Technology explained: Variable Turbine Geometry
Turbocharger use exhaust gas to power a turbine-driven pump that forces air into the intake manifold at a greater-than-atmospheric pressure (hence the term ‘forced induction’).
The bigger the turbocharger, the greater the pressure entering the engine, resulting in a greater air/fuel mixture and more power. However, continually bolting on larger turbos in order to seek power gains leads to the turbocharger’s inherent fault: lag.
In order to counter this, smaller turbochargers (with their inherently lighter turbines) take less force to spool up, resulting in increased response. Yet, due to their smaller size, they are unable to keep up with the engine’s demand for more air at greater speeds.
In order to provide the best of both worlds, though, Porsche has been using variable turbine geometry technology on its turbocharged engines since 2005.
While this solution found its way into turbo diesel engines over 20 years ago, the higher exhaust gas temperatures found in Porsche’s forced induction petrol motors (around 1,000 degrees Celsius) made implementing this solution difficult.
However, thanks to 21st Century material technology and Porsche’s use of an additional water-cooling system (with an after-run pump) made VTG possible on the first generation of 997 Turbo.
Inside the body of a Porsche VTG turbocharger, sitting around the outside of the turbine, are a collection of guide vanes. The position of these electronically controlled blades can be adjusted depending on the engine speed.
At low rpm, the Motronic ECU system causes the guide vanes to be tilted until they are almost flat, creating a small gap through which the exhaust gas passes. By being forced through a small gap, the gas is accelerating, spinning the turbine with greater force than a non-VTG turbo.
This enables the turbocharger to ‘spool up’ faster, resulting in improved low-end response. Once the boost level has reached 1 bar (in the 997 Turbo), the guide vanes are opened via the electrically driven adjuster within 100 milliseconds.
This creates a large area through which the exhaust gas is driven, improving the turbocharger’s breathing at high engine speeds and negating the need for a bypass valve. This allows the turbocharger to keep operating efficiently, resulting in the Turbo’s famous flat torque curve.
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