Development of Variable Two-stage Turbocharger for Passenger ...

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
Development of Variable Two-stage Turbocharger for
Passenger Car Diesel Engines
*1 Turbocharger Engineering Department, General Machinery & Special Vehicle Headquarters
BYEONGIL AN *1 TAKASHI SHIRAISHI *1
To cope with the increasingly stringent emission regulations of European Union (EU)
countries, conventional turbochargers are being increasingly replaced by variable geometry
(VG) turbochargers. Currently, passenger car diesel engines require high low-speed torque,
high power, and a quick transient response. This report introduces the development of a variable
two-stage turbocharger, which will be used mainly for passenger car diesel engines, as well as
our design technology ensuring its performance and durability, and the engineering techniques
used to achieve its practical utility.
|1. Introduction
In European Union (EU) countries, diesel engine passenger cars are used widely because of
their favorable fuel consumption; in fact, the number of diesel cars is believed to have surpassed
the number with gasoline engines. With the anticipated recovery from economic recession, the
popularity of diesel cars and turbocharger demand are expected to increase in newly developed
countries, such as the BRIC (Brazil, Russia, India, and China) and Asian countries. With the
progress in countermeasure techniques for exhaust gases, the emission regulations in EU countries
have been strengthened significantly. To meet these requirements, increasing numbers of variable
geometry (VG) turbochargers are being installed in diesel engine passenger cars. The VG
turbocharger can generate the required boost pressures at all engine operating ranges, and increase
the torque, as well as improve the fuel consumption and reduce the particulate matter (PM). It can
also control the exhaust gas recirculation (EGR) by adjusting the exhaust pressure, and is effective
at reducing nitrogen oxides (NOx).
To meet the increasingly stringent regulations, turbocharger manufactures are developing
new supercharging systems utilizing more advanced technologies. Passenger car diesel engines
require high low-speed torque, high power, and a quick transient response. This report introduces
the development of a variable two-stage turbocharger, which will be used mainly for passenger car
diesel engines, as well as our design technology ensuring its performance and durability, and the
engineering techniques used to achieve practical utility.
|2. Evaluation of the variable super charging system
To meet future exhaust gas regulations, a new technology is required to improve
performance characteristics significantly. Table 1 shows the comparison of variable turbocharging
systems applicable to current engines and their characteristics. Figure 1 shows the evaluation
results for different variable turbocharger systems. This shows that a variable two-stage
turbocharger is currently the best option for diesel engines when it comes to balancing the needs for
high low-speed torque, high power, and a quick transient response without major alterations.
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Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
Table 1 Comparison of variable turbocharging systems
Turbocharger system Advantages Disadvantages
VG turbocharger with ・ Low-speed torque increase
・ Durability must be secured.
variable compressor (Wider compressor range) ・ Complex control
Mechanical
・ Better transient response
・ Complicated packaging
Supercharger(S/C) ・ Low-speed torque increase
・ Engine must be modified.
+ turbocharger
・ Complex control
・ Better operability/transient response ・ High-speed motor inverter is necessary.
Electrical compressor ・ Low-speed torque increase
・ Noise problem
+ turbocharger ・ Higher power
・ Complicated packaging
・ Complex control
Variable
turbocharger
・ Better transient response
two-stage
・ Low-speed torque increase
・ Higher power
・ Complicated packaging
Turbocharger System
Low-Speed
Torque
(Response)
Rated
Power
Exhaust
Gas
Cost
Easy to
Mount
Technical
Problems
VG Turbocharger 0 0 0 0 0 0
VG Turbocharger with
Variable Compressor
Mechanical
Supercharger (S/C)
+Turbocharger
Electrical Compressor
+Turbocharger
Variable Two-stage
Turbocharger
+ 0 0 - 0 --
+++ 0 0 -- -- --
+++ + + -- - --
++ + + - - 0
Figure 1 Evaluation results for different turbocharger systems 0 : Same level as the VG
+ : Better than the VG
- : Worse than the VG
|3. Development of a variable two-stage turbocharger
3.1 Working principles and control method
The variable two-stage turbocharger described here can changeover between single- and
two-stage turbocharging, and consists of large and small turbochargers, activation of an exhaust gas
flow control valve between the low-pressure stage turbine inlet and exhaust manifold, and a bypass
valve at the high-pressure stage compressor inlet. When the engine speed is low, both the high- and
low-pressure turbochargers are activated. When the engine speed is increased, the flow rate of the
high-pressure turbocharger is reduced by adjusting the exhaust gas flow control valve. Finally, only
the low-pressure turbocharger is used as a single- stage turbocharger.
Figure 2 shows a schematic of the variable two-stage turbocharger that is controlled in the
following manner:
Mode 1: Complete two-stage turbocharging with all of the control valves closed at ultra-low
engine speeds.
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Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
Mode 2: The exhaust gas flow control valve is adjusted at low and intermediate speeds. In
this mode the compressor bypass valve and the waste gate valve are closed.
Mode 3: The low-pressure stage turbocharger solely works as one-stage turbocharging while
the high-pressure turbocharger is in an idling.
Mode 4: The waste gate valve is controlled when the boost pressure cannot be adjusted in
Mode 3.
