Characterization of High Pressure Natural Gas Injections...

18th International Sy mposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016

Characterization of High Pressure Natural Gas Injections for Stratified Operation of a Spark Ignited Engine by Simultaneous Fuel Concentration and Air

Entrainment Measurements

W. Friedrich1,*, R. Grzeszik1, M. Helmich1,2, T. Bossmeyer1, M. Wensing3

1: Robert Bosch GmbH, Corporate Research, Germany 2: Karlsruhe Institute of Technology (KIT), Institute of Internal Combustion Engines (IFKM), Germany

3: FAU Erlangen-Nuremberg, Institute of Engineering Thermodynamics (LTT), Germany * Correspondent author: [email protected]

Keywords: PIV/LIF, CNG-DI, engines, jets, sprays, stratified operation, high pressure, direct injection, jet interaction

ABSTRACT

In a study using an optically accessible single-cylinder research engine, contraction and deflection effects of the gas

jets during a high pressure natural gas direct injection out of a multi-hole injector are detected. These effects

complicate the injector design, especially for the stratified operation. To control the mixture formation and to

provide an ignitable mixture at the spark plug position, a fundamental understanding of the different effects which

occur during an injection is required. The air entrainment velocity is a major factor on the mixture formation process

as well as on the interaction between the single free jets.

Two measurement techniques, Particle Image Velocimetry (PIV) and laser-induced fluorescence (LIF) are utilized to

simultaneously determine the entrainment velocity and the fuel concentration during the injection. High velocity

gradients between the slow charge motion and the fast velocities in the jet region and inside the jet make the choice

of the interframing time for the PIV measurements difficult. A reasonable compromise is found to resolve both the

velocity of the charge motion and the spray induced flow which is most important to quantify the air entrainment

velocity. This air entrainment velocity is plotted over a line parallel to the jet contour to compare the spray-induced

flow for different rail pressures.

The effect of rail pressure on the entrainment velocity is determined at different crank angle positions. Low rail

pressure (50 bar) results in an entrainment velocity of 15 m/s, while up to 35 m/s are found for high rail pressure

(290 bar). A correlation with peaks in the entrainment velocity course parallel to a free jet contour and the presence

of shock barrels is established. These peaks are formed due to pressure fluctuations in the free jet which evidence

the existence of Mach disks. They arise by reason of a supersonic flow due to an expansion of the gas from

overpressure to the cylinder pressure. During the injection, various effects occurred. While the end of injection is

already reached and the injection is completely collapsed for the highest rail pressure, the lowest rail pressure still

has a stable cone angle.


1. Introduction

Natural gas is an attractive alternative to conventional liquid fossil fuels, not only due to the

beneficial hydrogen-to-carbon-ratio of its principal component methane (CH4). Its availability is

not limited because besides the fossil sources there is the possibility of the regenerative natural

gas production and thus to close the CO2 loop. The two most common ways are the production

from regenerative electrical energy (power-to-gas) and from biomass (van Basshuysen et al.

2015). Contemporary natural gas cars are running with a port-fuel injection (PFI). Due to its low

density the natural gas displaces a considerable share of the intake air and thus decreases the

volumetric efficiency of the engine. This results in a torque disadvantage compared to the gaso-

line counterpart. Hence natural gas is still not well-established as an alternative fuel. A gas direct

injection (DI) would suppress this disadvantage. An additional increase of the system pressure

allows prospects regarding the combustion process.

A significant efficiency potential has been shown for stratified operation using a high-pressure

natural gas direct injection (Friedrich et al. 2015). Corresponding optical measurements of the

mixture formation revealed deflection effects of the multi-hole injector’s individual free jets.

These deflections are either caused by an interrupting flow (charge motion) or by a mutual

attraction of the individual free jets (jet-jet-interaction). While the individual free jets are still

distinguishable from each other at the early stage of an injection, they contract in the further

progress of the injection. The air entrainment into the individual free jets effects the mutual

attraction. The scope of this paper is to characterize the transition between a non-contracted and

a contracted injection. For the investigation presented, Particle Image Velocimetry (PIV) and

laser-induced fluorescence (LIF) are utilized simultaneously. LIF is used to quantify the air/fuel-

ratio inside a planar layer and serves for the definition of the contour of the injection. PIV serves

for measuring velocity vectors in the same planar layer. Combining both measurement

techniques allows quantifying the air entrainment velocity into the spray plume by plotting the

normal air velocities over a defined line at the jet contour.

