Development of the "hybrid
turbo," an electrically assistedturbocharger
SEIICHI IBARAKI*1
YUKIO YAMASHITA*2KUNIO SUMIDA*1
HIROSHI OGITA*3YASUAKI JINNAI*3

If automotive engines are to achieve better environmental properties and drivability, they must be equipped withmore advanced systems for operation control. Electrical operation systems are now being incorporated in engine
designs for this purpose. The "hybrid turbo" is an electrically assisted turbocharger with a high-speed motor generator built in. When operating in the low-speed engine mode, the hybrid turbo can achieve better combustion, purer
exhaust gas, improved torque response, and torque enhancements with motor assistance. In the high-speed engine
mode, it can operate with very high efficiency by achieving regeneration of electric power in excess of the exhaust gas
energy. For these reasons, the electrically assisted turbocharger is expected to be adopted as the turbocharger of thefuture. In recent engine tests, a new hybrid turbo developed by Mitsubishi Heavy Industries, Ltd. (MHI) has been
proven to operate with improved engine torque and enhanced response compared to a conventional turbocharger.
The effectiveness of this hybrid turbo has thus been confirmed.
cation, a new generation of VG turbochargers (VariableGeometry Turbocharger) capable of changing turbine
capacities in accordance with engine loads have rapidly
come into wide use(1). Automakers are installing diverse
electrical equipment and introducing more electroniccontrol systems in their cars, both hybrid and conventional, and 36 V is being introduced as a battery powersource. Electrical operation systems such as electronically controlled actuators are also being increasingly
incorporated into turbochargers. And with progress inpower electronics, automakers can now create hybrid
generators by fusing turbochargers with newly developed
motor generators that run at ultrahigh speeds of 100 000
rpm and over. If the adoption of 36 V for the battery
power source becomes widespread, an electric load ofseveral kWs or more can be used. Thus, the turbochargerof the future may soon be a hybrid.

1. Introduction
In line with efforts to conserve the global environment,
regulatory authorities around the world have steadily
continued to tighten their control of automobile emissions and fuel consumption. As of this writing, almost
all diesel-powered vehicles are equipped with turbochargers to enhance the performance of the engines.
Turbochargers are also being increasingly applied ingasoline engines as a means to downsize the engines,
lighten their weights, and improve their efficiency(1).
Diverse engine control strategies to improve operation environments and operation performance aresteadily being introduced. In response to this diversifi-High-speed motor generator
CompressorTurbine

Air supplyExhaust gas

Air supply

2. Classification of electrically assisted charging systems

PowerTwo types of electrically assisted charging systems
are equipped with ultrahigh-speed motor generators.
The first, the "hybrid turbo," is a conventional turbocharger with a high-speed motor generator built in.
Figure 1 shows the hybrid turbo system and Fig. 2 shows
the prototype. In a conventional turbocharger, a turbine driven by the exhaust gas from the engine turns acompressor on the same shaft, supplying high-pressure
air to the engine. The motor generator incorporated into
this type of turbocharger assists the turbo when the engine speed is low and the exhaust gas energy isinsufficient, thereby improving the transient responsedelay (turbo lag), the Achilles heal of the conventional turbo.

Inverter
Exhaust gas

EnginePower

BatteryFig. 1 System diagram of hybrid turbo
The high-speed motor generator in the hybrid turbo gives andreceives electricity to and from the battery through an inverter,in accordance with the engine load.

*1 Nagasaki Research & Development Center, Technical Headquarters*2 Nagoya Research & Development Center, Technical Headquarters*3 General Machinery & Special Vehicle Headquarters

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 43 No. 3 (Sep. 2006)

1Compressor

High-speed motor
TurbineHigh-speed motor generator

Compressor

Fig. 3 Electrically operated compressor
External appearance of the electrically operated compressor prototype.
The centrifugal compressor is driven by the high-speed motor.

Fig. 2 Hybrid turbo
External appearance of the hybrid turbo prototype. A high-speed
motor generator is incorporated between the compressor and turbine.

Increase in engine
thermal efficiency

Increase inengine output

In the high-engine speed zone, on the other hand, excess
exhaust gas energy can be recovered as electric powerand either stored in the battery or used to assist theengine through the ISG (Integrated Starter Generator).
The second type of electrically assisted charging system is basically a high-speed motor driving a
supercharger without the use of exhaust gas. The motor torque drives the compressor used to supply
high-pressure air to the engine. Figure 3 shows a prototype of an electrically operated compressor combining acentrifugal compressor and high-speed motor(1).

