Mitsubishi Heavy Industries Technical Review Vol. 58 No. 2 (June 2021)
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Development of Turbocharger for
Next-generation Hybrid Electric Vehicles

MAKOTO OZAKI*1

TOYOTAKA YOSHIDA*2YUTAKA FUJITA*2

TAKAYA FUTAE*3

TAKASHI ARAI*4

YUKI YAMADA*2

In recent years, with the great efforts toward carbon neutrality as a measure against global
warming, it is predicted that the share of hybrid electric passenger cars will increase. The
operating range of internal combustion engines for hybrid electric vehicles is limited comparedwith conventional ones and turbochargers are also required to be developed to address thislimitation. Mitsubishi Heavy Industries Engine & Turbocharger, Ltd. (MHIET) has developed
high-efficiency compressors that meet the requirement of hybrid electric vehicles, vaned nozzle
turbines designed for higher efficiency, ball bearings designed to reduce mechanical loss andlow-cost turbine housing materials that are designed by using data science and has been
proposing to automobile manufacturers.

|1. Introduction
As a countermeasure against climate change caused by global warming in recent years,
efforts toward carbon neutrality have been accelerated in various countries around the world. In theautomobile industry, CO2 emissions regulations are planned to be tightened and in Europe, theregulations are expected to be tightened by 2030, in which the regulation value will be reduced by
37.5% compared with that in 2021. Due to this tightening of regulations, the introduction ofvehicles with lower CO2 emissions (plug-in hybrid electric vehicles (PHEV), battery electricvehicles (BEV), fuel cell electric vehicles (FCEV), etc.) is being promoted in various countries andsales of internal combustion engine vehicles will be prohibited in European countries byapproximately 2030 to 2040. On the other hand, reserves of raw materials (lithium, nickel andcobalt) for batteries installed on electric vehicles are limited and it is difficult with current battery
technology to realize a scenario of converting all vehicles to electric vehicles. There is also anotherproblem in that electric vehicles cannot be considered as zero CO2 emissions unless they are usedin areas where renewable energy power generation has become widespread. One possible realistic
scenario is to continue to produce automobiles equipped with a lower-fuel consumption internal
combustion engine with its operating range limited by using a hybrid system while promoting thespread of BEVs, like the proposal made in the "Energy Saving and New Energy Vehicle
Technology Roadmap 2.0" announced by the China Society of Automotive Engineers (SAE-China)
last year. Figure 1 shows the hybrid electric vehicle systems that are expected to become
widespread in major global markets. In Europe, the United States and China, parallel mild hybrid
electric vehicles with a 48 V power supply are expected to become the most popular. On the otherhand, in the Japanese market, due to the fact that hybrid electric vehicles have been mass-produced
ahead of other countries, it is expected that power split strong hybrid electric and series strong
hybrid electric vehicles will occupy a large share. In order to respond to this technological trend,*1

Chief Staff Manager, Engineering Department, Turbocharger Division, Mitsubishi Heavy Industries Engine & Turbocharger,*2

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

Ltd.
*3

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

*4

Chief Staff Researcher, Manufacturing Technology Research Department, Research & Innovation Center, Mitsubishi HeavyIndustries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 2 (June 2021)
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we are developing turbochargers which matches to the internal combustion engine installed on
hybrid electric vehicles. This report describes the development status.

Figure 1 Comparison of share of hybrid electric vehicle systems in major markets(according to FEV Consulting GmbH)|2. Changes in performance requirements for turbochargers due touse of hybrid system
Vehicle hybrid systems are roughly classified into a parallel hybrid system that uses both amotor and an engine to drive the vehicle and a series hybrid system that uses a motor only to drive
the vehicle and an engine dedicated only to electric power generation. (A power split system
distributes the engine output for driving and electric power generation and is classified as anintermediate between the parallel system and the series system. The turbocharger operating point of
the power split system is close to that of the series system, so its explanation is omitted in thisreport.)
The parallel hybrid system uses the engine as the main driving power source and the motor
as the sub driving power source to produce the driving force required by the vehicle, in order to set
the engine operating points within a high-efficiency range and improve the vehicle fuel efficiency.
For example, when the vehicle is running at a low speed, the engine is stopped and the vehicle isdriven only by the motor in order to avoid low-load engine operation, which is inefficient. Whenthe vehicle is quickly accelerating, the engine is assisted by the motor in order to use ahigh-efficiency middle-torque range and improve the vehicle fuel efficiency. Figure 2 illustrates,on a conventional downsized turbocharged engine torque diagram, the engine operating range thatis not used when the vehicle is driven only by the motor and the engine operating range that is not
used when the engine is assisted by the motor. Changes in the full load operating points of theturbocharger compressor in this case are shown in Figure 3. The compressor pressure ratio at low
flow rate operating points is reduced by the reduction of the maximum required torque at lowengine speeds and the maximum flow rate is also reduced by the suppression of the maximumoutput. Due to the reduced required torque at low engine speeds, it is possible to use alarge-capacity turbine that focuses on high engine speed and high output, so the improvement of
the turbine efficiency and fuel efficiency during high-speed running in which the engine is used as
the main power source is expected.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 2 (June 2021)
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Figure 2 Operating range of engine
installed on parallel hybrid electricvehicle

