injection pressure as a means to guide air …pdim15++diesel... · momentum transfer heat transfer...

| | 10th & 11th of March 2015| 1

INJECTION PRESSURE AS A MEANS TO GUIDE AIR UTILIZATION IN DIESEL ENGINE COMBUSTION

H. Dembinski, Scania AB Sweden

H.-E. Angstrom, KTH Stockholm, Sweden

E. Winklhofer, AVL List GmbH, Austria

London, March 10 and 11, 2015

Goteborg, November 26, 2015

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MOTIVATION

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THE QUESTION

t = 0.2 ms aSOI

How can higher injection pressure end up with lower engine out soot ?

injection pressure / soot

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THE QUESTION

How can higher injection pressure end up with lower engine out soot ?

Can we understand the mechanisms ? If so – how to exploit them ? Which kind of analysis would we require ?

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1.Injection pressure – spray at nozzle exit

2.Spray interaction with in-cylinder gas

Momentum transfer

Heat transfer

3.Ignition

4.Premixed and diffusion flames

Soot formation

Soot oxidation

5.Enhancing soot oxidation

6.How things come together

pressure temperature flow

7.Summary

CONTENT

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1. Injection pressure - spray

PRESSURE – GEOMETRY - FUEL FLOW - SPRAY

Fuel injection pressure

injector internal flow

Such flow is visualized in 2D model nozzle tests.E. Winklhofer et al.: „Basic flow processes in high pressure fuel injection equipment“, ICLASS 2003

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Internal flow is visualized in 2D model nozzle tests.Liquid into liquid injection (Diesel)

White: liquid phase (Diesel)Black: gas phase (cavitation bubbles) at Diesel vapor pressure << 1 bar

needle

body

30 bar

1 bar1 bar

Pin= 400 bar

needle

liquid

liquid

Shear layer

cavitation

boundary layer

cavitation

boundary layer

cavitation

Fuel flow is subject to cavitation in local shear and boundary layers.

High cross flow velocity gradientsforce static pressure to drop belowvapor pressure.

Driving parameters are:• Geometry• Velocity gradients• Pressure• Vapor pressure of fuel


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Pressure field at inflow into nozzle hole

Fuel pressure is discharged withinfractions of a millimeter at theentrance to the nozzle hole.

Geometry influence on local staticpressure – and hence on cavitation - ishighest in areas of high pressure drop.

Measurements were done as Diesel fluid was just below cavitation limitA B

Sharp (A) and

round (B) inlet


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Pin= 60 120 825 bar

laminar

turbulent

turbulent and

cavitating

spray

cavitation

fuel

80

0 µ

m8

00

µm

air

Liquid into air: 2D model nozzle to seeinternal flow together with spray.Average of 30 events

A nozzle flow – spray experiment:

At high injection pressure we see• Well developed cavitation down to nozzle hole exit• Atomizing spray with highly stable spray cone

angle

Note the dimensions: 0,8 mm nozzle hole + 0,8mm free spray

Conclusion: high injection pressure stabilises spray cone angle near nozzle exit.


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Fuel injection pressure

exit velocity:

Vspray/VBernoulli ~ 0.9

0 1 ms 2

-2

2

0

Z = 15 mm

sp

ray d

ia.

-m

m

1500 bar

spray diameter fluctuation

Spray observation in optical Diesel research engine:

Spray diameter fluctuation measurement todocument spray targeting and spray fluctuation in far field

2. Spray interaction with in-cylinder gasMomentum transferHeat transfer


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Length - time and Diameter - time shadow traces of Diesel sprays in

an optical engine

Spray tip propagation and spray core length.

Strahlschatten

20 mm

10

0

0 1 ms 2

Start of Injection End of Injection

Spray shadow 800 bar

Spray targeting and spray diameter

(spray angle) fluctuations.

0 1 ms 2

-2

2

0

-2

2

0

-2

2

0

500 bar

800 bar

1500 bar

Z = 15 mm

Spray observation in opticalDiesel research engine:

Spray diameter fluctuationmeasurement to document spray targeting and spray fluctuation in far field

Conclusion:

• high injection pressurestabilises spray targeting.

• Spray diameter fluctuationsappear at ever higherfrequency



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Schlieren imaging in optical research engine, 500 rpm

Spray in hot compressed in-cylinder gas

Heat transfer from gas into spray with resultant• fast expansion of spray vapor plume• and self ignition

T = 930 K

p = 53 bar


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Prail = 300 bar Prail = 800 bar Prail = 1100 bar

Average spray contours at 0.40 ms after SOI. Nozzle: 1x 0.115 mm


Schlieren imaging in optical research engine

Spray in hot compressed in-cylinder gas

Heat transfer from gas into spray withresultant• fast expansion of spray vapor plume• and self ignition

• Injection pressure enhances fuel vaportransport

Spray

core

Spray

vapor

cloud


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3. Ignition

Time sequence shows• sprays before ignition, • combustion of premixed fuel vapor in „blue flame“

pockets, • and start of diffusion combustion

Ignition and spray – flame interaction in optical engine, 85 mm bore, p = 60 barCombustion chamber is externallyilluminated, high speed camera records ofone cycle.

0 µs 50 µs 100 µs 150 µs

µs

PRESSURE – GEOMETRY - FUEL FLOW – SPRAY – EVAPORATION - IGNITION

Full glass optical piston

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3. Ignition

Time sequence shows• combustion of premixed fuel vapor in „blue flame“ pockets, • and start of diffusion combustion

• Note that blue premixed flame is only visible at ignition forup to 100 µs (0,6 deg CA at 1000 rpm)

Ignition and spray – flame interaction in opticalheavy duty engine, 127 mm bore, p = 153 bar

PRESSURE – GEOMETRY - FUEL FLOW – SPRAY – EVAPORATION - IGNITION

A

Piston bottom window

80 mm piston

window dia.

