A Second Law Perspective on Critical IC
Research for High Efficiency Low
Emissions Gasoline Engines
University of Wisconsin Symposium on Low Emission
Technologies for IC Engines
June 8-9 2005
J.T. Farrell, J.H. Farenback-Brateman, C.H. Schleyer,
J.G. Stevens, and W. Weissman (presenter)
ExxonMobil Research and Engineering
Outline
• Availability Analysis Methodology and Simulation Tools
• Engine / Vehicle Simulations
– PFI Lean high CR stratified (SIDI) Ultra-Lean High CR Boosted (ULBDI)
– Mid-size vehicle fuel economies in US city and highway cycles
• Outline of Other Areas for Efficiency Gains
• Hurdles and Research Challenges
• State of the Art Fuel Chemistry and Gaps
– Autoignition
– Burn rate
• Summary
Availability Analysis Methodology
Availability = Work Available vs. Standard Conditions;
Allows Determining Maximum Achievable Potential and Where Losses Occur
Reversible
Reversible
Chemistry
Isothermal
Reversible
Expansion
Cooling
T, P, μ*
T0, P, μ
T0, P0, μ
T0, P0, μ0
Carnot
Cycle
WSensible
T0 1
T0 Tˆ C p dTˆ
T
WS =
* chemical potential
+
WExpansion
WChemical
+
WC =-G0 (T0)+RT0 {ni,pln (P0/Pi,p)
- ni,rln(P0/Pi,r)}
WE = -RT0 ln(P0 / P)
Simulation Tools
Engine specs:
• Geometry
• Valve timings, burn parameters, manifold press., lambda, etc.
Modified MIT engine simulator
incorporating availability
algorithms; CHEMKIN for thermo.
GT- Power engine simulator
for air flow and turbocharging
Efficiency, fuel consumptions,
availabilities over range of engine speeds
and torques
ADVISOR
(DOE/NREL)
Engine work outputs
and availability balances
Drive cycle
fuel economies
Max. torque over range
of engine speeds
Driving cycle, vehicle
characteristics,
transmission specs
Engine Cases
Engine
Power Comp.
kW
Ratio
Liters
# of
Cylinders
Lambda
Min/Max
PFI
143
10.5
3.0
6
1.0 / 1.0
SIDI
ULBDI
143
143
12
12
2.9
2.4
6
4
1.0* / 1.7
1.7 / 4.0
*
stochiometric
only at WOT
• Stochiometric PFI Operation Allows Use of TWC to Meet Emissions Specs
• SIDI and ULBDI Engines Cases Unconstrained by NOx Limits During Lean Operation
– Shows efficiency possible if NOx controlled by combustion innovations
(e.g., HCCI) and/or aftertreatment advancements
Availability Breakdown for Base PFI Engine
Available Energy, % fuel
100
Cylinder Heat Losses
80
Combustion Irreversibilities
60
Exhaust to Ambient
Fluid Flow Losses
40
Mechanical Losses
20
Brake Work
0
0
20
40
60
Power, % Peak
80
100
• Large Availability Losses in Low Load Region Are Due to Throttling and
Low
Power Output per Stroke Relative to Throttling and Mechanical Losses
T
• About 20% Lost Throughout by Conversion
dQ of Chemical Energy to Heat
Wlost = mT0
SIDI vs PFI
PFI Engine
100
Available Energy, % fuel
Available Energy, % fuel
100
SIDI Engine
Cylinder Heat Losses
80
Combustion Irreversibilities
60
Exhaust to Ambient
Fluid Flow Losses
40
Mechanical Losses
20
Cylinder Heat Losses
Combustion Irreversibilities
80
Exhaust to Ambient
Fluid Flow Losses
60
40
Mechanical Losses
20
Brake Work
Brake Work
0
0
0
20
40
60
Power, % Peak
80
100
0
20
40
60
Power, % Peak
• Operating Lean vs. Throttling Increases Efficiency
– Reduces cylinder heat , exhaust availability and fluid flow losses
• Higher CR Reduces Exhaust Losses
• Combustion Losses Increase at Low Load/Lean Conditions due to
Lower T and Higher m
dQ
Wlost = mT0
T
80
100
Fuel Consumption Impacts: Through to ULBDI
2.0
15% peak power
3% peak power
Cylinder Heat
Losses
0.3
Combustion
Losses
0.2
Exhaust
Losses
0.1
Fluid Flow
Friction
1.5
Cylinder Heat
Losses
Combustion
Losses
1.0
Exhaust
Losses
Fuel Consumption (g/s)
Fuel Consumption (g/s)
0.4
0.5
Fluid Flow
Friction
Brake Work
Brake Work
0.0
0.0
PFI
SIDI
ULBDI
PFI
SIDI
• Comparison of PFI, SIDI, and ULBDI Fuel Consumption at Representative
Speed/Load Conditions Shows:
– Reduction in cylinder heat and exhaust losses due to higher CR & leaner
operation for SIDI and ULBDI vs PFI
+ Added benefits in these areas and in fluid flow losses for ULBDI due
downsizing and increased enleanment at low load
– Combustion losses increase slightly in ULBDI due leaner operation
ULBDI
