November 9th 2015
University of Wisconsin Engine Research Center
COMPARISON OF VARIABLE VALVE ACTUATION, CYLINDER DEACTIVATION AND INJECTION
STRATEGIES FOR LOW-LOAD RCCI OPERATION OF A LIGHT-DUTY ENGINE
Anand Nageswaran Bharath, Yangdongfang Yang, Rolf D. Reitz, Christopher J. Rutland
University of Wisconsin Madison – Engine Research Center
North American GT-User Conference 2015 November 9th 2015
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University of Wisconsin Engine Research Center
Acknowledgements • In addition to our advisors Prof. Rolf Reitz and Prof. Christopher Rutland, and our
sponsors DERC, we would also like to thank the following people: • Reed Hanson, Shawn Spannbauer & Chris Gross for experimental data • Current & Former ERC Colleagues: Dipa, Jae-Hyung Lim, Yifeng Wu, Nitya
Kalva, Jian Gong & Xingyuan Su • Paramjot Singh & Daniel Schimmel from Gamma Technologies • Joshua Leach, ERC Systems Administrator • Mike Andrie
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Motivation • High CO & UHC emissions at low-load operation of RCCI (and LTC
regimes in general) due to low combustion temperatures • At low-load operation, exhaust gas temperatures are insufficient to light
off the Diesel Oxidation Catalyst (DOC) • Other strategies for DOC light-off:
1. Using electrical heater requires large electrical currents for rapid catalyst warm-up (150 – 250 A), placing high power demand on vehicle electrical power supply components (Laing, Socha)
2. Varying exhaust gas composition to include hydrogen to lower light-off temperature, but at least one cylinder has to run rich to generate hydrogen, increasing PM emissions (Katare et al.)
• It can therefore be seen that increasing exhaust temperatures would be a good way to achieve DOC light-off and improve catalyst efficiency.
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• In our previous work, we showed that Early Exhaust Valve Opening (EEVO) with fully flexible VVT raised exhaust gas temperatures sufficient for DOC light-off
• However, there was a significant deterioration in fuel economy
• Should a cam phaser be used instead due to cost & complexity?
• What about other methods, e.g. post-injection, combustion phasing, cylinder deactivation, etc.?
Bharath et al., ASME ICEF 2014, Paper No. ICEF2014-5534, Oct. 2014
Compare EEVO, combustion phasing, post-injection & cylinder deactivation in raising exhaust gas temperatures for DOC light-off, and impact of each strategy on fuel economy & emissions at a near-idle load point using coupled GT-Power and KIVA-3V simulations.
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Simulation Setup – Engine & DOC Specifications
Engine Schematic used in GT-Power Model
Bore 82 mm Stroke 90.4 mm No. of Cylinders 4 Displacement 1.9 liters Compression Ratio 16.7
Turbocharger Variable Geometry
EGR High Pressure EGR Loop
Computational grid Specifications Cells at IVC 9,738 Cells at TDC 3,528 Radial resolution (mm) 4 Azimuthal resolution (deg.) 2.9
Stock Piston Geometry with re-entrant bowl
Catalyst Specifications Volume (L) 0.6 Substrate Dimension D: 90 mm X L: 90 mm Substrate Material Metallic CPSI/Wall Thickness 200/50 micron Loading/Washcoat 90 g ft−3 Platinum Heat Shield Yes
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Simulation Setup – Data for Model Calibration
1 Bar BMEP 4 Bar BMEP ** Intake Manifold Pressure (Bar) 1.006 1.06 Fuel Energy per Cylinder (J) 275.1 563.3 Engine Speed (rev/min) 1,500 Gasoline Quantity (mg/cyl/cyc) 3.525 10.5 Diesel Quantity (mg/cyl/cyc) 2.619 2.1 Gasoline Start of Injection (Deg.) -227.4 Diesel Start of Injection (Deg.) -40 -45 Diesel Fuel Rail Pressure (Bar) 400 480 EGR Fraction (%) 49.9 0
Data for Engine System Model Calibration
Parameter Value Cylinder Liner Temp. 390 K Cylinder Head Temp. 440 K Cylinder Wall Temp. 440 K
Temperature Boundary Conditions for Heat Transfer in KIVA-3V
**NOTE: 4 Bar BMEP case used for cylinder deactivation study
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Simulation Setup - Calibration Procedure
Experimental HRR Profile
𝑃𝑖𝑖𝑖 ,𝑇𝑖𝑖𝑖 & Gas Component Mole Fractions
KIVA-3V
IMEP, BMEP, Intake Air Flow Rate, Volumetric
Efficiency, etc.