Figure 3 All-load operating points ofcompressor for hybrid system

On the other hand, the series hybrid electric vehicle uses the engine only for electric powergeneration, so the most fuel-efficient operating range can be used for producing the required output
without depending on the engine speed. Figure 4 shows, on a conventional downsized
turbocharged engine torque diagram, operating points of an engine installed on a series hybridelectric vehicle. For a conventional downsized turbocharged engine, plane operating range area isrequired. However, in the case of a series hybrid electric vehicle, its operation can be changed aline. Changes in the full load operating points of the turbocharger compressor in this case areshown in Figure 3. Since only operating points with good engine fuel efficiency are used, theoperating ranges of both the compressor and turbine are narrowed.
As described above, when a hybrid system is adopted, the operating ranges of the engine andturbocharger tend to be narrowed for the purpose of reducing fuel consumption. Due to thenarrower operating range, it is expected that the efficiency of the turbocharger can be improved.
The following chapters explain the efficiency improvement of components: the compressor, turbine
and bearing.

Figure 4 Operating range of engine installed on series
hybrid electric vehicle|3. Improvement of compressor performance
In the past, centrifugal compressors used for passenger car engines were required to havehigh performance in the medium and high specific speed ranges, so designs for reducing secondary
flow loss and separation loss by using three-dimensional impellers and optimization of bladeloading have been considered(1)(2). However, engines for hybrid electric vehicles, which have been

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rapidly developed in recent years, require higher efficiency of low specific speed centrifugal
compressors that do not have a wide operating range. Therefore, we worked on the development of
a high-efficiency centrifugal compressor that reduces friction loss and mixing loss, which are themain loss structures in the low specific speed range.
Figure 5 is an appearance of the new designed centrifugal compressor. With the assumed
operating points of the hybrid electric vehicle engine as the design points, the inlet design of thiscompressor was optimized to reduce leakage flow, shock waves and collision loss, while the outlet
was designed to reduce wake, mixing loss and downstream friction loss. Figure 6 gives theperformance test results. This figure shows that the new designed centrifugal compressor has
achieved higher efficiency.

Figure 5 Appearance of high-efficiencycentrifugal compressor

Figure 6

Performance test results of newly designed compressor

Figure 7 plots the performance test results of our centrifugal compressor on a chart with thespecific speed on the horizontal axis and the maximum efficiency on the vertical axis. This figure
shows that the new designed compressor has achieved significantly higher efficiency in the low
specific speed range. Based on this compressor, we will try to improve the efficiency by applying avaned diffuser with stator blades therein and optimizing the blade shape in the future.

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Figure 7 Specific speed and maximum efficiency|4. Improvement of turbine performance
Today’s turbochargers for passenger cars have used vaneless nozzle turbines with no stator
blades in the flow path for guiding exhaust gas to the rotor blade and variable vaned nozzle turbinesthat can adjust the turbine flow rate with variable stator blades, in order to adopt the wide operating
range. On the other hand, engines installed on hybrid electric vehicles, especially series hybrid
electric vehicles, are required to have higher performance while its operating range is significantlylimited. In order to manage such a requirement, a vaned nozzle turbine with stator blades in theflow path for guiding exhaust gas to the rotor blade, which are used in power generation diesel
engines, is optimal.
Figure 8 presents an appearance of the vaned nozzle turbine designed for hybrid electricvehicles. One of the remarkable of this turbine is that there are stator blades between volute
passage (scroll) and the rotor blade for turning the exhaust gas flow. The stator blades reduce theflow deviation in the scroll circumferential direction and form the optimum flow for the rotor bladein order to improve the efficiency. At the same time, the expansion of exhaust gas is performed
mainly on the vaned nozzle to reduce the friction loss in the scroll. In addition, since the stator
blades of this turbine are not moved, it is possible to set the clearance at the tip of the stator blade,which is adopted for variable vanes of variable geometry turbine to avoid vane stick, to zero andthereby improve the efficiency.

Figure 8Appearance of vaned nozzle turbineMitsubishi Heavy Industries Technical Review Vol. 58 No. 2 (June 2021)
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Figure 9 shows the improvement in the aerodynamic efficiency of the vaned nozzle turbine
compared with that of a vaneless nozzle turbine. Adoption of the vaned nozzle turbine enables tohave the better turbine efficiency by about 10%. One of the other advantages is the design
flexibility with which the efficiency can be further improved by customizing the nozzle vane
opening, shape and blade specifications according to the targeted required operating point.

Figure 9 Improvement in aerodynamic efficiency due toadoption of vaned nozzle turbine(Comparison of pure aerodynamic efficiency)|5. Improvement of bearing system
Since a hybrid electric vehicle does not use the low load range of the engine, in which thefuel efficiency is poor, the engine operates a rapid load increase when restarted after idling stop.
According to this operation, the turbocharger rotation speed also rising rapidly, causing a delay inlubricating the turbocharger. As a countermeasure to this, we considered the adoption of ballbearings that can be used in no oil supply conditions. Figure 10 shows the cross-sectional view ofit.