2. Methodology

Laser-induced fluorescence

To investigate the mixture formation in a defined layer inside the combustion chamber, a well-

known measurement technique (tracer laser-induced fluorescence) (Eckbreth 1988, Schulz & Sick

2005) was employed. It is based on the excitation of molecules to a higher energy level due to


photon absorption. By emitting another photon of a higher wavelength (fluorescence), the

molecule can leave the excited molecular state. As methane and nitrogen are spectroscopic pure

for the excitation wavelength of 266 nm, the well-defined tracer substance triethylamine (TEA) is

added. In Fig. 1, the excitation wavelength and the fluorescence spectrum of TEA are depicted

including the filter transmission spectrum.

Intern | CR/AED2-SP | 20.10.2014 | © Robert Bosch GmbH 2014. Alle Rechte vorbehalten, auch bzgl. jeder Verfügung, Verwertung,

Reproduktion, Bearbeitung, Weitergabe sowie für den Fall von Schutzrechtsanmeldungen.

Promotion Gemischbildung CNG-DI

1

250 300 350275 325

norm

aliz

edsi

gnal

inte

nsit

y(-

)

wavelength (nm)

tran

smis

sion

(%)

1.0

0.8

0.6

0.4

0.2

0

100

80

60

40

20

0

triethylaminefilterexcitation

Fig. 1 Fluorescence emission spectrum of TEA for 266 nm excitation and filter transmission spectrum.

The signal calibration procedure works as follows: First, a background image is taken without

any tracer, it serves for the correction of fluorescent contaminants and dark current. Second, a

calibration image with defined equivalence ratio and homogeneous mixture is made. The

homogeneous mixture is guaranteed by using the PFI injector, positioned 0.5 m upstream the

suction tube (see Fig. 3). And third, the measurement of the high pressure direct injection is

performed. The three steps are conducted for each crank angle position investigated. To

compensate statistical error in the recordings, 50 images are taken for each crank angle position.

By applying the post-processing described by Mederer, quantitative results can be obtained

(Mederer et al. 2012).

Particle Image Velocimetry

The PIV is a non-invasive measurement technique to quantify the flow velocity inside a 2-

dimensional layer. Based on the assumption that small and light particles which still have to

scatter enough light to be mapped on the camera chip, follow the fluid flow without any lag, the

air velocity can be calculated from a particle movement detected. In general, particles are


disperse materials and differ in the phase limit from the ambient, continuous medium. For the

engine application, they should stand the compression temperature and not be abrasive or

corrosive.

Buschbeck (2013) and Neubert (2015) effectively utilized graphite to quantify the in-engine

charge motion velocity. The challenge in the present work is to quantify both the slow charge

motion velocity (~ 15 m/s @ 2000 rpm) and the very high velocity in a high pressure natural gas

jet (< 466 m/s). Therefore, an interframing time t variation was conducted in the range of

150 ns < t < 10 s. At the very low interframing times (150 ns < t < 300 ns), the particle

movement with 0.05 – 0.1 pixels was too low to calculate a velocity vector. Choosing t >> 1 s

results in: Fast particles in the gas jet leave the laser light sheet and thus cannot be tracked

anymore or two correlation peaks are calculated, one for the slow and one for the fast flow

velocities. A decrease of t leads to an overlap of peaks for more than one velocity and hence a

determinate result, which represents the averaging of the velocities. The absolute value was

verified by comparing it with an interframing time t = 10 s resulting in a pixel shift of

~ 4 pixels for 15 m/s. Thus the interframing time t was adjusted to 1 s to obtain a definite

intensity gradient for each 32 x 32 pixel interrogation window.

3. Experimental Setup

The investigations presented in this paper were performed using an optically accessible single-

cylinder direct injecting spark ignition engine with the characteristics presented in table 1. With

its silica glass cylinder and piston, it allows a view into the combustion chamber from two

spatial directions. The injector position is central and the spark plug is located between the two

exhaust valves, as depicted in Fig. 2. In order to facilitate the description of the injection, the jets

are numbered in Fig. 2, where number 1 is the central jet, number 2 and 3 are pointing into the

direction of the spark plug and number 4 to the inlet side.

