Development of Compactand High-performance
Tu r b o c h a rg e r f o r 1 , 0 5 0 ° CExhaust Gas
KOJI MATSUMOTO*1

MASAKI TOJO*1YASUAKI JINNAI*1

NORIYUKI HAYASHI*1
SEIICHI IBARAKI*2

The control of global exhaust gas emissions and the legislation concerning fuel consumption have promoted the reduction of thesize of engines for passenger automobiles, and as a result, the engine output per unit cylinder displacement is increasing. As theturbocharger is an effective way of making high-output engines smaller, the demand for turbochargers is increasing, and technologicalimprovements are required. The exhaust gas temperature of gasoline engines is rising, giving better combustion efﬁciency, and thisrequires the timely development of compatible turbochargers. In the face of this global demand, Mitsubishi Heavy Industries, Ltd.
(MHI) has developed, and has started delivering to customers, compact and high-performance turbochargers capable of operating at
1,050°C, the world’s highest level of exhaust gas temperature.

With the demand for gasoline engine turbochargers isexpected to increase, and engine operating conditions are
expected to become more severe (e.g., increased exhaustgas temperatures), a turbocharger capable of dealing withhigh exhaust temperatures is urgently required.

1. Introduction

2. Features of turbochargers for gasoline engines
The exhaust gas temperature of a gasoline engine is
generally about 200°C higher than that of a diesel engine,
and thus requires a more sophist icated t urbocharger
that can withstand the higher temperature. Exhaust gas
temperatures are further increasing to cope with recent
em issions rest r ict ions a nd t he dema nd for sma ll but
powerful engines. The requirement to handle exhaust gas
temperatures as high as 1,050°C has already been identiﬁed(see Fig. 2).

1100: Fuel consumption
legislation in
the State of California: Japanese fuelconsumption in 2015
: Fuel consumption
legislation in
the European Union

210
201 /km200
190
180
169.8 /km
170
160
161 /km150
137.5 /km
140130
127 /km120
120/km
110
100
2000
2005
2010
2015
2020
YearExhaust temperature (°C)

CO2 emission ( /km)

To cope with present environmental restrictions, small
displacement engines equipped with a turbocharger areincreasingly used in place of naturally aspirated large displacement engines, while maintaining a similar level ofengine output. This so-called “downsizing” trend helps toincrease the number of turbochargers required. In the past,turbochargers for gasoline engines were only used on a few
models of high-performance sports cars produced in small
quantities for a limited number of customers. However, therecent wave of downsizing has prompted turbochargers to beinstalled on an increasing number of gasoline engines.
The sharp increases in the price of gasoline and the strict
control of CO2 emissions (see Fig. 1) are the reasons behindthe strong drive for better fuel consumption.11050
1000
950
9008501995

20002005

2010

YearFig. 2 Transition of gasoline engine exhaust gas temperature
for MHI’s customers

Fig. 1 Transition of CO2 emission levels regulated by areas*1 General Machinery & Special Vehicle Headquarters*2 Nagasaki Research & Development Center, Technical Headquarters

1

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 3 (Sep. 2008)

In a f uel-injected engine where f uel is injected intothe intake port, it is general practice to set the engine
compression ratio lower than that of a naturally aspirated
engine in order to avoid knocking. However, due to thecooling effect of the vaporization heat of the fuel spraythat suppresses knocking, the direct-injection engine canmaintain a compression ratio comparable to that of thenaturally aspirated engine. However, this raises the turbine
inlet pressure and increases the load on the turbocharger. 2
While the demand for gasoline engine turbochargers isincreasing, the functional requirements such as high exhaustgas temperature and pressure require urgent developmentof a durable turbocharger. The following functions must beevaluated to verify the durability requirements.
• Strength of the turbine wheel• Strength of the turbine housing• Optimization of the wastegate valve mechanism
The next section discusses these factors in detail.

