BEAUTIFUL VIEW

Figure 11.2 shows a cutaway of a turbocharger. Turbochargers consist of an exhaust gas-driven turbine and a radial air compressor mounted at opposite ends of a common shaft (Fig. 11.3) and enclosed in cast housings. The shaft itself is enclosed and supported by the center housing, to which the compressor and turbine housings are attached. The turbine section is composed of a cast turbine wheel, a wheel heat shroud and a turbine housing, with the inlet on the outer surface of the turbine housing. It  functions as a centripetal, radialor mixed-inflow device in which exhaust gas flows inward, past the wheel blades, and exits at the center of the housing. The expanded engine exhaust gas is directed through the exhaust manifold into the turbine housing. The exhaust gas pressure and the heat energy extracted from the gas cause the turbine wheel to rotate, which drives the compressor wheel. The Ni-based super alloy Inconel 713C (see Table 11.1) is widely used for the turbine wheel.4 A typical microstructure is shown in Fig. 11.4. GMR235, which reduces costs by increasing the iron content, is also used. For much higher temperatures, Mar-M247 is used. The response and combustion efficiency of the wheel in acceleration is related to the inertial moment, a function of the weight. The lower the weight, the lower the inertial moment  and therefore the faster the response. Ceramic wheels5 have been developed, but low toughness means that the blades must be thick, making it less easy to adjust the weight. A wheel made from the intermetallic compound TiAl by investment casting has been marketed.6 It has a specific gravity of 3.9 g/cm3, which is much lower than ordinary titanium alloy, and a tensile strength as high as 600 MPa at 700 °C.

The compressor section is composed of a cast compressor wheel, a backplate and a compressor housing, with the inlet at the center of the compressor housing. It is a centrifugal or radial-outflow device, in that the air flows outward, past the wheel blades, and exits at the outer edge of the housing. The rotating compressor wheel draws ambient air through the engine’s air filtration system. The blades accelerate and expel the air into the compressor housing, where it is compressed and directed to the engine intake manifold through ducting. The compressor wheel does not have such a high heat resistance requirement, so a cast aluminum wheel (C355) is widely used. A cast Ti-6Al-4V alloy is also used for heavy-duty commercial diesels.

The science and technology of materials in automotive engines

Hiroshi Yamagata

Woodhead Publishing and Maney Publishing

on behalf of

The Institute of Materials, Minerals & Mining

CRC Press

Boca Raton Boston New York Washington, DC

WOODHEAD PUBLISHING LIMITED

Cambridge England

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