2.5: +3 kW assist: +2 kW assist: +1 kW assist

2.0+100 %1.5
1.00500

Engine torque (Nm).
.
.

2 000

1.20
1.10

3 000+12%

1.05
1.00

2 500

: +3 kW assist: +2 kW assist: +1 kW assist

1.15+8%0500

3. Development of hybrid turbo
3.1 Development objectives
The development objectives for the hybrid turbo weredefined through performance studies based on the engine simulation(2).
10 % improvement of fuel efficiency in the low-enginespeed zone
50 % improvement of engine torque in the low-enginespeed zone
70 % reduction of turbo lag during acceleration
The relationship between required motor output andengine performance was estimated in the simulated operation of a 2L diesel engine equipped with an intercooler.
The motor efficiency was assumed to be 90 %, and thesimulation data for the turbine and compressor weretaken from data on actual operation tests.
Figure 4 shows the simulation results. Engine torqueas a steady state characteristic was improved by 50 %with a 1 kW motor assist and by 100 % with a 2 kW
motor assist. The results also show an 8 % improvement in fuel efficiency with the 1 kW motor assist and a
12 % improvement with 2 kW. According to the assessments of transient characteristics, the turbo lag wasreduced by 50 % with a 1.3 kW motor assist and by 70 %with 2.0 kW. Based on these simulation results, the output of the prototype motor was set at 2.0 kW.

+50 %
1 000 1 500

1 000 1 500 2 000
Engine speed (rpm)

2 500

3 000Assist-70 %-50 %

300200Start ofassist: Conventional turbocharger: +0.5 kW assist: +1.3 kW assist: +2.0 kW assist

1000234567Time (s)Fig. 4 Engine simulation results
The hybrid turbo enhances the torque and thermal efficiency in thelow-engine-speed zone and improves the overall response.

3.2 Structure and characteristics
The prototype hybrid turbo was structured with ahigh-speed motor generator placed between the compressor and turbine. With the motor generator incorporated
into the hybrid turbo, increases in the shaft length andoutside diameter were inevitable. Fortunately, theseincreases could be minimized by carefully designing theshapes and positioning of the bearing, magnet rotor (rotor), and stator to keep the engine installation spaceequivalent to that in the conventional turbochargers.
Reliability and reduced cost were secured by incorporating the same turbine and compressor used inconventional turbochargers, as well as other conventional turbocharger parts with proven performance.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 43 No. 3 (Sep. 2006)

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The magnet rotor of the motor generator is positioned
in the overhang shaft system in the back surface of thecompressor, and the critical speed of the shaft system
has been reduced compared with that in conventional
turbochargers. To accommodate this design we adopted
a full float bearing, a component proven to be highly effective for restraining shaft vibrations at high-speed
rotations in conventional turbochargers, and we held
down the resonance magnification by tuning the bearing gaps, etc. A rotor vibration analysis and verification
test confirmed that this design was free of problems.
3.3 Power electronics
3.3.1 Ultrahigh-speed motor generator
A permanent magnet synchronous type was adopted
as the ultrahigh-speed motor generator. The rotor magnet and stator were reduced in size by incorporating aneodymium rare-earth magnet with strong magnetism
and relying on concentration winding, a winding methodthat can be used to reduce the size of the winding wireend. By keeping a relatively large gap between the rotor and stator, we managed to reduce the inductancewhich disturb the rapid change of current. Carbon fiber reinforcement was used to retain the magnet against
the centrifugal force of the rotor, fulfilling a crucial design requirement for the ultrahigh-speed motor
generator.
If the motor generator is downsized, the density of
the heat generation rises sharply due to motor loss andhinders the cooling. When cooling is insufficient, excessive rise in the winding wire temperature and rotor
magnet temperature cause a deterioration of insulation
performance and irreversible demagnetization respectively. Another factor that cannot be ignored in motor
generators is loss of the eddy current in the rotor. Thisis an especially important factor in the ultrahigh-speed
motor generator and must be reduced by any means necessary. Based on the results of the magnetic fieldanalysis, we found that a 6-slot structure for the stator

