2023 | 101
Fundamental study of the effect ofstratified NH3 injection system fornitrogen compounds reduction
Emission Reduction Technologies - Exhaust Gas Aftertreatment Solutions

Hiromichi Oba, Japan Engine Corporation
Kazuhisa Ito, Japan Engine Corporation
Chikara Matsuda, Japan Engine Corporation
Yasuhisa Ichikawa, National Institute of Maritime, Port and Aviation Technology
Yoichi Niki, National Institute of Maritime, Port and Aviation Technology
Koji Takasaki, National Maritime Research InstituteThis paper has been presented and published at the 30th CIMAC World Congress 2023 in Busan,
Korea. The CIMAC Congress is held every three years, each time in a different member country. The
Congress program centres around the presentation of Technical Papers on engine research anddevelopment, application engineering on the original equipment side and engine operation andmaintenance on the end-user side. The themes of the 2023 event included Digitalization &Connectivity for different applications, System Integration & Hybridization, Electrification & Fuel Cells
Development, Emission Reduction Technologies, Conventional and New Fuels, Dual Fuel Engines,
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ABSTRACTGreen House Gases (GHG) reduction is unprecedented challenge marine industries have everconfronted with. In the research for high reduction in GHG, NH3 and H2 attract world’s attention as
one of alternative fuels. Since these are carbon free fuels, adoption of these fuels for internal
combustion engines is expected to be one of the promising solutions to lead a pathway towards
carbon free shipping. Hence, we are working on the development of NH3 combustion engines and H2
combustion engines. Development of H2 combustion engine is in progress based on our experience,
knowledge, and results of the Dual Fuel engine development. In this paper, the development status of
the NH3 combustion engine is explained in detail.
As characteristics of NH3, it has high latent heat that cools the surrounding atmosphere at
evaporation, high ignition temperature, which requires high ignition energy, slow combustion speed,
and poor flame retention. In addition, from the combustion of NH3, N2O may be emitted, which has
about 300 times higherã€€greenhouse effect than CO2.
To overcome these problems, Stratified Fuel Injection System with NH3 and diesel fuel oil is proposed
to control the combustion of NH3. Spray from the Stratified Injection System has sandwiched layers of
NH3 between diesel fuels. First layer (diesel fuel) acts as pilot fuel to ignite following NH3 layer andlast layer (diesel fuel) contributes to the stable combustion of prior NH3 layer and also reduces NH3
slip and generation of N2O.
This paper describes detailed concept of stratified injection system with NH3 and diesel fuel and astrategy, which supports combustion of NH3, and is an effective solution for reduction of GHG
emission.
As a fundamental research work, observations of spray formation and combustion tests are carried out
to confirm the effect of this system using a newly developed constant volume combustion chamberthat simulates the condition of a real engine. The data is useful to complete the design of low-speed
two-stroke engine applying this system.

1

INTRODUCTION

To reduce greenhouse gas (GHG) emissions,hydrogen (H2) or ammonia (NH3) has recently
gained an attention as a carbon-free fuel forinternal combustion engine.
In case of using ammonia as fuel, there are
disadvantages compared to fossil fuel due to thecharacteristics. It has high latent heat that cools thesurrounding atmosphere at evaporation, highignition temperature, which requires high ignitionenergy, slow combustion speed, and poor flame
retention. In addition, from the combustion of NH3,
nitrous oxide (N2O) may be emitted, which has
about 265 times higher greenhouse effect than
CO2 [2].
To solve these problems, Japan engine corporation(J-ENG) is developing a stratified ammonia
injection engine that injects fuel oil (FO) andammonia in a stratified manner. “FO” includes
heavy fuel oil, gas oil, low sulfur oil or hexadecane,such as used as diesel oil. The concept is to apply
J-ENG's existing stratified water injection
technology to inject liquefied ammonia into theengine.
This allows the first FO layer (pilot FO) to act as an
ignition source, efficiently combusting the ammonia
that is subsequently injected. Furthermore, thecombustion activation by the terminal FO (post FO)
can reduce the production of unburned ammonia
and N2O.
Currently, J-ENG is preparing an experiment using
the single cylinder test engine. It is designed basedon knowledge of previous studies [2][3]. It isfinancially supported by Japanese Government,
Namely, Green Innovation Fund of New Energy
andIndustrial
Technology
Development
Organization (NEDO).
In this paper, we will present previous studies [2][3]as a basic study of stratified ammonia injection. In
the reference, direct imaging of the spray flame andshadowgraphs of the non-combustible spray weretaken. In the direct imaging of the spray flames, it
is confirmed that the strong luminescence isobserved at the early stage of combustion. At the
middle stage of combustion, luminescence area
was not observed, but luminescence was reobserved at the end stage. It has relationship withthe timing of post FO injected. In the noncombustion spray experiments, the spray shape of
the ammonia layered spray is found to be similar tothe spray outline shape of the FO spray.

