KAYABA TECHNICAL REVIEW No. 64 APR. 2022

Technology
Explanation

Research on Technology to Control Air Bubble
Content in Hydraulic Fluid
KITAMURA Yoshiaki, KODERA Yasuhiro
Abstract
In hydraulic pumps used in Continuously Variable
Transmission (CVT) of automobiles, air is mixed from thesurface of hydraulic fluid due to vibration of the hydrauliccircuit, and also the dissolved the air appears in theworking oil due to the cavitation. In CVT pumps, 10% tomore than 30% of air bubbles are contained in the hydraulic fluid in the hydraulic circuit under the operating environment. These bubbles cause many problems such as
pump discharge performance reduction, as well as degradation related to vibration, noise, and durability.
Therefore, when developing a pump, it is necessary toevaluate its performance under actual operating conditions, taking into account the bubble content. However, ingeneral, there are no devices or methods to stably adjust
the bubble content.
1

Introduction

A hydraulic fluid usually contains several percentagesof air dissolved therein under atmospheric pressure. Insidea hydraulic pump like the one shown in Photo 1, thehydraulic circuit may have a local decrease in pressure tocause cavitation, generating voids or air bubbles (Fig. 1).
Tiny bubbles appearing in this way will stay in the fluidfor a certain period of time and may need considerable
time to be dissolved in the fluid again or float up to collapse. The air can also be mixed into the hydraulic fluid
from the outside of the fluid due to entrainment (sloshing)that occurs when the hydraulic circuit or the oil tankvibrates. The hydraulic circuit for continuously variabletransmissions (hereinafter "the CVT") of automobiles is atypical example of those exposed to frequent opportunities for air intrusion in this mechanism. It is said that thistype of hydraulic fluid contains 10% to, in some case,even more than 30% bubbles. The fluid with a high bubble
content looks whitish as shown in Photo 2 compared towhen it has a low bubble content. These bubbles containedin the fluid are often known as the third contaminant 2) ofmachinery, next to the particulates and water, and may
cause the machinery to have vibration, noise or mechanical failure. The presence of air bubbles in the hydraulicfluid reduces the apparent elasticity of the fluid to impair

To solve this problem, KYB has developed a device andmethod to control the bubble content in the hydraulicfluid. First, we focused on a method using a swirling flow
as a technique for removing bubbles from oil, and fabricated a bubble separation device. Second, we analyticallyand experimentally confirmed that the bubble separation
effect of this device was sufficient.
Furthermore, we installed the bubble separator in theactual pump test equipment and verified whether the
bubble removal effect could be obtained. Finally, to automatically control the bubble content, we verified it waspossible to obtain the desired bubble content ratio bycontrolling the opening and closing of the valve on theoutlet side of the bubble separator. In this report, theresults of the above verification are described.

the responsivity of the system. Research 3) has verified that
aggressively decreasing the bubble content of the fluid
will eventually lower the amount of air dissolved in thefluid to reduce cavitation. Taking into account the fact that― 17 ―

Photo 1

Example of KYB hydraulic pump

Cavitation

OUT

Fig. 1IN

How cavitation occurs (example)

Research on Technology to Control Air Bubble Content in Hydraulic Fluid

operation (closing/opening) can be controlled to adjust
the bubble content of the fluid leaving the separator. Furthermore, the report explains the adequacy of this methodby introducing the results of an experimental verification
test.

Design and Fabrication of a Bubble
Separator

2(a) 0% bubble content(b) 10% bubble content
Photo 2

Bubbles in a tank in a hydraulic circuit

one of the major causes of failures of hydraulic machinery
elements is erosion caused by cavitation, reducing the
bubble content of the hydraulic fluid will definitely help
prevent machinery failures.
A question: what is the fluid's bubble content that
degrades the machinery performance to eventually causea failure? Such a quantitative indication has not yet been
determined. This is because you can barely measure or
adjust the bubble content of the fluid in real time at a highaccuracy under the present circumstances. Conventionally, the bubbles in the fluid have been removed predominantly by negative methods, such as spontaneous air purgeof the oil tank or addition of anti-foam agent to the fluid.
In such situation, modern automobiles are required tohave intermittent runs or stops of their engine more frequently for higher fuel efficiency. Accordingly, hydraulicpumps connected to the engines need to be started or
stopped frequently as well. Air bubbles contained in thehydraulic oil are likely to stay or merge in various locations within the hydraulic circuit, which may cause theautomobiles to generate noise or may make the hydraulicsystem unstable during the initial stage of the start-up
process. The need to determine the effect of the bubble
content of the fluid and to take measures has become
urgent.
To respond to the need, we first tackled technical development to implement techniques for stable adjustment of
the bubble content of the fluid. This report proposes a
bubble separation device using a swirling flow and amethod to control the bubble content of the fluid entering
the separator. The separator has an air purge valve whose