Mid-Size Vehicle MPG Estimates
US CITY
US HIGHWAY
AVERAGE
PFI
22
28
25 (Base)
SIDI
27
34
30.5(1.2x)
ULBDI
33
41
37(1.5x)
50% FE improvement = 33% FC benefit
Other Areas for Efficiency Gains
“Adiabatic”
Engine Design
ULBDI Medium Power Example
Cylinder Heat Losses
Combustion Losses
Turbocompounding,
Turbogenerator,
Bottoming Cycles
mT0
Exhaust Heat Losses
Fluid
Flow
Low Friction
Designs/Lubes
New Concepts That
Minimize Combustion
Loss Integral:
Friction
dQ
T
Hybridization
to Narrow
Operating Range
Output Work
• Burn Width Reduction Has Small Effect
– HCCI burn width (~ 5 CA) vs SIDI (~ 40 CA) yields 1-2 % benefit
– Main HCCI advantage is lower NOx
• HCCI Approaches That Control Burn Rate Through Wall Cooling Will Suffer
Efficiency Debits Due to Increased Heat Losses
Research Needs for Creating Future Fuel / Engine /
Aftertreatment Systems
Current Areas of
Research Focus
• High CR/Lean
Operation
• Turbocharging
• HCCI
Major Hurdles
• NOx Control
• Ignition Timing,
Knock & Noise
Control
Research Needs for
Overcoming Hurdles
• Chemistry and Fluid Dynamics
– End gas & HCCI auto-ignition
– Burn rate in various T/P/
composition profiles
• Lean De-NOx Catalysis
Longer Term
• Adiabatic Engine
• Advanced Exhaust
Recovery Concepts
• Low Cost /High Energy
Recovery Concepts
• Innovations From Various Fields
Outside Engine Area; e.g., High
Efficiency Thermopiles
• Reduced Combustion
Losses
• New System
• Address Through Effects in
Work-loss Integral
Fraction Availability Destroye
Constant Volume Combustion Losses Similar for
Gasoline and Diesel Range Fuels
0.35
cetane
T0 = 700K
Pi = 16 bar
iso-octane
butane
0.30
0.25
H2
0.20
CH 4
0.15
0.10
0.2
0.4
0.6
0.8
Equivalence Ratio
1.0
• Entropy Produced During Constant Volume Combustion Similar for ~ C4 and
Larger Molecules
– Initial mixture properties approach pure air (fuel mole fraction < 0.02)
– Final mixture composition (CO2, H2O, N2) and temperature effectively same
• Largest Fuel Effects on Reducing Availability Losses Will Be Indirect
– Enabling strategies (HCCI, high CR SI) that minimize other availability losses
1000
Cylinder Pressure (bar)
Gasoline HCCI
MON
30
Diesel HCCI
600
0
10
20
Pressure (bar)
30
80
20
paraffin + naphthene
15
paraffin + aromatic
paraffin + olefin
10
T/P History During
Compression Stroke
400
100
25
RON
800
120
Fuels with same
RON,MON
dQ/dt (J/deg)
Temperature (K)
Engine Advancements Move Autoignition Chemistry
into New Regions
60
40
20
5
0
40
-20
-10
0
10
Crank Angles (degrees)
20
• Fuel Ignitability Key Property for Future Engines
• SI Engines
– High CR and turbocharging demand increased knock resistance at peak load
– Turbocharging, internal EGR move autoignition requirements outside
RON/MON region
• HCCI
– Fuel structure effects (ignition kinetics) more prominent
– Wide variability in HCCI approaches precludes definition of a single metric
• Increased Focus on Understanding Kinetics Constitutes Key Need
Fuel Burn Rates in Advanced Engines Depend on
Interactive Effects of Chemistry and Fluid Dynamics
Promoting Lean Burn at High Pressure Will Help to Maximize Fuel Economy
• SI Engines
– Effect of high pressure on lean burn limit is incompletely understood
– Flame speed effects may be dramatically different at very high pressure
• HCCI May Not Be “Flame-less”
– Inhomogeneity / staged ignition can give rise to combination of autoignition +
flame propagation
Temperature (K)
late ignition
flame
Cylinder Radius (cm)
early ignition
time
Cylinder Radius (cm)
Summary
• SI Concepts Currently Being Developed Have Potential to Increase Fuel
Economy by Factor of ~ 1.5
• Main Challenges are NOx Control, Knock, HCCI Operability
• Significant Potential for Further Efficiency Improvements
– Heat transfer, exhaust losses and combustion irreversibilities are key areas
to address
• New Fuel / Engine / Aftertreatment Combinations Open Areas For Innovation
– Improved understanding of the engine chemistry and fluid dynamics related
to auto-ignition and burn rate
– Aftertreatment catalysis
– New system concepts (low irreversibility combustion, adiabatic engine, etc.)