Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)60

Development of Automotive Turbochargers
Contribute to Environmental Load Reduction

MAKOTO OZAKI*1

MOTOKI EBISU*1

YOSUKE DANMOTO*1

YOUJI AKIYAMA*2

TAKAO YOKOYAMA*3TADASHI KANZAKA*4

In recent years, further performance enhancement of supercharging systems has become
necessary in response to environmental regulations getting stricter. Major countries plan to makeregulations on CO2 emissions from automobile engines even stricter. In the sector ofturbochargers, development that can contribute to improvement in the thermal efficiency of engines
is required. We have developed the fundamental elements of a turbocharger including a turbinewith enhanced performance in the low-speed ratio region, a compressor with enhanced efficiency
in the vicinity of surging, and a bearing, for which mechanical loss was reduced by optimizing theload, and supercharging systems including a VG turbocharger for gasoline engines and an electriccompressor. These developed elements and systems are supplied to automobile manufacturers assamples.

|1. Introduction
In recent years, environmental regulations have been getting stricter as a measure against
global warming. In Europe, CO2 emissions from automobile engines will be regulated to 95 g/km
in 2021, and therefore, automobile manufacturers all over the world are working on developing
technologies to deal with the regulation. Supercharging systems represented by turbochargers
contribute to improving the fuel efficiency of automobiles, and are effective for reducing CO2emissions. Because the supercharging systems have good compatibility to direct injection, they
have been increasing their share as direct injection supercharged gasoline engines. In fact, amongautomobile gasoline engines for automobiles newly released in 2015, the proportion of direct
injection supercharged engines increased even in Japan, where hybrid vehicle sales are strong. Inthe North American market, a clear tendency of shifting to small displacement engines and anincrease in the proportion of direct injection supercharged engines has been seen. In addition, over70% of engines installed in new vehicles in the European market, which is the home of direct
injection supercharged gasoline engines, are direct injection supercharged engines.1
Automobile manufacturers have been developing various technologies for improving the fuelefficiency of direct injection supercharged gasoline engines. The past trend was improvement in thefuel efficiency mainly by downsizing and downspeeding of engines. In recent years, efforts tofurther improve fuel efficiency have also been progressing through adoption of cooled EGR(exhaust gas recirculation), mirror cycle using variable valves, lean combustion, HCCI
(homogeneous charge compression ignition), and so forth. Keeping in step with such development
trends for engine technologies, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd. (MHIET)
is developing systems including variable capacity turbochargers and electric compressors, inaddition to component technologies for turbines, compressors, bearings, and so forth, to deal withthe strict requirements for superchargers. This paper presents the state of our ongoing developmentof turbochargers for automobiles.
*1
*2
*3
*4

Chief Staff Manager, Turbocharger Division, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.
Turbocharger Division, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.
Chief Staff Manager, Fluid Dynamics Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.
Fluid Dynamics Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)61|2. Development of turbines
Downspeeded supercharged engines are required to enhance the torque in thelow-engine-speed region. For performance enhancement of such engines, a different design fromordinary radial turbines is necessary also in terms of a turbocharger because the effects of theexhaust pulsation are significant when the engine rotation speed is low. One of the dimensionlessparameters that represent a turbine efficiency characteristic is a speed ratio (U/C0 where U is thecircumferential speed of the turbine and C0 is the theoretical isentropic speed). Many ordinary
radial turbines have an efficiency peak at a point around U/C0=0.65. When the engine rotation
speed is low, performance at an operating point with a low U/C0 is important because of the effects
of exhaust pulsation. Figure 1 shows the rate of the turbine output at various speed ratios to thewhole turbine output in one cycle of a 1.6-liter direct injection gasoline engine under full loadconditions. When the engine rotation speed is high, the highest turbine output is obtained at a pointaround U/C0=0.55. When the engine rotation speed is low, however, the highest turbine output isobtained at a point around U/C0=0.4. Therefore, it can be confirmed that the performance in thelow U/C0 region is important.

