Development of Variable Two-stage Turbocharger …

1 Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010) 1 *1 Turbocharger Engineering Department, General Machinery & Special Vehicle Headquarters Development of Variable Two-stage Turbocharger for Passenger Car Diesel Engines 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.

2 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.

3 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 1shows 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.

4 Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010) 2 Table 1 Comparison of Variable turbocharging systems Turbocharger system Advantages Disadvantages VG Turbocharger with Variable compressor Low-speed torque increase Wider compressor range Durability must be secured. Complex control Mechanical Supercharger(S/C) + Turbocharger Better transient response Low-speed torque increase Complicated packaging Engine must be modified. Complex control Electrical compressor + Turbocharger Better operability/transient response Low-speed torque increase Higher power High-speed motor inverter is necessary. Noise problem Complicated packaging Complex control Variable Two-stage Turbocharger Better transient response Low-speed torque increase Higher power Complicated packaging Turbocharger System Low-Speed Torque (Response)Rated PowerExhaust Gas Cost Easy to Mount Technical Problems VG Turbocharger 0 0 0 0 0 0 VG Turbocharger with Variable Compressor+ 0 0 - 0 -- Mechanical Supercharger (S/C)

5 + Turbocharger +++ 0 0 -- -- -- Electrical Compressor+ Turbocharger +++ + + -- - -- Variable Two-stage Turbocharger ++ + + - - 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 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.

6 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. Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010) 3 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 .

7 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 Mitsubishi Heavy Industries Technical Review Vol.

8 47 No. 4 (December 2010) 4 Operating area Exhaust flow control valve Compressor bypass valve Waste gate valve Mode 1 Closed Closed Closed Mode 2 ControlledClosed Closed Mode

9 3 Open Open Closed Mode 4 Open Open Controlled Figure 4 Control map of the Variable Two-stage Turbocharger 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 .

10 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 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 .