Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
7Development of Electric Supercharger toFacilitate the Downsizing of Automobile Engines

YUKIO YAMASHITA*1

SEIICHI IBARAKI*2

KUNIO SUMIDA*１

MOTOKI EBISU*3

BYEONGIL AN*4

HIROSHI OGITA*5

To satisfy both environmental regulations and drivability requirements, high-level controlof automobile engines is currently in demand, and electrification is in progress. An electricsupercharger, in which a supercharging compressor is driven by a high-speed motor instead ofan exhaust turbine or belt, eliminates turbo lag by making use of the motor’s high-speedresponse. An engine with an electric supercharger offers comparable fuel consumption to anaturally aspirated engine, and is expected to facilitate the downsizing of engines. In this paper,
we introduce the electric supercharger developed by Mitsubishi Heavy Industries, Ltd. (MHI),including bench test results exhibiting a high-speed response of 1.0-s acceleration time to attaina compressor operating point rated at 2 kW, 140,000 rpm.

|1. Introduction
With the ongoing movement toward global environmental protection, regulations controllingthe exhaust emissions and fuel consumption of automobiles are being enforced. Turbochargers
have improved the performance of diesel engines; currently, almost all diesel vehicles are equippedwith turbochargers. More and more gasoline engines are also being fitted with turbochargers todecrease weight and increase efficiency1.
In recent years, engine controls have become widely diversified for the sake of both theenvironment and operating performance. Variable geometry (VG) turbochargers, which can vary
the turbine capacity in response to the engine load, are becoming increasingly popular. Also,electrification (and the installation of various electrical devices in automobiles) has produced thepresent generation of hybrid and electric vehicles, and progress continues. Batteries are also
expected to improve in response to this trend. Current electrification projects impacting
turbochargers include the application of electronic control actuators. Moreover, advances in powerelectronics have led to the development of ultra high speed motors/generators with a capacity of
100,000 rpm or higher, which are now available for use in turbochargers.
In an electric supercharger the compressor is directly connected to an ultra high speed motor,
and the compressor boost pressure is controlled by the motor speed, independent of the exhaustturbine. This supercharger is expected to contribute to lower fuel consumption, the reduction of
exhaust loss, and the downsizing of engines.

|2. Classification of electric turbochargers
Electric turbochargers equipped with ultra high speed motors are classified into two types.
One of these is a hybrid turbocharger2, in which an ultra high speed motor is contained in aconventional turbocharger. The conventional turbocharger supplies pressurized air to the engine
with a compressor rotated by a turbine driven by engine exhaust. The motor/generator integrated*1Nagasaki Research and Development Center, Technical Headquarters*2

Manager, Nagasaki Research and Development Center, Technical Headquarters*3

Manager, Turbocharger Engineering Department, General Machinery & Special Vehicle Headquarters*4Turbocharger Engineering Department, General Machinery & Special Vehicle Headquarters*5

MHI Sagami High-tech Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
8into the turbocharger assists the rotation under low-speed engine conditions, when the exhaustenergy is insufficient, and improves the weak point of the transient response delay of theturbocharger (i.e., turbo lag).
Under high-speed engine conditions, excess gas energy from exhaust is recovered aselectricity and is utilized for charging the battery or assisting an engine with an integrated starter
generator (ISG).
The other type of electric turbocharger with an ultra high speed motor is the newly developed
electric supercharger, which does not utilize exhaust. The centrifugal compressor supplies
pressurized air to the engine solely in response to the torque of the high-speed motor that drives it.
Figure 1 is a diagram of the electric supercharger system, and Figure 2 shows the prototype. The
electric supercharger is also effective for constructing a two-stage turbocharger combined withanother turbocharger. Figure 3 shows the results of a response analysis of a 1.4-liter gasoline
engine accelerating to the target torque. The transient time has been reduced by 35% compared tothat of a conventional turbocharger.

Figure 2 Electric superchargerThe prototype of the electric compressor is shown. The ultra highspeed motor drives the centrifugal compressor.

