Forced Induction

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 release the exhaust pressure in the turbocharger, saving the need of wastegate.


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 turbocharging 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.

Everybody knows mechanical superchargers are good for low end output but short of efficiency at high rev, while exhaust turbochargers works strongly at high rev but reluctantly at low rev. For decades engineers dreamed of combining supercharger and turbocharger together. This was tried once in history – the 1985 Lancia Delta S4 rally car. The car was successful in motorracing, but the technology never extended to production.

In 2005, Volkswagen finally introduced a production unit to its Golf 1.4 TSI. Called "Twincharger" system, it is actually developed by supercharger maker Eaton. It connects a supercharger and a turbocharger in series.

At low rev, the supercharger provides most of the boost pressure. The pressure it built up also speeds up the turbocharger so that the latter can run into operating range more quickly.

At 1500 rpm, both chargers contribute about the same boost pressure, with a total of 2.5 bar. (If the turbocharger work alone, it can only provide 1.3 bar at the same rev.)

Then the turbocharger – which is optimized for high-rev power – started taking the lead. The higher the rev, the less efficient the Root-type supercharger becomes (due to its extra friction). Therefore a by-pass valve depressurize the supercharger gradually.

By 3500 rpm, the turbocharger can contribute all the boost pressure, thus the supercharger can be disconnected by an electromagnetic clutch to prevent from eating energy.

In the 1.4-litre Golf, the Twincharger system produces 170 horsepower and 177 lbft of torque. That's equivalent to a 2.3-litre normally aspirated engine but it consumes 20% less fuel.

Copyright© 1998-2005 by Mark Wan
AutoZine Technical School
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