Mitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)
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Development of Hydrogen and Ammonia Engine thatContributes to Decarbonized Society
YUSUKE IMAMORI*1

SOTA WATANABE*2

KENGO TANAKA*1

TOMOHIRO NOGUCHI*2

NAOYUKI MORI*3

SATOSHI YAMADA*4

In order to achieve a low-carbon and carbon-neutral society, Mitsubishi Heavy Industries
Engine & Turbocharger, Ltd. (MHIET) is developing reciprocating engines that use hydrogen andammonia as fuel. For hydrogen/natural gas mixed combustion engines, we verified with actual
equipment the combustion characteristics when the ratio of hydrogen mixed with natural gas isincreased to 35 vol%, and have clarified the conditions for avoiding abnormal combustion such as
knocking and pre-ignition and determined specifications for future commercialization of theengine. For hydrogen combustion engines, we conducted numerical simulation and verification
using a single cylinder testing engine to select operating conditions and component specifications
that will achieve high power output and high efficiency while avoiding abnormal combustion, andare currently promoting the multi-cylinder engine design by developing the selected conditions andspecifications. For ammonia/diesel oil mixed combustion engines to be used in marine engines, we
researched for an appropriate combustion system that can significantly reduce greenhouse gases
including N2O by using numerical simulation and have confirmed an 82% reduction compared toconventional engines through verification using a small test engine.

|1. Introduction
MHIET has been providing diesel and gas engines with excellent performance, reliability,and environmental characteristics for power generation, marine, and vehicle applications. In orderto achieve a low-carbon and carbon-neutral society, we are developing triple hybrid power
generation (1)-(3), adoptability of carbon-neutral fuels, and employment of CO2 capture systems (4), (5)to further increase customer value with improvement of environmental characteristics and energy
transition in mind (6). This report introduces the development status of our new engines that usehydrogen and ammonia as carbon-neutral fuels.
As for hydrogen, we have determined the specification configuration and operating
conditions of engines that can stably generate electricity using hydrogen mixed fuel without major
specification changes to existing natural gas combustion engines. Furthermore, in order to realize
stable combustion operation with 100% hydrogen fuel, we are developing hydrogen combustion
engines with high efficiency and stable combustion without greenhouse gas emissions by utilizing
flow and combustion numerical simulation technology.
As for ammonia, which is a promising carbon-neutral fuel for the shipping industry, we havedetermined operating conditions and component specifications that enable stable combustion offlame-retardant ammonia ignited by light oil while significantly reducing emissions of N2O, which
has a high greenhouse effect, and are currently working on further improvements. This reportintroduces the status of these developments.

*1

Chief Staff Researcher, Combustion Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Chief Staff Manager, Engine Technology Development Department, Engine & Energy Division, Mitsubishi Heavy Industries Engine*3

Combustion Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*4

Manager, Combustion Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

& Turbocharger, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)
2|2. Development of hydrogen/natural gas mixed combustion engines
In Japan, a government policy of “aiming to achieve carbon neutrality by 2050” waspresented. The use of hydrogen, which is the key to achieving carbon neutrality, may start with theuse of hydrogen/natural gas mixture. Therefore, we worked with Toho Gas Co., Ltd. to develop ahydrogen/natural gas mixed combustion engine using our existing natural gas engine, andconfirmed the impact on combustion performance when a hydrogen/natural gas mixture is used andthe effect of reducing greenhouse gases (7).

2.1

Specifications of hydrogen/natural gas mixed combustion engine

For the test, we used the GS6R2, our leading gas engine model installed at Toho Gas's
Technical Research Institute. The engine specifications were basically unchanged from those of thenatural gas engine, and the adaptation was made by adding a hydrogen supply system. Figure 1
shows the appearance and main specifications of the engine. The combustion system ispre-chamber type, which achieves stable lean combustion.

Figure 1

Appearance and main specifications of engine

Figure 2 shows the air and fuel supply system. The fuel supply line supplies a mixture ofhydrogen and natural gas (city gas 13A) from upstream of the turbocharger. The mixing ratio isadjusted by a flow control valve installed in the hydrogen supply line. The power output inhydrogen/natural gas mixed combustion operation was set to 100% of the power output in natural
gas operation.

