improvement of spray characteristics in port injectors...generally, fuel injected by an injector...

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Keihin Technical Review Vol.2 (2013)

※Received28June2013,Reprintedwithpermission,fromSAEpaper2012-32-0071(JSAEpaper#20129071).Copyright©2012SAEInternationalandSAEofJapan.FurtheruseordistributionofthismaterialisnotpermittedwithoutpermissionfromSAEInternationalorSAEofJapan.

ポート噴射インジェクタより噴射される燃料噴霧は，エンジンの出力や燃焼効率に強い影響をあたえる．よって燃料を小さな油滴にする微粒化と，エンジンより受ける温度や負圧などの環境変化に依存しない正確な燃料供給が求められている．本報では，ニードルバルブとのシート部下流の徹底した圧力損失（エネルギーロス）の低減と噴孔位置の適正化による微粒化手法，及びシート部下流のデッドボリューム削減と燃料通路長短縮による温度や負圧の変化に依存しないインジェクタを紹介する．

Key Words: Fuel Injector, Atomization

Improvement of Spray Characteristics in Port Injectors※

ポートインジェクタにおける噴霧特性の向上

INTRODUCTION

Lately, there have been growing demands for

internal combustion engines for motorcycles and

other applications to have lower emissions, better fuel

economy and higher performance insusceptible to use

environments. This is because of skyrocketing fuel

prices and greater use of fuel injection systems under

stricter global-scale emission control regulations in

various countries including developing countries. As

part of such demands, injectors need to atomize fuel

spray for lower emissions and better fuel economy

and to minimize a variance in flow rate without being

susceptible to changes in temperature and negative

pressure so as to ensure a higher performance that is

unaffected by the use environments.

Our development focused on the following three

improvements for injectors.

(1) Atomization

(2) Minimization of change in temperature and flow

rate characteristics

(3) Minimization of negative pressure and under-seat

Technical paper

Junichi NAKAMURA＊１ Akira AKABANE＊２ Koji KITAMURA＊１ Yuzuru SASAKI＊１

中　村　順　一赤羽根　　明北　村　浩　二佐々木　　譲

flow rate characteristics

Various atomization techniques and flow rate

stabilization techniques have been developed and

put into practice. Among the currently available

techniques, this development focused on and modified

the structure located under the valve seat. Effects

of under-seat flow and pressure were clarified in

our existing injector structure so as to improve and

modify a flow path from the seat to the nozzle orifice.

1. Overview of injector for small motorcycles

Fig. 1 represents the structure of our gasoline

injector. When current flows through a coil, a core

of injector is vacuumed and a valve train integrated

with the core is lifted to open the valve as shown

in the figure. Next, fuel pressure applied by a fuel

pump delivers fuel through the opened seat and the

fuel is sprayed through multiple nozzle orifice laid

out in the plate.

On Fig. 2, the under-seat flow in the existing

＊１DevelopmentDepartment3,R&DOperations　　＊２DevelopmentDepartment1,R&DOperations

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Improvement of Spray Characteristics in Port Injectors

structure shows that fuel passes through the seat

toward the center of axis, then comes down to the

vertical hole area, and radially flows from the center

of axis to the counter bore. After entering the counter

bore, the fuel flows laterally along the plate located

under the counter bore. The plate has multiple nozzle

orifice laid out to deliver the required flow rate. At the

nozzle orifice in the plate, the laterally flowing fuel

rapidly changes its flow direction. Such rapid flow

direction change separates and pulls off the fuel from

the inner walls of nozzle orifice and forms a fuel film

with external air trapped in the space produced by

separation prior to spraying and diffusion.

Since the existing structure has a long fuel flow

path from the seat to the nozzle orifice and a large

dead volume, fuel often tends to drip out without

being atomized immediately after the valve opening

and closing stages in which a full fuel pressure is

not reached. Furthermore, fuel may be sprayed with

an improper particle size at the start of spraying

if the counter bore is filled up with fuel and fuel

separation is instable, such as when a change in fuel

temperature or negative pressure causes a variance

in dripping rate.

Fig. 4 represents a photo of the spray at the start

of injection. This photo shows that there are large

droplets at the start of injection. And, the graph of

spray particle size vs. time plotted in Fig. 5 reveals

poor atomization caused by a large particle size

of fuel dripping out at the start of injection. Also,

the distribution curve shown in Fig. 6 signifies the

distribution of large particle sizes.

Hole

Separate In inner wallsof nozzle orifice

Low High

Counter bore

Nozzle orifice

Plate

Flow direction

Seat

DeadVolume

Fig. 2 Fuel flow direction and pressure distribution

Fig. 3 Velocity in counter bore (flowing to the nozzle orifice)Model: 10-nozzle orifice

Nozzle orifice

Fig. 4 Current dripping

Initial dripping at injection

Fig. 1 Cross-section view of current Injector

Seat, valve

Fuel pressure

Moving Core

Power Supply

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As a long fuel path from the seat to the nozzle

orifice causes severe pressure loss, as shown in Fig.

