The supercharging or turbocharging has now been established by most manufacturers, in small and large displacement engines.
From the system with a turbocharger to the combined three in the new diesel engine in the BMW and to the Audi which is experimenting with electric turbo (with a supporting role in the base supercharged). But quite often, the demands for increased power created the need for a system with two turbochargers.
Known as the «Twin Turbo» ( Twin Turbo or Bi- Turbo) , usually one turbocharger is smaller than the other and running at low rpm (smaller size means less inertia. less spinning). As the revs rise and the small turbo can not push more air, the largest engaged. Note that the two turbochargers are in sequence and then rotate (in passive role of the large at low rpm) and exhaust flow changes a flap (flap). From the medium and after rpm there is sufficient volume of gas to move directly impeller and the larger turbo without lag (turbo lag).
As mentioned above, thus described a dual stage (2 – Stage), which is in a sense changes in serial arrangement. One of the first engines of this technology was the BMW 535d diesel system with BorgWarner and then followed by several others in a representative example of the new two-liter bi-turbo of the Opel.
Here to say that the first turbo purely serial device, and there was practically parallel operation. For example, the engine of Subaru Legacy (1992) in a turbo worked both low and high together. However, it could work both with low, or even a passive one that worked up by the flow of exhaust gases regulates an on/off damper. Of the first models with serial turbo old cut was the Mazda RX-7 Mk3, the Porsche 959, the latest generation Toyota Supra, the Subaru Legacy 1990s etc.
In parallel provision Twin Turbo both turbochargers can operate independently and simultaneously (ie in parallel) eg as a V engine with a turbocharger to each scale/number of cylinders. This is a simpler construction arrangement in relation to the serial. The Maserati was one of the first companies with large production engines with parallel turbo while the most famous is the N54’s BMW (2006), with 6-cylinder in-line and one turbocharger per three cylinders. Of course, the double parallel turbo find application in sports cars, since the Ferrari F40, the Lotus Esprit V8, the Nissan 300ZX etc.
At the point, the Twin- Scroll system saw from the age of Mazda RX-7 in the early 1990s or the Mitsubishi Evo (1996), but only recently used in smaller engines for normal road cars: from the exceptional the two-liter Renault (2002 in Laguna, Megane, Avantime) up the remarkable four-cylinder of 1,6 lt evolved by groups of BMW/Peugeot-Citroen. We have to say that we should not confuse the logo TwinPower as there now integrates all the BMW turbo engines, regardless if it’s Twin Turbo or Twin-Scroll (or both as in the case of V8 engine).
The operating principle of Twin-Scroll is relatively simple to capture. Unlike the simple single turbo where the exhaust driven all together into a lumen output to drive the turbocharger, the Twin Scroll divides the output into two parts. Thus, a typical cylinder exhaust cylinders 1-3 can be driven by one and the exhaust passage 2 of the other four. Here, note the two exhaust streams fall onto the turbocharger but separated by a diaphragm that is not spinning or suction. With Twin Scroll succeed continuous flow and rotating turbo and this leads translates into smooth operation, without tantrums and practical with almost zero lag (at least not perceived).
The turbocharger with variable geometry vanes (VTG or Variable Turbine Geometry) have the ability to alter the rotational speed and the air flow depending on the position of the blades. Widely used in diesel engines and the only company that has implemented a system of VTG in gasoline is the Porsche (the 997 Turbo). The reason that VTG can not find wide application in Otto engines are the highest exhaust temperature that destroys long blades (in 1989, Honda produced the Wing Turbo who left later).