Figure 2

2.2

Air and fuel supply system (hydrogen/natural gas mixed combustion engine)

Test result of hydrogen/natural gas mixed combustion engine

Figure 3 shows the various engine performances when the hydrogen mixture ratio isincreased with the air excess ratio and ignition timing kept unchanged. Figure 4 shows thecomparison of in-cylinder pressure and the rate of heat release for 0% and 35 vol% (12.5% in terms

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)
3of heating value) hydrogen mixing. The vertical axis of each graph in Figure 3 indicates the ratio tothe item at the base ignition timing and the hydrogen mixing ratio of 0%. Since hydrogen has ahigher burning velocity than natural gas, mixing hydrogen accelerates the heat release, shortens theheat release period, and improves the degree of constant volume. As the heat release period
shortens, the average in-cylinder gas temperature rises and the cooling loss increases due to theincreasing temperature difference between the cooling wall surface and the gas, but the thermal
efficiency improves due to the greater effect of improving the degree of constant volume. On theother hand, increasing the hydrogen mixing ratio also increases NOx emission concentration, so
measures such as delaying the ignition timing and adjusting the air-fuel ratio are necessary.
Since knocking and pre-ignition are likely to occur when the hydrogen mixture ratio isincreased, the specifications for commercialization will be determined after confirming the range ofhydrogen mixture ratio and air excess ratio that can avoid the occurrence of these abnormal
combustions through testing.

Figure 3 Various performances with respect to hydrogen mixture ratio

Figure 4 Comparison of internal cylinder pressures andrate of heat releaseMitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)
4|3. Development of 100% hydrogen combustion engine
We are developing hydrogen combustion engines fueled by 100% hydrogen and emit noCO2. Through joint research with National Institute of Advanced Industrial Science andTechnology (AIST) Fukushima Renewable Energy Institute, we have demonstrated the conditions
for stable and highly efficient combustion of hydrogen, which burns at a high rate, using a single
cylinder testing engine (8). In addition, we are designing the multi-cylinder engine by making fulluse of numerical simulation methods for related elemental technologies such as flow, combustion,and strength, which we have accumulated in our research and development activities.

3.1

Single cylinder testing engine

We are in the process of evaluation for determining appropriate specifications andcombustion conditions of hydrogen engines, utilizing a single cylinder testing engine. Figure 5
shows the main specifications and operating conditions of the single cylinder testing engine. As an
example, Figure 6 shows the effect of optimizing the air excess ratio λ in hydrogen combustion.
Because hydrogen combusts very quickly, the ignition timing range is narrow with an air excess
ratio λ equivalent to that for natural gas due to the limitation caused by knocking avoidance.
However, by making the air excess ratio appropriately lean, the combustion range can be extended
and thermal efficiency and brake mean effective pressure can be improved. Currently, we are using
this single cylinder testing engine to verify the proper specifications of compression ratio, intake
valve closing timing, etc.

Figure 5 Main specifications and operating conditions of hydrogen combustion
single cylinder testing engine

Figure 6
Example of result with single
cylinder testing engine

3.2

Study through analysis related to multi-cylinder engine design

We use CFD (Computational Fluid Dynamics) analysis together with the single cylinder
testing engine in selecting suitable combustion chamber specifications for the port injection andMitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)
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open chamber spark ignition system of 100% hydrogen combustion engines. First, Figure 7(a)shows the results of evaluating the fuel concentration uniformity of mixture in the cylinder for the
adoption of port injection in the equivalent ratio contour diagram. This indicates that themacroscopic cylinder internal flow and squish effects are changed by optimizing the combustion
chamber geometry to reduce locally rich fuel regions. Next, Figure 7(b) shows the result of
evaluating the flame propagation in the cylinder under conditions where such mixture distribution
exists. The presence of a region with extremely slow flame progression increases the risk of intense
knocking in the region around it. By optimizing combustion chamber specifications in
consideration of these factors, we are working to further improve the thermal efficiency andmaximum power output.

Figure 7 Example of CFD analysis of mixture formation in cylinder and flame
propagation combustion

3.3

Design of a multi-cylinder engine

As with the hydrogen/natural gas mixed combustion engine described in chapter 2, we aredeveloping 100% hydrogen combustion engines based on our GSR lean burn gas engine series thathave been marketed (Figure 8). In the case of hydrogen combustion, the combustion rate is evenfaster than that in hydrogen/natural gas mixed combustion, and the risk of abnormal combustion
such as backfiring, pre-ignition, and knocking is higher. Therefore, there are issues of stabilizing
combustion and reducing the risk of abnormal combustion. The following sections are describing
technologies to address these issues.

Figure 8

Hydrogen combustion engine

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)
6(1)

Combustion stabilizing
To suppress the high combustion rate in 100% hydrogen combustion, the conventional
pre-chamber spark ignition system used in the GSR series was changed to a open chamberspark ignition system (Figure 9). In addition, the compression ratio, intake valve closing
timing, air excess ratio, and other factors were reviewed through the verification using thesingle cylinder testing engine described in section 3.1 to optimize the combustion rate andstabilize combustion.