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Simulation Setup – Calibration Results
1 Bar BMEP 4 Bar BMEP Experiment Simulation Experiment Simulation
BMEP (Bar) 1.10 1.06 4.00 4.15 Intake Air Flow Rate (kg/h) 41.11 41.86 89.8 93.9 Intake Manifold Gas Temp. (K) 348.1 351.1 312.2 323.0
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Simulation Setup – Calibration Results
Platinum Dispersion Factor used: 1.45%
Experimental Data for DOC Model Calibration DOC Model Calibration Results BMEP (Bar) 1 2 3 4
Operating Conditions Exhaust Gas Temp. (K) 410 421 456 484 Intake Air Flow (kg/h) 88 87 87 89 DI (mg/inj) 3.864 4.278 4.264 3.396 PFI (mg/inj) 2.812 4.178 6.7 9.49 Engine Speed (rev/min) 1,500
Emissions Pre-DOC UHC (ppm) 1,192 1,006 1,108 1,091 Post-DOC UHC (ppm) 1,128 1,182 1,019 147 Pre-DOC NO (ppm) 5.5 8.56 4.81 6.26 Pre-DOC CO (ppm) 7,005 5,820 2,458 1,432 Pre-DOC 𝐻2𝑂 (%) 2.3 3.07 4.05 4.69 Pre-DOC 𝐶𝑂2 (%) 2.8 2.54 3.98 4.7 Pre-DOC 𝑂2 (%) 16.96 15.35 14.3 13
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Combustion Phasing Study • Gasoline/diesel ratios from 10% iso-
octane/90% n-heptane to 90% iso-octane/10% n-heptane, with 5% intervals between ratios
• Global fuel reactivity denoted by global PRF Octane Number:
•𝛹𝑃𝑃𝑃 =𝑚𝐶8𝐻18∙100+𝑚𝐶7𝐻16∙0
𝑚𝐶8𝐻18+𝑚𝐶7𝐻16
• No combustion for 𝛹𝑃𝑃𝑃 > 75 as fuel
reactivity too low • Combustion efficiency decreases with
increasing 𝛹𝑃𝑃𝑃, in turn leading to increased UHC and CO emissions.
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Combustion Phasing Study • Exhaust gas temperatures before DOC were
below the DOC light-off temperature of 457 K. • Hence UHC and CO conversion efficiencies
were poor for all iso-octane/n-heptane ratios • In conclusion, varying combustion phasing not
effective for DOC light-off. • Also, peak pressure rise rates may be a
problem at advanced combustion phasing.
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Post-injection Study •Definition of Post-injection for this study: • Subsequent injection of fuel into the combustion chamber during the expansion
stroke following the main injection •Equation for mass of additional fuel required:
• �̇�𝑒 + �̇�𝑓 ∙ ℎ457 − �̇�𝑒ℎ442 = �̇�𝑓 ∙ 𝑞𝐿𝐿𝐿,𝐶7𝐿16
•Assumptions in aforementioned equation: 1. Thermodynamic properties of exhaust gas same as air 2. Complete combustion of post-injected fuel 3. All additional post-injected fuel used to increase exhaust gas temperature to light-
off temperature, and none lost to heat or converted to work.
• From equation, additional fuel required: 4.11 𝑚𝑚 ∙ 𝑠−1 (0.082 𝑚𝑚 ∙ 𝑐𝑐𝑐−1 ∙ 𝑐𝑐𝑙−1 or 1.34% of total fueling rate)
• 2nd injection at injection timings from 40 degrees ATDC to 90 degrees ATDC, with 10 degree intervals.
• Additional fuel did not burn so exhaust gas temperatures did not increase.
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VVT System Comparison Study for EEVO
• Compare engine & DOC performance for fully flexible VVT and cam phaser at 80 degrees ATDC EVO timing: o 80 degrees ATDC gave lowest
increase in BSFC using EEVO for fully flexible VVT
o No external EGR used for cam phaser case, as EGR accomplished internally using zero valve overlap
o 22% EGR fraction was initial result obtained from GT-Power to calculate the mole fractions for KIVA.
