Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)
1Electric Supercharging Technology for Automobiles

NAOMICHI SHIBATA*YASUAKI JINNAI*

3

YUKIO YAMASHITA*

1

BYEONGIL AN*

2

MOTOKI EBISU*

45In the automobile environmental regulations of each country, significant fuel consumption
reduction and cleaner exhaust gas are required under conditions closer to real driving. To satisfy
environmental regulations, which are becoming increasingly strict every year, powertrainelectrification has been advancing lately. Various types of electric motor vehicles including hybridvehicles, plug-in hybrid vehicles, 48V mild hybrid vehicles (MHEV) with engines, electric vehicles(EV), and fuel cell vehicles (FCV) without engines are currently on the market. Mitsubishi HeavyIndustries Engine & Turbocharger Co., Ltd. (MHIET) has contributed to fuel consumption
reduction and cleaner exhaust gas through turbochargers. Utilizing these results, we havedeveloped electric supercharging technology applicable to future powertrains and proposed it toautomobile manufacturers. In this paper, MHIET presents this electric supercharging-relatedtechnology.

|1. Introduction
To satisfy environmental regulations for automobiles, vehicle electrification is required. Amongthese efforts, European automobile manufacturers are advancing the market introduction of 48V
MHEV featuring high-efficiency technology of the vehicle auxiliary equipment by raising the rated
voltage from the conventional 12V to 48V. MHIET is developing a two-stage electric turbocharger
system with an electric supercharger and an electrically-assisted turbocharger as an electricsupercharging system applicable to 48V MHEV, and has proposed them to automobile manufacturers
as effective technologies for improving powertrain efficiency and for cleaner exhaust gas.
From the next section, other electric supercharging-related technologies are explained
focusing on evaluation cases of engines equipped with 48V electric superchargers.

|2. 48V Electric supercharger
Table 1 shows the specifications of the 48V electric supercharger developed by MHIET. The
electric supercharger realized compactness and light weight by installing a motor coaxially with acentrifugal compressor and integrating the motor and power electronics. The electric supercharger
is electrically connected to the vehicle ECU*1 by CAN*2 communication. The power performanceand environmental performance of the powertrain are improved by controlling the boost pressure atfast response. The maximum motor output is 5 kW, and the response time from stop to 90% of themaximum rotational speed is 0.3 seconds or less, and the continuous operation time is 30 seconds.
Furthermore, continuous operation becomes possible by suppressing the output to 3 kW. To
achieve the above performance, the motor, power electronics and bearings are cooled by water. In
consideration of the engine room mounting, a grease packed ball bearing without oil lubrication isadopted.
*1 Engineering Department, Turbocharger Division, Mitsubishi Heavy Industries Engine & Turbocharger Co., Ltd.
*2 Chief Staff Manager, Engineering Department, Turbocharger Division, Mitsubishi Heavy Industries Engine & Turbocharger Co., Ltd.
*3 Deputy Director, Engineering Division, Turbocharger Division, Mitsubishi Heavy Industries Engine & Turbocharger Co., Ltd.
*4 Chief Staff Manager, Engineering Department, Turbocharger Division, Mitsubishi Heavy Industries Engine & Turbocharger Co., Ltd.
*5 Manager, Electricity & Applied physics Research Department, Research & Department Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)2Table 1 48V Electric supercharger specifications
ItemInput voltage
Motor type
Maximum speed
Maximum pressure ratio
Maximum
OutputContinuous
Response time
Cooling method
Bearing typeSpecification
48V
Flux Switching Motor
80,000 rpm
1.7
5 kW (30 s)
3 kW< 0.3 s (0 to 90% rpm)
Water cooling
Grease packed ball bearing

A flux switching motor has the advantages of no magnets, low cost and high efficiency. (1)Figure 1 shows the configuration of the two-stage electric turbocharger. The intake side ismade to be a two-stage layout of an electric supercharger and a turbocharger. The electricsupercharger can output the boost pressure and the transient response when the engine load is low
and the turbocharger rotation speed is low. When the engine load rises, the turbocharger can satisfy
the required boost pressure. Then the electric supercharger output is lowered and the bypass valve
is opened. The advantages with an electric supercharger are shown below:・Improvement of engine power performance (Improvement of engine low-speed torque andtransient response)・Supply of boost pressure required for fuel consumption reduction engine technology (Millercycle*3, EGR*4, etc.), and improvement of fuel consumption by enabling further turbocharging
downsizing.