7 resulting in a drop in fuel pressure applied by a

fuel pump before reaching the nozzle orifice, the

particle size of sprayed fuel eventually becomes

large. This is due to the lower effectiveness of air

entrapment and diffusion during fuel separation from

the hole wall as a consequence of a rapid change of

flow direction in the nozzle orifice.

Fig. 5 Change in particle size over time

Part

icle

siz

e [µ

m]

Time [ms]

0

50

100

150

200

250

0 2 4 6 8 10

Current model

Fig. 6 Particle size distribution of current model

0

5

10

15

20

0 100 200 300 400 500

Freq

uenc

y [%

]

Particle size [µm]

Current modelCurrent model

Fig. 7 Pressure loss and velocity of under-seat

0

10

20

30

200

250

300

350

Vel

ocity

(m

/s)

Pres

sure

(kP

a)

measurement positionValv

e sea

t hole

abov

e hole

sSea

t

Current-Pressure

Current-Velocity

Current-Pressure

Current-Velocity

Fig. 8 Change in the temperature and flow rate characteristics

-15.0%

-10.0%

-5.0%

0.0%

5.0%

10.0%

15.0%

10°C 30°C 50°C 70°C 90°C

Flow

rat

e ch

ange

[%

]

Fuel temperature [°C]

Current modelCurrent model

Fig. 9 Change in the negative pressure and flow rate characteristics

-20%

0%

20%

40%

Flow

rat

e ch

ange

[%

]

Pb [mmHg]0 200 400 600

Current model

Meanwhile, there was an issue of a change in the

temperature and flow rate characteristics. After the

valve closes, a lower pressure in the dead volume

boils the fuel. As the fuel cubically expands it is

pushed out. At higher fuel temperatures, a large dead

volume under the seat causes a variance in injection

rate in the event of a change in temperature (Fig. 8).

A change in negative pressure could also cause

a variance in the injection rate due to the aforesaid

effects of dead volume. At a higher negative

pressure from an engine, fuel flows out from the

dead volume (Fig. 9).

2. Approach to resolve issues

In light of the existing issues described under

section 1, an approach was selected and determined

based on the three mechanisms of injectors.

(1) Atomization: Shorter flow path from the seat

to nozzle orifice; minimized pressure loss and

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facilitated separation in the nozzle orifice (liquid

film forming)

(2) Minimization of change in temperature and flow

rate characteristics: Smaller dead volume

(3) Minimization of change in negative pressure and

flow rate characteristics: Smaller dead volume

2-1. Atomization mechanism

Generally, fuel injected by an injector splits in

the process shown in Fig. 10. While the mechanism

shown in Fig. 10 is a generally known droplet

mechanism, instability of this process could be a

possible cause of failed atomization. As shown in

Fig. 2, the flow in the cylinder called a counter bore

goes into the nozzle orifice but the pattern of such

flow into the nozzle orifice varies depending on their

layout. This may prevent the optimum liquid film

from forming. As fuel separation in the nozzle orifice

depends on the intensity of the lateral flow above

the nozzle orifice, instability of one flow direction

obstructs the separation. Such instability means it is

hard to ensure a stable particle size in each nozzle

orifice as the liquid film forming is a passive process

that induces separation in the nozzle orifice and uses

air trapped in the separation area in the nozzle orifice.

As a method to ensure stable atomization, it was

determined to form an active flow that would induce

liquid film forming on the inner walls of nozzle

nozzle orifice.

2-2. Mechanisms of temperature/Negative pressure

and under-seat flow rate characteristics

Fig. 11 represents a vapor pressure curve

of gasoline. This graph signifies that gasoline

can remain in a liquid state at an atmospheric

pressure of 101.3 kPa and starts vaporizing at its

vapor pressure of 53.3 kPa or lower even under

atmospheric pressure. It is also shown that gasoline

starts vaporizing at a temperature of 38°C or higher

even in the atmosphere. This suggests that the

characteristics can be improved by setting a pre-spray

pressure under the injector valve seat through the

nozzle orifice to meet the requirement stated below.

The distribution of under-seat flow pressure in an

existing injector is shown below. As seen in Fig. 11,

the pressure decreases in the dead volume under

the seat and in the larger-diameter counter bore

to or below the aforesaid requirement. Apparently,

reducing the pressure loss through an improved dead

volume is effective as a countermeasure.

2-3. Method of approach and effectiveness

Atomization

Fig. 12 represents the improved under-seat layout

of this development. The developed layout shortens

the flow path from the seat to the nozzle orifice

and reduces the dead volume, which minimizes fuel

pressure loss before it reaches the nozzle orifice in

the plate. It also allows fuel flowing from the seat

Fig. 11 Vapor pressure curve of gasoline

0

20

40

60

80

100

-100 0 100 200

Vap

er p

ress

ure

(kPa

)

Temperature (°C)

101.3kPa

53.3kPa53.3kPa

24°C

Fig. 10 Process of atomization

Liquid filmforming(Thin film)

Liquid columnforming

Liquid dropletforming(Atomization)

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Fig. 15 Initial dripping at injection