subjected it to a high-temperature tensile test and a creep
rupture test to supplement the material characteristics data
before conducting turbine wheel stress analysis (see Fig. 3)
and creep analysis. Since estimation of the actual turbine
blade temperature is critical to the creep life evaluation,
an infrared thermometer was used to measure the blade
temperatures of the rotating turbine wheel. An over-speed
burst test and a creep rupture test were conducted on an
actual turbine wheel to verif y its reliabilit y. Figure 4
shows photographs of the turbine wheel after the creep
rupture test at an exhaust gas temperature of 1,050°C andthe highest rotational speed. The figure shows a creep
rupture starting at the root of the blade at the trailing edgeof the turbine, which is a high stress location determined byprevious analyses. Furthermore, comparison test with theconventional Inconel turbine wheel showed that the creep
life of the MarM turbine wheel is about three times as long.

3. Technologies to cope with high exhaust
temperaturea
3.1 Turbine wheel
Inconel is a standard material for turbine wheels. This
nickel-based heat-resistant alloy, however, does not meet
MHI’s criteria for anti-creep characteristics at 1,050°C,
M H I has decided, t herefore, to use Ma rM, a si m i la rnickel-based heat-resistant alloy with greater strength
at high temperat ure. W hile MHI has used MarM forthe turbochargers in competition vehicles such as rallycars, this is the first time that MHI has used MarM in
mass-produced vehicles. Since MarM is more difﬁcult tocast and more likely to have minute casting defects thanInconel, MHI uses hot isostatic pressing (HIP) on MarM
to remove casting defects and to homogenize the materialstructure. HIP is a metal treatment technology that uses
an iner t gas such as argon as a pressur i zing medium
and utilizes the synergistic effect of a pressure normallygreater than 10 0 MPa and a temperat ure higher than
1,000°C.
Before using MarM in mass-produced vehicles, MHI

Enlarged view of ruptured portion
Fig. 4 Turbine wheel creep rupture test at an exhaust gas
temperature of 1,050°C and maximum rotational speed

3.2 Turbine housing
MHI has an array of turbine housing materials to meetcustomer exhaust gas temperature requirements, including
ductile cast iron with an allowable temperature of 700°C
and austenitic stainless cast steel that resists temperatures
exceeding 1,000°C (see Fig. 5). Many turbine housings

Allowable temperature (°C)

1 200
Mises stress
HighLow

Enlarged view oftrailing edgeof turbine blade root
Fig. 3 Turbine wheel stress analysis (Mises stress)

1 000
800700

750

780

HiSi
DUC

SiMo
DUC

930

950

D5Ferritic
heatresistantcast steel

1 000

1100

600
400
2000

DUC

Austenitic
stainlessheatresistantcast steel

Fig. 5 Turbine housing materials and allowable temperatures

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 3 (Sep. 2008)23.3 Wastegate valve
The wastegate valve bypasses exhaust gas to control
the air pressure entering the engine. Since the wastegate
valve must operate without lubrication while being exposed
to an exhaust gas of higher temperature, its resistance toseizure is particularly important.
MHI’s development included a high-temperature wear
test of the wastegate valve material on the test bench
shown in Fig. 8 to verify its anti-seizure characteristics.
The results suggested a combination of materials with antiseizure characteristics that exceed those of conventional
materials (see Fig. 9).
In addit ion, a st r uct ural analysis was conducted tooptimize the wastegate valve mechanism to reduce theof recent gasoline engine t urbocha rgers a re made ofaustenitic stainless cast steel. Optimization of turbine
housing profiles is as important as material selection inensuring the strength at elevated temperatures. T heengine operating conditions expose the turbine housingsto repeated heating and cooling, making them subject
to possible cracking through thermal fatigue. To avoid
t her ma l cracks, MH I conducted a n ex haust gas f low
analysis (see Fig. 6), a heat transfer analysis between
the exhaust gas and the turbine housing, and a turbine
housi ng t her ma l st ress a na lysis, a l l under t ra nsient
op e r a t i n g c ond it ions , c or re s p ond i n g t o t he e n g i ne
operating conditions, to locate the high-stress regions (seeFig. 7). In addition to using austenitic stainless cast steel,eliminating the structural discontinuity from the turbine
housing and the use of a more uniform wall thickness
helps to prevent the generation of cracks.