Figure 1
Electric supercharger
systemThe ultra high speed motor on the electricsupercharger rotates the compressor in
response to engine requirements for intake
air pressure, receiving electricity from thebattery through the inverter.

Figure 3 The results of the response analysis of the engine
The torque response of a 1.4-liter gasoline engine with electricsupercharger|3. Development of the electric Supercharger
3.1

Development concepts

To be applicable to current vehicles, the concepts are as follows:
・ The electric power source is 12 V DC.
・ The compressor, motor, and inverter are integrated into one body.
・ No separate fan or water-cooling device is used for cooling.

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
93.2

Construction and features

The electric supercharger prototype consists of a centrifugal compressor impeller and ahigh-speed motor rotor, supported with a ball bearing for improved response and reduced friction.
Grease lubrication eliminates the need for lubrication oil piping. To improve the transient responseby reducing rotor inertia, the rotor has been kept as small in diameter and as long as possible.
Compressor intake air is utilized to cool the motor and inverter. Because the motor andinverter are upstream of the compressor intake, they are cooled whenever the electric supercharger
rotates. The cooling airflow varies with the compressor working point and is favorable to high heat
reduction in the motor and inverter when the flow volume is large. Using loss calculations based on
finite element method (FEM) magnetic field analysis and circuit simulation, and using flow
velocity distribution calculations in the cooling flow channel based on computational fluid
dynamics (CFD) analysis, the temperature increases in the motor and inverter can be predicted, andthe flow channel can be improved. Figure 4 shows the results of the CFD flow velocity distribution
calculations. The cooling-channel pressure loss can also be calculated to improve the channel shapeand to avoid excessive increases in the power required for the motor and inverter.

Figure 4 Flow distribution calculations for motor cooling air by CFD
Calculation of cooling air velocity in the motor which is located in the path of inlet airflow of the compressor

3.3

Power electronics technology

3.3.1 Ultra high speed motor
The motor is a three phase permanent magnet synchronous type. Samarium-cobalt
rare-earth magnets (which are strongly magnetic and heat resistant) are used for the rotor tominimize the size of the motor. The magnets are held in place against rotor centrifugal force(which is especially important in ultra high speed motors) by surrounding them withcylindrically laminated, carbon-fiber reinforced plastic (CFRP) sheets. In carbon-cylinder
manufacturing, flaking and expansion are avoided by considering the fiber direction andtrimming the edge of the sheet. The rotor was spin-tested and verified at 1.2 times the rated
rotational speed without breakage. Figure 5 shows a sectional view of the rotor retention ringafter the increased temperature test with the motor running. The rotor appears robust, without
deformation or carbon flaking. The stator coil has a single turn per phase and a star connection.
A single turn per phase is the only way to realize the range of values for the back electromotive
force (EMF) constant and the inductance that provides the required torque under low-voltage
ultra high speed conditions. Figure 6 shows an external view of the stator.

Figure 5 Sectional view of the rotor-retention
ring after the operating test
The rotor appears robust, without deformation or carbon
flaking.

Figure 6Motor stator

The coil has a single turn per phase to satisfy
low-voltage ultra high speed specifications.

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
10The rated output of the motor is 2 kW, 140,000 rpm. To achieve a high-speed response,
instant double overload is permitted. By FEM magnetic-field analysis of the motor, using theelectric-current waveform acquired from the inverter circuit simulation, the loss (including theeddy current in the magnet) is calculated.
3.3.2 Inverter
The inverter is a general pulse-width-modulation (PWM) voltage source type. A
field-effect transistor (FET), which allows high-speed switching, is used as a semiconductor
switching device. A general-purpose microcomputer is employed for the motor control. Thefundamental electrical frequency exceeds 2 kHz, and the processing speed of themicrocomputer is insufficient to apply rigorous field-oriented control. A voltage-to-frequency(V/f) control that includes a partial closed-loop current control has been adopted. Rotational
position sensor such as Hall-effect devices and resolvers are not used, and a sensorless control
with position estimation based on the detected current has been adopted. To avoid loss ofsynchronization or over current caused by a position estimation error in a high-speed response,a current-phase correction control based on the current and rotational speed has also been
employed.