Figure 2 Schematic of the variable two-stage turbocharger
Figure 3 shows the compressor operating map of the variable two-stage turbocharger. The
variable two-stage turbocharger allows application over a wide range using two compressors with
high- and low-pressure stages. It also allows matching of the two-stage turbocharging at low and
intermediate engine speeds, and matching in single-stage turbocharging at high speeds with
increased flexibility. Figure 4 shows the control map of the variable two-stage turbocharger. By
utilizing a small flow rate compressor in the high-pressure stage and a large flow rate compressor
in the low-pressure stage, a broader compressor operating range is available. During engine
acceleration, all of the exhaust gas flows into the small turbine and the transient response is
improved markedly. At high-speed operation, direct flow to the large turbine allows appropriate
matching and a high engine power.
Figure 3 Compressor operating map of the variable two-stage turbocharger
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Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
Operating
area
Figure 4 Control map of the variable two-stage turbocharger
Exhaust
Compressor
flow control
bypass valve
valve
Waste gate
valve
Mode 1 Closed Closed Closed
Mode 2 Controlled Closed Closed
Mode 3 Open Open Closed
Mode 4 Open Open Controlled
3.2 Features of our product
With recent advances leading to higher engine power and torque, the demand for passenger
cars with 2-liter engines has increased in the Euro 5 and Euro 6 markets. Consequently, a compact
design applicable to 2-liter engines is essential for developing the variable two-stage turbocharger.
Together with the development of each component, to reduce size and improve control
performance, which are major issues for the variable two-stage turbocharger, development had
achieved the following three targets:
(1) Improved controllability over the range of variable two-stage turbocharging
(2) Compact size by the installation of a bypass passage on the high-pressure compressor
(3) A compact, easy-to-assemble design that integrates the turbine housing of high-pressure stage
and valve case
Figure 5 outlines points in the development of a variable two-stage turbocharger.
Development point A is the spherical flow control valve and seating ring that allow mild linear
control of exhaust flow in accordance with the valve opening aperture in variable two-stage
turbocharging, which is an improvement over the conventional ON/OFF valve control. Point B
involves the addition of a bypass passage on the high-pressure compressor cover and the
integration of a bypass valve driven with a poppet. With this, a compact design is achieved with the
elimination of unwanted piping. Point C is the integration of the turbine housing of the
high-pressure stage and valve case of the exhaust flow control to improve the ease of assembly.
Figure 5 Points developed for the variable two-stage turbocharger
3.3 Evaluation of engine performance and durability
A 2-liter class diesel engine was used for the evaluation test. Table 2 shows the
specifications of the engine and turbocharger. Figure 6 shows the rig test used for an engine with
the variable two-stage turbocharger.
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Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
Table 2 Specifications of the engine and turbocharger
Engine specifications 2.0L class diesel
Max torque (N•m) 340 (@2,000 rpm)
Rated power (kW) 110 (@4,000 rpm)
High-pressure stage turbocharger Low-pressure stage turbocharger
Turbo specifications TD025S TD04H
Waste gate valve No Yes
Figure 6 Test rig for an engine with a variable two-stage turbocharger
The test engine has four cylinders and the common rail injection system. The increase in the
low-speed torque was intended while keeping the high-speed engine power. The performance was
tested at engine speeds between 1,000 and 4,000 rpm at 250-rpm intervals. During the test,
temperature, pressure, and engine speed were measured to analyze the engine and turbocharger
performance, while controlling the exhaust flow, compressor bypass, and waste gate valves. As
limiting conditions, the exhaust temperature and pressure, and outlet temperature and pressure of
the high-pressure stage compressor were configured. After completing the test, a variable two-stage
turbocharger was analyzed and compared with a standard VG turbocharger. Figure 7 shows the
results of the engine full-load performance test. The torque was increased 56% at 1,000 rpm and
45% at 1,250 rpm, as compared to the VG turbocharger. The difference at intermediate to high
speeds of 2,000 rpm or more was small, partly because of the limitations of the Engine Control
Unit; however, the torque might be increased in the high-speed range with high-speed focused
matching. Figure 8 shows photographs of the rig test and control valve after the high-temperature
endurance test. The valve operating endurance test involved 300,000 cycles consisting of low
temperature for 3 seconds and high temperature for 3 seconds to simulate real engine operation.
After the endurance test, no obvious problems were seen with the valve moving parts or actuator.
Figure 7 Results of the engine full-load performance test
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Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
Figure 8 Photographs of the test rig and control valve after the high-temperature
endurance test
Figure 9 shows photographs of the turbocharger after a low-cycle endurance test consisting
of 1,800 cycles of idling for 5 minutes and full-load for 5 minutes, simulating 300 hours of real
engine operation. The endurance test verified the robust performance and durability. Currently,
sample turbochargers have been supplied to customers for field evaluations before starting mass
production.
Figure 9 Photographs of the turbocharger after a low-cycle endurance test
|4. Conclusion
A variable two-stage turbocharger that increases both low-speed torque and full-load engine
power showed a satisfactory, quick transient response as a promising supercharging system. This
project established proprietary technology that can be distinguished from the systems made by
other manufacturers. Its performance and durability were verified in a test diesel engine for 2-liter
class application. We are also expanding the technology to other classes of displacement engine.
We hope that this development will contribute to advances in MHI turbocharger products.
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