Intern | CR/AED2-SP Li | 04.07.2014 | © Robert Bosch GmbH 2014. Alle Rechte vorbehalten, auch bzgl. jeder Verfügung, Verwertung,


Homogen_ausmagern_schicht_mager

n=2000min-1, pmi=4bar IR-Messung ZZP 4

2

inte

nsity

(-)

Spk

25° BTDC

IV

IV EV

EV

10 mm

10 mm

0

20

50

60

80

100

-50

-100

a b

1 2 34

Fig. 2 IR absorption measurement representing a the methane injection

at 25° CA BTDC with numbering of the jets.


The suction tube’s layout is designed to generate a moderate tumble motion. For calibration of

the LIF fluorescence signal, a defined homogeneous mixture is generated using a conventional

Bosch NGI 2 port fuel injector as drafted in Fig. 3.

Table 1 Characteristics of the optically accessible direct injection spark ignition engine.

displaced volume 454 cc

stroke 86 mm

bore 82 mm

compression ratio 10.5:1

number of cylinders 1

number of valves 4

exhaust valve open 126° ATDC @ 0.15 mm lift

exhaust valve close 360° ATDC @ 0.15 mm lift

inlet valve open 358° BTDC @ 0.15 mm lift

inlet valve close 138° BTDC @ 0.15 mm lift

Instead of methane, nitrogen is used for the investigations of the mixture formation for security

reasons. A comparison between methane and nitrogen injections with 50 bar and 100 bar rail

pressure into a pressure chamber (p = 4 bar, T = 250 °C) is presented in Friedrich et al. (2015).

Both methane and nitrogen are not fluorescent. The nitrogen is seeded with TEA by leading it

through a pressure reservoir which is filled with liquid TEA, like performed by Mohamad

exploiting the principle of the Drechsel bottle (Mohammad et al. 2010).

For the excitation of the tracer molecules, a laser light sheet has to be coupled into the

combustion chamber, as shown in Fig. 3. The laser beam is emitted by a pulsed Nd:YAG-laser

whose output is frequency-quadrupled to a wavelenth of 266 nm. Each pulse has an energy of

90 mJ and a pulse length of 5 ns at a fixed repetition rate of 10 Hz. To ensure that solely laser

light of the wavelength 266 nm is lead to the combustion chamber, a Pellin-Broca-prism is used.

To normalize the image intensity subsequently for each individual image, the laser energy is

monitored. The laser beam is converted to a light sheet and introduced into the cylinder from

below. It is lead through the silica glass piston forming the vertical layer between each of the

inlet- and exhaust valves in line with the injector and the spark plug. A lense in the glass piston

expands the laser light sheet to the hole width of the combustion chamber.

The optics are arranged to profile the laser light sheet with a thickness of about 0.5 mm inside

the combustion chamber. An intensified camera (LaVision NanoStar P43), equipped with an UV-


senisitive objective (f = 100 mm) records the fluorescence signal in the spectral region of TEA,

excited by a wavelength of 266 nm, as illustrated in Fig. 1.

Intern | CR/AED2-SP Fri | 23.09.2014 | © Robert Bosch GmbH 2014. Alle Rechte vorbehalten, auch bzgl. jeder Verfügung, Verwertung,


Quantel Brilliant B2ω4ω

Ergebnisaustausch CNG-DI

Pellin-Broca-

Prism

energy-

meter

collecting lense

f = 1000 mm

cylindrical lense

f = 150 mm

beam splitter

5% reflexion

Nanostar

N2

pressure controller

TE

A

DaVis-Computer

intake air

suction exhaust

1

BigSky

Palas RGB 1000 D

PFI

sCMOS

LLS-Optics

Fig. 3 Experimental setup for the simultaneous PIV/LIF measurement

For the PIV measurements, a Palas RGB 1000 D particle disperser ensures a constant particle

seeding of the intake air. The particles are illuminated using a double-pulse BigSky Laser, which

emits two consecutive laser pulses at 532 nm with a pulse length of 6 ns each. Another laser light

sheet is formed using a LaVision laser light sheet optic with a cylindrical lense (f = -20 mm) and a

collecting lense (300 mm < f < 2000 mm). It is superimposed with the 266 nm laser light sheet by

leading it through a dichroitic mirror in such a way that the 266 nm is reflected and the 532 nm is

transmitted. Both laser light sheets are exactly at the same position inside the combustion

chamber and the light scattered by the particles is detected by a 5 MP LaVision sCMOS double

frame camera, equipped with an objective with f = 105 mm.

