20

Air flow rate m3 at 200C

Figure 7.22 Air flow ranges of Mitsubishi MET-SE and SEII series turbochargers

Introduced in 2001 to boost two-stroke engines powering large containerships, the MET90SE design has an air flow range some 30

Air flow rate m3 at 200C

Figure 7.22 Air flow ranges of Mitsubishi MET-SE and SEII series turbochargers per cent higher than that of the 83SE (formerly the highest capacity model) but is only 10 per cent larger. The higher capacity from a modest increase in dimensions was achieved by proportionally enlarging the rotor shaft and developing a new compressor impeller wheel and turbine blades. Expanding the capacity of such wheels and blades without compromising reliability and performance is a key challenge for turbocharger designers, Mitsubishi notes.

Different compressor impeller materials—such as titanium and cast steel—have been investigated as an alternative to the forged aluminium traditionally used in MET turbochargers. A pressure ratio of 4 with an aluminium impeller was nevertheless achieved thanks to an impeller air cooling system and a redesigned compressor blade. By reducing the impeller backward curvature angle from 35 degrees to 25 degrees the pressure ratio could be raised without the need to increase the impeller peripheral speed.

Mitsubishi successfully developed a cast stainless steel compressor impeller in preference to an aluminium alloy component for its MET-SH turbocharger. Raising the compressor pressure ratio to the higher levels increasingly demanded by the market dictates attention to compatibility between the impeller strength and design and the aerodynamic design. Mitsubishi cites, respectively, the problems of higher impeller peripheral speed, elevated impeller blade temperature and increased impeller blade surface load; and the problem of optimizing the aerodynamic characteristics of the higher speed air flow in the impeller.

Limitations in resolving all these problems are presented by an impeller of aluminium alloy, a material which Mitsubishi has traditionally used for MET turbochargers because of its high specific strength and excellent machin ability. Running the aluminium alloy impeller at temperatures above the ageing treatment level (190-200°C), however, threatens a reduction in material strength and should be avoided. Depending on the suction air temperature, the impeller blade could reach this temperature level when operating at a pressure ratio of around 3.7.

An aluminium alloy forging also experiences progressive creep deformation at temperatures of around 160°C and above, calling for consideration in design to preventing the stress from becoming excessive at impeller blade areas subject to high temperature.

The stainless steel impeller casting for the SH series turbocharger delivered improved high temperature strength and made it possible to raise the maximum working pressure ratio to 4 and higher, without reducing the impeller peripheral speed. Improved material strength allowed the impeller blade to be smaller in thickness than its aluminium alloy counterpart, despite the higher pressure ratio and increased peripheral speed. This feature in turn made it possible to increase the number of blades while ensuring the necessary throat area, hence improving the impeller's aerodynamic characteristics. The stainless steel impeller of the MET42SH turbocharger, the first production model in the MET-SH series, features 15 solid-formed full and splitter blades.

A higher density impeller casting called for special consideration of the rotor stability. The rotor shaft of the SH series turbocharger is larger in diameter than that of the SD series, with the compressor side journal bearing located outside the thrust bearing to increase the distance between the two journal bearings. Tests showed that, despite the higher mass of the stainless steel impeller, the 3rd critical speed for the rotor is around 50 per cent higher than the maximum allowable speed, underwriting rotor security.

A turbocharger total efficiency of 68 per cent was logged by the MET42SH version fitted with impeller blading having a large backward angle and operating at a pressure ratio of 4.3 on a diesel genset.

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