Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)27

Technologies for Improving Power Generation Efficiency
and Utilizing the Cooling Water Energy ofthe KU30GSI Gas Engine for Power Generation
HIROSHI YOSHIZUMI*1 SHOJI NAMEKAWA*2
MICHIYASU ISHIDA*3MINORU ESAKI*4

HAJIME SUZUKI*5

JUNNOSUKE ANDOU*6

Against the backdrop of the reduction of environmental impact and the development of shalegas, natural gas resources have attracted a great deal of attention as a primary fuel for the future.
In its product lineup, Mitsubishi Heavy Industries Engine and Turbocharger, Ltd. (MHIET) offers ahigh-speed GSR gas engine covering 305 to 1,500kW and the mid-speed KU30GSI model covering
3,650 to 5,750kW, both of which are applicable to small-scale distributed power generation,
co-generation in facilities or plants and dedicated utility power generation. This paper will
describe the technology for increased efficiency of the KU30GSI-Plus model, which achieved
power generating efficiency of 49.5%, and technologies relating to the all-steam recovery systemthat is designed to utilize the engine cooling water by raising its temperature so that it could be
used as a heat source for steam recovery.

|1. Introduction
Natural gas resources have attracted attention as a future primary fuel due to the increased
need for the reduction of environmental impact and an alternative to oil. On the other hand, interest
in distributed power sources, which are quite useful in times of disaster, is expected to growfurther.
Under such circumstances, gas engines, due to their excellent startup performance, load
following capability and flexible applicability to waste heat recovery, are utilized for variousapplications including Combined Heat and Power (CHP) generation with a wide range of power
output. In particular, mid-speed gas engines have achieved excellent power generation efficiency, aquick startup within 10 minutes and high-efficiency waste heat regeneration utilizing
high-temperature exhaust gas, which resulted in a total efficiency of over 80%. MHIET regards our
mid-speed gas engines as one of the best choices for providing high-efficiency energy solutions.
MHIET’s KU gas engine series models include the micro pilot-ignited KU30GA(MACH-30G) brought to the market in 2002, which is based on the spark-ignited KU30G gas
engine released in 1990, but is further sophisticated in terms of its power output, efficiency andendurance. The KU30GA was highly evaluated for its high reliability and performance and morethan 150 units were sold.
In 2009, based on the KU30GA engine with its extensive track record, MHIET adopted thespark-ignition method and released the KU30GSI gas engine (total efficiency-focused model) onthe market. The product focused on overall efficiency including waste-heat regeneration and had
excellent compatibility with a wide range of applications.
Furthermore, in 2012, in order to meet increased customer needs for power generating
efficiency, MHIET released the KU30GSI-Plus gas engine (power generation efficiency-focusedmodel), adding a new model in our product lineup that would satisfy a wide range of needs for*1*2*3*4*5*6

Engine & Engineering Divison, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.
Deputy General Manager, Engine & Engineering Divison, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.
Chief Staff Manager, Engine & Engineering Divison, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.
Manager, Engine & Engineering Divison, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.
Chief Staff Manager, Engine & Engineering Divison, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.
Combustion Research Division, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)28
1, 2

overall efficiency and power generating efficiency. The power generating efficiency has beenfurther improved in the KU30GSI-Plus engine. In 2015, the same model with an increased
efficiency of 49.5% was released.3, 4
Meanwhile, MHIET successfully commercialized an all-steam recovery system that collects
steam even from warm waste water, which has not been utilized effectively until now, from
non-utility generation facilities in Japan, as a system product jointly developed with 3 otherentities* in May 2015, with the top "power generation + steam recovery" combined efficiency
achieving 71.1%.
This paper will describe the excellent features of the KU30GSI gas engine and its
technologies for improved power generating efficiency, as well as those for the extensive
utilization of waste heat as typified by the all-steam recovery system (technologies utilizing energyfrom cooling water).
*Tokyo Gas Co., Inc., Mitsubishi Heavy Industries Engine and Turbocharger, Ltd., Miura Co., Ltd. andKobe Steel, Ltd.

|2. Overview of KU gas engine
The four stroke-cycle KU30GSI model pre-chamber lean-burn gas engine with spark ignitionis equipped with 12 to 18 cylinders and covers power output of 3,650 to 5,750kW. Table 1illustrates the principal specifications of the KU30GSI and KU30GSI-Plus engines.
Table 1 Principle specifications
Engine type
Number of cylinders
Bore/Stroke
Speed
Generator output*
Generation efficiency*
NOX (O2=0%)
Engine mass

KU30GSImm
min-1
kW%ppmkg

46.5

KU30GSI-Plus
12－18
300×380
720/750
3650－5750
49.5< 32040000－60000*The table above is based on the ISO3046 conditions and MHIET-recommended standard gas.