4. Results and Discussion

Results of the simultaneously acquired measurements of the mixture formation and the flow

field of the air entrained in the gas injection are presented. For the stratified operation, it is

fundamental that an ignitable mixture is formed during a very short time. Therefore, the amount


of air sucked into the gas jet has to be maximized. The flow field created by the charge motion

and the spray-induced flow plays the major role in the mixture formation process. To get an

impression of the influence of the injection pressure on the flow, the flow velocity is quantified

for different rail pressures.

First, the correlation between the two measurement techniques is established. Second, the

characteristics of a flow field and mixture distribution are discussed by means of a simultaneous

PIV/LIF measurements for prail = 100 bar. And third, the dependency from the spray-induced air

velocity of rail pressure between 50 and 290 bar is investigated.

Correlation between PIV & LIF

In Fig. 4, a simultaneous PIV/LIF measurement at prail = 100 bar with start of injection (SOI) at

30° CA BTDC is shown. The operating point is at 2000 rpm and represents a 4 bar indicated

mean effective pressure (IMEP) investigated at a single-cylinder research engine with a global

air/fuel ratio = 3.2. The flow velocity is plotted in false colors from 0 to 30 m/s, while is

given in gray scales.

Up to the crank angle position given solely jet 1 is visible with its propagating downstream part,

which is pushing the airflow and the upstream part, a steadier region, where the air is sucked

into the free jet. In Fig. 4, the vortex region, which separates these two parts, is marked with the

red arrow. In this position and around the spray tip, the gradient is low, indicating a

proceeded mixture formation process. The flow velocities in this position reach values > 30 m/s.

-25° CA ATDC

IV EV

SOI = -30° CA ATDC, ti = 2120 uS, prail = 100 bar, pmanifold = 996 mbar

10 mm300 320 340 360 380 400 420 440 460 480 500

240

260

280

300

320

340

360

380

Fig. 4 Simultaneous PIV/LIF measurement at 2000 rpm with SOI = 30° CA BTDC, prail = 100 bar at 25° CA BTDC with zoomed vortex region.


Simultaneous PIV/LIF visualization wall-guided combustion

The first crank angle position, depicted in Fig. 5 (25° CA BTDC) shows the tumble center of the

charge motion on the right side of the combustion chamber, marked with a clockwise turning

blue arrow. This tumble motion influences the central gas free jet significantly. On the left side of

the free jet, the transition between the air sucked into the free jet and the air pushed away by it is

clearly visible, separated by a vortex (red arrow) as described above. By contrast, on the right

side, this transition is not recognizable. In this position, the tumble motion is dominant. The air

velocities next to the spray-root and at the spray front reach 30 m/s outside the free jet. The

supersonic velocity inside the free jet, cannot be resolved by the PIV, resulting in random arrow

directions from the spray-root position until 10 mm downwards. Further downstream, inside the

free jet velocities up to 150 m/s are measured. For the visibility of the slower air velocity, the

displayed range is limited to 30 m/s.

At the next shown crank angle position (23° CA BTDC), the free jet 1 has already impinged on

the piston, where it spreads radial symmetrically, forming a torus vortex (red arrows), which

pushes the air back onto the free jet. Still, the highest velocities appear in the region of the spray-

root and now, at the torus vortex. The anticlockwise turning torus vortex on the right-hand-side

additionally accelerates the clockwise turning tumble charge motion resulting in a higher

entrainment velocity on the right side of the free jet compared to the left side. A second jet

resulting from the free jets 2 and 3 is visible meanwhile and further developed at 21° CA BTDC.

Comparing this position and 19° CA BTDC shows two distinct effects: First, the torus vortex has

adsorbed the tumble motion’s angular momentum and is pushed to the combustion chamber

roof and, second, the angle between jet 1 and the second jet is decreased due to their mutual

attraction. This effect is increased by an orthogonal velocity component of the airflow onto the

second jet induced by the torus vortex which has moved further towards the cylinder wall. This

velocity component additionally accelerates the air sucked into the second jet (yellow arrows).

