As most of us have most likely already observed, turbocharging is present in the majority of new vehicles sold today. With reliability issues kinked out of older examples, the turbo is the way to go if you like efficiency. A basic principle, more dense air into the engine means efficient combustion, leading to the downsizing of engines, and resulting in lighter vehicles further achieving better mpg.
Sure, we all love the turbo, but we all hate turbo lag. Big turbos are great for power, but the problem is at low engine speeds (RPMs) the turbo has a tough time spinning back up to the appropriate speed.
A common solution to the problem is sequential twin-turbocharging. This basically means that there are two turbochargers, one little one that can spool up quickly, and one big slow one for pushing air into the engine. The little one spools up to help the big one turn faster. Hence the name, sequential. While sequential turbocharging is great in many ways, it almost doubles the complications for the engine. A lot of twin turbo engines have proven unreliable, or at least less reliable. It is also costly to produce and takes up more space in the engine bay.
Which brings us to the next level of turbocharging; the Variable Geometry Turbo, or VGT. This ingenious technology has been around since the 1980's, with the first production vehicle seen with VGT sold by Honda on its Legend sedan, equipped to a 2.0L V6. But Audi is responsible for producing the technology on a large scale manufacturing process. In fact, all of Audi/Volkswagen TDI engines feature this technology on the exhaust side of the turbo.
So how does it work? What does it do? VGT technology virtually solves the problem of turbo lag. It does this by incorporating small vanes in a circle around the compressor wheel's blades on the exhaust side of the turbo. These vanes can be adjusted in and out of the way, extending the blades, and providing a more direct and centralized current of air to spin the turbo the right speed as RPMs increase. Turbos, when running too slow fail to create sufficient, if any boost. When turbos run too fast, they choke they engine and lack power increase. So to keep the optimum speed for the turbo, the vanes adjust via an actuator arm controlled often by a servo mechanism. (Paultan.org)
VGTs are usually only found on diesel engines because gasoline burns hotter, and requires heat resistant materials or a cooling system, which is both costly and inefficient overall. This technology may remind you of Nissans "Variable Compression Turbo" engine, that adjusts the distance of the pistons from the cylinder head by means of an actuator arm that shifts the crankcase.
Audi states "Variable turbine geometry (VTG) technology builds up torque smoothly and without delay, even at low engine speeds". Audi has perfected this technology, thus mass-producing it and achieving another 4-5% efficiency.
Here is how Audi explains it: "As the volume of exhaust gas increases, or if less boost pressure is required, the turbine blades return to a flatter angle. The cross section of the inlet increases, and a portion of the exhaust gas flows past the turbine rotor. The turbine turns more slowly, and boost pressure is reduced. This effect is particularly important when the turbocharger has to push through large exhaust flows at high engine loads and speeds. Without VTG technology, the useful speed range would be limited, and the engine would be less efficient."