could reduce the rotor eddy current loss in our prototype (2)(3). We also improved the cooling system with theuse of forced oil cooling for the stator and forced air cooling for the rotor. Figure 5 shows the external appearanceof the prototype and stator.
3.3.2 Inverter
The voltage-source PWM (Pulse Width Modulation)
inverter is widely used as an inverter to drive permanent magnet motor generators. However, in thishigh-speed application, the fundamental electric frequency reaches several kHz. Therefore it is difficult toform good sinusoidal current waveforms using the voltage-source PWM. As an alternative, we adopted apseudo-current source inverter in the prototype(2)(3). Thisapproach is favorable for the control of ultrahigh-speed
motors as it enables the straightforward control of thecurrent values and phase. The inverter circuit is madeup of a buck-boost chopper that controls current values
and a 6-step inverter circuit that controls the current
phase. Switching between buck and boost voltage in thebuck-boost chopper makes it possible to operate the inverter in two directions: motoring and regeneration. We
also managed to omit the use of external position detectors such as the hall element and resolver, as the magnet
rotor rotation position required for stable torque generation is detected based on the waveform of the induced
voltage generated in the stator. We are forced to rely on
the open loop for control, however, as the induced voltage is small and difficult to detect during startup and at
extremely-low speeds.
To power the inverter, we expect to use 12 V in thebattery for general passenger cars, 24 V for commercial
vehicles, and 36 V for future applications. For the prototype, however, our chief concern was to confirm theperformance and effectiveness of the hybrid turbo itself.
Thus, we initially set the voltage as high as 72 V andused integral multiples of the voltage in the existing
batteries.

High-speed motor
generator
TurbineCompressor

Fig. 5 Conditions of the test on the hybrid turbo, and the external appearance of the motor stator
Hybrid turbo during the engine testing. A 6-slot concentration winding method is adopted for the motor statorto downsize the component and reduce loss.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 43 No. 3 (Sep. 2006)3

Table 1 Comparison between the turbocharger speed and discharge pressure ratio before the motor assist and after
the motor assist

Compressor dischargepressure (kPa)

9080

4.6 s

7060501.9 s

4030
2010: With assist: Without assist

Turbocharger
speed (rpm)Before assist 40 000

81 000 100 000 122 000

After assist

85 000109 000 121 000 138 000

Compressor
dischargepressure ratio

Before assist

1.05

1.21

1.33

1.50

After assist1.23

1.41

1.51

1.66

0024681012160 000

4.0 s

140 000
120 000180

Engine torque (Nm)Turbocharger speed (rpm)

-2

1.3 s

100 000
80 00060 000: With assist: Without assist

40 000
20 000160140120: Variable geometry turbocharger: Hybrid turbo
Motor assist output: 2 kW
173.3
+16 %
(+24.2 Nm)
149.1
142.4
+18 %
Torque is(+21.8 Nm)
improved by 16 % to 18 %.
120.6

0-202

468

1012100
900

Time (s)

1 000

1 100

1 200

1 300

Engine speed (rpm)Fig. 6 Comparison of the transient response of the turbocharger
speed and discharge pressure ratio with the motor assist
and without motor assist
The motor assist greatly shortens the start-up times for the turbocharger
speed and compressor discharge pressure.

Fig. 7 Comparison of the steady torque of the engine between
the hybrid turbo and variable geometry turbocharger
Torque is improved by 16 % to 18 % at low engine speeds with themotor assist.

