Approach to High Efficiency
Diesel and Gas Engines
TATSUO TAKAISHI*1AKIRA NUMATA*23

KATSUHIKO SAKAGUCHI*4

RYOUJI NAKANO*To prevent global warming, the social need to improve the thermal efficiencies of engines and thereby reduce CO 2 emissions isbeing increasingly acknowledged. As a comprehensive engine manufacturer, Mitsubishi Heavy Industries, Ltd., (MHI) has beenworking on various technological developments to address the increased demand for dealing with environmental and energy issues
regarding clean emission gases and reduced CO2 output. This paper outlines MHI’s approaches to improving the thermal efficiencyof small and large diesel and gas engines to enable the clean and effective use of fuel oil and natural gas as valuable human assets.

1. Outline of the development and high-efficiency
performance of engines*1 Nagasaki Research & Development Center, Technical Headquarters*2 General Machinery & Special Vehicle Headquarters*3 Yokohama Dockyard & Machinery Works
21*4 Kobe Shipyard & Machinery Works

40
Thermal efficiency (%)

MHI has developed and currently manufactures piston
engines, with the world’s widest output, ranging from small
gasoline engines (less than 1 kW) to large, low-speed diesel
engines (68 MW class). Moreover, MHI is a comprehensive
engine manufacturer that produces major components suchas exhaust turbochargers and fuel injection systems. These
are used in engines for a wide range of products, including
vehicles and ships, various mobile industrial machines,
and cogeneration and electricity generation equipment.
Thus, MHI plays an important role in supporting worldwide
economic development.
However, piston engines have encountered environmental
and energy problems, such as air pollution caused by exhaustgas and global warming caused by CO 2 emissions. The
company is striving to utilize and evolve its wide technologicalcapabilities toward solving these problems, and is working
on developing engines that respond to the diverse andsophisticated needs of its customers.
In view of its improved thermal ef f iciency, which isdirectly related to reduced CO 2 emissions, a piston engine
is inherently advantageous in its working principles andcharacteristics. In a piston engine, a sequence of strokes,
including compression, combustion, and expansion, takes
place repeatedly within one cylinder, making up a hightemperature pressure cycle. The maximum temperature
constraints of the working media inside the cylinder are notvery severe due to the unsteady cyclic combustion. A piston
engine has another advantage in that its thermal losses canbe reduced because of the slow gas flow, leading to higher
thermal efficiencies. This will be discussed below. As aresult, the highest thermal efficiencies are realized in smallto medium-sized engines, as shown in Fig. 1.

Gas turbine combined engine
Large, slow-speed marine diesel engine

5030Medium-speed
marine diesel engine
High-speed
diesel engine(for trucks)
Gas engine

Gas turbine

Nuclear power
generation

Four-stroke
gasoline engine(for passenger cars)

20

10Thermal power
generation

Two-stroke gasoline engine

00.001Engine output(higher calorific value criteria)
0.01

0.1

1Output (MW)

10100

1000

Fig. 1 Thermal efficiencies of various types of small- to medium-sized
diesel and gas engines

An outline of the company’s efforts toward improving thethermal efficiency of diesel and gas engines is given below.

2. Improving the thermal efficiency of diesel engines
2.1 Four-stroke diesel engines for power generation
2.1 and industrial use
To respond to the various needs of customers, MHI
provides a wide lineup of four-stroke diesel engines, ranging
from 5 kW to 15 MW output.1 The fuels used include a wide
range of fuel oil, such as gas oil, kerosene, marine diesel
oil, and bunker fuel oil. Partly due to the recent rise in
crude oil prices, reductions in specific fuel consumption andimprovements in thermal efficiency have become essential
technological requirements to reduce running costs in
response to the strong demand from customers. To respond
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)

Mean effective pressure (MPa)to the increasing demand for improved thermal efficiency
a nd env i ronment a l compat ibil it y, M H I is work i ng on
improving the thermal efficiency of its medium-sized diesel
engines used for power generation.
In addition to the SU3 and MARK-30B engines, with outputranges of 2.0 to 4.0 MW and 5.2 to 8.1 MW, respectively,
an in-line 2.7-MW six-cylinder engine has recently beendeveloped; both its output and thermal efficiency are better
than in conventional engines. The SU3 and MA R K-30B
engines have attained generation efficiencies of 44.1% and46.8%, respectively, which are better than any other diesel
engine in the world in this class. To achieve a high thermal
ef ficiency, the thermal ef ficiency and output of both ofthese engines were improved simultaneously by adopting
a common design concept that included an increase in themaximum cylinder and fuel injection nozzle pressures, theadoption of a long stroke, and the use of a high-pressure ratio
turbocharger. By introducing electric control technology,
mainly to the fuel injection system, MHI plans to produce adiesel engine that can satisfy not only the economic needs ofits customers, such as a high thermal efficiency and output,
but also is compatible with environmental concerns, such as
reduced NOx and particulate matter emissions.
For further CO2 reductions and improvements in thermalefficiency, the combustion performance of the engine itself
must be improved and breakthrough technologies are

