Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)77

High-speed Model Predictive Control for
Next-Generation Turbocharging System

YUSUKE YASHIRO*1

VISSUTA JIWARIYAVEJ*2

YUKIO YAMASHITA*3 TORU HOSHI*4KIYOSHI TERAKADO*5 SEIICHI IBARAKI*6

As the use of turbocharged engines has expanded in recent years, turbocharging systems,
such as two-stage turbochargers, have become more complex, and as a result, the issue ofcontrolling such systems has become more challenging. This paper presents a high-speed optimalcontrol method developed to address this issue. This model predictive control contains an internalmathematical model of the engine and turbocharger, allowing it to predict system behavior. By
adopting this model predictive control and with some further algorithm improvements, this control
system can now be executed in real-time. Simulation results show that, compared with conventional
systems, an automotive supercharged engine with the proposed method could attain boost pressure
response improvement of approximately 50%. In addition, the basic operation of this control
method was verified using an in-house engine. With our progress in this development, Mitsubishi
Heavy Industries, Ltd. (MHI) is aiming to actively participate in the development of
next-generation products such as two-stage electric turbocharging systems in the future.

|1. Introduction
With the trend of downsizing automotive engines in recent years, the number of engines
equipped with a turbocharger system consisting of a turbine and a compressor has been increasing.
For this reason, competition has become increasingly intense, and thus further enhancement ofengine performance is required to survive in the market. For the enhancement of turbochargers,
more sophisticated turbocharging systems such as VG (variable geometry) turbochargers, which
are turbochargers equipped with a variable nozzle, and two-stage turbocharging systems, which arecomposed of two turbochargers and a bypass valve, have been put on the market. In addition,
electric compressors driven by an electric motor instead of a turbine are being researched as
next-generation products1. With such a trend, the increase in the number of controlled points in aturbocharging system caused by further system sophistication is inevitable. In response to thissituation, we developed a turbocharger control method that uses model predictive control (MPC) to
optimally and simultaneously control multiple controlled points. By using a model with enhanced
calculation speed to perform optimization, this control method for a more sophisticatedturbocharging system has become possible to be implemented in real-time. The results show animprovement in response by 30% or more in comparison with conventional turbochargers.
Furthermore, this method not only controls the turbocharger, but can also be used in combination
with an engine simulation system such as GT-Power(R) (Note) etc., to expedite turbocharger matching,which is a process to select the turbocharger best suited to the customer’s engine. This paper
presents a summary of MPC, its application in turbocharger control, and the verification results ofthe control performance obtained in simulations and bench testing.
(Note) Engine performance calculation software widely used in the automobile industry*1*2*3*4*5*6

System Technology Development Department, ICT Solution Headquarters
Electricity & Applied physics Research Department, Research & Innovation Center
Chief Staff Manager, Electricity & Applied physics Research Department, Research & Innovation Center
Fluid Dynamics Research Department, Research & Innovation Center
Combustion Research Department, , Research & Innovation Center
Chief Staff Manager,, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)78|2. Advance turbocharging system
Figure 1 and Figure 2 show the two turbocharging system configurations that are describedin this paper. The two-stage VG turbocharging system shown in Figure 1 is equipped with one
large and one small VG turbocharger, and therefore there are four controlled points in total,
consisting of two variable nozzles of the VG turbochargers and two bypass valves for switching
between single-stage turbocharging and two-stage turbocharging. On the other hand, Figure 2
shows the two-stage electric turbocharging system, in which one of the turbochargers is replaced byan electric compressor that can be driven at an arbitrary timing by the electric motor, unlike aturbocharger that is rotated passively by exhaust gas. This two-stage electric turbocharging system
is a next-generation product that is superior in response compared with a two-stage VGturbocharger, and it contains three controlled points including the variable nozzle, the torque of theelectric motor and the bypass valve.
It is necessary to appropriately control the multiple controlled points to maximize thepotential of the two systems, and we hope such optimal control ability of turbocharging systems
will be included in the evaluation during the engine development phase by our customers, such asengine and automobile manufacturers. The next section explains the turbocharger selection process
and the control simulation technique that was developed to meet the speed of customerdevelopment.

Figure 1 Configuration diagram of two-stage VG turbocharger
This figure shows the layout of the two-stage turbocharging system with twoVG turbochargers combined with a bypass valve.

Figure 2 Configuration diagram of two-stage electric turbocharging system
The low-pressure stage is replaced with an electric compressor with no turbine.

|3. Application of MPC to turbocharger control
Figure 3 shows the diagram of the turbocharger control logic using MPC. The necessary
inputs of MPC are the physical quantities measured in the intake and exhaust manifolds of theengine and the target boost pressure. MPC uses these inputs as the initial value of the current timeMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)79

step and calculates the predicted physical quantities of future time steps using the mathematicalmodel of the system. From these predicted physical quantities and the optimization calculation
based on cost function, the optimal array of control commands is generated and output as an actualcontrol input (operating amount) to each controlled point. The challenge in realizing MPC lies in its
high computational load due to the optimization required in each of the control periods. However,adopting the mean value engine model (MVEM) as an internal model enables the faster calculationof the physical quantities necessary for MPC, and therefore real-time implementation for actual
devices has become possible.
The benefits of the adoption of this method are that rather than individual optimization foreach controlled point, it performs optimization for the overall system, and based on the constraints
set for optimization, the risk of using a device exceeding its design limit value can be avoided inadvance. These benefits allow us to compare the maximized potential of various sophisticatedturbocharging systems, and enable us to promptly offer customers turbocharging systems best
suited to their engines.

