed on high pressure side for BS-IV applications.
4. Boundary conditions for CFD analysis
The CFD analyses are carried out to find the flow uniformity for catalytic converter and cooling efficiency for C-EGR. All the meshes were generated in ICEM CFD and CFX solver is used for solving the governing equations.
4.1. Close Coupled/ Underbody-Catalytic Converter
A grid independence study is carried out for the given exhaust system model and mesh with 5.1 million tetrahedral cells is found to be adequate for capturing the results accurately.
At the exterior walls of the catalytic converter heat transfer due to thermal convection and radiation is taken into account. The Convection is taken into account by a heat transfer Coefficient of 10 W/ (m2 K). All proposed designs in this paper are analyzed in CFX with appropriate boundary conditions and final design is validated against experiment. In the case of uniform velocity profile it is assumed that at every position on the inlet face the exhaust gas enters the monolith with the mean velocity of each time step. This is to improve the light-off behavior of the catalytic converter. Characteristics for efficient flow defines high velocity flow area where the minimum flow velocity is 65% of the maximum flow velocity and the low velocity area as the area where the minimum flow velocity is 35% of the maximum flow velocity .By rule of the thumb, the high velocity area must utilize 40% of the catalyst frontal area and the low velocity flow must utilize about 90% of the catalyst front face of the substrate. The normalized distribution (with a mean velocity of 1 m/s) is multiplied with the mean velocity of each time step. For the quantification of the uniformity of the velocity distribution, the uniformity index is usually used. Uniformity-Index(UI):
dAVV
VAUI
2
11
Where,
– Uniformity Index (UI)
A – Area of sectional plane
V – Velocity at various data points on sectional plane
V - Area averaged velocity at sectional plane
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4.2. Following boundary conditions are used for the Catalytic Converter analysis
1. Turbulence Model: k-epsilon
2. Working Fluid: Exhaust Gas (Density=0.4393 kg/m3, Viscosity = 3.75e-5 Pa.s)
(ii). Viscous Coefficient: 2.57e07 1/m2 (approx CFX Linear resistance of 959.4 kg/m3-s)
4.3. Following boundary conditions are used for the C-EGR analysis
At the exterior walls of the EGR tubes heat transfer due to thermal convection and radiation is taken into account. The Convection is taken into account by a heat transfer Coefficient of 200 W/ (m2 K). All proposed designs in this paper are analyzed in CFX with appropriate boundary conditions and final design is validated against experiment. Total pressure for this analysis is 3 bars and gas inlet temperature is 680°C. Further the properties of air are used for this analysis with mass flow rate of 37 kg/hr Substrate dimensions selected for BS- IV The substrate and catalysts investigated in the present study are described in Table 1. Substrate used is 400Cpsi, 4.5 mil wall thickness.
Table 1. Substrate used for BS-IV LCV applications
RESULTS & DISCUSSION
Ceramic DOC
GSA (1/m)
PGM loading
BS-IV
CC- Catalytic Converter
2870 50 g/Cu.ft
UF-Catalytic Converter
2870 25 g/Cu.ft
Fig 2. Core length vs. Outlet Exhaust gas reduction temperature with 13 tubes.
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For achieving the cooling efficiency greater than 81% i.e. outlet gas temperature lesser than 200C, cooler pipe length should be greater than 300 mm which is shown in Fig2.
From the figure 3. For achieving the cooling efficiency greater than 81% i.e. outlet gas temperature lesser than 200C, exhaust gas pressure drop is 7mbar for pipe length 300mm this will affect the EGR mixing.
From the figure 4. For achieving the cooling efficiency greater than 81% i.e. outlet gas temperature lesser than 200C, cooler pipe length should be greater than 200 mm.
From the figure 5. For achieving the cooling efficiency greater than 81% i.e. outlet gas temperature lesser than 200C, exhaust gas pressure drop is 7mbar for pipe length 200mm. This will also affect the EGR mixing.
Fig 3. Core length vs. Outlet Exhaust gas Pressure drop for 13 tubes.
Fig 4 Core length vs. Outlet Exhaust gas reduction temperature with 19 tubes.