MARK-30B
SU3SU1200rpm

SU1000rpm

10.0
9.0
Mean piston speed(m/s)

MARK-30B
SU3
KU30A
SU
1800

1400

2.8
2.6

SU32.4

MARK-30B

2.2
2.0
1.8

SU

KU30A

38 40 42 44 46 48
Electricity generation efficiency(%)
Mean effective pressure (MPa)

11.0

KU30A

required to reduce the friction loss of various parts, forexample, the piston rings, improve the cycle efficiency, andrecover the energy lost in the exhaust gas.
2.2 Low-speed, two-stroke marine diesel engines
W hile economic ef f iciency continues to be an issue(reduced f uel consumption and the use of a relatively
inexpensive heavy fuel oil), a strong demand exists for a
substantial reduction in the polluting elements of the exhaustgases to protect the global environmental and prevent marine
pollution, as well as a reduction in CO2 emissions to prevent
global warming. Various technological developments arecurrently being pursued to address these challenges.
From their beginning, large, low-speed, two-stroke marine
diesel engines have had a high level thermal efficiency ofover 50% (lower calorific value criteria), which is higher
than in other engines and turbines. This high thermal
efficiency is obtained because of the large cylinder diameter,long stroke, high air excess ratio, and low revolution speed.
These factors are assumed to contribute to the reductionof various losses and the realization of a cycle that isthermodynamically close to ideal. From the viewpoint ofcombustion, a slow gas flow inside the cylinder is achieved asstatic compression and expansion are enabled in the piston
engine. Therefore, although the working media reach hightemperatures and pressures, the heat loss can be reduced.
The temporal constraints of unsteady combustion in lowspeed, two-stroke diesel engines are not as severe as insmall, high-speed diesel engines. Because of this, the gas
f low speed inside the cylinder can be reduced, which isadvantageous from the perspective of reducing the heat loss
and increasing the thermal efficiency.
Based on this background, MHI has developed the UEC
Eco-Engine in response to the need for large, low-speed
marine diesel engines by introducing new electronic controltechnology. 2 The conventional mechanically controlledsystems, i ncludi ng t he f uel i nject ion, ex haust va lue,
starting, and cylinder lubrication systems, are electronicallycontrolled in this engine so that the operation timing andfuel injection rate can be freely changed according to theengine load, ambient conditions, and fuel properties. As aresult, the trade-off relationship between the NOx emissions
and fuel consumption rate can be greatly improved, as
shown in Fig. 3. In comparison with conventional cams,
this engine reduces NOx emissions by 10 to 15 % when theNOx priority mode is selected and the fuel oil consumption
is kept constant, and improves the fuel oil consumption by 1
to 2% when the fuel oil consumption priority mode is selected
and the NOx emissions are kept constant. Thus, MHI canprovide marine diesel engines that satisfy the economicefficiency required by customers while being friendly to theglobal environment.
To attain further CO 2 reductions, additional studies arerequired to increase the thermal efficiency of the engine
itself and to recover the energy lost in the exhaust gas
and the waste heat in the exhaust gas and cooling water.

1000

2.8
2.6
2.4

MARK-30B
SU32.2KU30A
2.0SU
1.8140 160 180 200 220 240 260

Fuel injection nozzle pressure Maximum cylinder pressure
(bar)
(bar)
Fig. 2 Typical performance values for conventional engines and theSU3 and MARK-30B engines, including thermal efficiency,
mean effective pressure, injection pressure, and maximum
cylinder pressure

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)22

Fuel consumption rate

Conventional cam engine

Values within normal operation load range

UEC Eco–Engine

Current practical operating point(assumed)-10–15%-1–2%

NOx priority mode
Fuel consumption rate priority modeNOx emissions
Fig. 3 Relationship between fuel consumption rate and NOx emissions
A comparison of the trade-off relationship between the fuel consumption rate and NOxemissions indicates that an electrically controlled engine is superior to a conventional camengine.