2.1 Bubble Separator Using a Swirling Flow
Fig. 2 shows the operating principle and structure of abubble separation device that uses a swirling flow to draw
bubbles, which are distributed throughout the hydraulicfluid, and puts them together to be separated and removed
by a simple mechanism. Using a swirling flow, the separator can draw bubbles distributed in the fluid and build
them up around the central axis in a time as short asapproximately 100 msec. The separator can remove anddischarge these collected bubbles from a purge outlet tothe outside by utilizing a slight difference in pressure
between the fluid outlet and the purge outlet. 4) To implement this, a control valve should be installed downstreamof the purge outlet to adjust the air purge by valve opening/closing. Controlling the period of time for the valve to beopen or closed and/or the operation timing will adjust thevolume of bubbles to be separated from the fluid. In thisdevelopment, we began to fabricate the separator forbuilding a system that can control the bubble content of
the fluid at the fluid outlet of the separator.
Bubbles + Fluid

Fluid outlet

Tapered pipe

Fluid inlet
Purge outlet

Fluid

Bubbles

Fluid inlet
Fluid

Bubbles
Bubbles + Fluid

Fig. 2

Operating principle and structure of bubble separator

2.2 Simulation of Bubble Separation
For the method to be verified in this research, we set up
a target range of the bubble content between 1% and 40%.
To achieve this goal, the bubble separator must provide
adequate bubble removal performance within the range.
Since the performance of the separator heavily depends
on the geometry of the fluid inlet and the diameter of thepurge outlet, 5) it is very important to select appropriate
parameters for flow conditions in designing the separator.
To verify the operating principle and select optimal
geometry parameters, we used the computational fluid
dynamics (CFD) to simulate and verify the bubble separation. For the purpose of CFD, we used a fluid duct shownin Fig. 3 and simulation conditions listed in Table 1.
Within the target range, 20%, 30% and 40% bubble― 18 ―KAYABA TECHNICAL REVIEW No. 64 APR. 2022

content levels were selected as examples for analysis
purpose.
Fluid inlet

Air purge outlet

L2

L1

φD3

h
L3

w

φD1

φD2

Fluid outlet

D

L

Fig. 3

Fluid duct for computational simulation

Table 1

Computational simulation conditions

Item
Fluid flow at inlet (L/min)
Size of bubble particles (mm)Density (g/cm3)
Physicalproperties ofDynamic viscosity
hydraulic fluid(mm2/sec.)
Bubble content (%)

Value
30
0.1
0.8474
9.189
20, 30, 40

the center of the duct for any of the bubble content levels
as shown in Fig. 4(b). It was verified that the bubbles
flowed towards the purge outlet efficiently to be separated
and removed from the fluid.
As a result of the CFD, an optimal set of geometry
parameters for the bubble separator was obtained asshown in Table 2. Fig. 5 shows the resultant bubble
removal rate of the separator with its fluid inlet sized tobe 2.18 mm (width) x 4.35 mm (height), its air purge
outlet sized to have an inside diameter of φ4.4 mm, andits fluid outlet sized to have an inside diameter of φ16 mm
for the bubble content cases of 20%, 30% and 40%. Notethat the bubble removal rate here refers to what percentage of the bubbles have been removed from the fluid thatis discharged from the fluid outlet, not from the air purge
outlet. The CFD proved that the simulated bubble separator with the optimal geometry parameters delivered abubble removal performance as high as 90% or more forall the cases. These results were used as design parameters for an experimental bubble separation device.
Table 2The bubbles entering the separator via the fluid inlet
has a higher flow rate around the perimeter of the inlet
port and a lower flow rate in the center of the same asshown in Fig. 4(a). The figure represents how a swirling
flow occurs. The volume fraction of the bubbles for the
bubble content levels of 20%, 30% and 40% is higher in

Bubble flow rate (m/sec.)