Figure 1 Rate of turbine output at various speed ratios
to whole turbine output in one cycle(1.6-liter direct injection gasoline engine: Result of one-dimensional
engine simulation)

MHIET changed the turbine blade design from the conventional radial one to a mixed flow
one to enhance efficiency in the low-speed-ratio region, and obtained good results. Figures 2 and 3
show the external view of the new mixed flow turbine and its efficiency characteristics with respect
to the speed ratio U/C0.
In addition, the new mixed flow turbine described above was installed on a 1.6-liter direct
injection gasoline engine and then its torque characteristics were measured. The obtained results
were good and indicated an enhancement of the torque in the low engine rotation speed region.
Figure 4 shows these results.

Figure 2 External view of newmixed flow turbine

Figure 3 Characteristic with respect to speed
ratio of new mixed flow turbine

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)
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Figure 4 Verification result of torque characteristic ofengine equipped with new mixed flow turbine|3. Development of compressors
For downsized supercharged engines that generate the maximum torque in the low engine
rotation speed region, a different design from ordinary radial compressors is required. In the designof ordinary compressors, the design point is often set at the efficiency peak point to enhance
performance. In the design of recent supercharged engines, however, the performance at theoff-design point is important. In particular, efficiency at the operation point at a low engine rotation
speed, which is in the vicinity of surging, is important. MHIET made the design in the light of theabove-described background, and obtained good results. Figure 5 shows the CFD result. For thenew blade, the flow of the rotor blade outlet at the operation point near the surging point was madeto be uniform to enhance efficiency.
Figures 6 and 7 show the external view and the performance test results of the new blade.
The low-pressure ratio side surge flow amount and choke flow amount remain at the same level,
but the efficiency at the same flow amount point in the vicinity of the surging is enhanced.

Figure 5 Comparison of flow conditions of rotor blade outlet
at operation point near surging point (90% blade height,
circumferential speed of 270 m/s)

Figure 6 External view of newcompressor

Figure 7 Performance test result of newcompressorMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)
63

Figure 8 shows the one-dimensional simulation results of a 1.6-liter direct injection gasoline
engine to which the new blade was applied. It was confirmed that the new blade has an effect thatenhances the torque at a low engine rotation speed.

Figure 8 Result of one-dimensional engine simulation|4. Improvement of bearing performance
During transient response from a low engine rotation speed, the exhaust gas flow amount issmall and therefore the amount of energy that can be recovered by the turbine is small. The bearing
loss increases roughly in proportion to the square of the rotation speed. Therefore, the bearing loss
has a large effect in a low-speed activation where the energy recovered by the turbine is small. We
have also been improving continuously the bearing performance. Figure 9 shows the main
improvement items. After performing a review of various design parameters, optimization of thebearing load capacity, and enhancement of the oil drainability, the bearing loss was measured. The
obtained results confirmed that the loss was reduced by 20% in comparison to the conventional
bearing as shown in Figure 10.

Figure 9

Improvement items of bearing system

Figure 10 Measurement result of bearing loss

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)64

In addition, one-dimensional simulation of the engine was executed to verify that theimproved bearing described above influences the transient responsiveness of the engine. Figure 11shows the simulation results. Thus, it was confirmed that the time necessary to reach 90% torquewas reduced by 8% in the range between idling at 1,500 rpm to WOT (whole open throttle).

Figure 11 Analysis result of one-dimensional engine
simulation (1.6-liter direct injection gasoline
engine, 1,500 rpm)