Figure 3

Turbocharger control using MPC

This figure shows the calculation executed in MPC..

|4. Verification results
4.1

Simulation verification

In order to verify the boosting response enhancement effects of MPC, third-gear tip-in
operation was simulated using the MVEM of a 2.0-liter diesel engine. Figure 4 shows the block
diagram of the simulator. In the simulation, the engine of interest was also modeled by MVEM toenable real-time operation of the whole simulation including the control. As a result, the optimum
turbocharger could be selected rapidly from the turbocharger lineup. The calculation speed in thecase of a simulation using the combination of MVEM and MPC was approximately 1200 times
faster in comparison with that of commercially available simulators using a detailed engine model.
In addition, the simulator contains a vehicle model consisting of the vehicle weight, the gear
ratio, etc., and enables the evaluation of the engine torque required for third-gear tip-in operation
where the engine torque target value is increased in a stepwise manner with the transmission heldin third gear, as well as the resulting change in the engine rotation speed, while taking into
consideration the transient characteristics.
Figure 5 shows the simulation results. The two-stage VG turbocharging system and thetwo-stage electric turbocharging system can improve the boost pressure response time byapproximately 30% and 50%, respectively, in comparison with a single-stage VG. The two-stage
VG turbocharger is equipped with an additional small VG turbocharger in the high-pressure stageMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)
80

that is optimally controlled by MPC, and therefore attains excellent boost pressure response.
Moreover, the two-stage electric turbocharging system can improve boost pressure response byeven more than that of the two-stage VG turbocharger due to the use of an electric compressor witha rotation speed that can be increased independently of the exhaust gas.

Figure 4

Block diagram of simulator

This figure shows the relation of the control method described in this paper to the overall simulator.

Figure 5 Results of third-gear tip-in simulation
This figure shows the response improvement as a result of using the two systems described in thispaper in comparison with an existing single-stage VG turbocharger.

4.2

Verification test with small engine for construction machine

For the verification of MPC on an actual device, an engine bench test was performed.
Figure 6 shows an overview of the test facility. In the test, a two-stage VG turbocharging system
was equipped on a 2.2-liter diesel engine for a construction machine, which require exhaust gas
reduction similarly to passenger cars. For the control of the actual turbocharger, a rapid-prototype
controller that can embed the control program used for simulation directly into actual equipment
was used. As described in section 3, the embedded control program uses MVEM and therefore canbe executed in real-time for the test.
Figure 7 shows the test results of the test case where engine control is constant speed
control, while the load is increasing for an engine with a two-stage VG turbocharging system.
When MPC was applied, the drop in engine speed was recovered approximately 50% faster thanthat of the case where the control using a VG opening map, constructed from VG openings thatmaximize the steady-state fuel efficiency, was applied. Figure 8 shows the controlled operation of
the two variable nozzles. To increase the boost pressure, MPC sends the command to close thevariable nozzle to the optimal opening degree, and therefore the engine speed drop is recovered in
the minimum amount of time.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)
81Figure 6Test facility and test engineThis figure shows a photograph of the test facility used for the bench test
and the 3D-CAD drawing of the test engine.

Figure 7 Bench test results (engine speedand torque)This figure shows the behaviors of the engine speed
and the torque from the bench test results. It is
indicated that the engine speed decreased when theload was increased and the recovery time differed
between the control methods.

Figure 8 Bench test results (controlledoperation of high-pressure stage VG
and low-pressure stage VG)
This figure shows the controlled operation of two VG
turbochargers from the bench test results. The
difference between the controlled operations of MPC
and the compared map control is indicated.

|5. Conclusion
MHI has developed an optimal control method for more sophisticated turbocharging systems
that integrates MVEM and MPC and is capable of high-speed calculation. For the performanceverification of the method, simulation was performed using a model including a two-stage VG
turbocharging system and a two-stage electric turbocharging system, and significant improvement
in response was verified. In addition, through bench testing with a small engine for a construction
machine, basic control operation was verified and the improvement in response was confirmed.
With this development, MHI can offer customers a complete solution for turbocharging
systems that are best suited to the customer's engine combined with the optimal control scheme.
This will also contribute to the development of engine systems that will become increasingly
complicated in the future. We are also putting our best efforts toward enhancing our ability to meetthe needs of customers in other areas, in addition to the response described in this paper, such as
the optimization of fuel efficiency and emissions. Furthermore, we plan to extend the use of theMPC method to products other than turbochargers, as well to enhance product competitiveness.
This control method was developed in cooperation with the University of Cambridge (UK).
We would like to express our appreciation to Professor Emeritus Nick Collings, Professor Emeritus
Keith Glover, and Doctor Paul Dickinson for their contributions.

References
1.
2.
3.

An, B. et al., Development of Two-stage Electric Turbocharging system for Automobiles, Mitsubishi Heavy Industries
Technical Review Vol. 52 No. 1 (2015) pp. 76-81
Yashiro, Y. et al., Development of Model Predictive Control ｆ or Two-Stage Turbocharging System, 20thSupercharging Conference 2015 paper, pp. 267-288 (2015)
Paul, D. et al., On-Engine Validation of Mean Value Models for IC Engine Air-Path Control and Evaluation, The 19th
IFAC World Congress paper (2014)