visual constant(VCVCC).

volumecombustion

vessel2. CONCEPT OF STRATIFIED INJECTION
J-ENG has designed a stratified ammonia injection
system by applying its existing stratified water
injection technology. The stratified water injection
system is a NOx reduction technology by injecting
a fuel oil (FO) and water at once to lower thetemperature during combustion. Focusing on the
point of injecting FO and a different liquid at thesame time, we thought that if ammonia wasliquefied, the technology could be converted to afuel injection system that injects FO and ammonia
at the same time.
There are other advantages of stratified injection of
FO and ammonia: the layer of FO on the tip side
can be used as a pilot FO and as an ignition source
for ammonia. Furthermore, the layer of FO on theterminal side can be expected to improve ammonia
combustion at the end of the combustion period
and reduce slip ammonia and N2O production.
Pilot injection of FO for diesel combustion of fuelswith high self-ignition temperatures such as
methanol or LPG has been used in the past, but thestratified ammonia injection is characterised by thatsuch pilot fuels are injected again (post-injection) at
the end of the ammonia injection duration.
Figure 1 shows the reaction temperatures at which
each emission is formed in the flame as
investigated by Okafor et al [1] using anexperimental combustion apparatus. According to
the figure, the formation temperature range of N2O
is found around 1300 K. That is a lower
temperature range than the normal FO whose
flame temperature is above 2000 K.
In this figure, the peak of N2O production is lower
than in other emissions, but considering the global
warming potential of about 265 times that of CO2,
it is necessary to avoid this temperature range andto ensure that N2O is reduced to as low as
possible.
It is imagined that post-injection by means of thestratified ammonia injection system, it may beeffective in reducing not only unburnt ammonia but
also N2O, by activating the ammonia spray flame
again and raising the flame temperature.

In addition, we report the results of Rate of Heat
Release (ROHR) and exhaust gas analysis ofcombustion test using hexadecane as FO in a
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following yellow ammonia. This is advantageous as
the ammonia could be ignited by multi-points fromthe periphery of spray.

Figure 1. Profiles of emissions in the oxidation of
CH4-NH3-air mixtures (ENH3 =0.7, ϕ = 1.1) in alaminar plug flow reactor at various oxidation
temperature and a constant arbitrary residence
time of 0.3 s [1]Figure 4(b) shows the last part of injection. The blue
colored post FO is injected after the yellow coloredammonia. It can be seen that the post FO
penetrates into the central part of spray by its own
momentum. It is supposed that the post FO could
activate combustion from internal region of theammonia spray. As a result, post FO is expected tobe effective in reducing unburnt ammonia and N2O
production as discussed later.

In actual operation, an ammonia injection channel
is connected to the middle of the FO channel insideof the injector. Schematics of stratified ammonia
injection system in actual engine is shown in Figure
2. It consists of fuel oil pump, fuel injection valve
and ammonia injection pump.
Figure 3 shows working procedure of the stratified
ammonia injector.
(A) Before the start of injection, FO channel insideof the injector is filled with FO. Ammonia is filled up
to the connection of FO channel.

Figure 2. Schematics of stratified ammonia
injection system in actual engine.

(B) Ammonia discharged from the ammonia
injection pump forms a layer, of FO and ammonia
inside of the FO channel.
(C) To inject stratified FO and ammonia, fuel oilpump discharges FO. Then, the FO and ammonia
in the FO channel are pushed and injected into theengine. At first, FO is injected so it behaves as pilot
FO. After that, ammonia is injected continuously.
(D) As the fuel oil pump continues injection, allammonia that was filled inside of the FO channel
will be injected. At the end of injection, FO is
injected as post fuel.
The quantity of ammonia can be controlled byactuating the lift of the ammonia injection pump,and the total amount of FO and ammonia can becontrolled by actuating the lift of the fuel oil pump,thus controlling the total heat value and ammonia
calorific ratio.

Figure 3. Working procedure of the stratified
ammonia injector.

Figure 4 shows a simple CFD simulation on thespray formation process of the fluid injected as
described above. The red and blue fluids are
assumed to be FO and the yellow fluid ammonia.
Figure 4(a) shows that the red pilot FO injected firstis pushed out to the periphery of spray by the
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Figure 4. CFD analysis result of Stratified Injection.

