Energy 59 (2013) 233e239

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Energyjournal homepage: www.elsevier.com/locate/energy

Effect of nano air-bubbles mixed into gas oil on common-rail
diesel engine
Yasuhito Nakatake a, *, Shintaro Kisu a, Kenta Shigyo a, Toshihiko Eguchi b,
Takashi Watanabe cabc

Department of Mechanical Engineering, Kurume National College of Technology, 1-1-1 Komorino, Kurume, Fukuoka 830-8555, Japan
Aura-tech Company Limited, 1725-2 Tsuhukumoto, Fukuoka 830-0047, Japan
Department of Transport Mechanical Engineering, Kurume Institute of Technology, 2228-66 Kamitsu, Kurume, Fukuoka 830-0052, Japana r t i c l e i n f o

a b s t r a c t

Article history:
Received 28 November 2012
Received in revised form
24 May 2013
Accepted 23 June 2013
Available online 22 July 2013

The nano air-bubbles mixed into gas oil are used for the energy saving and environmental load reduction ofdiesel engine. The nano air-bubbles were mixed into the fuel line continually by a super miniature ejectortype mixer (outer diameter: 20 mm, length: 34 mm) which can make nano and micro sizes. After the micro
air-bubbles were separated with the nano air-bubbles in a mixing tank, diesel engine performance test witha common-rail injection system was experimented. The results showed that 3% reduction in break means
speciﬁc fuel consumption, 1% rise in charging efﬁciency and a slight reduction in the density of exhaustsmoke etc. These were caused by mixing the nano air-bubbles into gas oil. It is conﬁrmed that the nano airbubbles had advanced and activated combustion by a physical and chemical action.
Ó 2013 Elsevier Ltd. All rights reserved.

Keywords:
Diesel engine
Liquid fuelNano bubble
Speciﬁc fuel consumption
Environmental pollutant

1. Introduction
Recently, improvement in fuel efﬁciency of vehicles has been
drastically required due to the aggravation of problems of theenvironment and energy, such as global warming, tight oil demand,
etc. The demand for diesel passenger cars that are fuel-efﬁcient andemit less greenhouse gases has increased centering around west
Europe, where the sales share accounted for more than 56% in 2011[1]. Meanwhile, in order to cope with the emission control regulations becoming more severe, the increase in the after treatment costof gases emitted from diesel engines has become a large task. One ofits solutions is a fuel pretreatment, that is to say fuel reforming.
Conventionally, various studies are being done on fuel additives toimprove diesel combustion and decrease harmful emissions. Incylinder strategy to reduce local temperatures and consequently
the NOx production rate is the injection of water in emulsion with thefuel [2e16]. Most engine experiments and numerical studies using
water emulsion fuel technique show that the NOx reduction isaccompanied with a large reduction of PM and soot emissions. At agiven fuel injection rate, the use of WDE leads to an increase of the* Corresponding author. Tel.: þ81 942 35 9369; fax: þ81 942 35 9321.
E-mail address: nakatake@kurume-nct.ac.jp (Y. Nakatake).
0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.energy.2013.06.065

total injected mass, of which a consequence is an increase of themixing rate between fuel and air, thus reducing local fueleair ratios
and consequently PM production [3,10,12]. Author et al. have proceeded with research on simultaneous reductions of fuel consumption and poisonous exhaust gases by mixing gas oil with air-bubbles
micronized using air as a means of fuel improvement [17e20]. As aresult, the fuel consumption was reduced by 2e14% and reductions of
exhaust smoke and NOx were also conﬁrmed, by mixing the gas oilwith micronized bubbles, or nano bubbles, to the extent where stable
driving is possible without air entrainment in the engine.
As the diesel engine which was used in this research was a jerk
fuel injection type, it is unknown whether the gas oil mixed withnano air-bubbles favorably affects the common-rail injection system that is most commonly used now.
Therefore, in this research, a new common-rail injection system
was introduced and its effect on the high pressurization of gas oil
mixed with nano air-bubbles was discussed experimentally.
2. A summary of micro and nano air-bubbles mixed into gas oil
Practical applications of micro and nano bubble technology haverecently become to attract people’s great concerns in wide varietyof areas in advanced and conventional science and technologies,
such as energy conversion and heat removal devices, diagnosis by