Comparing 23° CA BTDC with 19° and 15° CA BTDC in Fig. 5 shows clearly the time-dependent

evolution of the cone angle enveloping jet 1 and the second jet and the curvature of the second

jet. This effect will be further investigated in the following sections after a correlation between

the air velocity and the equivalence ratio is established.


tumble motionvortex

resulting flow

Fig. 5 Selected crank angle positions of a simultaneous PIV/LIF measurement at 2000 rpm with SOI = 30° CA BTDC, prail = 100 bar.

Comparison of normal air velocities – rail pressure variation

To get a better insight into the flow field, the results of PIV measurements, averaged from 100

double images, are plotted with the resulting flow velocity from 0 to 60 m/s in false colors for


50 bar < prail < 290 bar. The crank angle positions shown are chosen on the basis of engine-

relevant crank angle positions for the prail = 100 bar operating point. According to this, the

ignition was initiated at 24° CA BTDC and start of combustion (SOC) was at 18° CA BTDC.

Another crank angle position is showed (12° CA BTDC) representing a steady-state injection for

50 bar < prail < 150 bar.

For the first crank angle position plotted in Fig. 6, 24° CA BTDC, the tumble center is still present

for all rail pressures. The higher pressures are pushing this center farther to the cylinder wall. At

prail = 50 bar, jet 1 has not yet reached the piston. As already seen in the simultaneous PIV/LIF

measurement at prail = 100 bar for jet 1 at 25° CA BTDC, the characteristic vortex, marked with a

red arrow, is separating the upstream and downstream part of the jet on the left side. On the

right-hand-side, due to the dominating charge motion, there is only an upstream part

recognizable.

At prail > 50 bar, jet 1 has already impinged on the piston spreading radial symmetrically,

forming a torus vortex (red arrows). A distinct influence of the charge motion is recognizable by

comparing the left and the right side, where the tumble supports the air entrainment, resulting

in higher entrainment velocities. The influence of the second jet is visible at this crank angle

position from prail = 150 bar (yellow circles) on. The higher rail pressure additionally leads to a

wider free jet resulting from a bigger expansion zone and higher velocities inside the free jet

(Rist 1996). With increasing rail pressure, the velocities in jet 1 are increasing as well as the

velocities in the impingement area and in the torus vortex.

prail = 50 bar prail = 100 bar



0 10 20 30 40 50 60





0 10 20 30 40 50 60

Fig. 6 Flow field at 24° CA BTDC for 50 bar < prail < 290 bar.

To compare the air entrainment into the gas injection, a line was defined next to the central

injection as plotted in Fig. 7. The orthogonal velocity component n along this line, which is,

assuming constant air density, directly proportional to the air entrained, is plotted for the

investigated rail pressures at characteristic crank angle positions in Fig. 8, Fig. 10 and Fig. 12.

Fig. 7 Position of the entrainment line inside the combustion chamber.

The normal velocities, plotted in Fig. 8 over the line as described in Fig. 7, are showing an

increasing trend with the rail pressure for 100 bar < prail < 200 bar and are decreasing with the

distance zs from the injector tip. The highest normal air velocities are found for 250 bar rail

pressure. Especially for the higher fuel pressures maxima in the velocities vs. the position zs are

visible. The distance between these peaks correlates with the following equation (1) (Rist 1996):

1*43.1*

4.0

cyl

inj

p

pDL (1)

In the equation from Rist the distance L describes the length of a beam cell between two Mach

disks, D the injector’s hole diameter and cyl

inj

p

p the pressure ratio between the injection and the


cylinder. The different parts of the expansion-compression-zone during the expansion, the

further expansion and the following compression due to the oblique shock and the straight

shock affect the static pressure along the jet, thus the entrainment velocity which leads to these

characteristic peaks. As a consequence of losses behind the straight shock, the lower rail

pressures do not show peaks because the number of shock cells formed is that small that the

distance does not reach the evaluation line.

0 2 4 6 8 100

5

10

15

20

25

30

35

40

zS [mm]

un [

m/s

]

Mehr-Loch-Injektor, nMotor

=2000 min-1, pRail

=200 bar

50 bar

100 bar

150 bar

200 bar

250 bar

290 bar

L

L

Fig. 8 Normal air velocities at 24° CA BTDC for different rail pressures.