Flow velocityHighLow

Exhaust manifold

Turbine housing

Fig. 8 High-temperature wear
test bench

Fig. 6 Results of ﬂow analysis (ﬂow velocity distribution)

Thermal stress
HighLowSeizure

Mild scratch(a) Combination ofconventional materials(b) Combination ofmaterials usedin actual productFig. 9 State of sliding surfaces after high-temperature
wear tests

Fig. 7 Example of thermal stress analysis of turbine housing

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 3 (Sep. 2008)

3Valve

Shaft
JointBushLever

Fig. 10 Wastegate valve structuralanalysis modelFig. 12 Turbocharger with unitized exhaust manifold(unitized casting)wear. Figure 10 shows the structural analysis model.
Using the dimensions of each member of the mechanism
as parameters, the simulation took place while applying
variable loads equivalent to the exhaust pulsation to evaluate
the severity (slide amount and load) of the sliding section.
Figure 11 shows an example of the calculated results. This
analysis provided MHI with dimensions that reduced theseverity of the wear, and this design was incorporated into
the end products.

4. More compact and higher performance

MorewearLesswearSeverity ofwear generation

To design a simple engine compartment layout, a compact
turbocharger design is required. The demand for compact
turbochargers is increasing because of the requirement toshorten the catalytic activation time at engine startup andto reduce the thermal capacity of the exhaust gas passage
that reaches the catalyst. Since the catalyst must be hot
to be active, it is necessary to reduce the amount of heatabsorbed by components on the exhaust gas passage suchas the exhaust manifold and turbine housing. To meetthis requirement, MHI has been delivering turbochargers
in which the exhaust manifold and turbine housing are
combined in a single cast part (see Fig. 12). To make theturbocharger even more compact and lightweight, MHI has
developed a production method, jointly with a customer,
whereby the exhaust manifold is press-formed from a sheetof stainless steel and welded to the cast turbine housing (seeFig. 13 Turbocharger with unitized exhaust manifold(weld-fabricated structure)

Fig. 13). The exhaust manifold is double-walled, and thisgreatly reduces its thermal capacity.

5. Conclusion
To meet the requirements of the increasingly stringent
legislation on exhaust emission and f uel consumption,
MHI has concentrated its ef for ts on high-temperature
exhaust gas turbochargers for gasoline engines. MHI has

Dimensions for reducing wear

Small
MediumLarge

Small
MediumLarge

Small
MediumLarge

Small
MediumLarge

Dimension A

Dimension B

Dimension C

Dimension DFig. 11 Example of calculation of wear severity by structural analysis

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 3 (Sep. 2008)

4successfully developed, and has started to deliver, compact
high-performance turbochargers capable of dealing withan exhaust gas temperature of 1,050°C, the highest in theworld. By conducting further thermal stress analysis andstructural analysis of sliding mechanisms to evaluate design
before prototyping, MHI is meeting exacting customer needs
and reducing development time to deliver better automobile
environmental performance and driving pleasure.

Koji Matsumoto

Masaki Tojo

Noriyuki Hayashi

Seiichi Ibaraki

Yasuaki Jinnai

References
1. Yoshiro Tsur uha ra, Fuel E conomy Reg ulat ions in Europe,
Automotive Technology, Vol.1 (2008) p.122
2. L ut ter mann, C. et al., BM W High P recision Fuel Inject ion
Conjunction with Twin-Turbo Technology: a Combination for
Ma x i mum D y na m ic a nd H igh Fuel Ef f icienc y, S A E Paper
2007-01-1560

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 3 (Sep. 2008)5