Optimal geometry parameters for bubble separator
Geometry parameter

Value [mm]Inside diameter of tapered pipe section φD1
Inside diameter of fluid outlet φD2
Inside diameter of air purge outlet φD3
Total Length L
Length of fluid inlet pipe L1
Length of tapered pipe section L2
Length of fluid outlet pipe L3
Width of fluid inlet w
Height of fluid inlet h

30.0
16.0
4.4
150.0
20.0
30.0
100.0
2.18
4.35

40

Bubble removal rate [%]

32
24
16
8
0

(a) Distribution of bubble flow rate(for fluid with 40% bubble content)

120
100

99.8

60
40
20
0

40%
Fig. 540%
bubblecontent

0.0

0.2

0.4

0.6

0.8

1.0

(b) Volume fraction of bubbles
Fig. 4 Computational simulation of bubble flow

90.2

80

20%
bubblecontent
30%
bubblecontent

94.8

30%
20%
Bubble content [%]Results of simulation of bubble separation

2.3 Separator Design and Fabrication
Based on the results of the CFD analysis stated in theprevious section, we designed and fabricated a bubble
separation device to be used in experiments of controllingthe bubble content of the hydraulic fluid. Fig. 6 shows the
bubble separator. The separator was designed in modulars
so that the geometry parameters including the fluid inlet― 19 ―

Research on Technology to Control Air Bubble Content in Hydraulic Fluid

size, the fluid outlet size and the air purge outlet size canbe adjusted later to be suited to the test conditions. Piecesof this modular device can be first inserted into a casing
and then fixed together from both sides to be a one-piece
structure. The peripheral casing and inserts are made oftransparent acrylic.

test made it difficult to achieve precise control of the flowrate at the purge outlet by varying the degree of openingof the valve. Unfortunately, just slightly opening the valve
at the purge outlet actually caused the separator to deliver
the bubble removal effect more than expected, resulting ina substantially lower bubble content of the fluid at thefluid outlet. However, it was verified that the bubble
content of the fluid at the fluid outlet can be controlled bychanging the degree of opening of a valve installed at thepurge outlet.

Peripheral
casing(Air purge outlet)

Fluid outlet(Fluid outlet)

Purge outlet

Bubble separator

InsertNeedle valve

Fluid inlet

Fig. 6

Design and fabrication of bubble separator

Bubble mixing

Evaluation of Performance of Bubble
Separator

Hydraulic circuit for testing of manual control of
bubble content(Purge outlet)

Fig. 7(Fluid outlet)(a) Purge outlet closed(Purge outlet)

To verify the bubble separation performance of thefabricated separator, a preliminary test was first conducted.
Fig. 7 shows the hydraulic circuit used in the preliminary test. The bubble separator was installed on the fluid
circuit. A manually operated needle valve was mounted
on the circuit line of the purge outlet side of the separator
and a Coriolis-type flowmeter was on the circuit line of
the fluid outlet side of the separator. The hydraulic fluid
was forcefully mixed with bubbles from the air blown by
an air compressor located on the discharge side of thepump. While the air was being blown into the fluid, theneedle valve was manually controlled to deliver the fluid
from the separator at different bubble content levels. The
bubble content of the fluid was determined by measuring
the density of the fluid at the fluid outlet against the degreeof opening of the needle valve. 6) The fluid pressure at thefluid inlet of the separator was maintained at 0.5 MPa.
Photo 3 shows what happened inside the bubble separator during the preliminary test. With the purge outlet fully
closed, the fluid looks whitish with great amounts of air
bubbles over the entire duct in the separator as shown in
Photo 3 (a) (Bubble separation with the purge outlet
closed). With the purge outlet fully open, the air bubbles
flowing into the separator from the fluid inlet quickly
flowed toward the purge outlet. Few bubbles stayed in theseparator for a while and then flowed toward the fluid
outlet as shown in Photo 3 (b) (Bubble separation with thepurge outlet open).
Fig. 8 shows the results of the preliminary test: therelationship between the degree of opening of the needle
valve and the bubble content of the fluid at the fluid outlet.
In fact, the specifications of the needle valve used in the

M(Fluid outlet)

3

Manualcontrol

Coriolis-type flowmeter― 20 ―(b) Purge outlet open
Photo 3Inside the bubble separator

KAYABA TECHNICAL REVIEW No. 64 APR. 2022

Now we have verified that using the bubble separator in
the actual pump test unit enables quick, stable adjustmentof the bubble content of the fluid in the circuit.

