0 50 100 150 200 250 300 350 400 450 5000

30

60

90

120

150

180

Pre

ssure

(bar

)

Compression side

Driver side

0 50 100 150 200 250 300 350 400 450 5000

0.5

1

1.5

2

2.5

3

Time (milliseconds)

P

osi

tion

(met

ers

from

pis

ton t

o c

yli

nder

end)

~ 50 m/s

~ 90 m/s

Increasing Engine Efficiency through Extreme Compression

S.L. Miller, M.N. Svrcek, K.-Y. Teh, C.F. Edwards

Advanced Energy Systems Laboratory, Department of Mechanical Engineering, Stanford University

Motivation Basic Design Current Work: Non-Combusting Experiments

Future Work

VR/200

VR/10

VR

VP = VR

VP/200

VP/10PP = 1 atm

Wadia, max

WAtk,

CR =10

WOtto,

CR =10

Wlost, Otto

CR =10

WAtk,

CR =200

Wlost, Atk

CR =200

Sgen, CR =10Sgen reduction

A

B C

D

F

Wlost, Atk

CR =10

E

-25 0 25 50

-20

0

20

40

60

80

S - SP,0 (kJ/kgfuel-K)

U -

UP,

0 (

MJ/

kg

fuel

)

Ideal Otto and Atkinson cycles for stoichiometric propane/air on a U-S diagram.

Compression ratios shown include 10, 20, 50, 100, and 200:1.

Special thanks to GCEP (Global Climate Energy Project) for funding this research

effort and to Scott Sutton and Ken Hencken for their valuable design and fabrication

advice.

At a compression ratio of 200:1, the work lost due to combustion irreversibility

can be reduced to one-half that from the 10:1 compression ratio case.

At 100:1 compression ratio we can potentially realize simple cycle efficiencies

near 60% – significantly higher than current devices1. The goal of this project

is to design and build a device that will test the feasibility of increasing

efficiency by using extreme compression.

Non-Combusting Experimental Goals:

• To refine components such that we can:

• Predict and repeatably achieve a given compression ratio

•Achieve the desired high piston speeds

•To develop a piston position sensing system to determine the volume profile

• To design the larger infrastructure required to run such a large experiment

safely

0

20

40

60

100

Xfuel/LHV

100 101 102

80

Compression Ratio

Cy

cle

Eff

icie

nci

es (

%)

CI

SI

1st Law (per LHV)

70 - 80% of 1st Law Eff.

Ideal Expansion (Atkinson):

1st Law (per LHV)

70 - 80% of 1st Law Eff.

Symmetric Expansion (Otto):

1Heywood, Internal Combustion Engine Fundamentals, 1988. p.196

One of the most substantial loss mechanisms in current, simple-cycle,

unrestrained, reactive engines is combustion irreversibility. A large fraction

(~20%) of the exergy of the fuel resource can be destroyed during the

combustion process. The goal of this project is to substantially reduce the

combustion irreversibility thereby increasing the overall efficiency.

New design choices are required to construct a device capable of these high

compression ratios. Post-combustion pressures are greater than 1000 bar, while

post-combustion temperatures are greater than 3000 K. A few of the obstacles

and their design implications include:

Exergy destroyed due to unrestrained reaction at different initial states for

stoichiometric propane/air modeled as an ideal gases. Includes the effects

of variable specific heats, reaction, & dissociation.

140

50

60

70

80200

10050

2010 1

300

500

1000

1500

2000

V / VP,0

Compression RatioF

roze

n R

eact

ant

T (

K)

U -

UP,

0 (

MJ/

kg

fuel

)

10-3 10-2 10-1 101 102 103

25% Xfuel

20%

18%

16%

14%

12%100 b

ars

10 b

ars

0.1

bar

s

Isobaric Reactant

Heating

Isobaric Reactant

Heating

Isentropic Reactant Compression 300 K, 1 atm

A concept drawing of how a high-compression ratio, free-piston engine would work in

practice. Diagram taken from Van Blarigan and Aichlmayr, “Optimized Free Piston Engine

Generator”, DOE National Laboratory Advanced Combustion Engine R&D Merit Review

and Peer Evaluation, April 2005.

• Typically the higher temperatures lead to greater heat transfer losses

→ Design engine with a low surface-to-volume ratio at 100:1 CR

(e.g. a long stroke)

→ Increase expansion speed to extract sensible energy as work

before it is transferred out as heat (aim for piston speeds of ~Mach

0.3, ~100 m/s at room temperature)

• High pressures lead to very high forces

→ Use two pistons to balance the forces

• Pre-mixed or early injection strategies will react too early

→ Use high-pressure direct injection system to phase combustion

We are currently manufacturing the new high-pressure combustion

section that attaches to our cylinder. The section will :

• Be tested and rated to at least 1400 bar

• Be capable of optical access through the base of the vessel

• Contain five high-pressure fuel injectors

Concept Drawing of Engine Design with Work Extraction

Experimental Design

A photograph of the

experimental setup in our

lab at Stanford. We are

currently testing the

repeatability of achieving

various compression ratios.

Next, we will incorporate

our high-pressure test

section to start combusting.

Combustion

Chamber

High Pressure

Gas Driver

High Pressure

Gas Driver

M = 0.3 M = 0.3

We have designed an experimental apparatus to study the feasibility of

achieving reduced combustion irreversibility by performing the reaction at

high internal energies. The apparatus contains:

Basic concept drawing of device design

Current Experimental Results:

Pressure data are captured by quartz dynamic pressure transducers. Position

data are captured by magnetic variable reluctance sensors placed at intervals

along the cylinder bore.

The above data are for a driver air pressure of 10.3 bar, resulting in:

• Peak pressure of 190 bar, corresponding to an isentropic, adiabatic

compression ratio of 46

• Mean piston speed during first compression of ~60 m/s

These results are in the range of our modeling predictions, and indicate that the

basic device design is capable of achieving high compression ratios and piston

speeds.

Having successfully tested

the basic functionality of the

device, we are currently

iterating on the piston design

to refine repeatability and to

characterize gas blowby.

The piston design at right

produced the data shown on

this poster.

A large, high-

pressure air

reservoir,

capable of

providing

driver air

pressure up

to 68 bar

A fast (~20 ms opening time)

helium-driven poppet valve

to allow repeatable,

controllable introduction of

driver air to cylinder

An ~ 8 ft. long cylinder bore

to achieve low surface to

volume ratios at TDC,

reducing the effects of heat

transfer

A high-pressure vessel

attached to the end of the

cylinder to withstand the high

pressures at TDC. It will be

replaced by an even higher

pressure combustion vessel

with optical access.

A free-piston architecture

that allows for high piston

speeds