At the next depicted crank angle position, 18° CA BTDC in Fig. 9, free jet 1 has impinged on the

piston. For prail = 50 bar, jet 1 forms the torus vortex likewise, marked with red arrows. The

tumble charge motion is still present (blue arrow) for the lowest investigated rail pressure while

it has already transferred its angular momentum to the torus vortex and has been pushed

against the combustion chamber’s roof for the higher pressures. At prail = 100 bar in Fig. 9, the

torus vortex is accelerating the air sucked into the second jet, resulting in an intensified

orthogonal velocity onto these jets (yellow arrows). The flow induced by the second jet (jets 2/3)

is meanwhile formed for all investigated rail pressures. With increasing rail pressure, the air

between jets 1, 2 and 3 (black circle) is displaced faster which leads to an intensified contraction.

This effect is intensified by the anticlockwise turning torus vortex on the right side (red arrow),

increasing with the rail pressure and leading to a completely collapsed spray for the rail

pressures 250 and 290 bar.





0 10 20 30 40 50 60


The graph in Fig. 10 shows that the entrainment velocity at the spray-root is almost twice as high

for 100 bar compared to 50 bar rail pressure. In the further progress of the examination line, the

value assimilates. The slower expansion of the torus vortex in case of 50 bar is supported by the

higher vortex velocity here. Except the entrainment velocity for prail = 250 bar, because of the

early EOI at 17° CA BTDC, the remaining rail pressures are on the same level between 25 and

28 m/s in the spray-root area. In the further progress, they are in the expected order with the

exception of 250 and 290 bar where the EOI and fluctuations influence the air velocity.


0 2 4 6 8 100

5

10

15

20

25

30

35

40

zS [mm]

un [

m/s

]

Mehr-Loch-Injektor, nMotor

=2000 min-1,a =-18°KW

50 bar

100 bar

150 bar

200 bar

250 bar

290 bar


At the next presented crank angle position, 12° CA BTDC, the tumble charge motion is adsorbed

by the torus vortex at prail = 50 bar in Fig. 11. The spray-induced flow by the second visible jet

(jets 2/3) is generating a converse flow direction to the anticlockwise turning torus vortex. This

generates a region on top of this torus vortex with a clockwise flow direction, marked by a

yellow arrow. The black circle marks the area between jets 1, 2 and 3 again, which indicates that

the injections at 50 bar < prail < 150 bar are not yet completely collapsed. An increased cone angle

can be examined for the 100 bar rail pressure. Even as the normal velocity component by the

anticlockwise turning torus vortex should support the mutual attraction of the jets, the injection

has not yet collapsed. For the rail pressures 100 bar and above, the tumble charge motion is

completely adsorbed by the dominating jet induced flow at 12° CA BTDC. While the injector is

already in the ballistic region (EOI is at 10° CA BTDC for prail = 150 bar), the air velocities in the

jet-root region are smaller compared to prail = 100 bar.

At the rail pressures 200, 250 and 290 bar, the injection has ended, but the jet momentum is still

big enough to keep the torus vortices stable. Even in case of prail = 290 bar, where EOI is already

at 19° CA BTDC, the vortices with velocities above 60 m/s are still present at 12° CA.





0 10 20 30 40 50 60


The normal velocities at 12° CA BTDC, plotted in the graph in Fig. 12, are still in the same order

for 50 and 100 bar rail pressure at the jet-root. 100 and 150 bar rail pressure are matching for the

reason of the injector needle throttling the rail pressure short time before EOI. Except for the jet-

root region, the normal velocities for 250 and 290 bar rail pressure are matching perfectly and are

both on a low level between 5 and 9 m/s. The prail = 200 bar entrainment velocity is constantly

higher because of the late EOI at 14° CA BTDC. At zs = 7 mm, it assimilates with the higher

pressures before it rises again to a magnitude of 9 m/s.



The rail pressure variation shows different phenomena regarding the influence on the charge

motion and the mutual attraction of the free jets leading to a collapsed injection. An increasing

penetration velocity by increasing rail pressure strengthens the torus vortex, formed after the

impingement on the piston. The vortex expands with a higher velocity and thus dominates the

flow inside the combustion chamber in a shorter time. Also the higher penetration velocity leads

to a faster displacement of the air between the jets. An increasing expansion zone with the rail

pressure plus a higher entrainment velocity cause an increased jet-jet-interaction.

5. Conclusions and Outlook

Contraction effects in between different jets in a high pressure natural gas injection with a multi-

hole injector were investigated. Air entrainment into the single gas free jets was found as a major

factor supporting mutual attraction between single jets, which can result in a collapsed structure.