Figure 9(2)

Air and fuel supply system (hydrogen combustion engine)

Abnormal combustion risk avoidance
The 100% hydrogen combustion engine is equipped with additional cylinder internalpressure sensors compared to the conventional GSR series (Figure 9). The cylinder internalpressure sensors quickly detect abnormal combustion such as pre-ignition and knocking andstop the fuel supply to avoid the risk of engine damage due to abnormal combustion. In
addition, for the hydrogen combustion engine, the conventional method of feeding fuelupstream of the turbocharger and supplying premixture through the turbocharger, which is usedin the GSR series, is changed to a method of supplying fuel from the intake port of eachcylinder (port injection method). By applying this method, the area where premixture exists in
the intake system is limited to a minimum, and the risk of engine damage in the event of
backfires can be reduced. We are proceeding with the design of multi-cylinder engine that
addresses the issues as described above, and have a plan to begin verification tests on theperformance and reliability using the multi-cylinder engine from fiscal 2023 onward.

|4. Development of ammonia/diesel oil mixed combustion engine
We are developing engines that can use ammonia, which is attracting attention as one of the
new fuels for achieving carbon neutrality. Since ammonia can be produced from hydrogen, and canbe liquefied easily by pressurizing and the volume is reduced, it is expected to be applied to marine
engines, etc., where the fuel tank capacity is limited. The following sections describe the results ofa test using our small engine to verify the greenhouse gas emission reducing effect.

4.1

Issues with ammonia combustion

Figure 10(a) shows the fuel supply system to the combustion chamber. Based on aconventional diesel oil fueled engine, this system has a configuration that supplies ammonia gas tothe intake system so that the ratio of diesel oil and ammonia can be adjusted as desired according tooperating conditions. The base engine is a small four-cylinder diesel engine equipped with acommon-rail injection system that allows the injection amount and timing of diesel oil to be set
freely, as shown in Figure 10(b) and Figure 10(c). There are issues with ammonia combustion:
more unburnt combustible content is emitted because ammonia is more difficult to ignite thanhydrogen or natural gas and has a lower combustion rate; nitrous oxide (N2O), which has aMitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)
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greenhouse effect about 300 times greater than carbon dioxide, is produced by thermal
decomposition of ammonia; and nitrogen oxide (NOx) is abundantly generated. As a result ofpredicting the gas mixture distribution in the combustion chamber by combustion simulation(Figure 11), it was found that a large amount of unburnt ammonia is retained at the edges, so
promoting combustion in the area where the diesel oil flame cannot reach is an issue.

Figure 10

Overview of ammonia/diesel oil mixed combustion engine

Figure 11

Ammonia combustion simulation result

4.2

Test result of ammonia/diesel oil mixed combustion

Figure 12 shows an example result of ammonia/diesel oil mixed combustion test using an
actual engine. As a result of reducing the air excess ratio λ aiming to increase the combustion rate
to promote the combustion of ammonia, the unburnt ammonia concentration in the exhaust
decreased as shown in Figure 12(a). Also the N2O concentration decreased, which can be attributed
to the reduction in the amount of unburnt ammonia, one of the N2O generation factors, asmentioned above, and to the increased temperature in the combustion chamber as a result of thedecrease in λ, promoting consumption of N2O. Furthermore, the accelerated combustion ofammonia enabled stable combustion even with a higher ammonia mixed combustion rate (ratio of
the heat quantity of ammonia to that of the entire fuel) (Figure 12(b)), increasing the mixed
combustion rate from the initial setting of 75% to 94%. Figure 12(c) shows the change in thegreenhouse gas emission rate. The increase in ammonia mixed combustion rate and the reduction of
N2O resulted in an 82% reduction in greenhouse gas (GHG) emissions compared to the diesel oil
combustion.
The negative effects of the decrease in λ include increased NOx and heat load on the
combustion chamber and exhaust system components. We will continue to find appropriate settings
through simulations and engine tests and accelerate the development of ammonia engines that
contribute to achieving carbon neutrality.

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)
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Figure 12

Test result of ammonia/diesel oil mixed combustion|5. Conclusion
We are currently conducting research and development of hydrogen/natural gas mixed
combustion engines, 100% hydrogen combustion engines, and ammonia/diesel oil mixed
combustion engines for use in distributed power sources and marine main/auxiliary engines, with aview to the low-carbon and carbon-neutral society that is expected to be realized in the future.
Among them, the hydrogen/natural gas mixed combustion engines are now in the final stage ofcommercialization with the aim of improving the environmental burden and increasing customer
value by bringing them to market at an early date. In addition, we will contribute to MISSION NET
ZERO, in which MHI Group aims to achieve carbon neutrality by 2040, and to the realization of asustainable society, through the timely development and commercialization of hydrogen
combustion engines and ammonia combustion engines in response to the status of hydrogen andammonia supply infrastructure, their penetration into the fuel market, and the needs of society andcustomers.

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