(a) Current model (b) New model

Fig. 12 Cross-section of new injector under seat and flow direction

HighLow

Fig. 13 Comparison of layout

(a) Current model (b) New model

seat

Abovenozzle orifice

Abovenozzle orifice

seat

Valve seathole

Valve seathole

Above nozzle orifice

Table 1 Countermeasure

CURRENT1.2mm3

3.9mmDead volumeLength of flow path

NEW0.5mm3

1.2mm

Fig. 14 Comparison of pressure and velocity

0

10

20

30

200

250

300

350

Vel

ocity

(m

/s)

Pres

sure

(kP

a)

measurement position

New-PressureCurrent-PressureNew-VelocityCurrent-Velocity

New-PressureCurrent-PressureNew-VelocityCurrent-Velocity

Valve s

eat h

ole

abov

e hole

sSea

t

to meet the flow returning from the center of the

axis above the nozzle orifice, and drags fuel against

the inner wall of the nozzle orifice, forming a void

in the center of hole and facilitating liquid film

forming in the nozzle orifice. In addition, a smaller

dead volume allows for the aforesaid under-seat flow

immediately after spraying, which can minimize

initial and end dripping.

Fig. 12 illustrates a flow line of under-seat flow

as identified through fluid analysis of this layout.

As seen in Fig. 12, liquid-film-formed fuel at the

entrance to the nozzle orifice can actively form a

liquid film of fuel sprayed and induce atomization.

Temperature/Negative pressure and under-seat

flow rate characteristics

And as shown in Fig. 13, 14, the developed

injector can keep the under-seat pressure at a vapor

pressure of gasoline or higher and thus neither a

pressure decrease nor boiling occurs in the injector.

In addition, the pressure decrease point is located at

a post-spray point. These achievements minimize the

effects of variance in gasoline remaining in the dead

volume, which results in minimization of any change

or variance in the flow rate in comparison with the

currently achievable level even at high temperature

and/or negative pressure.

Moreover, a smal ler dead volume reduces

the time required to fill the dead volume with

fuel and contributes to both minimum (Table 1)

initial dripping and better injection responsiveness

(Fig. 15).

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Table 2 Result of countermeasure

NEW CURRENT RESULT

64µm 85µm Improvement25%Atomization (S.M.D)

2% 9% Improvement77%

Temperature flow rate characteristics

+11.7% +28% Improvement58%

Negative pressure flow rate characteristics

Fig. 16 Comparison of change in particle size over time

0

50

100

150

200

250

0 2 4 6 8 10

Part

icle

siz

e [µ

m]

Time [ms]

New model

Current model

Fig. 17 Comparison of change in Particle size distribution

0

5

10

15

20

0 100 200 300 400 500

Freq

uenc

y [%

]

Particle size [µm]

New modelCurrent modelNew modelCurrent model

Fig. 18 Comparison of change in the temperature and flow rate characteristics

-15.0%

-10.0%

-5.0%

0.0%

5.0%

10.0%

15.0%

10°C 30°C 50°C 70°C 90°C

Flow

rat

e ch

ange

[%

]

Fuel temperature [°C]

New model

Current model

New model

Current model

Fig. 19 Comparison of Change in the negative pressure and flow rate characteristics

-20%

0%

20%

40%

0 200 400 600

Flow

rat

e ch

ange

Pb [mmHg]

New model

Current model

New model

Current model

3. Result of countermeasures

This section describes the Result of countermeasures

incorporated in the developed injector. The developed

layout was more effective in the areas listed below

(Table 2).

This can a lso be observed in d is t r ibut ion

(Fig. 16, 17).

Temperature/Negative pressure and under-seat

flow rate characteristics improvements also minimize

changes in temperature and flow rate characteristics,

CONCLUSIONS

This developed injector can reduce any variance

in flow rate in the event of a change in temperature

in different engine use environments or a change

in negative pressure in different engine running

modes and can improve the atomization of fuel

to be sprayed. Under changing market situations

in the future, manufacturers will need to develop

and offer environmentally friendly products at a

moderate price particularly in developing countries

where sales of motorcycles and automobiles are

expanding. And along with a full-fledged growth

of FI for motorcycles, these advantages can make

a great contribution to lower emissions and better

fuel economy, not only in developed but also in

developing countries.

and in negative pressure and under-seat flow rate

characteristics (Fig. 18, 19).

－15－


We sincerely appreciate all support we recieved

from everyone. We continue challenging the further

new technology so that it can contribute to lower

emissions and better fuel economy. (NAKAMURA)

Authers

Junichi NAKAMURA Akira AKABANE Koji KITAMURA

Yuzuru SASAKI

REFERENCES

(1) Daisuke Matsuo, Akihiko Haramai, Kazuhiko

Sato, Minoru Ueda: Development of a Small

L ow - c o s t F u e l I N J E C TO R t o O ve r c o m e

Diversificat ion of Requirements in Global

Markets, SETC Paper 20097055

(2) Mitsutomo Kawahara, Kenichi Saitoh, Kazuhiko

Sato: Reduction of operation noises of Injector

for small motorcycle. SETC Paper 20119625

improvement of spray characteristics in port injectors...generally, fuel injected by an injector...

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