40
30

Cooler

20
Bubble separator

1000

Fig. 8Return line

Bubble content
at fluid outlet [%]50

HeaterDrain line

51015
Degree of opening of needle valve
at the purge outlet [%]Reservetank

Manually controlled bubble content

Bubble content
measuring device

Tank
Suction line

Evaluation of Performance of Bubble
Separator on Actual Machine

To verify that the bubble separator developed can also
be applied to evaluation of actual pumps, we carried out
an experimental test on the bubble separator installed onan actual pump test unit. Note that the bubble separator
used for this test was another unit made of metal, not theoriginal separator.
Fig. 9 shows the hydraulic circuit of the actual machine
test equipment. In order to determine the effect of the
bubble content of the hydraulic fluid on the pump, adevice to mix the air into the hydraulic fluid in a tankupstream of the target pump to be evaluated was provided
to increase the bubble content of the fluid. This air mixing
device was designed to be able to control the air discharge
rate for mixing. The bubble separator was installed downstream of the target pump. On the purge outlet line aneedle valve was provided to be able to be opened orclosed freely in the same manner as for the preliminary
test. On the fluid outlet side an orifice of any round shape
was inserted as a back pressure control. Further downstream, an inline impedance-type bubble content measuring device was installed to measure the bubble content inreal time. The oil temperature was maintained at 100°C bya cooler and a heater provided on the circuit.
As an example of verification of the bubble removal
effect, Fig. 10 shows changes in bubble content of thefluid over time for the case of a constant mixing rate of
bubbles in the tank, a fluid outlet size of φ10, and a pumpdischarge rate of 18 L/min. At the time point of 0 seconds,
the separator has its purge outlet closed and the duct contains the fluid with bubbles mixed in as sufficient asaround 30% with the mixing tank being operated. Whenthe purge outlet is opened, the bubble content immediately
dropped and became stable at around 10% about 4 minutes
later.
This bubble content level was obtained when the bubble
mixing device in the tank and the bubble separator wereboth operated. The air content of the hydraulic fluid may
be adjusted to any given level by controlling the air mixing
rate and/or the air purge rate of the bubble separator.

Target pump

Hydraulic circuit of actual machine test equipment

Fig. 940

Start bubble removal
Bubble content [%]4

Air mixing

30

Stable air content

20

10

0

0

120

240

360

480

600

Time [sec.]

Fig. 10 Bubble content of hydraulic fluid on actual machine

5

Evaluation of Performance of Automatic
Bubble Content Control

After verifying that controlling the degree of opening of
the purge outlet of the bubble separator can adjust the
bubble content of the hydraulic fluid in the circuit, we
further tried to determine the possibility of automaticallycontrolling the bubble content to any given level. To thisend, we installed an on-off control solenoid at the purge
outlet and carried out a test to control the bubble content
of the fluid at the fluid outlet by adjusting the on/off timeor the switching timing of the solenoid.
Fig. 11 shows the hydraulic circuit used in the control
test. The bubble separator was installed on the fluid circuit
similar to the one shown in Fig. 7. A Coriolis-type flowmeter was installed at the fluid inlet and a bubble content
measuring device installed at the fluid outlet. Then, thebubble content of the fluid was measured in real time forvarious cases. Air mixing into the fluid was attained bykeeping the suction port of the pump open to the atmosphere. An on-off control solenoid was installed at the― 21 ―

Research on Technology to Control Air Bubble Content in Hydraulic Fluid

purge outlet of the separator. The solenoid control(opening/closing) was implemented by a sequencer using
a relay according to the deviation of bubble content measurements at the fluid outlet from the target level.
Fig. 12 shows an example of the results of the bubble
content control tests. When the incoming fluid had about
29% bubble content, the outgoing fluid had about 28%
bubble content with the purge outlet closed. Then, thetarget bubble content was set to 18%. With the on-off
control solenoid operation frequency 1s and the duty ratio
between 10% and 100%, the bubble separator automatically controlled the duty ratio to attain the target level as
indicated by the figure. Table 3 shows the average bubble
content for each duty ratio and its relative deviation fromthe target bubble content for a certain period of time (19.2
seconds to 25 seconds) after the bubble content became
stable at around the target level. In general, the smaller the
duty ratio is, the lower the control accuracy and responsivity are. However, this test demonstrated that the relative deviation from the target level was within 9.4% forsmall duty ratios, proving a certain level of control accuracy.
For some duty ratio settings, however, the bubble
content may fail to be stable at around the target level andmay continue fluctuating in some cases. This is because
the on-off solenoid at the purge outlet can only deliver
low responsivity while the bubble separator has highly
Data logger
Bubble separator

Bubble content
measuring device

Relay

Coriolis-type flowmeter

Sequencer

M

Fig. 11

Average [%]Relative deviation [%]

10

19.7

9.4

20

18.8

4.4

30

17.9

0.5

40

16.8

6.7

50

17.3

3.9

60

18.0

0.0

70

18.6

3.3

80

18.6

3.3

90

18.6

3.3

100

18.6

3.3

responsive bubble removal performance. This can beprobably improved by using a more responsive proportional valve.

