carried out as part of Low Carbon Vehicle Technology Project using a candidate SI engine. 2011 European Ricardo Software User Conference, 5/04/2011. 2 ...
Using Wave for reducing emissions for REEV application Dr S. Aleksandrova, Prof S. Benjamin (Coventry University, UK)
REEV applications • EV (traction) batteries currently take several hours to fully charge electric vehicle usage is limited • Range Extended Electric Vehicles operate in full electric mode on short distances (~80% of car mileage) and switch to generator mode on longer trips • In most cases REEV engines use less fuel (and generates less CO2) than conventional car engines because: – engine is smaller - to only cover average power needs – operates at a more efficient speed/load
• Preliminary investigation of REEV engine emissions is being carried out as part of Low Carbon Vehicle Technology Project using a candidate SI engine 2011 European Ricardo Software User Conference, 5/04/2011
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REEV applications
(V-SIM simulation, courtesy of Ricardo)
• engine is on for a fraction of the driving cycle • mostly running in "steady-state" • several cold starts may be present depending on the strategy 2011 European Ricardo Software User Conference, 5/04/2011
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REEV applications • engine operating in a more efficient regime high speed/load high emissions • emissions have to meet standards for both charge-sustaining and charge-depleting tests: "The emissions test results shall comply with the limits under all specified test conditions of this Regulation" (Reg 83, Annex 14, part 1.4)
The weighted average often cited De - electric range, Mdepl and Msust - emissions in charge-depleting and charge-sustaining tests
is used for communication purposes only • test results for the candidate engine studied show that NOx break-through is a problem in steady-state, while CO and THC are more important during cold start 2011 European Ricardo Software User Conference, 5/04/2011
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Emissions trade-off 1000 BSFC / best BSFC power/max power
0.45
13.6
950 900
0.4
1.0076
800
Inlet manifold pressure
Power/ max power
850
1.0266
1.0076
0.35
1.0266
0.3 1.0456
0.25
1.0837
0.2
700 650
13.6 10.9
600 550
1.1407
0.15
750
Break-specific NOx emissions before catalyst [g/kWh]
1.2167 1.5209
450
1.9011
0.05 2000
2500 Engine speed [rpm]
6.99
3.4221
3000
3500
350
8.31
9.63
5.67
400
2.6616
1500
10.9
500
1.3308
0.1
12.3
4.35
1500
1750
2000
2250 2500 2750 Engine speed [rpm]
3000
3250
3500
• Region of optimum BSFC (lower CO2 emissions) coincides with high NOx levels • Engine operating strategy should be chosen to achieve low BSFC while keeping CO, HC and NOx emissions within legislation limits 2011 European Ricardo Software User Conference, 5/04/2011
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Full Wave model • Full Wave engine model was provided. It was used to understand the effect of engine operating conditions on the emissions • The engine modelling results agreed well with the test data:
2011 European Ricardo Software User Conference, 5/04/2011
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Full Wave model • Effect of AFR and ignition timing on NOx emissions
• Changing ignition timing from -18 deg (current operating point) to -8 deg reduces mass flux of NOx by 20% • Extra reduction can be gained by running the engine slightly rich (however effect on CO and HC conversion should be taken into account) 2011 European Ricardo Software User Conference, 5/04/2011
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Catalyst model Full model is impractical for catalyst studies Input: mass flow rate, gas temperature
Input: CO, NO, HC, O2, CO2, H2, H2O [ppm]
a simplified model used Output: CO, NO, HC, O2, CO2, H2, H2O [ppm] R-CAT model
Input: outer wall temperatures (found to have little effect on conversion) 2011 European Ricardo Software User Conference, 5/04/2011
Output: monolith temperatures (with heat generated by the exothermic/endothermic reactions) 8
Model kinetics • Most 1D models focus on the three main reactions
• Including extra reactions is beneficial for controlling oxygen conversion and improving correlation between the model and test results for a range of engine speeds/loads H2 oxidation reaction was included (H2O and H2 concentrations approximated from typical exhaust composition)
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Model kinetics Reaction rates can be described by Arrhenius equation, e.g. for CO oxidation: A - pre-exponential factor E - activation energy R - gas constant D - inhibition term [CO], [O2] - species mole fractions
• •