Fully Flexible VVT Cam Phaser IVC Pressure (Bar) 1.17 1.15 IVC Temperature (K) 390 388 EGR Fraction (%) 45 22*
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VVT System Comparison Study for EEVO
Baseline Case Fully Flexible VVT Cam Phaser BMEP (Bar) 0.84 0.77 0.44 PMEP (Bar) 0.15 0.17 0.48 Intake Air Flow Rate (kg/h) 39.15 43.27 64.12 Pre-DOC Exh. Temp. (K) 453.8 463.3 472.45 EGR Fraction (%) 49.9 45 22 DOC Exh. Flow Rate (kg/h) 40.21 44.02 65.22 BSFC (𝑚 𝑘𝑘ℎ−1) 553 607 1,053
• Larger hot residual gas quantity for cam phaser case causes increased heat loss during compression stroke, giving higher pumping work.
• Lower EGR for cam phaser leads to higher flow rate through catalyst, thereby lower conversion efficiencies for cam phaser case.
Fully Flexible VVT Cam Phaser UHC Conversion (%) 94.9 89.7 CO Conversion (%) 99.6 98.1
Engine Performance Comparison
DOC Performance Comparison
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Cylinder Deactivation Study
• Cylinders 2,3 and 4 were motored while cylinder 1 was fired. • Actuators added to cylinders 2,3 and 4 to cut off fueling and keep valves closed • Heat release rate & fueling for 4 Bar BMEP at 1,500 rev/min operating point
used in cylinder 1
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University of Wisconsin Engine Research Center
Cylinder Deactivation Study All Cylinders Firing Only Cylinder 1 Firing Baseline Case BMEP (Bar) 4.15 0.68 0.84 FMEP (Bar) 1.04 1.04 1.04 PMEP (Bar) 0.24 0.05 0.15 Gross IMEP (Bar) 5.43 1.77 2.03 Intake Air Flow Rate (kg/h) 93.9 22.8 39.15 Pre-DOC Exhaust Temp. (K) 500.15 472.45 453.8 DOC Exhaust Flow Rate (kg/h) 96.18 23.34 40.21
All Cylinders Firing Only Cylinder 1 Firing Baseline Case UHC Conversion (%) 79.8 99.6 0 CO Conversion (%) 93.4 99.9 18
Engine Performance Results
DOC Performance Results
• Much lower flow rate through DOC and the higher than light-off temperature results in near complete conversion of UHC & CO for cylinder deactivation.
• Also, significantly reduced pumping work due to closure of intake and exhaust valves in motoring cylinders. (BSFC: 351 𝑚 𝑘𝑘ℎ−1, best of all 4 strategies)
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Conclusions 1. Varying combustion phasing via changing gasoline/diesel ratio did not raise
exhaust gas temperatures sufficiently to reach DOC light-off temperature, so catalyst efficiency was poor.
2. 1.34% of fueling rate required for post-injection to raise exhaust gas temperatures to 457 K for DOC light-off. (Lower limit, assuming no heat loss and no conversion of additional fuel to heat.) However, additional fuel did not burn.
3. EEVO using either cam phaser or fully flexible VVT raises exhaust gas temperatures high enough to light-off DOC, but significant deterioration in fuel economy observed (607 𝑚 𝑘𝑘ℎ−1 for fully flexible case & 1,053 𝑚 𝑘𝑘ℎ−1 for cam phaser case.)
4. Cylinder deactivation the best strategy for near-idle operation for LTC, because of superior fuel economy of 351 𝑚 𝑘𝑘ℎ−1, and near complete conversion of UHC and CO by DOC due to lower exhaust flow rate and high exhaust temperatures.
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Future Work • Planning of experiments to verify and validate the cam phaser and fully
flexible VVT strategies. • Simulation & Experimental studies of cylinder deactivation strategies:
• 2-cylinder operation (fire either cylinders 1 & 4, or cylinders 2 & 3) • Transient 1-cylinder operation that takes into account firing order • Noise, Vibration & Harshness (NVH) issues to determine stresses on
engine
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