Engine Efficiency and Power Density: Distinguishing Limits from LimitationsChris F. Edwards
Stanford University  
Limits are imposed by the  resource, environment, and  physics governing transfers 
and transformations.
Limitations are introduced  by the choice of devices and  processes—i.e., by the  architecture of an engine.
Chemical Resource
Restrained Reaction
Electrostatic  Work
Batch  Expansion
Flow Work
Unrestrained Reaction
Batch  Expansion
Flowing  Expansion
Efficiency, Effective Compression Ratio,  and Ideal Architectures
• 7080% Ideal Work, 6075% Peak Pres., 7590% Peak Temp.
10 0
10 1
10 20
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Fuel-air Atkinson cycle Fuel-air Otto cycle 70-80% of Otto cycle Jaguar AV133, 5.0 L DISI gasoline, ULEV2
Volvo TG103/G10A, 11.8 L SI natural gas Cummins 6BT5.9-G6, 5.9 L Diesel, turbocharged, Tier 1
Cummins QSM11-G4, 10.8 L Diesel, turbocharged, Tier 3 Volvo Penta TAD734GE, 7.2 L Diesel, turbocharged, Tier 2
RCCI gasoline + Diesel, Gross-Indicated PCI gasoline, Gross-Indicated FPEC Diesel, Gross-Indicated
Equivalence and Compression Ratios
• Efficiency and peak pressure require use of significant compression with a dilute mixture.
Use of LowTemperature Combustion
• Use of LTC to control NOx emissions limits work  output to 67 bar IMEP (56 bar BMEP).
Atkinson?  Not LTC?
• Efficient, pressurelimited, highoutput operation might be  achievable with optimal expansion and nondilute mixtures.
Spanning Exergy to Engines
Limits are imposed by the  resource, environment, and  physics governing transfers 
and transformations.
Limitations are introduced  by the choice of devices and  processes—i.e., by the  architecture of an engine.
Chemical Resource
Restrained Reaction
Electrostatic  Work
Batch  Expansion
Flow Work
Unrestrained Reaction
Batch  Expansion
Flowing  Expansion
9
FreePiston Architecture for High CR
• Balanced forces, no bearing loads • Long stroketobore ratio for low 
surfacetovolume ratio • Short residence time at min. V
• Can use linear alternator for work  extraction (van Blarigan/Aichlmayr)
Gas driver Gas driver
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Diesel Combustion at High Compression
CR = 30:1, 1050 K CR = 100:1, 1550 K
#2 Diesel, 1 ms injection, EOI at TDC
Initial Diesel Efficiency Results
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Ideal 1st-law efficiency Ideal cycle minus air experiment losses Experimental indicated efficiency
Diesel #2 φ = 0.27 - 0.30
Limited to DieselStyle Combustion?
• Premixed combustion is sootless. • Premixed lean combustion is very efficient. • Premixed stoich combustion has high power density. • Premixed stoich combustion permits use of a TWC, and  therefore very low NOx emissions.
• To accomplish this at high CR, autoignition must be  held off until the minimum volume.
• Might be able to hold off autoignition by: – Choice of fuel (e.g., methane/NG, methanol) – Active cooling of the charge
Temperature Control of Autoignition
• Lowering the initial gas temperature by 50 K lowers the  temperature at 100:1 by 210 K.
• Ignition occurs at the desired volume.
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ρ/ρ0
– Volumetime profile from  experimental data
– GRI 3.0 chemical kinetics
Two Methods of Cooling
P = 2.17 atm
This is a common turbocharger / intercooler, with 2.17 atm  manifold  pressure.
Experiments w/Intercooling
• Experimental method: – Charge compressed part way, remaining at wall T – Usual rapid compression starts from that point – Intercooling P chosen for ignition just after TDC
10-2 10-1 100100
• Peak Efficiency:  57% (Includes comp. work.)
Measured Combustion Efficiency
10
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4
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~1% Loss in  Combustion Efficiency
Emissions in Context of TWC
1J. Chiu, J. Wegrzyn, and K. Murphy, SAE Paper 2004012982 2 I. Saanum, M. Bysveen, P. Tunestal, and B. Johansson, SAE Paper No. 2007010015.
60:1 CR, 1.028 φ
– Vaporization rate matches  injection rate of real  injector
– Startofinjection chosen to  avoid gas saturation
• Inject liquid during compression (water has good properties). • Vaporization draws sensible energy from the gas, thus lowering the 
temperature.
Start water injection
10-1 100 100
SOI Model EOI
• More water needed  (8% vs. 1%)
• 10% decrease in  efficiency (53%)
• Limited by the  injector setup:
– Stratification – Slow vaporization
~ 5000 bar/CAD for  intercooling approach
~ 80 bar/CAD for water  injection approach
61 62 63 64 65 0
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Intercooling Water injection
• Maximum rate of pressure  rise, translated to slider crank at 1800 RPM:
TakeAway Messages
• Exergy sets an absolute limit for the work from a resource in a  specified set of surroundings.  If you are not aspiring to approach  this limit, please adjust your thinking.  (Suspension of disbelief!)
• The physics of the various energy transfer and transformation  processes that can be invoked sets additional limits.  Take these  seriously and change the processes used if necessary.
• The architecture you choose for your engine introduces limitations  based on both the processes involved and the devices used to  implement them.  Track the exergy destruction through these devices  to know how well you are doing.
• If you are not doing well (exergy efficiency below 50%), consider  changing the set of processes, as well as improving the devices.  Also  consider using nontraditional devices to implement the processes.
• The key to improvement is to know where you stand.  (Absolutely!)
Engine Efficiency and Power Density:Distinguishing Limits from Limitations
Exergy to Engines
Equivalence and Compression Ratios
Use of Low-Temperature Combustion
Initial Diesel Efficiency Results
Limited to Diesel-Style Combustion?
Temperature Control of Autoignition
Two Methods of Cooling
NOx Emissions vs. GRI3.0
Evaporative Cooling?
Stoichiometric with EGR
Optical imaging system