Activation energy barrier E is the minimum energy required to get the reaction started Pre-exponential factor A characterises the number of collisions between molecules and depends on the active catalytic surface area. A and E will vary for different catalysts they need to be adjusted for each application model calibration is needed using test data
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Test data for calibration • • • •
Test data obtained by MIRA Ltd Two types of tests carried out: steady-state and cold start Steady state tests: - simulate how well the model performs in predicting post-light-off emissions and kinetic parameters Cold-start tests: - a bypass system was used for calibrating catalyst kinetic parameters (A and E) and for assessing how well the model simulates warm-up to light-off with constant concentrations at inlet - catalyst was warmed up to around 120 C - below the light-off temperature but high enough to ensure there was no condensed water present - once these conditions were achieved, the test started by opening the catalyst valve - for more detailed description see S. Benjamin & C. Roberts, Int. J. Engine Res. Vol. 5 (2004)
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Model calibration • •
Step 1: active catalyst surface area unknown adjust dispersion to get reasonable conversion fractions with default reaction rate parameters Step 2: change pre-exponential factors for four main reactions until reasonable agreement is obtained; perform independently for two sets of operating conditions (two points on the light-off curve). The following trends were noted and used: Reaction rate increased for
CO conversion
THC conversion
NOx conversion
CO THC
~
NOx
•
~
Step 3: calculate final parameters A and B = E/R using mid-monolith temperature at the two considered points:
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Steady-state: model calibration • Challenges: – In some cases negative values of exponential factor B resulted from the calculations – This issue was caused by significant reduction in conversion efficiency for both NOx and CO with temperature in test data – It was resolved by lowering NOx reduction rate and increasing reaction rates for H2 (less oxygen available oxidation reactions slow down)
• For engine speeds > 2000 rpm maximum difference with test results ~ 3% Test Data, 2500 rpm 800 mbar Wave model
1
1
0.8
Conversion fraction
Conversion fraction
0.8
Test Data, 3500 rpm 800 mbar Wave model
0.6
0.4
0.6
0.4
0.2
0.2
0
0 CO
THC
NOx
O2
CO2
2011 European Ricardo Software User Conference, 5/04/2011
CO
THC
NOx
O2
CO2
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Steady-state: catalyst parameters • Test data: NOx emissions in steady-state exceed Euro 5/6 standard conversion around 97% needs to be achieved existing aftertreatment system needs to be improved • A study was performed to investigate the most efficient ways to improve species conversion in steady-state
2011 European Ricardo Software User Conference, 5/04/2011
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Steady-state: catalyst length • •
Increasing catalyst length by factor 4 results in 6% improvement in both NOx and CO conversion while HC conversion drops by 1% Increasing catalyst length further showed no added benefit Similar trend was observed if increasing catalyst diameter 1
0.9
Base model Length x 2 Length x 4
0.9
0.8
0.8
0.7
0.7
0.7
0.6
0.5
0.4
0.6
0.5
0.4
0.6
0.5
0.4
0.3
0.3
0.3
0.2
0.2
0.2
0.1
0.1
0.1
0
CO
0
2011 European Ricardo Software User Conference, 5/04/2011
THC
Base model Length x 2 Length x 4
0.9
0.8
Conversion fraction
Conversion fraction
1
1 Base model Length x 2 Length x 4
Conversion fraction
•
0
NOx
15
Steady-state: cell density • Increasing catalyst cell density gave little improvement of species conversion
1
1 Base model 400 cpsi 600 cpsi 800 cpsi
0.9
0.9
0.8
0.8
0.7
0.7
0.7
0.6
0.5
0.4
Conversion fraction
0.8
Conversion fraction
Conversion fraction
0.9
1
Base model 400 cpsi 600 cpsi 800 cpsi
0.6
0.5
0.4
0.6
0.5
0.4
0.3
0.3
0.3
0.2
0.2
0.2
0.1
0.1
0.1
0
CO
0
2011 European Ricardo Software User Conference, 5/04/2011
THC
Base model 400 cpsi 600 cpsi 800 cpsi
0
NOx
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Steady-state: catalyst loading • Increasing precious metal loading by factor 4 gave 9% improvement in NOx conversion, 5% improvement in CO conversion but 7% reduction in HC conversion
1
0.9
Base model Loading x 2 Loading x 4 Loading x 8
0.9
0.8
0.7
0.7
0.7
0.5
0.4
Conversion fraction
0.8
0.6
0.6
0.5
0.4
0.6
0.5
0.4
0.3
0.3
0.3
0.2
0.2
0.2
0.1
0.1
0.1
0
CO
0
2011 European Ricardo Software User Conference, 5/04/2011
HC
Base model Loading x 2 Loading x 4 Loading x 8
0.9
0.8
Conversion fraction
Conversion fraction
1
1 Base model Loading x 2 Loading x 4 Loading x 8
0
NOx
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Cold start: model calibration •
Reasonable agreement between Wave model and test results:
1
1
0.8
0.8
0.7
0.7
0.6 0.5 0.4