This effect complicates the injector design, especially for the stratified operation of an engine.

Two measurement techniques were used to characterize the interaction of gas free jets in an

engine. The Particle Image Velocimetry (PIV) was used to quantify the air entrainment velocity

and the Laser Induced Fluorescence (LIF) to quantify the fuel concentration.

To characterize the influence of the rail pressure on the mixture formation, a rail pressure

variation was performed. The fuel mass was kept constant for an operating point at 2000 rpm

and 4 bar indicated mean effective pressure (IMEP) by adapting the injection time. The

entrainment velocity was plotted over a line at the left side of jet contour. Thereby, the distinct


effect of the injection pressure was determined. While the entrainment velocity in the jet-root

region is 15 m/s at prail = 50 bar, it reaches 35 m/s at prail = 250 bar in the steady-state region of

the injection. At the high pressures, pulsations of the velocity profile over the evaluation line

were detected whose periodicity correlates with a supersonic Mach disk’s distance and the

according expansion- and compression zones which have impact on the flow velocity.

During an injection with the present injector design, several effects occur in the same order, but

in shorter time scales for an increasing rail pressure. The effects are listed below:

- Entrained air into central jet influenced by tumble charge motion

- Free jets distinguishable from each other

- Central free jet impinging on the piston, radial expansion surrounded by a torus vortex

- Cone angle of whole gas injection decreases due to mutual attraction of the free jets

- Jet-induced flow dominates the charge motion, tumble accelerates the torus vortex before

it’s momentum is adsorbed

- Contraction intensified by the torus vortex pressing the free jets together.

The simultaneous PIV/LIF measurements are suitable to visualize, quantify and characterize the

mixture formation process of a multi-hole injector.

Investigating a perpendicular layer would give the possibility to further characterize the

interaction of the free jets, because every single free jet would be visible and not only resulting

jets (for jets which are not inside the vertical layer). An adaption of the combustion chamber’s

geometry as well as the injector’s hole geometry will be subject of further investigations.

References Buschbeck, M. (2013) – Laseroptische Analyse der zyklischen Schwankungen in einem Transparentmotor,

Dissertation.

Bruneaux G., Causse M. and Omrane A. (2011) – Air Entrainment in Diesel-Like Gas Jet by Simultaneous Flow

Velocity and Fuel Concentration Measurements, Comparison of Free and Wall Impinging Jet

Configurations, SAE Technical Paper 2011-01-1828, doi:10.4271/2011-01-1828.

Eckbreth, A. C. (1988) – Laser Diagnostics for Combustion Temperature and Species, Abacus Press.

Friedrich W., Grzeszik R. and Wensing M. (2015) - Mixture Formation in a CNG-DI Engine in Stratified Operation,

SAE Technical Paper 2015-24-2474, doi:10.4271/2015-24-2474.

Lee J., Nishida K., and Yamakawa M. (2002) - An Analysis of Ambient Air Entrainment into Split Injection D.I.

Gasoline Spray by LIFPIV Technique, SAE International.

Mederer, T., Wensing, M. and Leipertz, A. (2012) – Laser-Induced Fluorescence to Visualize Gas Mixture

Formation in an Optically Accessible Hydrogen Engine, COMODIA 2012, Fukuoka.


Mohammad, T.I., Harrison, M., Jermy, M. and How, H.G. (2010) – The structure of the high-pressure gas jet from a

spark plug fuel injector for direct fuel injection, J Vis 13:121–131, doi: 10.1007/s12650-009-0017-2.

Neubert, V. M. (2015) – Analyse der Zylinderinnenströmung in einem optisch voll zugänglichen 2V-Dieselmotor

mittels konventioneller und zeitaufgelöster Particle Image Velocimetry, Dissertation.

Schulz, C. and Sick, V. (2005) – Tracer-LIF diagnostics: quantitative measurement of fuel concentration,

temperature and fuel/air ratio in practical combustion systems, Progress in Energy and Combustion Science

31(1), 75-121, doi: 10.1016/j.pecs.2004.08.002.

Van Basshuysen R. (2015) - Natural Gas and Renewable Methane for Powertrains. In: Future Strategies for Climate-

Neutral Mobility, doi:10.1007/978-3-319-23225-6.

Date post:	28-Sep-2020
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

Characterization of High Pressure Natural Gas Injections...

Documents