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Y. Nakatake et al. / Energy 59 (2013) 233e239

ultrasound echo due to micro and nano bubble collapse, drug delivery systems, cleaning and sterilization by shock waves due to thecollapse of high-pressure micro and nano bubbles, production ofozone water, mining using physical absorption at gaseliquid interfaces, puriﬁcation and improvement of oil-contaminated soil
and polluted water by air supply (sea, lake, river etc), absorption ofcarbon dioxide gas, ﬁsh culture, etc [21].
Fig. 1 shows (a) photograph of the device and (b) internal
structure of an ejector-type micro and nano air bubble mixture
device. The size of this mixture device was 20 mm in outer diameterand 34 mm in length, and the material was brass [22]. The left side of
the ﬁgure is the inlet for fuel (hereafter referred to as gas oil) and theright side is the outlet of the gas oil mixed with nano air-bubbles.
The mechanism of mixing nano air-bubbles by this mixture
device is as follows: 1) It injects the gas oil pressurized by a pump at
a high speed using an injection effect or a nozzle, selfprimed air by
the negative pressure generated in the neighborhood of the outlet,
and form a ﬁlm-like gaseliquid interface. 2) The absorbed air isseparated and bubbled by the separation, turbulent mixing andshearing action of the gas oil. 3) The air-bubbles are further reﬁned
by the separation area formed in the sudden expansion region.
This is a method to prompt the atomization of the gas oil sprayby mixing nano air-bubbles into gas oil as nano air-bubbles [18],and accelerate the combustion by increasing the amounts of dissolved oxygen and ions [18e20]. As the size of the nano air-bubble
mixture device of this method is extremely compact, it is possible
to retroﬁt it to existent diesel engines.
When air-bubbles of the macro or micro-size entered in fuel, a fuelinjection pump and a nozzle caused an air entrainment and used only
air-bubbles of the nanosize here to become unstable in an engine.

Fuel ﬂow rate was measured by a 213 type of Max Machinery
Inc, smoke in exhaust gas by an opacimeter (light transmission typesmoke meter, MEXA-600SW of HORIBA), O2 concentration by LM-1of Grid, and noise by a CEM DT-8851 noise meter. In addition, thenoise meter was put in the distance of 1 m from engine side, andthe setting assumed a properties (a weighted sound pressure level).
3.2. In-line type nano air-bubbles mixture device

3. Experimental setup

In this experiment, two In-line type nano air-bubble mixture
devices were ﬁxed on the line supplying gas oil from the fuel tankto the engine. Fig. 4 shows a photograph of the mixture device. For
the purpose of measuring the amount of nano air-bubbles mixed
into the gas oil, two mixture devices were used.
The gas oil that ﬂowed from the fuel tank and was pressurized
by the nano air-bubble mixture device I selfprimes the air at theejector part, which is sent to a mixing tank installed on the fuel line,
as shown in Fig. 5. The ultra ﬁne bubbles formed in this mixture
device include micro bubbles and nano bubbles. Therefore, after the
bubbles of micro-size were separated at the mixing tank, the gas oil
mixed with nano bubbles is supplied to the engine. The gas oil
mixed with ultra ﬁne bubbles is referred to as “nano bubble gas oil”
hereafter in this research. When the ﬂuid level in the mixing tankrises and switches on a liquid level indicator, the ﬂow is switched tothe circulation line of the mixing tank. With this circulation, theefﬁciency of mixing nano bubbles is raised by increasing thenumber passing through the injector part. Mixture device II plays
the role to raise the amount of bubbles mixed into the gas oil bycirculating the gas oil in the mixing tank.
The amount of dissolved oxygen in the mixing tank wasmeasured by a DO meter B-507 for organic solvent of Iijima Electronics Corporation.

3.1. Experimental apparatus

3.3. Common-rail type fuel injection system

Fig. 2 shows the schematics of the experimental apparatus usedin this experiment, Fig. 3 shows a photograph of its appearance, andTable 1 shows the test engine speciﬁcations. The engine load androtation speed of this experimental apparatus can be controlledfrom a PC by software for the dynamometer (FC-design Co. Ltd.
DA50-UW [23]). Vital engine factors, such as fuel consumption andintake air mass, can be directly obtained and stored by PC.
A commercially available Yanmar Diesel NFD150-E remodeled
into a common-rail fuel injection system was used as the test engine (Table 1). As for the test fuels, ordinary gas oil (JIS No.2 gas oil,
Table 2, hereafter referred to as control) and gas oil mixed withnano air-bubbles were separately used through a three-way valve
ﬁxed in a part of the fuel pipe. In order for the effect of the amountof mixed nano air-bubbles to be measured, two devices, nano airbubble mixture device I and mixture device II, were ﬁxed on theline of the gas oil mixed with nano air-bubbles.