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4. Premixed and diffusion flamesSoot formationSoot oxidation

INJECTION PRESSURE – COMBUSTION

Diesel combustion movie

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HD DIESEL OSCE WITH

PISTON BOTTOM WINDOW

Fired operation for

15 cycles

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Topic: 20 bar IMEP

1400 rpmFired operation for

15 cycles

HD DIESEL OSCE WITH


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0 10 20 30 40 50 60 70 800

2

4

6

8

10

12

14x 10

4

CAD

tota

l soo

t (K

L*a

rea)

500 bar

1000 bar

1500 bar

2000 bar

1000bar

Injection at

500bar

deg CA

tota

l soot

-K

L*a

rea

500 bar1000

1500

2000

„metal engine“

In optical engine


„Metal engine“ soot measurement:• Lower FSN at higher injection pressure

„optical engine“ flame evaluation shows• Faster soot oxidation at higher injection

pressure

Soot oxidation needs flame (soot) – air mixing

Data show that injection pressure has significantinfluence on air utilization = soot oxidation.

How can this happen ?

KL is evaluated from high speed

flame movies with 2-color method


– AIR UTILIZATION

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How can it be that injection pressureimproves air utilization ?


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2000 bar

21

2000 bar

0 m/s 55 m/s

near end of injection 10 deg CA after end of injection

2000 bar


At ongoing injection

• Backflow of flames into combustionchamber center following spray – vapor -flame reflection on piston bowl wall

After end of injection

• Speed up of swirl motion in center of pistonbowl

HOW CAN INJECTION PRESSURE IMPROVE

AIR UTILIZATION?

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200 bar

2000 bar

22

2000 bar

0 m/s 55 m/s

near end of injection 10 deg CA after end of injection

2000 bar200 bar


A comparison with very low injectionpressure

Spray momentum is too small foreffective interaction with reflecting pistonbowl wall

Conclusion 1Injection pressure drives the flame back into areas of un-used air

HOW CAN INJECTION PRESSURE IMPROVE

AIR UTILIZATION?

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How can it be that injection pressureimproves air utilization ?

1. It introduces flame transport into areas with un-used air

2. And further: flame transport enhances turbulent motion


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FLAME MOTION AND TURBULENT KINETIC ENERGY

5. Enhancing soot oxidation

Flame (soot) transport

Flow field

Turbulence: data on local turbulent kinetic energy

500 bar,

FSN = 1,2

1000 bar

FSN = 0,5

At 8,5 EOI 12,5 16,5 21,5 deg CA

Significant rise of local turbulence with high

injection pressure…

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KINETIC ENERGY RELAXATION AFTER END OF INJECTION

𝐾𝐸~𝑢𝑥2 + 𝑢𝑦

2

2.


2

2.

50

0 b

ar,

FS

N =

1,2

10

00

ba

r

FS

N =

0,5

CAD

Kinetic energy vs CAD

Local Turbulent Kinetic Energy


Flame (soot) transport

Flow field

Turbulence

…and fast decay of turbulence at

end of injection

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KINETIC ENERGY RELAXATION AFTER END OF INJECTION


2

2.


2000 bar

Soot transport

Turbulence is driver for

soot – oxygen mixing…


2

2.

…and results in enhanced

soot oxidation

Soot

formationSoot oxidation

Note: high turbulence for effective soot oxidation is only available right at the end of injection.

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„Metal engine“ soot measurement:

• Lower FSN at higher injection pressure

„optical engine“ flame evaluation shows• Faster soot oxidation at higher injection pressure

Soot oxidation needs flame (soot) – air mixing

Data show that injection pressure has singificant influence on airutilization = soot oxidation.

How can this happen ?

Flame - summary

1. Transport soot to meet with air2. Use turbulence for fast soot – air mixing

Examples have shown• that and how both, transport and turbulence, are controlled by fuel injection and spray momentum reflection on piston bowl walls.• Effective soot oxidation must happen close to the end of injection to benefit from high temperature oxidation rates.

6. How things come together pressure temperature flow

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7. Summary


Spray: Spray turbulence and atomizaton are driven by cavitationCavitation is controlled by shear and boundary layer flow under influence of local spray hole geometryAt usual temperatures ( 100 °C) and injection pressures ( 500 – 2500 bar)„normal“ behaviour of Diesel sprays: liquid spray core of droplets and ligaments, fuel vapor anddiffusion flame

Flame:Temperature and pilot injection control ignition, soot formation in diffusion flame,

Soot oxidation:Injection pressure is most effective to control flame transport and turbulent mixing for fast oxidation

Analysis: in optical Diesel engines with realistic temperature, pressure and geometry parameters

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Thank you

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6 MINUTES TO STOP THE ENGINE, CLEAN THE COMBUSTION CHAMBER AND START AGAIN

Diesel engine movie

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HD DIESEL OSCE WITH


Fired operation for

15 cycles

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Topic: 20 bar

IMEP

1400 rpm

Fired operation for

15 cycles

HD DIESEL OSCE WITH


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THE RISK OF GLASS DAMAGE

MOUNTING

PRESSURE

TEMPERATURE

INERTIA FORCES

LOCAL CONTACT

UNKNOWN

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needle

body

1 bar

Pin= 400 bar

liquid

liquidFuel flow is subject to cavitation in local shear and boundary layers.




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needle

body

30 bar

1 bar

Pin= 400 bar

liquid

liquid

Shear layer

cavitation

boundary layer

cavitation

Fuel flow is subject to cavitation in local shear and boundary layers.




injection pressure as a means to guide air …pdim15++diesel... · momentum transfer heat transfer...

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