Fig 5. Core length vs. Outlet Exhaust gas Pressure drop for 19 tubes
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From the figure 6. By increasing the tubes to greater than 30 for 12 lpm of coolant flow hot spots are formed. The cooler efficiency is 88%.
From the figure 7. As the flow is increased hot spot formation is less with the above cooler. The cooling efficiency of 93 % is achieved.
From the figure 8. As the lpm is increased to 18, there is no hot spot formation. Cooling efficiency of 97% is achieved.
Fig 6 Temperature distribution for cooler with greater than 30 tubes at 12 lpm
Fig 7. Temperature distribution for cooler with tubes greater than 30 tubes at 15 lpm
Fig 8. Temperature distribution for cooler with tubes greater than 30 tubes at 18 lpm
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From the figure 9. As the number of tubes is reduced to 11with 12 lpm hot spots regions are found to be more and maximum of 72 % efficiency is achieved. Heat dissipation is 6.9kW.
From the figure 10. As the number of tubes is reduced to 11with 15 lpm, hot spots regions are found to be less and maximum of 79 % efficiency is achieved. Heat dissipation of 7.1 kW is achievable.
From the figure 11. As the number of tubes is reduced to 11 with 18 lpm, hot spots regions are found to be low and maximum of 85 % efficiency is achieved. Heat dissipation of 7.6 kW is achievable
Fig 9. Temperature across the pipe for 12 lpm coolant flow with 11 tubes
Fig 10. Temperature across the pipe for 15 lpm coolant flow with 11 tubes
Fig 11. Temperature across the pipe for 18 lpm coolant flow with 11 tubes
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From the figure 12. For 21 tubes with 12 lpm, hot spots regions are formed. Maximum of 79 % efficiency is achieved. Heat dissipation is 7.1 kW.
From the figure 13. For 21 tubes with 15 lpm, hot spots regions are not formed. Maximum of 84 % efficiency is achieved. Heat dissipation is 7.55 kW.
From the figure 14. For 21 tubes with 18 lpm, hot spots regions are not formed. Maximum of 88 % efficiency is achieved. Heat dissipation is 7.9 kW.
Fig 12. Temperature across the pipe for 12 lpm coolant flow with 21 tubes
Fig 13. Temperature across the pipe for 15 lpm coolant flow with 21 tubes
Fig 14. Temperature across the pipe for 18 lpm coolant flow with 21 tubes
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From the figure 15. For 21 tubes with 21 lpm, hot spots regions are not formed. Maximum of 92 % efficiency is achieved. Heat dissipation is 8.2 kW.
Fig 15. Temperature across the pipe for 21 lpm coolant flow with 21 tubes
Flow of water Flow of Inlet(t3) Outlet(t4) Temp. Diff Heat Dissipation Hot effectiveness
lpm Water in Temp. in Temp. in T2 = t4 - t3 Q
kg/s °C ( t3 ) °C ( t4 ) °C (KW) %
12 0.2 90 99.5 9.5 7.953 88.37
15 0.25 90 98 8 8.372 93.02
18 0.3 90 97 7 8.791 97.67
21 0.35 90 96 6 8.791 97.67
Flow of water
Flow of Inlet(t3) Outlet(t4) Temp. Diff Heat Dissipation Hot effectiveness
lpm Water in Temp. in Temp. in T2 = t4 - t3 Q
kg/s °C ( t3 ) °C ( t4 ) °C (KW) %
12 0.2 90 98.5 8.5 7.116 79.07
15 0.25 90 97.3 7.3 7.639 84.88
18 0.3 90 96.34 6.34 7.962 88.46
21 0.35 90 95.67 5.67 8.307 92.30
Table: 5 Hot effectiveness for 37 tubes without fouling factor
Table: 6 Hot effectiveness for 21 tubes without fouling factor
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Table: 2 Comparison of NOx levels for 13/30 tubes for EGR cooler
The uniformity index is modeled throughout the length of the ceramic substrate for close coupled catalytic converter. The uniformity index increased through the body of the depth flow substrate due to radial and longitudinal flows occurring through the body of the substrate. Computational fluid studies show the increase in uniformity index from the front face of the substrate to the body of the substrate. The flow is found to be very well distributed from the front face of the substrate. The flow is very stable in substrate and attained flow uniformity index () 0.82 (Figures 15).