Fig. 6 shows a photograph of the common-rail fuel injection
system. This system was assembled with commercially available
components. Table 3 shows the main speciﬁcations of the commonrail fuel injection system (FC-design Co. Ltd., Fi-CMR [23]). Thissystem has the ability of multiple injections up to 7 point injections
at the maximum injection pressure of 200 MPa.
3.4. Experimental method
A partial load experiment was conducted at a constant rate of2000 rpm. The total performance of the engine, the smoke inexhaust gas and remaining O2 were measured, when the brakemean effective pressure of atmospheric correction is 200w550 kPa.
The time of fuel injection was set as a single point injection 17
before top dead center, which was the rating before the engine was
modiﬁed to a common-rail type, and the injection pressure wasFig. 1. Ejector-type micro and nano air-bubbles mixture device.

Y. Nakatake et al. / Energy 59 (2013) 233e239

235

Fig. 2. Schematics of experimental apparatus.

increased to 80 MPa from 20 MPa of before modiﬁcation. The injection time of fuel for the above load was approximately 0.40e0.85 ms.
3.5. Error analysis
The measurement errors for the fuel ﬂow rate, engine torque,and engine speed are estimated to be about 0.5%, 0.2% and 0.1%,
respectively. Total error for brake speciﬁc fuel consumption isestimated to be about 0.55%, and is calculated by taking thesquare root to add the square of three errors. Therefore experimental data were sampled ﬁve times, and calculated approximate
curves such as the brake speciﬁc fuel consumption in the sameload condition.
4. Experimental results and discussions
4.1. Nano bubble gas oil
4.1.1. Bubble size
Fig. 7 shows the distribution of the particle sizes in gas oil
measured by a NanoSight M-10 of Quantum Design Japan, Inc.
Two peaks were observed in the control (309, 649 nm), possibly

caused by additive agents and coloring agents originally included
in gas oil. Nano bubble gas oil was analyzed after it was mixed for
10 min by a bubble mixture device and extracted from it. In theﬁgure, nano bubbles in gas oil are distributed in the range from
100 nm to 200 nm. It is reported that nano bubbles in water are
distributed around 100 nm [17]. So the distribution of particle
size of nano bubbles are resemble in water and gas oil. As thenumber of particles of the control was 0.52  108 particles/mL
and that of the nano gas oil was 1.07  108 particles/mL, the resultof the calculation shows that around 0.5  108 bubbles/mL ofnano bubbles were mixed. This amount of bubbles means that no
nano bubbles were included in a drop of fuel spray. If the amountof bubbles is increased to more than 1010 bubbles/mL, more than
one bubble will be included in every drop of spray, and therefore
better fuel efﬁciency and further decreases in environmental
burdens can be expected by using nano bubble gas oil. In the caseof common-rail injection system, the spray is more atomized
compared with jerk fuel injection system, it is necessary totake measures to increase the amount of mixing nano bubbles
furthermore.
4.1.2. Dissolved oxygen
The characteristics of micro and nano air-bubble mixed gas oil
become different according to not only the kinds of mixing device
and operating conditions, but also the time they are mixed (that isthe discharge rate (circulation) against container size and the lapseof time after the mixing stopped.) Therefore, an in-line type micro
and nano air-bubble mixture device to be installed in the fuel line

Table 1
Test engine speciﬁcation.

Fig. 3. Snapshot of experimental apparatus.

Engine modelType
Combustion
Combustion cavity
Number of cylinders
Bore  Stroke
Displacement volume
Compression ratio
Max power
Injection timing
Injector
Hole diameter  number
Injection pressure

Remodeled Yanmar Diesel NFD150-E
Water cooled 4-stroke
Direct injection
Toroidal type1
f96  105 mm
760 cc16.5
11.0 kW/2400 rpm
BTDC 17
Common-rail system
f0.12 mm  5
80 MPa236

Y. Nakatake et al. / Energy 59 (2013) 233e239

Table 2
Properties of JIS No.2 gas oil.
Cetane number
Net caloriﬁc value
Density@288 K
Viscosity@313 K
Pour pointC
H
O
SulferWater
50% Distillation temp.

