study of after-treatment challenges in hybrid vehicles ... · • partly due to higher density of...
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Study of after-treatment challenges in hybrid vehicles through system simulations
Zhiming Gao,
Kalyana Chakravarthy,
Stuart Daw
Oak Ridge National Laboratory
Sponsor : Lee Slezak Vehicle Technologies Program
U.S. Department of Energy
2010 DOE-CLEERS workshop April 22, 2010
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Introduction • Systems simulations
– Virtual prototyping using component models
– Component compatibility, integrated controls
– Inexpensive
• US DOE Vehicle Systems Analysis Technical Team (VSATT) – Powetrain System Analysis Toolkit (PSAT) developed at ANL
– ORNL is tasked with studying after-treatment options
• Current focus is primarily on hybrid vehicles – Fuels
– Operating modes (HCCI, PCCI/HECC, GDI etc.)
– Alternate aftertreatment options
– Battery charging strategies
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PSAT construct • Graphical interface written in C# • Component modules written in Matlab-Simulink, Stateflow • Model databases managed by XML • User defined components
Vehicle Configuration
Driving, braking & shifting
Powertrain Components
Simulation Setup and Run
User Define Components
Variable Solver Options
Post Analysis & Simulation Output options
Save & Reload Data
PSAT GUI
XML Model
Database User Front-End
Models
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ORNL contribution to VSATT • Objective : develop engine maps and emissions control device
models for simulating performance of conventional, hybrid and plug-in hybrid vehicles operating with gasoline, diesel and alternative fuels – Engine maps (steady state) – Transient engine warm-up model – Oxycat model – Lean NOx trap (LNT) model, regen strategies, aging/sulfation effects – 3-way catalyst (TWC) model – Diesel particulate filter (DPF) model – SCR model (for Cu-ZSM-5), dosing strategy
• Approach – Physically based models to deal with transients (move away from steady state
components of PSAT) – Generate/utilize public domain lab, engine dynamometer data for building maps
and models – Fill gaps in experimental data using predictions from analytical, computational
tools such as WAVE, GT-power or in-house software
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Engine warm-up model • Approach : physically based first order
non-equilibrium model (using data from steady state maps) to simulate engine transients
• Handles cold/warm start (important for hybrid vehicles) for both gasoline/diesel engines
• Sample results : Cold start UDDS cycle using a Mercedes 1.7L engine (A170 compact car)
• Published in IJER (Gao et al., Int. J. Engine Res., vol 11, 2010)
Mileage (mpg)
CO (g/mi)
HC (g/mi)
NOx (g/mi)
PM (g/mi)
Experiment 40.3 2.28 0.54 0.74 0.14
Simulation 40.4 2.29 0.54 0.89 0.12
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TWC model
• Model validation conditions – Data from a gasoline engine from vehicle
tests (supplied by an OEM) – UDDS cycle with a cold start
• Integrated emissions : – CO (g/mi) : 0.833 (exp) vs. 0.836 (model) – NOx (g/mile) : 0.156 (exp) vs. 0.157
(model) – HC (g/mile) : 0.139 (exp) vs. 0.148 (model)
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SCR model for Cu-ZSM-5 catalyst • 1-D transient simulink module
• Based on Chalmers/GM data/model
• NH3 adsorption/desorption
• 3 SCR reactions (NO, NO2, “fast”)
• NO, NH3 oxidation reactions
• No N2O (simplicity, same as in LNT model)
• Cu-ZSM-5 appropriate for low T applications
• O2 effect, HC inhibition of SCR reactions not included yet
• Urea thermolysis/hydrolysis also not included yet
• Predictions in line with published data on a recent commercial formulation
Points from experiments by Olsson et.al, Appl. Cat. B: Environ., 81(2008), 203-217. Lines from ORNL simulation.