Flow of water Flow of Inlet(t3) Outlet(t4) Temp. Diff Heat
Dissipation Hot
effectiveness lpm Water in Temp. in Temp. in T2 = t4 - t3 Q
kg/s °C ( t3 ) °C ( t4 ) °C (KW) %
12 0.2 90 97.76 7.76 6.497 72.18
15 0.25 90 96.87 6.87 7.189 79.88
18 0.3 90 96.12 6.12 7.685 85.39
21 0.35 90 95.45 5.45 7.985 88.72
Cooler performance with greater than 30 tubes
Cooler performance with 13 tubes
NOx 0.289 (g/km) 0.299(g/km)
THC +NOx 0.332 (g/km) 0.349(g/km)
Fig 15 Normalized velocity distribution at the center for Close Coupled Catalytic Converter
Table: 7 Hot effectiveness for11 tubes without fouling factor
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The flow is found to be very well distributed from the front face of the under floor substrate. The flow is very stable in substrate and attained flow uniformity index () 0.83 (Figures 16).
From the fig 17 HC, CO emissions is high before LOT, After LOT HC, CO is very well controlled with CC & UF DOC. With the help of 81% efficiency EGR cooler and progressive EGR valve the NOx is optimized to meet BS-IV norms.
EMISSION RESULTS
Table: 3 Emission Levels for BS-IV LCV
Fig17. BS-IV NOx, HC, CO Emission Trace
Fig 16. Normalized velocity distribution at the inlet for Under Floor- Catalytic Converter
Emission BS-IV Norms (g/km)
BS-IV Achieved (g/km)
HC+NOx 0.39 0.34
CO 0.63 0.26
NOx 0.33 0.26
PM 0.04 0.01
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From fig 18, BS-IV consist of close couple and under floor catalytic converter so back pressure is slightly higher than BS-III which has only catalytic converter, but both BS-III/IV as designed within target limit for LCV. CONCLUSION
The selected ceramic DOC substrate and catalytic converter design facilitate proper exhaust flow into the
catalyst. Ultra-thin wall and high cell density substrate offer the possibility to reduce the converter volume,
which provided obvious packaging advantage. Design optimization for the flow, facilitates early light-off
performance during cold start. As a result of CFD analysis of the after-treatment system design, facilitated to
reduce the PGM loadings of 50g/Cu.ft for CCCC and 25g/Cu.ft for UFCC with improved emission performance
by improving the uniformity index to 0.82. With the help of higher cooling efficiency EGR cooler the exhaust
gas temperature is cooled before induction which reduced in-cylinder combustion temperature to meet BS-IV
norms. EGR cooler with tubes greater than 30 provided best performance with 15 lpm of coolant flow. Further
with reduced power 13 tube EGR cooler achieved BS-IV NOx limit as compared to greater than 30 tubes cooler.
By considering the fouling factor, EGR cooler with tubes greater than 30 is selected for this LCV application.
REFERENCES
[1] Michael G. Campbell., Edward P. Martin., “Substrate Selection for Diesel Catalyst,”SAE 950372, 1995.
[2] Chandler.G.R, Phillips.P.R, Wikinins.A.J.J, “Diesel Oxidation Catalyst for Light-Duty Vehicles,” SAE paper 2000-01-1422.
[3] Susan C., Lauderdale, Seth T. Nickerson., “Impact of Ceramic Substrate Web Thickness on Emission Light-Off, Pressure Drop and
Strength,” SAE paper 2008-01-0808
[4] Julia Windmann., Joachim Braun., Peter Zacke., “Impact of the inlet flow distribution on the light-off behavior of a 3 –way catalytic
converter,” SAE paper 2003-01-0937
[5] Terry Alger., Barrett Mangold., “Dedicated EGR : A New Concept in High Efficiency Engines,” SAE paper 2009-01-0694
Fig 18. LCV Exhaust System Pressure drop results
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APPENDIX
DOC- DIESEL OXIDATION CATALYST
C-EGR- COOLED EXHAUST GAS RECIRCULATION
CCCC-CLOSE COUPLED CATALYTIC CONVERTER
UFCC- UNDER FLOOR CATALYTIC CONVERTER
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