3. EXPERIMENTAL SETUP
To verify the effect of our combustion concept, avisual constant volume combustion vessel(VCVCC) was developed to simulate combustion in
the cylinder of a large two-stroke marine engine.
Figure 5 shows the appearance and crosssectional view of the VCVCC, fabricated to conduct
basic combustion experiments for stratified
injection combustion. The distance from theinjection hole to the bottom of the combustion
space was 350 mm, and the diameter of the
combustion space was 150 mm. During thevisualization test, an observation window of 300
mm long and 96 mm wide was installed. The inchamber pressure was measured using a watercooled in-cylinder pressure sensor (Kistler, 6041
B). The heat release rate was then be calculated
from the in-chamber pressure history [3].

Figure 6 shows the entire experimental combustion
system. Air supplied from a high-pressure air
source at a controlled flow rate is heated by aheater and filled into a heated vessel. When theinside of the vessel reaches the target pressure, it
is sealed. FO is then injected and a combustion
experiment is conducted. 100 milliseconds after the
injection signal is issued, the exhaust valve is
opened to release the combustion gas into a buffertank that has been evacuated in advance. This
rapid depressurization freezes the chemical
reactions in the VCVCC. The composition of the
combustion gas in the buffer tank is analysed by
Fourier transform infrared spectrometer (FTIR)
analyser (Iwata Dengyo, Fast-2200) [2].
Figure 7 shows schematic of the NH3 stratified
injection system of VCVCC test system. The
system primarily comprises a fuel injection pump ofa pressure-amplifying piston type, a stratified fuelinjector, and an NH3 supply unit. The fuel injection
pump can adjust the injection pressure bycontrolling the working oil pressure and injection
period, by manipulating a high-speed hydraulicservo valve. The stratified fuel injector has the flow
channels for the FO and liquid NH3. The liquid NH3
supply unit can charge the required volume into the
FO flow channel. In addition, the fuel injection
pump and stratified fuel injector are equipped with
Non-return Valve 1 for the FO and Non-return
Valve 2 for the NH3, respectively. The spray tip of
the injector featured a single hole with a diameterof 0.7 mm [2].
Figure 8 shows appearance of test system,
schematics are shown in Figure 5.

Figure 6. Schematics
combustion system [2].

of

the

experimental

Figure 5. Schematics of the exterior and crosssection of VCVCC [3].

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period could be controlled by a high-speed
hydraulic servo valve, as shown in Figure 7 [2]. The
actual injection period was measured by the needle
lift sensor, as shown in Figure 7 [2].
In Case (A) shows the two-layer NH3 stratified
injection of the FO / liquid NH3. There is a pilot FO
at the tip, it contains higher injection energy of FO
than injection energy of ammonia. After that, theFO fraction gradually decreases at the middle ofinjection period, and lower FO fraction continues atthe end of injection period. The total injectionenergy of FO and ammonia is 51 kJ at the end of
the measurement range.
Figure 7. Schematic of the NH3 stratified injection
system [2].

In Case (B) (C), in which the NH3 injection volume
is decreased, the presence of FO at the tip and thedecrease of FO fraction at the middle of injection
period are the same as that in Case (A). There is
the difference from Case (A) in that the FO fraction
increase at the end of injection. Finally, there ishigher fraction of FO on the terminal side, whichmeans combustion will proceed with post FO, so
production of unburned ammonia and N2O will
decrease compared with the Case (A). The total FO
and ammonia injection energy of Case (B) is 55kJ
and of Case (C) is 57 kJ.
The total NH3 energy ratio of Case (A) was 64%,
Case (B) was 54% and that of Case (C) was 49%
in one NH3 stratified injection [2].

Figure 8. Photograph of VCVCC

4. EXPERIMENTAL RESULT
4. 1. Spray test, spray volume measurement
Figure 9 shows three patterns of energy injection
rates, and Figure 10 shows three patterns ofvolumetric injection ratio. Details of the injectedmass measurement system is shown in previous
studies [2]. In this section, we will re-introduce themeasurement method of injection rate.
The energy injection rates in terms of the lower
heating value of the FO and liquid NH3 were
estimated using the injection mass measurement
results at several injection periods during one NH3stratified injection. The stroke volume of the liquid
NH3 feed pump was fixed in advance. The injection
CIMAC Congress 2023, Busan

Figure 9. Three patterns of energy injection rates.
Modified from [2].