|5. Development of VG turbochargers
VG (variable geometry) turbochargers, which have stator vanes having a variable mechanism
located at the turbine rotor blade inlet nozzle part, can maintain high supercharging efficiency fromthe low engine rotation speed to the high engine rotation speed in comparison to W/G (waste gate)
turbochargers. Currently, the VG turbocharger is used in combination with a mirror cycle direct
injection gasoline engine in some cases, and is an effective method to improve the fuel efficiencyof engines. However, the VG turbochargers have a complicated variable mechanism, and therefore,a VG turbocharger for gasoline engines, the exhaust gas temperature of which is high, is very
expensive because the components need to be made of materials with high heat resistance. In
addition, because of knocking generated by the high supercharging pressure, the number ofgasoline engines that adopted the VG turbocharger was limited. As the knocking limit is improved
due to direct injection, cooled EGR, and so forth, however, the environment that allows adoption of
the VG turbocharger has been established. Therefore, we developed VG turbochargers adaptable togasoline engines. Figure 12 shows the external view of our VG turbocharger for gasoline engines.
Our VG turbocharger for gasoline engines was developed on the premise that expensiveheat-resistant alloy was not used in consideration of economic efficiency. In the design of the VG
turbocharger, the shape and materials were optimized by analysis and the functions of the variablemechanism could be satisfied even at the exhaust gas temperature of 950°C.2 Figure 13 shows theexample analysis for thermal deformation of a nozzle assembly executed in the development phase.
Based on the result of this analysis, the thermal fatigue strength and various clearances wereoptimized.
The developed turbocharger was durability-tested in the MHIET test specifications and it
was confirmed that the durability is enough for use for actual equipment.

Figure 12 VG turbocharger for gasoline
engineFigure 13 Example of thermal
deformation analysis of nozzle assemblyMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)
65

Figure 14 shows the calculation result of engine one-dimensional simulation that wasperformed to confirm an engine thermal efficiency improvement effect of a VG turbocharger
applied to a gasoline engine(2). Based on this result, it was confirmed that the VG turbocharger
installed on an engine equipped with cooled EGR could enhance the EGR rate under a full loadcondition in comparison to a W/G turbocharger. The result indicates that a VG turbocharger canenhance thermal efficiency, and it is expected that the VG turbocharger is effective for improving
the fuel efficiency of gasoline engines.

Figure 14 Comparison of thermal efficiency between
W/G turbocharger and VG turbocharger|6. Development of electric compressors
Above is described the improvement of the supercharging efficiency of turbochargers. We
have been developing an electric compressor to improve the fuel efficiency of vehicles with further
higher supercharging pressure and transient responsiveness. In addition, we developed asupercharging system having an electric compressor instead of the high-pressure or low-pressure
side turbocharger of an ordinary two-stage turbocharger.3 Figure 15 shows an example of theelectric two-stage turbocharger system. The advantages of the developed system over an ordinary
two-stage turbocharger are enhancement of the transient responsiveness, reduction of pumping loss
caused by lowering the engine exhaust pressure, shortening of the catalyst activation time period
after starting the engine through reduction of the exhaust side heat mass, and so forth.
For verification of fuel-efficiency-improving effects of the developed system, comparison toa 1.4-liter single turbocharged gasoline engine was estimated. As a result of this estimation, it wasverified that the fuel efficiency could be improved by 8% even with the engine displacement
reduced approximately 20% while maintaining an equal or higher steady engine torque. Also,transient performance was improved and the torque rising time period could be reduced byapproximately 50% as shown in Figure 16. Thus it can be said that the electric compressor is aneffective method to improve fuel efficiency.

Figure 15 System diagram of electric
two-stage turbocharger system

Figure 16 Comparison of transient
response timeMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)66

For electric supercharging, energy management is important. We believe that the hurdle ofenergy management to use an electric compressor will be lowered in the future due to performanceenhancement of on-vehicle power sources, increase of the voltage, and enhancement of the energy
recovery efficiency. In addition, it is also effective to combine the electric compressor withmethods to improve the fuel efficiency such as cooled EGR, mirror cycle, and lean combustion.

|7. Conclusion
We developed the fundamental elements of a turbocharger the meets the needs for reducing
environmental load including a turbine having enhanced performance in the low speed ratio region,
a compressor having enhanced efficiency in the vicinity of surging, and a bearing, for whichmechanical loss was reduced by optimizing the load, and supercharging systems including a VG
turbocharger for gasoline engines and an electric compressor. These are turbochargers that matchthe conventional engine development trend including downsizing and downspeeding. By
combining with the mirror cycle and the cooled EGR, the developed technologies can also deal
with rightsizing engines, aiming to improve the thermal efficiency of the engine, and are expected
to be developed actively in the future. We believe that the developed technologies will contribute toenvironmental protection. We are willing to disseminate the technologies described in this paper incooperation with automobile manufacturers to enable such technologies to be commercialized andused throughout the world.

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