Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
71

Development ofTwo-stage Electric Turbocharging system
for Automobiles

BYEONGIL AN*1

NAOMICHI SHIBATA*2

HIROSHI SUZUKI*3

MOTOKI EBISU*1

Engine downsizing using supercharging is progressing to cope with tightening global
environmental regulations. In addition, further improvement in fuel consumption is expected withsuch applications as ultra-high EGR, Miller cycle, and lean combustion. Mitsubishi HeavyIndustries, Ltd. (MHI) has developed a two-stage electric turbocharging system to balance better
drivability and improved fuel consumption by increasing the turbocharging pressure andimproving the transient response.

|1. Introduction
Engine downsizing/downspeeding through supercharging is progressing to cope withannually enhanced improvement in fuel consumption and exhaust gas. Downsizing through direct
injection and supercharging has been developed mainly in European countries where the CO2
regulations are the most stringent, and it has expedited the increase of the turbocharger installation
rate in other areas. Diesel vehicles are supposed to satisfy the CO2 and exhaust gas regulation
standards in 2021. However, gasoline vehicles are still not able to meet the standards even in thecase of low-fuel consumption vehicles with supercharged downsizing, and further measures arerequired. The adoption of WLTC (Worldwide harmonized Light duty driving Test Cycle) isplanned globally in and after 2017, and new regulations taking actual driving conditions into
consideration are being discussed. Turbochargers are required to provide a further boost pressure
and better response, as well as robust and easy to operate characteristics, for this purpose.
Existing turbochargers have a time-lag and EGR response delay, and proper control isdifficult. In addition to existing exhaust gas turbochargers, a turbocharger with electric power assist
utilizing the high-speed response of an electric motor, which improves both the fuel consumption
by significant downsizing and the transient response, is recently expected to be put into practicaluse.1
The electric compressor reported in this paper was developed by combining an inverter and ahigh-efficiency motor, which has a conventional 12V electrical system, a maximum power of 3kW,
and a maximum speed of 90000 rpm. In addition, evaluation testing was conducted as a two-stage
turbocharging system by combining it with an existing exhaust gas turbocharger. The turbocharger
satisfied the specifications required by the engine, and sufficient durability was verified. The
technologies being developed for mass production are also introduced in this report.

|2. Characteristics of Electric Compressor
For the development of the electric compressor, the concepts below were considered for
applicability to existing vehicle systems:
・The electrical system used is the conventional DC12V.
・The compressor, motor and inverter are integrated in one body.
*1

Chief Staff Manager, Turbocharger Engineering Department, Automotive Parts Division, Machinery, Equipment & Infrastructure*2

Turbocharger Engineering Department, Automotive Parts Division, Machinery, Equipment & Infrastructure*3Manager, Turbocharger Engineering Department, Automotive Parts Division, Machinery, Equipment & InfrastructureMitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)72

・A high-speed motor which allows operation at a maximum of 90000 rpm is to be
developed.
・The high-speed motor generates low noise, and can start from the stopped condition.
・A separately placed fan for cooling or a cooling system is not used.
The main targets of the electric compressor developed in this study are the improvement ofengine low-speed performance and turbocharging response by eliminating the turbo lag by
combining the electric compressor with an existing exhaust gas turbocharger. A photograph and thespecifications are shown in Figure 1. The electric compressor consists of the compressor part, themotor and inverter part, and the internal cartridge assembly part. The ball bearings are greaselubricated, and they support the compressor wheel and rotor of the high-speed motor. Cooling finsare provided for air cooling to control the temperature increase caused by the rise in thetemperature of the motor/inverter and bearings. The electric compressor specifications areactivation by a conventional 12V power source system, a maximum of motor output of 3kW, amaximum speed of 90000 rpm, and an acceleration time to maximum speed of 0.6 second.2

Motor max. output
Max. current
Max. rotational speed
Response
Pressure ratio
Flow rate

Figure 1

3.0kW
250Arms90000rpm< 0.6 s1.6
< 0.1 kg/s

Photograph and specifications of electric compressor|3. Development of Two-stage Electric Turbocharging System
Improvement in fuel consumption through the supercharged downsizing of gasoline engines
is progressing mainly in Europe. The improvement of combustion efficiency is necessary forfurther improvement in fuel consumption, and a higher supercharging pressure is required for thispurpose. However, the higher supercharging pressure is subject to the constraint of the knocking
limit and the reduction of exhaust gas energy. As such, a supercharging efficiency higher than thecurrent value is required. In the development of the two-stage electric turbocharging system, aninvestigation was conducted for comparison with an existing two-stage turbocharging system, andits validity as a supercharging system of future gasoline engines was evaluated. Figure 2 shows acomparison of two-stage turbocharging systems. The two-stage electric turbocharging system
combining the electric compressor and an existing exhaust gas turbocharger is the mostadvantageous from the perspectives of low speed performance, the improvement of transient
response, and the flexibility of installation.

