Opportunities and Challenges of Lean

Combustion in Automotive IC Engines

Russ Durrett

GM Global R&D

1

Intro – Diesel Advantage Over Gasoline

¶ Data from EPA show approximately

30% reduction in gallons/100 miles for

diesel (approx. 40% higher MPG)

¶ Why is a diesel engine more efficient

that a conventional gasoline engine?

¶ Common answers:

¶ No throttling losses

¶ Higher compression ratio

¶ These are contributors, but the lean

combustion process of the diesel

engine is the main factor leading to

the efficiency gain

2

Source: Light-Duty Automotive Technology,

Carbon Dioxide Emissions, and Fuel Economy

Trends: 1975 Through 2012, EPA, 2013

Laboratory 55/45 Fuel Consumption

vs. Vehicle Weight

approx.

30% red.

Intro – Diesel Advantage Over Gasoline

¶ Use cycle simulation to look at the independent effects of:

¶ Throttling

¶ Compression ratio

¶ Lean combustion

¶ Engine configuration:

¶ 2 liter, in-line 4 cylinder

¶ CR = 9.5

¶ Port fuel injected

¶ Wiebe heat release

¶ Woschni heat transfer

¶ Chen-Flynn friction

¶ 98% Comb. Efficiency

¶ 2000 RPM / 5 bar BMEP

operating condition

3

Intro – Diesel Advantage Over Gasoline

¶ Four cases modeled

1. Baseline case – throttled, stoichiometric, CR = 9.5

2. Un-throttled case - use EIVC to un-throttle the engine

3. High CR case - increase CR from 9.5 to 16.0

4. Lean case - increase lambda from 1.0 to 2.0

4

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

thrott stoich

CR = 9.5

EIVC stoich

CR = 9.5

EIVC stoich

CR = 16

EIVC lean

CR = 16

Pe

rce

nt

of

Fu

el E

nerg

y

Exh Chem.

Exh Thermal

Heat Transfer

Friction

Brake

Intro – Diesel Advantage Over Gasoline

¶ Four cases modeled

1. Baseline case – throttled, stoichiometric, CR = 9.5

2. Un-throttled case - use EIVC to un-throttle the engine

3. High CR case - increase CR from 9.5 to 16.0

4. Lean case - increase lambda from 1.0 to 2.0

¶ Effects are cumulative

for the 4 cases

¶ Bars show percentage of fuel

energy going to:

¶ Brake work

¶ Friction losses

¶ Heat transfer losses (coolant)

¶ Exhaust thermal losses

¶ Exhaust chemical losses

5

29

.0 %

30

.7 %

32

.7 %

37

.0 %

Intro – Diesel Advantage Over Gasoline

¶ Four cases modeled

1. Baseline case – throttled, stoichiometric, CR = 9.5

2. Un-throttled case - use EIVC to un-throttle the engine

3. High CR case - increase CR from 9.5 to 16.0

4. Lean case - increase lambda from 1.0 to 2.0

¶ Effects are cumulative

for the 4 cases

¶ Bars re-scaled to show equal

brake work for all cases

¶ This reflects the actual fuel

energy used in the 4 cases

6

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

thrott stoich

CR = 9.5

EIVC stoich

CR = 9.5

EIVC stoich

CR = 16

EIVC lean

CR = 16

Pe

rce

nt

of

Ba

se

lin

e F

uel E

nerg

y

Exh Chem.

Exh Thermal

Heat Transfer

Friction

Brake

94.5 %

88.8 %

78.6 %

Intro – Diesel Advantage Over Gasoline

¶ Four cases modeled

1. Baseline case – throttled, stoichiometric, CR = 9.5

2. Un-throttled case - use EIVC to un-throttle the engine

3. High CR case - increase CR from 9.5 to 16.0

4. Lean case - increase lambda from 1.0 to 2.0

¶ Effects are cumulative

for the 4 cases

¶ Bars re-scaled to show equal

brake work for all cases

¶ This reflects the actual fuel

energy used in the 4 cases

7

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

thrott stoich

CR = 9.5

EIVC stoich

CR = 9.5

EIVC stoich

CR = 16

EIVC lean

CR = 16

Pe

rce

nt

of

Ba

se

lin

e F

uel E

nerg

y

Exh Chem.

Exh Thermal

Heat Transfer

Friction

Brake

94.5 %

88.8 %

78.6 %

Intro – Diesel Advantage Over Gasoline

¶ Cumulative gains in brake

thermal efficiency:

¶ 6% from un-throttling

¶ 7% from increased CR

¶ 15% from lean combustion

¶ Total gain of 27%

¶ Over half of the gain comes from

the lean combustion process

8

0.0%

5.8%

12.6%

27.3%

0%

5%

10%

15%

20%

25%

30%

BT

E Im

pro

ve

me

nt

Ove

r B

as

eli

ne

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

thrott stoich

CR = 9.5

EIVC stoich

CR = 9.5

EIVC stoich

CR = 16

EIVC lean

CR = 16

Pe

rce

nt

of

Ba

se

lin

e F

uel E

nerg

y

Exh Chem.