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No luminescence is observed from 18 ms to 28 ms
after start of injection. This is a period of low Rateof Heat Release (ROHR) discussed in section 4.3. ,and ammonia combustion seems to be continuing,although not to the extent that luminescence isobserved. After that, luminescence is seen again.
The bright flame is seen in the area close to theinjector, and orange luminescence is seen in thearea farther away. It is thought that the ammonia isburning off assisted by the luminescence and
ROHR from the post-FO and the combustion of the
post-FO.

Figure 10. Three patterns of volumetric injection
rates.

4.2. Referral of direct imaging
Visualization tests were performed using
hexadecane as FO; visualization of FO-only
injection and visualization test results for stratified
ammonia injection are shown.

Figure 12. High-speed direct images of ammonia
stratified spray flame. Modified from [3].
The results of the shadowgraphs for the noncombustion condition are shown in Figure 13. Case(S) shows the shadowgraphs of FO-only spray. The
spray cone shape and spray length are generally
the same for the FO-only case and the ammonia
stratified injection case.

In the FO-only visualization test result is shown in
Figure 11. In this result, luminesced flame wasobserved at 9ms after start of injection. After that,the area of combustion increases and combustion
progresses.

Figure 11. High-speed direct images of FO-onlyspray flame.
In the case of ammonia stratified combustion,
injection rate of Case (B) is shown in Figure 12. The
first luminescence is seen at about 2.5 ms. It seesthat the pilot FO ignited at this timing. Subsequent
flashes continue until about 18 ms. The color of theluminescence is orange, different from the color of
the flame when only the FO is visualized, and isthought to be caused by the combustion ofammonia.

CIMAC Congress 2023, Busan

Figure 13. Shadowgraph images of ammonia
stratified spray and FO-only sprays under noncombustion conditions.

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4.3. Combustion test
A stratified combustion test was conducted using
hexadecane. Results are shown for hexadecaneonly, for NH3 pump stroke 40 mm (Case (A):ammonia calorific ratio is about 64%). NH3 pump
stroke 20 mm (Case (B): ammonia calorific ratio is
about 54%), NH3 pump stroke 15 mm (Case (C):ammonia calorific ratio is about 49%), where theinjection volume was measured at 4.1.
In this experiment, pressure and temperature
condition in VCVCC is 5 MPa and 783 K. Thistemperature is lower than that of the typical large
marine 2-stroke engine, and the condition is severe
for ammonia combustion with increased productionof unburned ammonia and N2O. This experimental
condition also causes reduce of ignitability, we use
hexadecane as FO, which has good ignition
properties, to ensure a stable experiment.
Figure 14 shows relationship between in-chamberpressure and heat release rate with respect to timeafter start of injection for Case (S), Case (A) andCase (C).
In Case (S), ROHR curve has mountainous
waveform with a single peak. It is similar tocommonly seen in typical conventional engines.
In Case (A), which has the higher ammonia fraction
in all period of injection, there is a large ROHR peakin the early period, which appears to be due to thepilot FO combustion. At the middle period of
combustion, the ROHR is lower than the early
period. In this section, ammonia, which has a lower
calorific value than FO, is injected. The ROHR does
not reach zero. It indicates that combustion
continues even higher fraction of ammonia, without
support of FO combustion. After that, clear ROHR
peak was not observed. The ROHR of terminal
period decrease gradually. Heat release continues
at about 50 ms after start of injection. At this time,the injection rates measurement result indicatesthat ammonia injection has already ended.
Figure 15 shows relationship between total of NO
and NO2 (NOx) production ratios with respect with
NH3 calorific ratio. Figure 16 shows relationshipbetween N2O production ratios with respect with
NH3 calorific ratio. Figure 17 shows relationshipbetween NH3 production ratios with respect with
NH3 calorific ratio. In the exhaust gas Case (A),unburned ammonia is about 0.03 mg/kJ and N2O
is about 2.3 mg/kJ. These data indicate thatammonia still remains in the VCVCC and that the
combustion is still continuing, even after the
injection is terminated. This state is regardless ofwhether the FO is also remaining in the VCVCC ornot.