3.1

Structure of Two-stage Electric Turbocharging SystemFigure 3 shows a schematic view of the two-stage electric turbocharging system. This
two-stage electric turbocharging system consists of an electric compressor, a waste gate
turbocharger, a bypass valve and other components. The electric compressor developed in thisstudy offers low noise, high efficiency and high-speed applicability through the use of a magnet
motor compared with the SR (switched reluctance) motors of other companies.3 In addition, thesystem is advantageous in terms of fuel consumption without electric current loss, as thecompressor can start from the stopped condition during transient operation. Table 1 shows acomparison of the advantages and disadvantages with the arrangement of the electric compressor.
The advantages and disadvantages are shown depending on the electric compressor positions(layout 1 and 2), and the appropriate piping is available according to the requirements. MHI
recommends layout 1, where the electric compressor is installed in the low pressure stage from theperspective of continuous operating hours and the mounting flexibility of the compressor.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)73
Two-stageturbocharger(Turbocharger+ Turbocharger)

Steady-state
performance
Transientperformance
Fuel consumption
Back pressure
Mountability
Thermal loadWeightCost
Remarks

Figure 2

Mechanical supercharger+ Turbocharger

Electric compressor+ Turbocharger

0000++++

000000+++--

・Effect on catalyst
temperature whenengine cold starting++++0・Limit of operating
・Modification oftime
clutch and engine are・Flexibility ofrequired
arrangement

0： Same level as
the two-stage
turbocharger＋：Better than thetwo-stage
turbocharger－：Worse than thetwo-stageturbocharger

Comparison of two-stage turbocharging system

Figure 3 Schematic view of two-stage electricturbocharging system
Table 1 Comparison of advantages and disadvantages with arrangement of electric compressor
Layout 1

Layout 2

Layout 3

Layout3.2

Advantages

・Improvement of mounting
flexibility on vehicle
・Increase of continuous
operating time

・Torque increase

・Max. torque

Disadvantages

・Slight reduction of total torque・Decrease of continuous
operating time・Decrease of continuous
operating time・Cost increase

Reliability Evaluation of the Two-stage Electric Turbocharging SystemFigure 4 shows the testing equipment on the engine. The engine used in this test is a 1.5 L
gasoline engine with a maximum torque of 230 Nm and a maximum output of 113 kW. Figure 5
shows a comparison of transient response at the engine speed of 1500 rpm. Figure 6 shows acomparison of engine exhaust pressure in transition. The two-stage electric turbocharger improved
the response by 43% compared with that of the existing two-stage turbocharger at 1500 rpm. InFigure 6, the two-stage electric turbocharger shows an improved engine exhaust pressure of 70%
compared with that of the existing two-stage turbocharger. With the improvement of the engine
exhaust pressure, the knock limit is improved through the reduction of gas remaining in theMitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)74

cylinder. This also affects the improvement of pumping loss and combustion instability. A
significant engine torque increase compared with that of a waste gate turbocharger can be achievedat an equivalent exhaust pressure, and improvement in fuel consumption and drivability withultra-high EGR can be expected. Figure 7 shows the change of exhaust temperature increase inengine cold starting. The test duration was 500 seconds including idling for 50 seconds simulating
engine cold start at the engine speed of 2000 rpm and torque of 40 Nm. From this result, thetwo-stage electric turbocharger shows an exhaust temperature approximately 100°C higher thanthat of the existing two-stage turbocharger. It was verified that the exhaust temperature could beraised rapidly in engine cold starting, and that it was effective for the activation of the catalyst. An
endurance evaluation test of the electric compressor/two-stage electric turbocharging system wasconducted, and the results are summarized in Figure 8. The endurance evaluation of the electriccompressor was conducted referring to the evaluation specifications of the engine alternator andstarter as the baseline. In addition, an endurance evaluation of two-stage electric turbocharging
system was conducted as a system on the engine. The system met the endurance conditions forautomobiles, and is currently in widespread use for evaluation testing by engine manufacturers.

