Internal Combustion Engine GroupInternal Combustion Engine Group

The effect of compression ratio on exhaust emissions from a PCCI Diesel engine

ECOS 2006

12-14 July 2006

Laguitton, Crua, Cowell, Heikal, Gold

• Introduction• Experimental set-up• Validation of single cylinder design• Strategy for low NOx, soot and FC• Conclusions

Content

Highly pre-mixed and cool combustion

IMPROVED AIR SYSTEM EFFICIENCY

INCREASED IGNITIONDELAY

INCREASED EGR RATES AND

TEMPERATURE MANAGEMENT

ROBUSTNESS CONTROL

IMPROVED AIR/FUEL MIXING

REDUCED COMPRESSION

RATIO COMBUSTION SYSTEM DESIGN

ADVANCED AIR/EGR SYSTEMS

ADVANCED COMBUSTION & AIR

PATH CONTROL

ADVANCED FIE TECHNOLOGY

COLD STARTTECHNOLOGY

REDUCE OXYGENCONCENTRATION

INCREASE EFFICIENCY+

Oxygen concentration

Source: MTZ 11/2002: Toyota

Temperature /(K)

Lo

cal

Eq

uiv

alen

ce R

atio

1000 1400 1800 2200 2600 3000

1

4

3

2

5

6

7

8

9

10

Soot formation area

NOx formation area

Combustion trend to more pre-mixed and lower temperature

Euro 4 – O2 Concentration Map

100 %

85 %

70 %

Level 3 – O2 Concentration Map

100 %85 %

70 %

Approach is to reduce the oxygen concentration characteristics over the engine speed and load operating area:– Oxygen concentration in the flame is reduced– Less NOx are formed

Time

Drive current

Rate of injection

Injection pulse

Start of combustion

SOC – Real SOI

Injectionperiod

0.00

2.00

4.00

6.00

8.00

10.00

0.00 5.00 10.00 15.00 20.00 25.00 30.00

Injection period (°CA)

SO

C -

re

al

SO

I (°

CA

)

100 % 66 % 50 %

Low NOx strategy

Improved air/fuel mixing to achieve low soot and good combustion efficiency Euro 4

Level 2

Level 3

Level of premixed fuel

Increasing Load

Trend is clear:- Injection durations reduced by increased

injection pressure and nozzle flow- Ignition delay increased by changes to

air/fuel, CR, intake temperature and EGR

Single cylinder engine facility

Single cylinder – Ricardo HYDRA:– 500cc swept volume (86mmx86mm)– 2.0L high-flow head– Variable swirl (1.0-3.5 Rs) – Compression Ratio 18.4:1 and 16.0:1– Off-engine HP pump + common rail– Delphi injector– Delphi nozzle library – EmTronix FIE controller – Reference ultra low sulphur diesel fuel

Test bed:– Horiba gas analyser MEXA 7100DEGR– AVL733 dynamic fuel meter– AVL415 variable sampling smoke meter– High speed data logger– Custom-built low speed data logger– TDM post processing

Piston-bowl cross-sections

Validation of single cylinder design

Full Load Results

0

10

20

30

40

50

1500 rpm2 bar

2000 rpm6 bar

2000 rpm16 bar

1000 rpmFull Load

2000 rpmFull Load

4000 rpmFull Load

EG

R R

ate

[%] Single

Multi

0

5

10

15

20

25

30

35

1500 rpm2 bar

2000 rpm6 bar

2000 rpm16 bar

1000 rpmFull Load

2000 rpmFull Load

4000 rpmFull Load

AF

R

Single

Multi

0

5

10

15

20

25

1500 rpm2 bar

2000 rpm6 bar

2000 rpm16 bar

1000 rpmFull Load

2000 rpmFull Load

4000 rpmFull Load

Net

IM

EP

[b

ar] Single

Multi

Part Load Results

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 5 10 15 20 25 30

NOx [g/h]

Sm

ok

e [

FS

N]

Single

Multi

1500 rpm, 2 bar

2000 rpm,6 bar 2000 rpm,

16 bar

2000 rpm,10 bar

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000 1500 2000 2500 3000 3500 4000

Engine Speed [rev/min]

Sm

ok

e [

FS

N]