CIMAC Congress 2023, Busan

It was confirmed that the stratified ammonia
injection system enables the pilot FO to ignite theammonia, and that even if the temperature drops
due to the heat of vaporization when ammonia is
injected. Ammonia combustion continues even
without FO combustion support, although it burns
at a slower rate for a longer duration.
Case (C) is a pattern in which the ammonia fraction
decreases at the end period of injection, resulting
in conditions in which post FO was expected tostimulate combustion and suppresses unburnedammonia and production of N2O.
There is a ROHR peak in the early period and lower
ROHR at the mid period of combustion. Trend of
ROHR of early period and middle period is almost
same as Case (A). However, there is a second
peak of ROHR not seen in Case (A) at 30ms after
start of injection. From the results of the injection
volume measurement test, this is the period of little
bit after FO injection ratio increases and injection
finished. Therefore, this second peak of ROHR
seems to be caused by the post FO combustion.
After that, heat release drops to almost zero
rapidly, so burnable substances such as ammonia
or FO are not remaining.
In the exhaust gas result, unburned ammonia
decreased to about 0.006 mg/kJ and N2O to about
0.1 mg/kJ. In conjunction with the result of ROHR,
even in an environment where ammonia is incombustion, post FO ignition is not inhibited, andthe desired effect, activating combustion, reducing
unburned ammonia and N2O production, areachieved.
Returning to Case (A), this comparison with Case(C) recognises the importance of the presence ofpost FO in combustion optimization, as post FO
supports the ammonia to burn up quickly and to
reduce the unburned ammonia and N2O
production drastically.
From these experimental results, J-ENG designed
a fuel injection system that was CFD analysed andoptimized. This is, an ammonia stratified injection
system that increases the ammonia calorific ratio
while retaining post FO with certainty. J-ENG is
preparing the stratified injection system for
examination in a test engine.

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Page 6

Figure 17. Relationship between NH3 production
ratios with respect with NH3 calorific ratio.

Figure 14. Relationship between in-chamberpressure and heat release rate with respect to timeafter start of injection. Modified from [2].

5. CONCLUSION
A stratified ammonia combustion test andvisualization test have been conducted by the
newly developed VCVCC.
As a fundamental research work, observations ofspray formation and combustion tests have been
carried out to confirm the effect of this system using
the VCVCC that simulates the condition of a realengine.
It has been confirmed that the first layer (FO) acts
as pilot fuel to ignite following NH3 layer
successfully and the last layer (FO) contributes tothe stable combustion of prior NH3 layer and alsosignificantly reduces NH3 slip and production of
N2O.

Figure 15. Relationship between total of NO andNO2 production ratios with respect with NH3
calorific ratio.

In the case of two-layer injection, it is imagined thatthe combustion ends with the ammonia spray andthe reaction of ammonia, which has lost
momentum, proceeds slowly at a lower
temperature, increasing N2O production.
On the other hand, if the after-burning part ofammonia is replaced by the last layer of FO, its
spray penetrates the ammonia flame and burn thewhole flame actively. Not only the higher
temperature combustion, but also the H radicals
produced by the combustion of the last layer may
also contribute to N2O reduction by promoting the
NO reduction reaction of NO + H = N + OH and theN2O consumption reaction of N2O + H = N2 + OH
[2].

Figure 16. Relationship between N2O production
ratios with respect with NH3 calorific ratio.

CIMAC Congress 2023, BusanAll the data is useful to improve the design of lowspeed two-stroke engine applying this system. This
successful low-emission cases was achieved by
the VCVCC described above, where the ammonia
calorific ratio was limited to about 50 - 60%, toPaper No. 101

Page 7

ensure a stable experiment. This limitation is due tothe lower temperature and pressure than that of thetypical large marine 2-stroke engine.
The authors plan to carry out running tests with atest engine, which surely achieve the higher
ammonia calorific ratio with improved combustion
environment, optimized stratified injection systemand superior control technology. The results of test
engine will be fed back into the design of the fullscale engine, which will be in service for theverification voyage after shop test completed.

Acknowledgments
This paper is based on results obtained from a
project, JPNP21501684-0, subsidized by the New
Energy and Industrial Technology Development
Organization (NEDO).

LITERATURE[1] Okafor EC, Kobayashi H, et al. Flame stability
and emissions characteristics of liquid ammonia
spray co-fired with methane in a single stage swirl
combustor. Fuel; 287: 119433 (2021).
[2] Ichikawa Y, Niki Y, Takasaki K, Kobayashi H
and Miyanagi A, NH3 combustion using three-layer
stratified fuel injection for a large two-stroke marine
engine: Experimental verification of the concept,
Applications in Energy and Combustion Science,
Volume 10, 2022, 100071, ISSN 2666-352X[3] Ichikawa Y, Niki Y, Takasaki K, Kobayashi H
and Miyanagi A, Experimental study of combustion
process of NH3 stratified spray using imaging
methods for NH3 fuelled large two-stroke marine
engine, Applications in Energy and Combustion
Science, [In press]

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