Figure 4

Testing equipment on the engine

Figure 5

Figure 6 Comparison of engine exhaustpressure
Test item
Rapid acceleration/
deceleration cycle endurance test
Random cycle endurance test

Vibration endurance test
Low- and high-temperature
operational test
Endurance test of thermal shock
Cycle endurance test on
engine test bench

Figure 8Comparison of transient response

Figure 7 Comparison of exhaust
temperature when engine cold starting
Test conditions and results- Successful result of driving endurance mode test ofpassenger car- Operating mode : Transient- Random vibration conditions (3 directions)- Successful result of vehicle installation condition test
- Test temperature (low <--> high)- No problem- 1.5L Gasoline engine- Low speed (electricity On) <--> High speed (Off)- Successful result of endurance target

Results of endurance evaluation test

Test equipment

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)75

3.3

Simulation Evaluation of Two-stage Electric Turbocharger

GT-Power, which is widely used as an engine simulator, was used for the comparison of theengine transient response and improvement in fuel consumption when LPL (Low Pressure Loop)
EGR is introduced. In addition, a mode fuel consumption simulation was conducted with a vehicle
simulation model, and the optimal control method of the two-stage electric turbocharging system
was discussed. The downsizing effect of the two-stage electric turbocharger was studied with
GT-Power simulation.
Figure 9 shows the engine downsizing effect. The figure shows the relation between thetransient response time and engine torque of the engines with a waste gate turbocharger and thetwo-stage electric turbocharger. The 1.1 L gasoline engine with the two-stage electric turbocharger
maintains stable performance, and improves the transient response by 42% compared with the 1.5L gasoline engine with a waste gate turbocharger. The results verify that the two-stage electricturbocharger allows the downsizing of the engine to 1.1 L while maintaining the transient response.

Figure 9Engine downsizing effect (GT-Power calculation)

Figure 10 shows a comparison of transient response change by EGR. Using the 1.5 L
gasoline engine with a waste gate turbocharger and no EGR as the base line, in the case of EGR at10%, the engine torque was decreased by 19%. Contrarily, the gasoline engine with the two-stage
electric turbocharger shows a 48% torque increase with no EGR, and a 17% torque increase evenwith EGR at 30%. These results show that the two-stage electric turbocharger allows EGR duringtransient operation, and significantly improves the fuel consumption while improving the transient
response. Vehicle models with naturally aspirated (NA) gasoline engines, gasoline engines with awaste gate turbocharger (WG T/C), and gasoline engines with the two-stage electric turbocharger(E-2 stage) were constructed, and the fuel consumption was compared in the automobile runningmode of Japanese JC08 and U.S. LA4 modes.

Figure 10 Comparison of transient response change
through EGR (GT-Power calculation)

Figure 11 shows the results. The 1.6 L gasoline engine with a waste gate turbocharger shows
an improvement in fuel consumption of approximately 10%, and the 1.2 L gasoline engine with thetwo-stage electric turbocharger shows an improvement of approximately 30% compared with the2.0 L gasoline NA engine. The total displacement can be made smaller through the turbocharger

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
76downsizing effect, and the friction loss, heat loss, pumping loss and exhaust energy loss can beimproved. As a result, the fuel consumption improved both in JC08 and LA4 modes. The WLTC
mode planned to be adopted mainly in Europe from 2017 assumes actual driving conditions, ahigher speed, and more acceleration/deceleration under a heavy load. As such, further improvementin fuel consumption can be expected.

Figure 11 Fuel consumption comparison of test mode
(JC08 and LA4)

|4. Conclusion
A high-efficiency, high-speed motor was developed under the current 12V power source
specifications. The improvement of boost pressure at an extremely low speed where the exhaust gas
amount is small, and improvement in fuel consumption through the application of EGR became
possible. In addition, the two-stage electric turbocharging system combined with an existingturbocharger verified further improvement in fuel consumption through a significant downsizing of
the engine, as well as the improvement of drivability from the perspectives of engine evaluation
testing and vehicle simulation. Moreover, the durability of the system as a two-stage electricturbocharging system was verified. The system is now expected to contribute to the expansion ofbusiness and to measures to strengthen environmental regulations through future marketing.

Reference1.

Development of Electric Supercharger to Facilitate the Downsizing of Automobile Engines, Yamashita, etal., Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (2010)
2. Byeongil, A. et al., Development of Two-Stage Turbocharger System with Electric Supercharger, 2012FISITA World Congress, F2012-A01-026.
3. Kazuwaki, N. A. et al., Development of High Speed Motor and Inverter for Electric Supercharger, 2013
SAE World Congress, doi:10.4271/2013-01-0931.