6

Future Prospects

We will apply the technology that controls the bubble
content of the hydraulic fluid developed in this research toevaluation of our pumps under actual operating conditions, thereby developing high-function and high-quality
products, although the control stability is still a challenge.
On the other hand, we aim to deploy the technology insuch a manner that our pump products including CVT
could be improved in performance by focusing the featureof the technology that can remove the bubbles containedin the fluid, which may be called "the third contamination" as stated in the beginning of this report.
By improving our capability of technical development
like this, we will be able to offer even-higher-efficiency
pumps that could improve the fuel efficiency of automobiles for instance. These products will help create a carbon
neutral society or achieve the sustainable development
goals (SDGs).

7

Duty ratio
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%

29
27
25
23
21
19

Target17
15

Duty ratio [%]Bubble content for various duty ratios

Hydraulic circuit for automatic bubble control test

Start control

Bubble content [%]Target bubble
content

Table 30

Fig. 125

1015Time [s]20

25

Automatically controlled bubble content

Concluding Remarks

This report has introduced the verification of the methodthat freely controls the bubble content of the hydraulicfluid with the bubble removal technology using a swirling
flow, toward establishment of such a bubble content
control system. The verification has produced the following conclusions:(1) A bubble separator has been developed and fabricatedthat can adequately remove air bubbles from thehydraulic circuit to obtain a bubble content between
1% and 40%.
(2) It has been verified that the bubble separator mounted
on the actual pump test unit can control the bubble
content in the hydraulic circuit quickly and stably.

― 22 ―KAYABA TECHNICAL REVIEW No. 64 APR. 2022(3) Testing has proven that adjusting the flow rate at thepurge outlet of the bubble separator can control the
bubble content of the fluid at the fluid outlet of theseparator.
This research was carried out in cooperation with Professor TANAKA Yutaka, Hosei University, who designed
the bubble separation device using a swirling flow, andAssociate Professor SAKAMA Sayako, Aoyama Gakuin
University (currently belonging to National Institute ofAdvanced Industrial Science and Technology), whocarried out various element tests. Finally, we would like totake this opportunity to sincerely thank them.

References
1) SAKAMA S, et.al: Air Bubble Separation and Elimination fromWorking Fluids for Performance Improvement of Hydraulic
Systems, Proc. IFPE 2014, Paper No. 27.1 (2014).

2) SEKIDO Makiko, MIZUMURA Atsushi: How to select and use
filters and their trouble cases, Basic Hydraulic Technology Class- How to select and use peripheral equipment, Japan Industrial
Publishing Co., Ltd. (2016).
3) GOTO H, SAKAMA S, SUZUKI R, TANAKA Y: Reduction of
Cavitation Damage by Elimination of Bubbles in Oil Reservoir,
Proc. FLUCOME2013, OSI-02-4. (2013).
4) TANAKA Y, et.al.: Visualization of Flow Fields in a Bubble
Eliminator, J. of Visualization, Vol. 4, No. 1, pp. 81-90. (2001).
5) SAKAMA Sayako, TANAKA Yutaka, SUZUKI Ryushi:
Research on design and evaluation of bubble separation devices(Report #2), Transactions of The Japan Fluid Power System
Society Vol. 45, No. 5, pp. 79-84. (2014).
6) GOTO Hiroyuki, SAKAMA Sayako, TANAKA Yutaka:
Measurement of bubble content of hydraulic oil using Coriolistype flowmeter, Transactions of FY2015 Annual Assembly
Lectures of the Japan Society of Mechanical Engineers (JSME),
No.15-1, S1150302, (2015).

Author 
KITAMURA Yoshiaki

KODERA Yasuhiro

Joined the company in 2013.
Mechanical Component Engineering
Sect., Basic Technology R&D
Center, Engineering Div.
Engaged in research anddevelopment of hydraulic pumps andpower steering

Joined the company in 2009.
Developmental Experiment Sect.,
Developmental Center, Engineering
Headquarters, Automotive
Components Operations
Engaged in development of hydraulicpumps― 23 ―