SAE 2009-01-0897
Steady State Response
ORNL Model
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Gasoline vs. diesel • SI engines
– Stoichiometric operation => higher exhaust T – Most emissions during cold start but faster light-off – TWC technology still evolving (low PGM, dual cat. Systems) – May need lean NOx control with GDI
• Diesel engines – Very lean operation => low exhaust T – More efficient than gasoline engines (due to high compression ratios) – After-treatment more challenging, technologies evolving – Unconventional modes (HCCI, MK, PCCI/HECC) proposed for emissions/efficiency – Fuel penalty associated with aftertreatment (LNT, DPF pressure drop/
regenerations)
• Hybrid vehicle pose additional aftertreatment challenges – Engine switches on and off several times during a drive cycle – Small engine size => lower exhaust temperature
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Gasoline vs. diesel with out aftertreatment
• Simulation parameters – Prius HEV (28% series, 72% parallel) – Hot start UDDS cycle – 1.3kWhr battery (charge 65%) – 1.5L stoichiometric gasoline engine
based on Miller cycle (map available in PSAT) with TWC
– 1.5L diesel engine (performance scaled down from a 1.7L Mercedes A170 map) with no valving adjustments, no NOx/PM control
• Results – 84.2 mpg diesel vs. 70.7 mpg gasoline
(SAE 2007-01-0281 reports 71.2 mpg) • Partly due to higher density of diesel
– Max efficiency : 41% diesel vs. 37% gasoline
– Ave. efficiency : 36% diesel vs. 34% gasoline
– Diesel offer 19% more mpg, 5.4% more energy efficiency (mainly due to compression ratio)
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Simulations in present study • Compare fuel economy, emissions of hybrid and plugin hybrid
electric vehicle (HEV and PHEV) with a similar conventional small car
• Gasoline engine (with TWC) and diesel engine with LNT and SCR – Conventional vehicles
• Honda Civic (1110kg) with a 2.0L engine from SAAB Biopower 9/5 flex-fuel vehicle with 2.7L TWC (gasoline based map used)
• Mercedes A170 (1090kg) with (its own) 1.7L diesel engine with 2.8L LNT/SCR
– Hybrid vehicles : Toyota Prius (28% series, 72% parallel) of 1450kg • Its own 1.5L stoichiometric Atkinson cycle based gasoline engine with a 2.0L TWC • 1.5L diesel engine (scaled down from Mercedes 1.7L engine) with a 2.4L LNT/SCR
– Battery • 1.3kWhr with 65% initial charge in HEV (charge sustaining mode, final charge almost
similar) • 5.0kWhr with 100% initial charge in PHEV (charge depletion mode), full battery charge
roughly translates to 20 miles (16kWhr battery in 1600kg GM Volt reported to last for 40 miles)
• Cost of electricity (about $0.1 to $0.15 per kWhr) not factored into mileage comparison
• Focus on CO, HC in addition to NOx (PM will be added later)
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Simulations in present study • Driving conditions
– Conventional vehicles and HEVs • 1 cold start UDDS cycle (~1370s, ~8miles) for gasoline vehicle • 5 consecutive UDDS cycles each with cold start (NOx/NH3 storage carry over from a
cycle to next) • 65% initial battery charge (final charge with in 2%-3% range)
– PHEV • 5 UDDS cycles in succession (engine on rarely during first 3 cycles) • 100% battery charge (5kWhr battery charge roughly translates to 20 miles of driving)
• Control strategies – LNT regeneration : optimal of 3 available
• 60s minimum lean phase • T > 150C (CO poisoning of Pt observed at low T) • Initiate regeneration when LNT-out NOx exceeds a threshold value • Rich pulse width adjusted to achieve required NOx conversion
– SCR urea dosing • T > 150C (urea conversion to NH3, SCR reactions kinetically limited at lower T) • Inlet NH3 (from urea) = inlet NOx (from engine map, no NOx sensor)
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Results and analysis
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Fuel economy
• Combustion efficiency is about 6% higher in case of diesel – Gasoline engine in HEV/PHEV is based on an Atkinson cycle – LNT and DPF add about 2% fuel penalty each – SCR has no fuel penalty
• 32.5% urea solution costs about same as diesel • Early studies show 1 gallon of urea solution needed for 18 gallons of fuel
• GDI modes can increase efficiency of gasoline engines
• Use of PCCI/HECC modes (when possible) increase efficiency of diesel engines only marginally
conventional HEV PHEV
gasoline 24.5* 67.3 113.3
Diesel with LNT
40.4 (2.1%) 78.8 (2.8%) 133.9 (1.9%)
Diesel with SCR
41.3 80.9 136.4
Numbers in red indicate fuel penalty associated with after-treatment
* Oversized engine optimized for E85
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Engine-out emissions • Engine
– on 35%-38% of the time in HEVs
– On 21%-23% of the time in PHEVs