Figure 1 Structure of two-stage electric turbocharger|3. Evaluation on engine bench
The performance test was carried out on an engine bench to confirm the performance andcontrollability of the two-stage electric turbocharger. The test engine is a 2.0 L gasoline engine
with a maximum torque of 422 Nm and a maximum output of 221 kW.
Figure 2 shows the steady-state performance of the engine. The two-stage electricturbocharger and the base turbocharger is compared with the torque ratio based on the steady fullload torque at the engine revolution speed of 2500 rpmAt the low engine speed of 1250 rpm, theengine torque of the two-stage electric turbocharger is improved by 122% in comparison with thebase turbocharger. At the engine speed of 1750 rpm, the engine torque is improved by 45% incomparison with the base turbocharger. Since the exhaust gas energy increases with the engine
speed increasing and the turbocharger output improves, the engine full load torque is obtained even
if the electric supercharger output is lowered. Further assistance is also possible, because there is amargin in the rotational speed and output of the electric supercharger at each engine rotationalspeed.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)3

Figure 2 Engine steady-state performance

Figure 3 shows the transient performance at an engine speed of 2000 rpm. The engine
torque response when the throttle valve was operated from idling to the full opening wasconfirmed. Since the intake piping of the two-stage electric turbocharger is longer than that of thebase turbocharger, the pressure loss at idling increases and the engine torque slightly decreases.
The two-stage electric turbocharger improved the transient response by 80% in comparison withthe base turbocharger. Although the electric supercharger could not be driven at the maximum
rotational speed and maximum output due to the convenience of the test engine equipment,
further assistance is possible and response time improvement is expected.
From the above, it was confirmed that steady-state torque and transient response at low
engine speed were significantly improved with the electric supercharger assistance by the two-stage
electric turbocharger. It is expected that the drivability of the vehicle will be improved while
maintaining the maximum output by enhancing the low-speed performance of the engine.

Figure 3 Engine transient performanceMitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)4|4. Vehicle driving simulation
The vehicle driving analysis was carried out with the 1D engine simulation tool. The
effectiveness in the vehicle driving mode of the two-stage electric turbocharger was confirmed.
Table 2 shows the analysis conditions and the driving mode. Using a gasoline 2.0 L vehicle model,
a comparison between a two-stage electric turbocharger and a base turbocharger was carried out.
WLTC (Worldwide harmonized Light vehicles Test Cycle) mode covering operations from urban
district driving to high-speed driving was selected. Then, Miller cycle and EGR were applied forengine fuel consumption reduction technology. Miller cycle is a technology to shorten thecompression stroke and substantially increasing the expansion stroke by controlling the engine
cylinder intake valve and to improve the engine thermal efficiency. EGR is a technology to reduce
the pressure loss in the throttle valve and improve the margin of knocking and the heat loss inengine combustion by recirculating engine exhaust gas to the intake side. These technologies areeffective for fuel consumption reduction. However, when these technologies are applied, engine
torque decreases because the fresh air intake decreases. In this simulation, the electric supercharger
and turbocharger were controlled to compensate for the engine torque reduction. Furthermore,
electric power was generated during driving so that the SOC*6 of the battery becomes equal before
and after the driving mode, and the fuel consumption was evaluated by adding the electric energy
consumption of the electric supercharger driving.
Table 2 Analysis conditions and driving mode of two-stage electric turbocharger
Items

Analysis Conditions

Driving Mode Diagram

1. Gasoline 2.0 L turbocharger
2. Gasoline 2.0 L two-stage
electric turbocharger(ElectricSupercharger+Turbocharger)
Driving WLTC (Worldwide harmonizedmodeLight vehicles Test Cycle)
・ Fuel consumption reductionengine technology: Miller cycle +Control EGR・ Equivalent SOC of battery
before and after driving
EngineFigure 4 shows the results of steady-state simulation of the engine with Miller cycle andEGR. The engine speed, engine torque and fuel consumption of the two-stage electric turbocharger
and the base turbocharger were compared. In the two-stage electric turbocharger, the engine
low-speed and high-torque range applied the Miller cycle and EGR is expanded with the electricsupercharger assistance in comparison with the base turbocharger, and the fuel consumption isreduced. Furthermore, the pumping loss*7 of the engine is reduced by positively using the electricsupercharger in the output range and exhausting without passing through the turbocharger, and thefuel consumption is reduced. Thus, two-stage electric turbocharger application was effective for theexpansion of the Miller cycle and EGR application range and pumping loss reduction.
Figure 5 shows the 1D engine simulation results of the engine output operating point inWLTC mode. In the two-stage electric turbocharger, the integrated fuel consumption including
the electric energy balance in electric supercharger driving was reduced by 1.0% compared to thebase turbocharger. It is considered that the application of Miller cycle and EGR and the pumping
loss reduction of the engine reduce the fuel consumption in the engine operating range. Although
the transmission control was calculated as the same, it is considered that further fuel consumption
improvement is possible by optimizing the control of the engine speed and engine torque toprovide good fuel consumption.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)
5Figure 4 Engine steady-state simulation (Miller cycle + EGR)

Figure 5 WLTC mode simulation|5. Introduction ofsupercharging

othertechnologies

relatedtoelectric

Table 3 presents other electric supercharging-related technologies for future powertrains.
The following is an outline of each development technology.
Table 3 Powertrains and Electric Supercharging-related Technologies
Powertrain

Electric
Supercharging-related
Technology

Usage[Usage]Two-stage electric・ Improvement of engine powerturbocharger
performance(Electric supercharger + ・ Application of technologies toturbocharger)
reduce engine fuel consumption
48V MHEV(Miller cycle, EGR, lean burn, etc.)
・ Energy regeneration(Electrically-assisted turbocharger)
Electrically-assisted[Characteristics]
turbocharger
・ Fast response
・ Small size, Light weight