Exh Thermal

Heat Transfer

Friction

Brake

Why Lean Combustion

¶ Increased dilution improves

isentropic efficiency by lowering

temperatures and increasing

gamma

¶ Switching from exhaust dilution

to air dilution improves isentropic

efficiency by increasing gamma

¶ Increased dilution improves the

indicated efficiency by lowering

temperatures and decreasing

heat losses

9

Source: Foster

Combustion Engine Efficiency Colloquium

DOE, 2010

Why Lean Combustion

¶ Increased dilution improves

isentropic efficiency by lowering

temperatures and increasing

gamma

¶ Switching from exhaust dilution

to air dilution improves isentropic

efficiency by increasing gamma

¶ Increased dilution improves the

indicated efficiency by lowering

temperatures and decreasing

heat losses

¶ Ignition and flame propagation

limit the potential of traditional

homogeneous, flame propagation

based combustion systems

¶ Operating lean with high levels of

dilution can improve vehicle-level

efficiency by about 15%

10

Source: Foster

Combustion Engine Efficiency Colloquium

DOE, 2010

Stoichiometric

w/o EGR

Stoichiometric

w/ EGR

Lean w/ EGR

Lean w/o EGR

Why Lean Combustion

¶ To maximize efficiency we must migrate to air dilution and use levels of

dilution beyond the limits of traditional homogeneous ignition and flame

propagation

¶ Operating lean with high levels of dilution can improve vehicle-level

efficiency by about 15%

11

Stoichiometric

w/o EGR

Stoichiometric

w/ EGR

Lean

w/ EGR

Isentropic

Indicated

13% 3%

Challenges of Lean Combustion

¶ Lean combustion offers a significant efficiency advantage as described

¶ However, there are also several challenges associated with implementing

the technology in a practical light duty automotive application:

¶ Combustion stability and robustness over a wide operating range

¶ Boosting system requirements

¶ Controls requirements (including sensors & actuators)

¶ Low exhaust temperature

¶ Advanced lean aftertreatment

¶ Cold start and transient operation

12

Lean Combustion Alternatives

¶ To maximize ICE efficiency it is necessary to operate lean (air dilution)

with overall dilution levels beyond the limits of traditional homogeneous

flame propagation combustion modes

13

Combustion

Technology Key Challenges

Lean SI – Gasoline Ignition and flame propagation limits along with

emission challenges

Lean SI Stratified Charge –

Gasoline Combustion robustness and emission challenges

SI-HCCI – Gasoline Combustion control and NVH challenges

Gasoline CIDI Combustion control and NVH challenges

RCCI CIDI Multi-fuel requirement and emission challenges

PCCI / LTC Diesel CIDI Emission and NVH challenges

Traditional Diesel CIDI Emissions challenges

The Combustion Control Challenge

¶ To maximize the fuel economy

potential of the ICE while

minimizing emissions we must

operate in a narrow range of

equivalence ratios and

temperatures

¶ We must avoid rich diffusion

flames

¶ We must avoid high temperature

homogeneous propagating flames

¶ We must maintain sufficient

temperature for complete

oxidations

¶ We must maintain these ideal

conditions over all operating

conditions

14

Source: Kamimoto and Bae– SAE 880423

0

1

2

3

4

5

6

500 1000 1500 2000 2500 3000

Eq

uiv

ale

nc

e R

ati

o (

ph

i)

Temperature (K)

NOx reduction

via dilution

Soot reduction

via increased

mixing

Soot formation

zone

NOx

zone

CO / UHC

oxidation

limit

The Lean Combustion NVH Challenge

¶ To meet the goals of advanced

lean combustion , the global

community is exploring a range of

LTC concepts – SI-HCCI, Gasoline

CIDI, PCCI, RCCI

¶ All these concepts are challenged

by operating domain constraints –

combustion issues at low loads &

temperatures and dilution/noise

issues at high loads

¶ All these concepts require

sophisticated injection and control

systems to regulate in-cylinder

conditions

¶ All these concepts are sensitive to

ambient conditions and fuel

properties

15

RCCI Mapping, Curran, Gao, Wagner,

Oak Ridge National Labs

The Exhaust Temperature Challenge

¶ Increasing the fraction of fuel energy that does useful work means

reducing the energy in the exhaust and this poses aftertreatment

performance challenges

16

The Exhaust Oxygen Challenge

¶ Increasing the extent of lean operation to enhance fuel economy poses

significant aftertreatment cost and robustness challenges

17

Exhaust Oxygen Content

Fuel E

ffic

iency

Lean NOx Trap

High PGM cost

Sulfur poisoning

Desulfation required

Narrow temperature

window Urea-SCR

Secondary urea tank with

injection system; high urea

consumption for gasoline

Urea solution freezing

Conventional TWC

Poor NOx efficiency with

DFCO/Lean-idle

Urea-Free SCR

Low PGM cost

No sulfur poisoning

No secondary tank

Conclusions and Future Research Needs

¶ Developing robust, cost-effective, lean combustion technologies for

automotive gasoline engines will be challenging but the fuel economy

benefits are significant

¶ In-cylinder emissions control is important

¶ Challenges for engine optimization:

¶ Robust combustion control over all operating conditions

¶ Robust emissions control over all operating conditions

¶ Good fuel consumption under real world driving conditions

¶ Low combustion noise

¶ Exhaust temperature

¶ This will require a coordinated effort between air handling, combustion,

aftertreatment and controls – a system optimization approach

¶ In order to do this work effectively it is important to focus research on

fundamental insights that have long-term value critical to achieving

upper-bound efficiency and lower-bound emissions

18

19

Questions?