MJ/kg
kg/mm3mm2/s
Cmass %
mass %
mass %
mass ppm
mass ppm

C

56
43.12
830
2.5
17.5
87.3
12.5
e
< 10
382
278

was experimentally produced for mounting on a real vehicle, andthe chemical characteristics of the micro and nano air-bubble
mixed gas oil produced by the device were examined.
Fig. 8 shows the changes with time of the amount of dissolved
oxygen in the mixing tank in the cases where the control, only one
nano bubble mixture device, and two nano bubble mixture devices
was/were used. As the degree of saturation of dissolved oxygenchanges depending upon temperature and the amount of saturated
dissolved oxygen in gas oil is unknown, the vertical axis of thegraph shows the volume of relative dissolved oxygen (DOR) in gas
oil non-dimentionalized by the volume of saturated dissolved oxygen in water, of which the temperature dependence is known.
The DOR in the case of the control is stable at around 50%.
Furthermore, because the DOR at the time when the nano bubble
mixing started was around 70%, the DOR of the control is presumed
to be in the range of 50e70%. The DOR became stable at 92% inaround 20 min when only one nano bubble mixture device wasused, and it became stable at 98% in around 10 min when two nanobubble mixture devices were used. These values are thought to belimited by the conditions of the mixture device and mixing in thisexperiment.
Shortening of burning time is predicted by increase of a partial
excess air factor because the amount of dissolved oxygen in fuelincreases.
4.1.3. GC/MS measurement
Fig. 9 shows the results of analyses of the control and nanobubble gas oil using a GC/MS (Gas chromatograph-Mass spectrometer). The horizontal axis shows the time and the vertical axis
shows the ion intensity. The upper side shows the control and thelower side shows the nano bubble gas oil. As gas oil is the mixtureof carbon hydrides, substances with different numbers of carbon

Fig. 4. Ejector-type nano air-bubbles mixer.

Fig. 5. Micro and nano air-bubbles mixed into the gas oil.

and of saturated and unsaturated bindings are mixed. The large
peaks of the graph show saturated hydrocarbon of the main components, e.g. C9 to C26 appear in proportion to carbon number, andbristle-like projections between these peaks are considered to bechemical compounds replaced with unsaturated hydrocarbon,
sulfur, nitrogen and other molecules as non-main components.
Although there is not enough energy to cut the carbon bond of themain chain cable corresponding to the large peaks, by mixing nanobubbles, it is possible that a replacement reaction reducing thefunctional group at the end to hydrogen occurs, and as a result,minor components decrease and the bristle-like non-main components decreases. As a result, improvement in the burning reaction of nano bubble gas oil can be expected by hydrogen atoms andoxygen atoms being added. Furthermore, if nitrogen atoms are
replaced by hydrogen atoms, its contribution to the reduction of
Fuel-NOx can also be expected.
4.2. Engine performance test
Fig. 10 shows the results of the engine performance test. The
horizontal axis shows a brake mean effective pressure, Pmec [kPa],
which is the engine load, and the vertical axis shows unit output,
BSFC (brake speciﬁc fuel consumption)[g/kWh] or fuel consumption volume per unit time, charging efﬁciency hc[%], smoke inexhaust gases or smoke [BSN], noise [dB] and temperature of
exhaust gases or Teg [ C] in the order from the bottom.
4.2.1. Fuel consumption rate
The fuel consumption rate was reduced by nano bubbles being
mixed into the fuel. The comparison of the case of the control andthat of nano bubble gas oil using one mixture device shows that theFig. 6. Appearance of common-rail system.

Y. Nakatake et al. / Energy 59 (2013) 233e239

237

Table 3Speciﬁcation of common-rail fuel injection system.
System model
Injector driver andpressure sensor
Common-rail systemand pressure regulator
Supply pump
Supply pump motor
Fuel cooling method
Injection timing

FC-design Fi-CMR [5]DENSO common-rail system(max. 200 MPa)
BOSCH common-rail system
BOSCH CP3
AC200 V 1.5 kW 4P
Air-cooling fuel conditioner
in a coolant tankMultiple injections (max.: 7 point inj.)

fuel consumption was reduced by 1.1% at the load average, and 2.7%
at the maximum. The comparison of the case of control and that ofnano bubble gas oil using two mixture devices shows that the fuelconsumption was reduced by 3.2% at the load average, and 6.2% at alow load time.
The reasons why the reductions of fuel consumption were larger
when the load was low or medium and two mixture devices wereused are as follows: The fuel consumption was from 0.013 to
0.033 L/min. at the load for the range from low load (200 kPa) tohigh load (500 kPa), and the calculation from the discharge quantity of the pump of the mixing tank (0.7 L/min) showed the gas oil
was circulating in the mixing tank approximately from 50 times (atlow load) to 20 times (at high load). When two mixture devices
were used, the number of circulation times became twice, and as aresult, the amount of nano bubbles mixed into the gas oil increased
and the fuel consumption was further reduced. This result suggests
the method of mixing bubbles and optimization of various conditions for the practical use of this system.
As the reasons why fuel consumption can be reduced by nanobubble gas oil, a chemical action can be presumed that the rate of
combustion advances due to the increase in local air-fuel mixing
ratio and the concentration of ion, and owing to the increase of
dissolved oxygen [18e20]. A physical effect can also be consideredas the advanced combustion speed caused by further micronizationof atomized gas oil due to the mixture of bubbles.
4.2.2. Others performance
The charging efﬁciency of the control is nearly constant in theneighborhood of 78% in all loading ranges. Meanwhile, in the caseof nano bubble gas oil, the charging efﬁciency tended to be slightly
improved, and compared with the control, was improved byapproximately 1%. It is considered that the combustion speed was
improved due to the effect of bubbles mixed into the gas oil, that is,from both chemical and physical actions, and as a result, theFig. 7. Particle/bubble size distribution in control and nano bubbles gas oil.

