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  This article is from From Autozine.org   Variable Turbine Geometry (VTG)

Variable Turbine Geometry technology is commonly used in turbo diesel engines in recent years. It is primarily used to reduce turbo lag at low engine speed, but it is also used to introduce EGR (Exhaust Gas Recirculation) to reduce emission in diesel engines. Here, we concentrate on the former advantage. Ordinary turbochargers cannot escape from turbo lag because at low engine rpm the exhaust gas flow is not strong enough to push the turbine quickly. This problem is especially serious to modern diesel engines, because they tend to use big turbo to compensate for their lack of efficiency. A Variable Geometry Turbocharger is capable to alter the direction of exhaust flow to optimize turbine response. It incorporates many movable vanes in the turbine housing to guide the exhaust flow towards the turbine. An actuator can adjust the angle of these vanes; in turn vary the angle of exhaust flow. Look at the following illustration: At low rpm: 

The vanes are partially closed, reducing the area hence accelerating the exhaust gas towards the turbine. Moreover, the exhaust flow hits the turbine blades at right angle. Both makes the turbine spin faster.  

At high rpm :
At high rpm the exhaust flow is strong enough. The vanes are fully opened to take advantage of the high exhaust flow. This also releases the exhaust pressure in the turbocharger, saving the need of waste gate. 

VTG on gasoline engines Although VTG technology is extensively used in diesel engines; it is very much ignored in gasoline engines. This is because the exhaust gas of gasoline engines could reach up to 950°C, versus 700-800°C in diesel engines. Ordinary materials and constructions are difficult to withstand such temperature reliably. In 1989, Honda produced a handful of Legend Wing Turbo, which employed a variable geometry turbocharger developed by itself. Its variable vanes (”wings”) were made of a special heat-resisting alloy, Inconel. Nevertheless, the experimental production run was never followed by mass production. In the next one and a half decade Honda simply gave up turbo charging in all its petrol cars. In the same 1989, Garrett produced a VTG turbocharger for use in the limited production Shelby CSX, a car derived from Dodge Shadow. However, only 500 cars were produced. Neither Chrysler group nor any other car makers would follow its footprints. As compression ratio increases, modern gasoline engines have exhaust temperature higher and higher. Experts estimated it could exceed 1000°C in the foreseeing future. Perhaps this is why VTG technology for gasoline engines never went into mass production. In 2006, BorgWarner finally developed a VTG turbocharger for use in Porsche 911 (997) Turbo. Both firms refused to reveal the technical details, but said it employed “temperature-resistant materials derived from aerospace technology”. Hopefully the technology breakthrough will finally bring VTG turbochargers into mass production gasoline engines.