Figure 10

Cross-sectional view of ball bearing

Ball bearings have the advantage of low mechanical loss compared with the floating bearings
used in conventional turbochargers, but have a risk of reduction in the vibration damping, increasein the shaft vibration and worsening the noise level. Therefore, we designed and applied a squeeze
film damper between ball bearing and bearing housing in order to improve vibration damping.
Figure 11 shows the mechanical loss measurement result and the turbocharger efficiency test
result. It shows that the adoption of the ball bearing reduced mechanical loss by about 50% (at 120krpm) and the turbocharger efficiency is improved by about 2% over the entire operating range.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 2 (June 2021)
7Figure 12 shows the turbocharger housing acceleration measured as an alternative value for shaft
vibration and turbocharger noise. In this figure, it shows that the turbocharger can operate up to therated rotation speed without a sudden increase in the acceleration along with an increase of theturbocharger rotation speed. It can also be seen from this figure that there is almost no difference
between acceleration data before and after the 250-hour durability test.
We validated that the ball bearing has less mechanical loss than conventional floating bearing
and has no problem in terms of the vibration characteristics. In the future, we will expand themodel variations based on these specifications.

Figure 11 Comparison of mechanical loss and overall turbocharger efficiency between
plain bearing and ball bearing

Figure 12 Comparison of turbocharger vibration acceleration
before and after durability test

|6. Development of materials
Hybrid electric vehicles have additional parts like a motor, inverter, battery, etc. comparedwith conventional passenger cars, and it have high production costs. Therefore, the price of theengine including the turbocharger needs to be reduced and it is indispensable to reduce the cost of
the turbine housing, which function is turning the exhaust gas flow by scroll and is the costliest
parts among the turbocharger components. We use austenitic stainless heat-resistant cast steel
providing DIN standard* as our standard material for the turbine housing of turbochargers forgasoline engines (Figure 13). Due to the countermeasure of high temperature exhaust gas
requirement from the customer, DIN materials (1.4848 and 1.4849) containing a large amount ofexpensive nickel have to be used, resulting in high cost.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 2 (June 2021)
8Figure 13 Heat resistant temperature and Ni content of austenitic
stainless cast steel conforming to DIN standard

In collaboration with Alloyed, which conducts heat-resistant alloy materials design business,etc., we have developed a low-cost turbine housing material with material properties equivalent to
the 1.4848 material using Alloys-by-Design, ABD®, a computational material design technology
owned by Alloyed. Figure 14 shows the alloy design procedure for the low-cost turbine housingmaterial. This method can estimate material properties from the combination of the relationships ofthe chemical composition, material composition and material properties with databases andcomputational engineering, facilitating the design of alloys in a shorter period of time and at alower cost compared with conventional trial-and-error or experimental approaches. After
estimating the performance of all alloys that can be combined within a wide range of chemicalcomposition of austenitic stainless steel, we selected chemical components that are cheaper than
1.4848 material and have the same material properties thereof (high-temperature strength, structural
stability and oxidation resistance). Figure 15 is an example of the prototype material test results ofthe two designed candidate alloy materials. It was confirmed that the materials have the same or
higher high-temperature strength than the 1.4848 material and that the material cost can be reduced
by about 10 to 30%. In the future, we will evaluate the castability and machinability of thecandidate material into the turbine housing shape and also conduct durability evaluation according
to the operating conditions of the engine in order to apply the material to series production.
* DIN standard: National standard of the Federal Republic of Germany established by Deutsches Institut
für Normung

Figure 14

Alloy design procedure for low-cost turbine housingMitsubishi Heavy Industries Technical Review Vol. 58 No. 2 (June 2021)
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Material test result of prototype|7. Conclusion
This report provided an overview of changes of the requirement for turbocharger due to theadoption of a hybrid system and described the newly developed high-efficiency compressor, vaned
nozzle turbine, low-mechanical-loss and low-noise ball bearings, as well as low-cost turbine
housing materials as technologies to be applied to turbochargers for next-generation hybrid electricvehicles. In recent years, attention to electric vehicles that do not have an internal combustion
engine has been increasing. However, there are still many technical issues to be solved and webelieve that internal combustion engines will continue to play an important role in the future.
Turbochargers can greatly contribute to reducing the fuel consumption of such internal combustion
engines. In the future, we will continue our efforts to improve the global environment by striving tospread the technologies described in this report in cooperation with automobile manufacturers.

References(1)(2)

Takao Yokoyama, et al., Advanced Aerodynamic Design Technologies for High Performance
Turbochargers, Mitsubishi Heavy Industries Technical Review Vol.54 No.1 (2017) p79-87
Yosuke Danmoto, Et al., Development of VG Turbocharger for Next-Generation Gasoline Engines,Mitsubishi Heavy Industries Technical Review Vol.56 No.2 (2019) p1-8