Multi

Single

Effect of compression ratio on NOx emissions

0.00

2.00

4.00

6.00

8.00

10.00

12.00

-10.0 -5.0 0.0 5.0 10.0

Main injection timing (deg CA ATDC)

NO

x m

ass

flo

w (

g/h

)

-0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

-60.0 -20.0 20.0 60.0 100.0

Crankangle (deg CA)

Insta

nta

neo

us h

eat

rele

ase

0.0

20.0

40.0

60.0

80.0

-60.0 -20.0 20.0 60.0 100.0

Pre

ssu

re (

bar

)

2000 rev/min 7.7 bar GIMEPLEVEL 2: CR 18.4 and CR 16.0:1

2000 rev/min 10.8 bar GIMEP

2000 rev/min 7.7 bar GIMEP

1500 rev/min 3.0 bar GIMEP

Reduced CR decreases NOx emissions especially at high loads. At low loads (1500 rev/min 3.0 bar GIMEP), slight improvements but combustion is already fully premixed, hence reduced benefits

Fixed calibration

Effect of compression ratio on auto-ignition delay

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

-10.0 -5.0 0.0 5.0 10.0

Main injection timing (deg CA ATDC)

An

gle

of

SO

C (

deg

CA

AT

DC

)

0.0

20.0

40.0

60.0

80.0

100.0

120.0

-10.0 -5.0 0.0 5.0 10.0

Ma

xim

um

dP

/dT

(b

ar/

ms)

30.0

40.0

50.0

60.0

70.0

80.0

90.0

-10.0 -5.0 0.0 5.0 10.0

Main injection timing (deg CA ATDC)

Pre

ssu

re a

t S

OC

(b

ar)

2000 rev/min 10.8 bar GIMEP

2000 rev/min 7.7 bar GIMEP

1500 rev/min 3.0 bar GIMEP

Reduced CR decreases in-cylinder pressures. Combustions occur later, increasing the level of premixed leading to higher maximum pressure variations but lower NOx

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

-25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0

Main injection timing (°CA ATDC)

NO

x (g

/kW

h)

F

C (

kg/h

)

-2.00

-1.60

-1.20

-0.80

-0.40

0.00

0.40

0.80

1.20

1.60

2.00

So

ot

(g/k

Wh

)

Potential operating zone

3.06 FSN

Responses at 43% EGR

Late injection strategy for low NOx and soot

Summary of single injection timing responses at 1500 rev/min 6.6 bar GIMEP

(43% EGR rate, 1000 bar rail pressure)

19.0:1 AFR17.0:1 AFR

High FC penalty with veryretarded single injections

DOE Model: Soot (g/h)

Test data for FC (kg/h)

DOE model: NOx (g/h)

NOx reduced by high EGR and low AFR Low soot and good fuel consumption is achieved

through improved air/fuel mixing- Low CR, swirl and rail pressure enhancement is

critical Good fuel consumption is achieved by optimising

50% burn after TDC. Late combustion is avoided by shortening combustion duration

DOE modelling

0.0

20.0

40.0

60.0

80.0

100.0

-60.0 -30.0 0.0 30.0 60.0

Crankangle (°CA ATDC)

Pre

ssu

re (

bar)

Insta

nta

neo

us h

eat

rele

ase

Combustion phasing for optimum fuel consumption

Good combustion efficiency:- Rapid combustion- Centred between 0 and 10 °CA ATDC

Test data for FC (kg/h)

This is a conflicting requirement with low NOx combustion strategies, which require slow and late combustion

A compromise to minimise impact on combustion efficiency is to operate:

- Slow combustion- Well phased combustion

Conclusions

A good comparison with multi cylinder baseline results was achieved

Ultra low NOx has been achieved through highly pre-mixed and cool combustion

At 1500 rev/min, 3.0 bar GIMEP - a twin early injection strategy achieved improved HC and CO results compared to a pilot + “late” main strategy

At 1500 rev/min, 6.6 bar GIMEP - testing showed that a late injection strategy was essential for low NOx. A single late injection with high EGR achieved the best overall results

With 16:1 CR, an early injection strategy was only beneficial below 3.0 bar GIMEP. Late, high pressure injection combined with EGR is recommended

With the combustion bowl geometry tested, 10 and 12 hole nozzles did not offer an advantage at rated power. Reduced spray penetration, bowl interaction and air utilisation was detrimental at the higher loads and speed