Fig. 8. Comparison of the dissolved oxygen content in control and nano bubbles gas oil
versus mixing time.

effective work ratio increased and the heat loss from the cylinderwall was reduced. From the results of this research, reduction of
brake speciﬁc fuel consumption in the high loading range is expected. However, the fuel consumption decreased in the low
loading range instead. These points are considered to be improvedby optimization of the bubble mixing method.
As one of the improving effects of fuel by bubble mixing, although
the reduction of smoke generated in the high loading range was
expected, no difference was observed between the control and thegas oil mixed with bubbles. This is because the fuel consumption was
greatly decreased in the low loading range and also because almost
no improvement in the combustion effect by bubble mixing wasobserved in the high loading range, as described previously.
Reduction in engine noise can be expected from the improvement of ignition delay by the nano bubble gas oil. The larger Pmec
became, the more noise was gradually reduced, and it was reduced
by approximately 3 dB for the change of Pmec ¼ 200e550 kPa. Thisis considered to be because the formation of pre-mixed gas was
prompted with lean mixer in a low load period caused by the
micronization of atomized gas oil and the rate of pressure riseincreased. The comparison between the control and nano bubble
gas oil showed that the engine noise decreased by 0.5 dB at a
maximum in the low load range when nano bubble gas oil wasused, compared with control. However, no signiﬁcant difference
was recognized.
The temperature of the exhaust gas when nano bubble gas oil
was used was low, by approximately 20  C at a maximum compared

Fig. 9. GC/MS analysis of control and nano bubbles gas oil.

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Y. Nakatake et al. / Energy 59 (2013) 233e239

condition is shown in Fig. 11. In the performance test of the preceding paragraph, the effect of fuel consumption reduction performed comparison of control and nano bubble gas oil at 200 kPa
load condition which appeared most notably. There are no clear
differences between them and both are almost the same. This isbecause the effect of fuel consumption reduction was very small, toeffect HHR and incylinder pressure. However, the fuel consumption
improving effect was accepted.
5. Conclusions
The engine performance caused by the difference of nanobubble mixing conditions using a newly introduced common-rail
injection system was experimentally discussed. The ﬁndings obtained through this experiment are as follows:
1) The bubble mixing effect by common-rail high-pressure fuelinjection system contributes to a reduction in fuel consumption
in the ranges of medium and low loads, and when nano bubbles
are mixed in a high concentration, the fuel consumption is
reduced by 3.2% at the load average and 6.2% at the max.
2) Nano bubble gas oil improved charging efﬁciency, temperatureof exhaust gas, smoke and engine noise approximately by 1% at
the max. compared to the control.

Fig. 10. Engine performance.

with the control. This is considered to be because the combustion
speed increased caused by the effect of bubble mixing the same as
for the charging efﬁciency described above, and as a result, the heatloss from the cylinder wall reduced and the temperature of theexhaust gas was also reduced.
In addition, also in the experiment using nano bubble gas oil,
unstable phenomenon or unusual noise etc. of the diesel engine
were not observed.
4.3. Combustion characteristics
The comparison of heat release rate (HRR) and cylinder gas
pressure for control and nano bubble gas oil at 200 kPa load

Fig. 11. Heat release rate and cylinder gas pressure with crank angle at 200 kPa loadcondition.

The bubble mixing effect, or its cost-effectiveness, remains to beclariﬁed for practical use.
Acknowledgment
This work was supported by the Grants-in-Aid for ScientiﬁcResearch 22560842.
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Nomenclature
ATDC: after top dead center (degree)
BSFC: brake speciﬁc fuel consumption (g kWh1)
DOR: relative dissolved oxygen
HHR: heat release rate (J degree1)
N: rotation speed of engine (rpm)Noise: engine noise (dB)
NOx: oxides per nitrogen
PM: particulate matter
Pmec: brake mean effective pressure (kPa)
Smoke: smoke in exhaust gases (BSN)
Teg: temperature of exhaust gases ( C)
WDE: water-in-diesel emulsion
hc: charging efﬁciency (%)