0.6 0.5 0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
0
50
100
150
200
250
300
THC conversion fraction, FastFID THC, Wave
0.9
CO conversion fraction, test data CO, Wave
Conversion fraction
Conversion fraction
0.9
0
350
0
50
100
1
800
0.9
750
250
300
350
650
NOx conversion fraction, test data NOx, Wave
Temperature [K]
0.7 Conversion fraction
200
700
0.8
0.6 0.5 0.4
600 550
450
0.2
400
0.1
350
0
50
100
150
200
250
300
350
Time [s]
2011 European Ricardo Software User Conference, 5/04/2011
Temperature pre-cat, test data Temperature mid-cat, test data Temperature mid-cat, Wave Temperature post-cat, test data Temperature post-cat, Wave
500
0.3
0
150 Time [s]
Time [s]
300 0
50
100
150
200
250
300
350
Time [s]
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Cold start: catalyst length •
Increasing catalyst length results in little change in light-off time and no significant improvement in conversion in steady-state (please note that a different catalyst and engine operating point was used for transient tests due to test result availability) 1
0.9
CO, base model THC, base model NOx, base model O2, base model CO, length x 4 THC, length x 4 NOx, length x 4 O2, length x 4
0.8
Conversion fraction
0.7
0.6
0.5 Light-off time (conversion 50%) 0.4
0.3
0.2
0.1
0
0
50
100
150
200
250
300
350
Time [s]
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Cold start: catalyst length • • •
Conversion of all species stops when all of the oxygen is used Species concentration profiles inside catalyst show that the reactions in the post light-off region mostly occur in the first 40% of the catalyst length Beyond that, there is no oxygen available for reactions and thus increasing catalyst length does not affect species conversion 1
CO THC NOx O2
0.9
Normalized species concentration
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0 0
0.1
0.2
0.3
0.4 0.5 0.6 Catalyst length fraction
2011 European Ricardo Software User Conference, 5/04/2011
0.7
0.8
0.9
1
20
Cold start: cell density • •
Little change in post light-off conversion fractions Light-off time increased (catalyst thermal mass increased)
1
0.9
0.8
Conversion fraction
0.7
0.6
0.5 Light-off time (conversion 50%) 0.4 CO, base model THC, base model NOx, base model O2, base model CO, 600 cpsi THC, 600 cpsi NOx, 600 cpsi O2, 600 cpsi
0.3
0.2
0.1
0
0
50
100
150
200
250
300
350
Time [s]
2011 European Ricardo Software User Conference, 5/04/2011
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Cold start: catalyst loading • •
•
Conversion of THC and CO is only affected slightly Higher precious metal loading means NOx reduction is happening faster in the first half of the catalyst and thus conversion reaches higher values before the oxygen runs out Light-off occurs around 20s faster for all species 1
0.9
0.8
Conversion fraction
0.7
0.6
0.5 Light-off time (conversion 50%) 0.4 CO, base model THC, base model 0.3
NOx, base model O2, base model CO, loading x 2
0.2
THC, loading x 2 NOx, loading x 2 0.1
0
O2, loading x 2
0
50
100
150
200
250
300
350
Time [s]
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Catalyst parameters: summary Wave modelling for the candidate engine showed that:
• Catalyst precious metal loading has most effect on the NOx conversion • Combination of measures may be needed to improve NOx conversion to meet regulations for the candidate engine • Modification of catalyst parameters may not be sufficient for required NOx reduction, engine tuning is needed as well (e.g. ignition retard or cooled exhaust gas recirculation) • Alternative APU strategy may need to be employed, e.g. running the engine at lower speed/part load at the expense of higher fuel consumption • Including oxygen storage reactions could be beneficial for analysis as lack of oxygen affects the reaction rates considerably Work on a different engine will be carried out and results compared 2011 European Ricardo Software User Conference, 5/04/2011
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Conclusions • Wave models help to understand the kinetic processes inside the catalytic converter, especially the relationship between conversion of different species • Wave model can be used to estimate the effect of changing catalyst parameters, both in steady-state and transient modes • Further work will be carried out in order to estimate the effect of 3-D flow distribution on the conversion and estimate the uniformity index for flow inside the catalyst (important for catalyst ageing) • Further work will be carried out to investigate the advantages of using EHC
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Acknowledgments This work is carried out as a part of Low Carbon Vehicle Technology Project funded by Advantage West Midlands and ERDF
led by 7 industry and research partners
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