3.4

Test results

The bench test results of the electric supercharger with modified compressor impeller and themotor and inverter described previously, are presented here. The compressor load was varied by
opening and closing the outlet valve in the compressor discharge pipe.
Figure 7 is a graph of the typical working-point measurements, showing the compressor
characteristics on a Total Pressure Ratio to Compressor Performance map. A 1.0 second response
from the idle speed of 10,000 rpm to the rated speed of 140,000 rpm is verified near the surge-line
side, the choking side, and on the impeller maximum efficiency line. Figure 8 shows the response
curve on the impeller maximum efficiency line. The motor output is 2 kW, and the speed is140,000 rpm at the working point.

Figure 7 Measured working
points of the compressor

A wide working range was tested
by adjusting the area of the openingof the compressor outlet valve.

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
11Figure 8 Quick response curve of the electric supercharger
A response time of ~1.0 second to the compressor working point of 2 kW, 140,000 rpm (the rated point for themotor) was verified.

Figure 9 shows one of the measured motor-current waveforms, illustrating rapid accelerationfrom the idle speed of 10,000 rpm to the rated speed of 140,000 rpm. Control is achieved from low
to high rotational speeds without loss of synchronization. Even at high rotational speeds, thecurrent waveform is kept close to sinusoidal by the PWM control drives the motor.
Figure 10 shows the temperature changes at the measured points on the motor and inverter.
All of the temperatures are below the allowable limit, and the previous FEM and CFD studies wereverified. The present thermal design is based on continuous operation, and reduced size is possible
for short-term or intermittent operation.
The shaft vibration was measured at the end of the compressor shaft, and the maincomponent (first order of rotational speed) did not exceed the limit during operation.

Figure 9

Motor current waveform

At high speeds, the current waveform was close to sinusoidal by PWM control

Figure 10

Temperature changes at the measured points on the motor and inverter

Temperature changes at the measured points during 2 kW, 140,000 rpm operation. All points had a temperature
below the calculated allowable limit.

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)12|4. Problems and future prospects
The bench test of the 12-V electric supercharger prototype demonstrated a high-speed
response of 1.0 second, and its effectiveness for eliminating turbo lag was experimentally verified.
For application to automobiles, the following items need to be addressed:
・ Durability verification of parts such as the bearing and the semiconductor element
・ Reduction of the dimensions, cost, and noise・ Establishment of design goals for the electric supercharger output specifications, and an
operational pattern to optimize the system and the corresponding control method
・ Establishment of an electric power management system in which the electric supercharger
works compatibly with other electrical equipment in vehicle.

|5. Conclusion
A prototype of the electric supercharger (a promising new turbocharger technology in theface of stricter exhaust and fuel consumption regulations) was produced and tested to verify its
technical possibilities. In addition to the electric supercharger, an electrically assisted turbocharger
and an electric compressor for fuel cells are being developed as applications of the ultra high speed
motor.

References
1.
2.
3.
4.

Ibaraki, S. et al., Numerical and Experimental Reliability Evaluation of the High Temperature Turbine
Wheel for Gasoline Engines, 13th Supercharging Conference, 2008.
Ibaraki et al., Development of the "hybrid turbo," an electrically assisted turbocharger, Mitsubishi HeavyIndustries Technical Review Vol. 43 No. 3 (2006)
Yamashita, Y. et al., Development of Electrically Assisted Turbocharger for Diesel Engine, 8thInternational Conference on Turbochargers and Turbocharging, IMechE Paper, p.147
Toshihiko Noguchi, et al., 220,000-r/min, 2-kW permanent Magnet Motor Drive for Turbocharger,
International Power Electronics Conference (IPEC-Niigata 2005)