The total efficiency-focused KU30GSI model has achieved improved power generating
efficiency and high exhaust gas temperature at the same time, realizing the top-class total efficiencyof over 80% compared with similar output gas engines, including power generating efficiency of46.5%, exhaust gas temperature of 395°C, NOx emissions of 320ppm (at 0% O2), making it mostsuitable for applications combined with exhaust heat regeneration such as CHP generation.
Furthermore, MHIET developed the power generating efficiency-focused KU30GSI-Plusmodel with enhanced power generating efficiency applicable to dedicated utility power generation
and Independent Power Producers (IPP). In 2015, the same model with improved power generating
efficiency of 49.5% (including power necessary for lubricating oil pump on engine) was launchedon the market.
The KU30GSI series models are utilized for various applications both at domestic andabroad, achieving field operating hours of approximately 30,000 hours. The KU series gas engines
as a whole has achieved a total of more than 7 million operating hours. The KU30GSI is ahighly-reliable gas engine which has 80% of its parts in common with other KU series models.
From the planning stage, the design was oriented to facilitating ease of access to installation,
wiring, plumbing, etc. Thus, the streamlining of on-site installation and a shortened work period forthe construction of power plants are targeted.

|3. High-efficiency gas engine power generating technology
In recent years, due to the application of the Miller cycle and improved technologies forin-cylinder flow prediction utilizing Computational Fluid Dynamics (CFD), the power generating
efficiency of gas engines has been rapidly advancing. MHIET has been working hard on the further
improvement of power generating efficiency while securing combustion stability, being very awareof the Japan-specific gas properties (methane number 65).

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3.1

Advanced Miller cycle

Upon the application of the advanced Miller cycle, we first confirmed the in-cylinderpressure fluctuation and the impact that the timing of when the intake valve is closed would haveon the thermal efficiency. Figure 1 demonstrates a comparison between Case A and Case B withlate intake valve closing timing, in both of which the air excess ratio and in-cylinder pressure are
maintained at a certain level. In Case A, Coefficient of Variation (COV) Pmax, indicating the rangeof the maximum in-cylinder pressure fluctuation, improves by about 50% compared with Case B,
and the thermal efficiency increases by about 1pt%. In Case A, although the intake valve opens
early and there is an increased period of time when the intake and exhaust valves are both open,
allowing unburnt gas to blow through, which might lead to a possible decrease in the thermal
efficiency, the combustion stability realizes improved thermal efficiency. Figure 2 shows thetime-series behavior of the peak firing pressure in both Cases A and B. There are the larger
fluctuation with frequent high peak pressure events in Case B, whereas Case A demonstrates
excellent combustion stability.
According to the test results above, the KU30GSI-Plus engine is optimized based on Case A,
where the intake valve closes early in order to achieve combustion stability. Such an optimization
focusing on reliability prevents excess temperature rise in the parts and components inside thecombustion chamber induced by knocking and other factors originating from unstable combustion.
It also contributes to an increased service life and long-term improved reliability of parts andcomponents, as well as allowing the gas engines to achieve sufficient reliability to changes inenvironmental conditions including load variation and temperature fluctuation of the cooling water.
The KU30GSI-Plus model engine achieves stable combustion, quick startup performance andexcellent in particular load following capability even during transitional periods when loads areincreasing/decreasing.

Figure 1 Miller timing effectFigure 2

Peak firing pressure based on miller timing

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3.2

Enhancement of turbocharger efficiency

In order to achieve high efficiency in Case A, where the intake valve closes early, it isnecessary to maintain high turbocharger efficiency when the pressure ratio increases. The
KU30GSI-Plus model adopts MHI’s high-efficiency MET42MB Turbocharger with the maximum
pressure ratio of 5.0. Figure 3 illustrates a cross-section of the MET42MB Turbocharger, whereas
Figure 4 demonstrates a comparison in efficiency before and after changes of specifications whenengine operated. In order to maximize the efficiency of the turbocharger, the specifications andcharacteristics are carefully selected and combined so that the turbocharger efficiency is improved
by about 2pt% in a high-pressure ratio region of 4.5 and over, which contributes greatly to theimproved power generating efficiency of the KU30GSI-Plus engine.

Figure 3 MET42MB Turbocharger

Figure 4

Increased turbocharger efficiency

3.3 Further reduction of dead volume areas
For the KU30GSI-Plus model, the occurrence of abnormal combustion and the generation ofunburnt gas are minimized through an understanding of the distribution of the air-fuel mixture andparts not contributing to combustion based on CFD analysis. Conventionally, by upgrading theshape of the cylinder head or reducing the dead volume areas around the intake and exhaust valves,the amount of unburnt gas (Total Hydro Carbon: THC) was successfully reduced by approximately
30% in 2012. Optimization utilizing CFD further realized the effective reduction of dead volume
areas inside the combustion chamber, achieving a total decrease in THC by about 60% combinedwith the other improvements described above in 2015. (Figure 5)
As explained so far, the KU30GSI-Plus engine focuses on stable combustion while
successfully increasing the power generating efficiency up to 49.5% (ISO3046).