3.4 Test results
3.4.1 Unit test results
We tested the hybrid turbo prototype by a unit test toconfirm the basic performance(2). The test was conducted
on a test bench for the turbocharger and an air source
was used to drive the turbine. A 72-V DC stabilized
power-supply was used to power the inverter over a long
period of stable operation and measurement.
Table 1 shows a comparison of the rotational speed of
the turbocharger and the compressor discharge pressure
ratio in the prototype before and after motor assist. The
motor was controlled to a constant torque of 0.16 Nm
throughout the test (equivalent to 120 000 rpm, 2 kW).
Figure 6 compares the transient response of the turbocharger speed and the discharge pressure ratio withand without motor assist. The effects of the motor assist are noticeable if we look at the values during
acceleration: the turbocharger took 33 % less time toreach 74 000 rpm and the compressor discharge pressure took 41 % less time to reach 26 kPa with the motorassist. The turbine torque became dominant after several seconds of acceleration, leaving the steadydifferences shown in Table 1.
3.4.2 Engine test results
The hybrid turbo was also tested by an engine combination test on the test bench (2) using a 2L-class
direct-injection diesel engine equipped with an intercooler. This type of engine usually incorporates thevariable geometry turbocharger. During the testing, thehybrid turbo was controlled to a constant torque of 0.16
Nm at 120 000 rpm or less and to a constant output of
2.0 kW at 120 000 rpm or more.
Figure 7 compares the steady torque of the engine
with the hybrid turbo used and with the variable geometry turbocharger used. The steady torque of the engine
improved by about 17 % at engine speeds of 1000 rpm
and 1200 rpm. This effect was thought to be due mainly
to the higher air-fuel ratio.
Figure 8 compares the transient response of the turbocharger speed with the hybrid turbo and with thevariable geometry turbocharger. The engine begins in ahigh-idle condition, and the engine output and output of
the hybrid turbo motor both increase as the engine loadincreases. The turbocharger speed accelerated more
quickly with the use of the hybrid turbo than with thevariable geometry turbocharger. The overall acceleration performance with the hybrid turbo was also
confirmed to be equal to or better than that with thevariable geometry turbocharger.

4. Challenges and outlook for the future
We have experimentally confirmed that the hybrid
turbo effectively improves engine torque and eliminates
turbo lags when applied to automotive engines. In thefuture we will work on the following tasks toward thedevelopment of an in-vehicle hybrid turbo.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 43 No. 3 (Sep. 2006)

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40

2 000
Start of assist

30

1 500

20

1 000

10500

0

Turbocharger speed (rpm)4567Time (s)89120 000
100 000

2 000

80 000Start of assist

1 500

60 0001 000

40 000500

20 0004567Time (s)89.
.

010@ Engine speed: 1 000 rpm
3 000: Hybrid turbo: Variable geometry turbocharger
2 500: Inverter output

140 000

Establish a technology for ultrahigh-speed motors andinverters that can be realized electrically and thermally based on the specifications for the existingin-vehicle 12-V and 24-V batteries, or for the 36-V batteries expected to be widely adopted in the future
Establish a method to control the hybrid turbo as partof an optimum system in combination with an engine
and exhaust gas aftertreatment device
Establish a power supply system comprised of thehybrid turbo working effectively with other in-vehicle
electric components

5. Conclusion
To help us adapt to the ever control of automobile
emissions and fuel consumption in today's regulatoryenvironment, our group has tested and confirmed theeffectiveness of the prototype of a hybrid turbo expected
to be adopted as a new technology for turbochargers. Inaddition to the hybrid turbo itself, we are also developing an electrically operated supercharger and anelectrically operated compressor for fuel cells based on
the application of the electrically operated supercharger,
as products to which we can apply an ultrahigh-speed
motor generator (1). Henceforth we hope to use our turbocharger and ultrahigh-speed motor drive technologies
to further improve the environmental performance of carsand enhance the pleasure of driving.

Inverter output (W)Boost pressure (kPa)50.
Inverter output (W)@ Engine speed: 1 000 rpm
3 000: Hybrid turbo: Variable geometry turbocharger
2 500: Inverter output

60010Fig. 8 Comparison between the transient response of the turbocharger speed and boost pressure with the use of thehybrid turbo and with the use of the variable geometry
turbocharger
The hybrid turbo has a transient performance equal to or better thanthat of the variable geometry turbocharger.

References(1) Ibaraki, S. et al., Turbocharger R & D in MHI, Journal ofthe Gas Turbine Society of Japan, Vol. 33 No. 4 (2005) p.288(2) Takahashi, I. et al., A Super High Speed PM Motor Drive
System by a Quasi-Current Source Inverter, Industry Applications, IEEE Transactions, Vol. 30 Issue 3 (1994)
p. 683Seiichi Ibaraki

Yukio Yamashita(3) Takata, Y. et al., 220000 r/min -2 kW PM Motor Drive for
Turbochager (in Japanese), IEEJ Trans. IA. Vol. 125 No. 9(2005) p.854(4) Yamashita, Y. et al., Development of Electrically Assisted Turbocharger for Diesel Engine, 8th International Conference
on Turbochargers and Turbocharging, ImechE Paper, p.147

Kunio Sumida

Hiroshi Ogita

Yasuaki Jinnai

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 43 No. 3 (Sep. 2006)5