Engine Efficiency and Power Density: Distinguishing Limits from Limitations

Engine Efficiency and Power Density:

Distinguishing Limits from Limitations

Chris F. Edwards

Advanced Energy Systems Laboratory Department of Mechanical Engineering

Stanford University  

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

Lorentz Work (MHD)

Exergy Classification Architecture Engines

Efficiency, Effective Compression Ratio,  and Ideal Architectures

• 70‐80% Ideal Work, 60‐75% Peak Pres., 75‐90% Peak Temp.

10⁰ 10¹ 10²

0 10 20 30 40 50 60 70 80 90 100

Effective Compression Ratio (ρ/ρ

0)

First Law Efficiency (%)

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 Low‐Temperature Combustion

• Use of LTC to control NOx emissions limits work 

output to 6‐7 bar IMEP (5‐6 bar BMEP).

Atkinson?  Not LTC?

• Efficient, pressure‐limited, high‐output operation might be 

achievable with optimal expansion and non‐dilute 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

Lorentz Work (MHD)

Exergy Classification Architecture Engines

Van Blarigan/Aichlmayr

Free‐Piston Engine Concept

9

Free‐Piston Architecture for High CR

• Balanced forces, no bearing loads

• Long stroke‐to‐bore ratio for low  surface‐to‐volume ratio

• Short residence time at min. V

• Can use linear alternator for work  extraction (van Blarigan/Aichlmayr)

Gas driver Gas driver

Free pistons Combustion 

chamber

0 20 40 60 80

0 0.2 0.4 0.6 0.8 1

Time (ms) Volume (V/V 0)

Free-piston experiment Slider-crank

10 Sample bag

Condenser Vacuum  pump HC analyzer

Heated  sample line

Band  heaters

Stanford Free‐Piston, Extreme 

Compression Apparatus

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

0 20 40 60 80 100

20 30 40 50 60 70 80

Compression Ratio

Efficiency(%)

Ideal 1^st-law efficiency

Ideal cycle minus air experiment losses Experimental indicated efficiency

Diesel #2

φ = 0.27 - 0.30

Limited to Diesel‐Style 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 NO

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.

0 20 40 60 80 100

200 400 600 800 1000 1200 1400 1600

ρ/_ρ

0

T (K)

T_start = 250 K T_start = 298 K

Model:

– Adiabatic compression 

– Homogeneous, stoichiometric  methane‐air charge

– Volume‐time profile from  experimental data

– GRI 3.0 chemical kinetics

Two Methods of Cooling

Compressor

J‐T  valve Cooling air

Refrigerant

Reactants at T₀, P₀

1 2

Engine Products

Compressor Cooling air

Reactants at T₀, P₀

1

2

Engine 3

Products

1 2 3 4

250 300 350 400

ρ/ρ

0

T (K)

S S

2

P = 1 atm

1

1 2 3 4

250 300 350 400

ρ/_ρ

0

T (K)

S

S

3 2

1

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^-1 10⁰

10⁰ 10¹ 10² 10³

Volume / Full Cyl. Volume

Pressure (bar)

• Fuel:  premixed methane‐air

• Effective CR: 

35 to 90:1

• Equivalence ratio:

0.96 to 1.04

• Peak Efficiency:  57%

(Includes comp. work.)

Measured Combustion Efficiency

18

NO _x , HC, and CO 

Premixed Emissions  w/Intercooling

0.96 0.98 1 1.02 1.04 1.06 0

10 20 30 40

Equivalence Ratio

Specific Emission (g/kW-hr) _{~ 35:1 CR}

~ 60:1

~ 80:1

0.96 0.98 1 1.02 1.04 1.06 0

0.5 1 1.5 2

Equivalence Ratio

Specific Emission (g/kW-hr)

~ 35:1 CR

~ 60:1

~ 80:1

0.96 0.98 1 1.02 1.04 1.06 0

2 4 6 8 10 12 14

Equivalence Ratio

Specific Emission (g/kW-hr)

~ 35:1 CR

~ 60:1

~ 80:1

NO

HC CO

NOx Emissions vs. GRI3.0

60:1 CR, methane

~1% Loss in  Combustion Efficiency

Emissions in Context of TWC

1J. Chiu, J. Wegrzyn, and K. Murphy, SAE Paper 2004‐01‐2982

2 I. Saanum, M. Bysveen, P. Tunestal, and B. Johansson, SAE Paper No. 2007‐01‐0015.

60:1 CR, 1.028 φ

21

Evaporative Cooling?

Model:

– Same as before, but with  water vaporization

– Vaporization rate matches  injection rate of real 

injector

– Start‐of‐injection chosen to  avoid gas saturation

• Inject liquid during compression (water has good properties).

• Vaporization draws sensible energy from the gas, thus lowering the  temperature.

10^-2 10^-1 10⁰

10⁰ 10¹ 10² 10³

V/V0

P (bar)

No cooling Water injection

3% mass fraction total water injected

Start water injection

10^-1 10⁰ 10⁰

10¹ 10² 10³

V/V₀

Pressure (bar)

Water injection Intercooling Water model for equal cooling

SOI Model EOI

Experiment EOI

Experiments w/Evaporative Cooling

22

• Achieved TDC phasing  up to 60:1 CR

• More water needed  (8% vs. 1%)

• 10% decrease in  efficiency (53%)

• Limited by the  injector setup:

–Stratification

–Slow vaporization

Reduction in Rate‐of‐Rise, Ringing

~ 5000 bar/CAD for 

intercooling approach

~ 80 bar/CAD for water  injection approach

61 62 63 64 65

0 200 400 600 800 1000

Time (ms)

Pressure (bar)

Intercooling

Water injection • Maximum rate of pressure  rise, translated to slider‐

crank at 1800 RPM:

Take‐Away 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 non‐traditional devices to implement the processes.

• The key to improvement is to know where you stand.  (Absolutely!)