Figure 5

Reduced unburnt gas|4. Technology utilizing energy from gas engine cooling water
The KU30GSI engine achieves a total efficiency of over 80%, taking into account theutilization of steam and warm water, which is widely used in co-generation plants. On the otherhand, warm water is not available throughout the year due to some restrictions on the side ofconsumers. Therefore, we cannot always benefit from the advantages of owning in-house power
generation facilities to the maximum extent.
In terms of the utilization of low-temperature waste heat such as warm water, theMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)31

conventional approaches for converting the heat into some form of usable energy include
applications of the Organic Rankin Cycle (ORC) or heat pumps. Under such circumstances, wehave come to think that raising the temperature of cooling water in an attempt to improve theefficiency of heat utilization in the auxiliary equipment would be quite significant. For instance, if
the temperature of cooling water discharged from the engine could be maintained at 110 to 120°C,
low-pressure steam similar to atmospheric pressure could be generated, which would allow us to
use a steam compressor in order to back up the steam from the exhaust gas boiler. This would
successfully provide valid heat utilization even when warm water is not available.
As shown in Figure 6, MHIET commercialized the "The gas engine cogeneration systemwith all steam recovery," a system, which compresses saturated steam generated by utilizing thecooling water from the KU30GSI engine as a heat source and adds it to the steam generated byexhaust gas boilers and other devices. This system achieved "power generation + steam recovery"
efficiency of 71.1% by increasing the amount of steam generated by more than 1.5 times (fromabout 3.3ton/h to about 5.1ton/h) in the 18KU30GSI engine (50Hz). MHIET began testing a single
cylinder engine of this system in 2011. Subsequently, we carried out testing on an actual multiple
cylinder unit in 2014 for verification prior to the launch in May 2015.
MHIET developed a new technology for raising the temperature of engine cooling water
higher than existing systems through a close examination of combustion and mechanical elements
in order to create a new kind of engine applicable to this system.
Generally speaking, when the temperature of cooling water rises, the combustion velocity
inside the engine cylinder increases, which requires extra care for the combustion stability.
Especially when utilizing fuel gas with a low methane number just like we do in Japan, in order tosecure the combustion stability, the power generating efficiency or power output based on the rated
capacity would sometimes need to be compromised.

Figure 6

The gas engine cogeneration system with all steam recovery

Figure 7 shows the heat release rate when the temperature of cooling water at the engine
outlet is raised from 93 to 120°C. When the water outlet temperature is 120°C, the combustion
accelerates and the peak of the heat release rate curve tends to be slightly high. This seems to be
due to the accelerated flame propagation caused by the increased wall temperature inside thecombustion chamber as mentioned above.
Figure 8 shows the thermal efficiency and COV-Pmax. When we keep the air excess ratio
and average peak firing pressure same, as the temperature of cooling water goes up, the thermal
efficiency improves, whereas the combustion fluctuation increases, indicating that there is a risk ofjeopardizing the combustion stability. In Case C, the thermal efficiency and combustion fluctuation
are both optimized, keeping the knocking risk to a minimum.
Although Case C indicates similar efficiency as Case A, we assume that this is due to thereduced cooling loss achieved by the increased temperature of the cooling water and the effects of
the reduced amount of unburnt gas realized by some improvements in the combustion efficiency.
Figure 9 shows data indicating the trend of an actual engine unit at the time of startup.
According to the trend of the combustion pressure, we can see that, during the rapid startup forMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)32

about 5 minutes until establishing the rated load, for about 15 minutes until the temperature ofcooling water reaches 120°C, and after the temperature settles at 120°C, no abnormal combustion
was detected, achieving operation with stable combustion.

Figure 7 Heat release rate before andafter temperature rise of engine
outlet cooling water

Figure 9Figure 8 Efficiency and COV Pmax before
and after temperature rise of engine
outlet cooling water

Trend before and after temperature rise of engine outlet cooling water|5. Conclusion
MHIET applied the efficiency-enhancing technology described in this paper to theKU30GSI-Plus engine and achieved a power generating efficiency of 49.5%. Such an improvementin power generating efficiency greatly contributes to the reduction of power generation costs andenvironmental burden, as well as effective energy use. By adding the KU30GSI/KU30GSI-Plusmodels to our product lineup, we believe that we will be able to provide the optimum solution not
only for heat and power generation applications including CHP, but also for mono-generation
which requires high power generation efficiency. The KU30GSI series gas engines offer highstartup performance, excellent load following capability and the technology for raising thetemperature of cooling water with a high level of flexibility. We strongly believe that we are able toprovide the optimum solution for each and every customer need.

References
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2.

Suzuki et al., CIMAC Congress 2013, Shanghai; Paper No.421
Suzuki et al., Technologies to Improve Efficiency, Utilize Exhaust Heat of KU30GSI Gas Engine for Distributed
Generation System, Mitsubishi Heavy Industries Technical Review Vol. 50 No. 3 (2013)
3. Takai, A., Latest Development Outline of Mitsubishi Lean Burn Miller Cycle Gas Engine, ournal of The Gas
Turbine Sosiety of JapanVol.44 No.5 (2016. 9)
4. Yoshizumi, CIMAC Congress 2016, Helsinki; Paper No.189