Power generation efficiency (%)

Since MHI also manufactures exhaust turbochargers, thecompany is working on developing a hybrid turbocharger.

3. Improving the efficiency of gas engines
Gas engines using natural gas or city gas as fuel produce
less exhaust gas emissions containing CO 2 , which causes
globa l wa r m i ng, a nd ot her env i ron ment a l ly adverse
elements, such as NOx, SOx, and PM. Thus, these areenvironmentally friendly engines.
MHI manufactures low-NOx, lean-burn gas engines witha wide output range from low-power GSA engines to highpower MACH-30G engines. 3 The GSR engines have a rangeof 280 to 845 kW, adopt the Miller cycle, and have attained
a generating end efficiency of 41.5%, which is the best in theworld for this class. The MACH-30G engine has a rangeof 3,650 to 5,750 kW, and has attained a generating end
efficiency of 44.5% by adopting pilot ignition instead of spark
plugs and using the Miller cycle.
3.1 High-speed GSR gas engines
MHI was the first company in the world to apply a leanburn system and the Miller cycle to gas engines with apower generation output of less than 1,000 kW. As shown inFig. 4, the CO2 emissions were reduced by 12% comparedto the company’s conventional engines. The CO2 emissions
were equivalent to 75% of those of diesel engines. The powergeneration of gas engines can satisfy customers requiringa large amount of hot water and steam, and those requiring
mainly power generation, who focus on improved generation
ef f iciency while responding to needs for reduced CO 2emissions.
B y c o nt i nuo u s l y a i m i n g t o a c h i e v e b o t h h i g h e r
thermal efficiencies and better reliability through further
improvements, MHI intends to contribute toward preventing
global warming while improving the economic efficiency ofits engines.
3.2 Medium-speed MACH-30G gas engine
Large gas engines with an electricity generation output
of over 1,000 kW require a lean-burn system that can attaina high thermal efficiency and low NOx emissions. The

45

Lean-burn-Miller
cycle gas engine

40
Lean-burngas engine

Dual-fuelgas engine

35
30

Three-waycatalyst gas engine

25
100200500

1000 2000

Simple cycle
gas turbine
5000 10000

Output (kW)Fig. 4 Improvement in gas turbine efficiency
A power generation efficiency of 40% was achieved by applying theMiller cycle to a lean-burn gas engine, yielding an efficiency higher
than that of gas turbines.

MACH-30G engine has also achieved stable combustion ofa lean mixture by adopting a micro-pilot ignition using anelectronically controlled common-rail fuel injection system.
I n add it ion to t he i mproved elect r icit y generat ion
ef f icienc y, a tot a l ef f icienc y above 8 0 % was at t a ined,
including the recovery of high- and low-temperature water,thus successfully responding to customers’ needs.
W hile cont inuously work ing to improve elect r icit ygeneration ef f iciency, MHI aims to study the possible
application of this engine to power generation plants that use
various types of gas, such as biogas and waste product gas,
and to thus contribute toward preventing global warming.

4. Conclusions
This paper has described the technological developmentsof diesel and gas engines that can directly contribute tothe reduction of CO 2 emissions to prevent global warming.
Based on predictions of continued growth in the world’senergy requirements and economic development, more
intense and accelerated efforts toward the reduction of
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)23

CO2 emissions are essential. While making the best of themany years of technological knowledge accumulation as an
all-around manufacturer of engines, MHI will continue toprovide products that can respond to society’s requirements
by introducing advanced sophisticated technology. Aiming
at achieving harmony between the earth’s environment andan affluent society, the company will continuously strive todevelop engine technology that responds to customers’ trust.

References
1. Tsunoda, A. et al., Newly-Developed Diesel Engines for Generator
Set, Mitsubishi Juko Giho Vol.40 No.4 (2003)
2. Sakaguchi, K. et al., Environmental Friendly Diesel Engine "UEC
Eco-Engine", Mitsubishi Heav y Industries Technical Review
Vol.41 No.1 (2004)
3. Tsunoda, A. et al., MIT SUBISHI Lean-burn Gas Engine withWorld's Highest Thermal Efficiency, Mitsubishi Heavy Industries
Technical Review Vol.40 No.4 (2003)

Tatsuo Takaishi

Akira NumataKatsuhiko Sakaguchi Yoshinori Kobayashi

Ryouji Nakano

Hiromi Ishii

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)24