• Emissions of gasoline engines roughly proportional to fraction of time engine is on
• HEVs produce more NOx in case diesel engines despite engine being on only a fraction of the time
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NOx emissions : conventional vs. HEV • Engines generally operate in high
efficiency mode in HEVs
• Gasoline engines operate almost always in stoichiometric mode – combustion temperature roughly the
same in conventional and HEVs
– Catalyst temperature little cooler in HEVs (intermittent cool down, lower emissions => lower heating due to reactions)
• High efficiency operation of diesel engine is associated with high operating temperatures – Average engine-out temperature,
catalyst temperatures higher for HEVs than conventional engines
– Higher NOx in case of HEVs
– PHEVs produce most of the NOx in last 2 cycles, where they are similar to PHEVs
gasoline
diesel
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Catalyst-out emissions
• HEVs powered by gasoline engine lead to more emissions than conventional counterpart (delayed light-off)
• CO, HC emissions from LNTs very high
• DOC is needed upstream on SCR to reduce CO, HC (not included in results shown)
• SCR performance very inadequate (NH3 slip is 0.02g/mile, 0.04g/mile, 0.07g/miles respectively for conventional, HEV and PHEV)
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Catalysts performance
• TWC performance lower in HEVs and PHEVs (delayed light-off)
• CO, HC conversion in LNTs very low (non-optimal regen)
• SCR performance low – Kinetically limited
• 60% (175oC), 80% (200oC) • Higher in HEVs
– not NH3 supply limited – Improvement with upstream DOC
(fast SCR instead of NO SCR) – May worsen with HC poisoning,
urea conversion are included
• NOx conversion by LNTs lower in HEVs than in conventional vehicles
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Intermittent operation effect on TWC
• Light-off is delayed by more than 100s in hybrid vehicles (relative to conventional vehicles) – Run engine at high temperature
(perhaps inefficient mode) till the catalyst lights-off in HEV, excess power can charge the battery
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Reductant utilization during LNT regen
• More than half CO (and HC) emissions from LNT are due to reductant slip during regeneration events – Partly due to non-optimal regen
strategy • Increasing rich pulse width beyond a
certain level has diminishing returns on NOx conversion (pre-lightoff NO emissions are not affected)
• Switch to lean operation when the cat-out flow is rich (UEGO sensor), i.e., non-constant rich pulse width
– Add a DOC (with stored O2) to reduce CO, HC slip during regen events
CO conversion in LNT on a conventional diesel vehicle
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NOx conversion in LNTs : conventional vs LNTs
• NOx conversion lower in HEVs than in conventional diesel vehicles – HEVs exhaust hotter on average – Much of the difference is due to delay in
first regeneration event in case of HEVs – regen not initiated until Tcat-out > 150oC
(based on observed CO poisoning during rich operation at low T)
• Regeneration actually efficient at 150oC with H2 but not with CO (with the Umicore GDI CLEERS catalyst)
• CO and H2 lumped into one species in the current LNT model (may need to be modified)
• Control strategies are rapid LNT warmup in early stages of engine operation in HEV – Engine typically operates in high efficiency
(high T) modes in HEVs, increase T further if possible
conventional
HEV
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Summary
• Combustion efficiency of diesel engines is about 6% higher than that of gasoline engines in all vehicles – LNT add about 2% - 3% fuel penalty (most of it used to reduce stored O2 than
NOx)
– SCR operational costs not easy estimated • (high quality 32.5% urea solution roughly same cost as diesel fuel) • Urea is used only to reduce NOx (no stored O2 as in LNTs) • Urea to NH3 conversion typically 50% - 70% for light duty vehicles
– DPF add about 2% fuel penalty (results not shown), higher in HEVs • Pressure drop effect of power negligible (regenerate when pressure drop reaches
7.5kPa, a strategy resulting in one regeneration every 40 UDDS cycles) • Engine can not be switched off in HEV/PHEV once regen starts
• HEVs/PHEVs may need rapid warmup strategies
• Need focus on low temperature performance of LNT/SCR systems
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Future work
• Use alternate (UEGO sensor based) regen strategy for better reductant utilization in LNTs
• Add DOC upstream of SCR catalysts – Oxidize CO, HC, NO
– Improve SCR performance
– HC poisoning (long term)
– Urea thermolysis/hydrolysis (long term)
• Simulate diesel vehicles with DPF
• Study effects of increased insulation