EVFCV[Usage]・ To extend the cruising range of EV
・ More freedom in charging timing
Micro Gas Turbine for[Characteristics]
Range Extender・ Capable of adapting to a variety offuels
・ Small size, Light weight[Usage]・ Supply air to the fuel cell stack[Characteristics]
Electric compressor
・ Small size, light weight, highefficiency, low noiseOverview

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)6

5.1. Electrically-assisted turbocharger
The electrically-assisted turbocharger installed coaxially with the motor and the turbocharger
is driven by the electric motor and the exhaust turbine. Since the turbine driving force is assisted by
the motor, boost pressure can be improved at low engine load with low exhaust gas energy andtransient response. Therefore, the low-speed torque and engine transient response are improved.
Since the size is not significantly different from a conventional turbocharger, the engine room
mounting is excellent, and it is also possible to regenerate the exhaust gas energy as electric powerby utilizing the motor as a generator in the operation range. At low engine speed, the response isimproved more than 80% in comparison with a conventional turbocharger. MHIET is developing
several kinds of electrically-assisted turbochargers such as a maximum output exceeding 5 kW
according to the turbocharger size.

5.2 Micro Gas Turbine for Range Extender
MHIET has been developing a gas turbine auxiliary generator for range extender EV byapplying turbocharger technology. The environmental impact of EV is small, and it is anticipatedthat the popularization will advance in each country in the future. However, there are still many
problems such as the high battery cost and long charging time. Therefore, by mounting a small
generator with an internal combustion engine in an EV, it is considered that the driving range canbe extended and the charging timing of the battery can be improved. A motor generator is installed
coaxially with a micro gas turbine developed based on a turbocharger. The turbine is driven by
combustion gas from the combustor, compressed air is supplied to the combustor by a coaxial
compressor, and surplus turbine output is recovered as electric power. Compared with a generator
using a reciprocating engine, a gas turbine is compact, provides high output and cleaner exhaustgas, so that it may be able to satisfy the exhaust gas regulations without a post-processing device.
Furthermore, the gas turbine has an advantage in that it can deal with various fuels simply bychanging the combustor. Since gas turbines and turbochargers have many parts with commonfunctions, it is possible to apply the turbocharger technology to the cost reduction of the system.
Compact and highly-efficient generators are considered to be an effective technology not only forEV, but also for the range extension of other mobility platforms such as commercial vehicles,aircraft, and ships, which are expected to be electrified in the future.

5.3 Electric compressor for FCVElectric compressor for FCV can control the boost pressure at fast response for the output
demand of the fuel cell stack. In addition, miniaturization, higher efficiency, and noise reduction
are also important elements, and MHIET has been developing an electric compressor for FCV
applying the turbocharger and electric supercharger technology. In addition, by adopting acentrifugal compressor with one shaft and two stages, high boost pressure and long life of thebearing are possible, while also lowering the motor rotational speed.

|6. Conclusion
This paper introduces the electric supercharging-related technologies that MHIET isdeveloping for powertrain electrification. Among these technologies, the possibility of powerperformance improvement and fuel consumption reduction in the electric supercharger for 48V
MHEV was confirmed through experiments and simulation. In the future, MHIET would like tocontribute to the realization of a sustainable society by providing a turbocharging systemcorresponding to various powertrain technologies.
*1 ECU: Electronic Control Unit. An electronic controller for engines.
*2 CAN: Controller Area Network. A communication standard between electronic devices. It is used forcommunication between electronic devices used in automobiles.
*3 Miller cycle: A technology that shortens the compression stroke of the engine by controlling the intake
valve of the engine and substantially increases the expansion stroke. The application of Miller cycle
improves the thermal efficiency of the engine and reduces the fuel consumption. However, engine torquedecreases because the inflow of fresh air decreases.
*4 EGR: Exhaust Gas Recirculation. To recirculate part of the exhaust gas of an engine to the intake side.
In gasoline engines, by applying EGR, fuel consumption is reduced through the reduction of pressureMitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)7loss in the throttle valve, improvement in margin to knocking, and improvement in terms of heat loss.
However, engine torque decreases because the ratio of fresh air to total air volume decreases.
*5 Turbocharging Downsizing: To achieve the same output as a large engine with a small engine with aturbocharger. Through this, the engine mechanical loss is reduced, and it is effective for fuelconsumption improvement.
*6 SOC: State Of Charge. The amount of charge on the battery.
*7 Pumping loss: Loss caused when the piston pushes out the exhaust gas in the exhaust process due topressure loss in the exhaust path.

References(1) Yukio Yamashita, et al., Development of High-speed Single-phase Motor for Automotive Turbocharger,
Mitsubishi Heavy Industries Technical Review Vol. 56 No. 1 (2019)(2) Byeongil An et al., Development of Two-stage Electric Turbocharging System for Automobiles,
Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (2015) p. 76 to 81(3) Yukio Yamashita, et al., Development of Electric Supercharger to Facilitate the Downsizing ofAutomobile Engines, Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (2010) p. 17 to 12(4) Y. Yamashita et al., Two-stage boosting system using 48 volt electric compressor, 22nd Supercharging
Conference 2017