Twin-scroll turbo
The first time I heard about twin-scroll turbo was in 1989, when the updated Mazda RX-7 Mk2 introduced this feature. It was employed to seperate the exhaust gas from the Wankel engine's two rotors in order to avoid interference. Anyway, twin-scroll turbo is also useful on 4-cylinder and 6-cylinder engines. Mitsubishi, for example, has been using it on its hot Lancer Evo since 1996. Renault used it on the 2.0 turbo engine of Avantime and Megane II Sport in the early 2000s. GM did the same to its 2.8 V6 turbo of Saab 9-3 Aero and Opel Vectra OPC in 2005. Then many manufacturers joined the camp. BMW is perhaps the keenest promoter of the technology. It used twin-scroll turbos on the 1.6-liter Prince engine of Mini (which also benefits countless of Peugeots / Citroens), 2.0-liter four-pot engine, 3.0-liter N55 straight-six and 4.4-liter V8. What makes twin-scroll turbo so attractive? The answer is quicker response and higher efficiency.
The first time I heard about twin-scroll turbo was in 1989, when the updated Mazda RX-7 Mk2 introduced this feature. It was employed to seperate the exhaust gas from the Wankel engine's two rotors in order to avoid interference. Anyway, twin-scroll turbo is also useful on 4-cylinder and 6-cylinder engines. Mitsubishi, for example, has been using it on its hot Lancer Evo since 1996. Renault used it on the 2.0 turbo engine of Avantime and Megane II Sport in the early 2000s. GM did the same to its 2.8 V6 turbo of Saab 9-3 Aero and Opel Vectra OPC in 2005. Then many manufacturers joined the camp. BMW is perhaps the keenest promoter of the technology. It used twin-scroll turbos on the 1.6-liter Prince engine of Mini (which also benefits countless of Peugeots / Citroens), 2.0-liter four-pot engine, 3.0-liter N55 straight-six and 4.4-liter V8. What makes twin-scroll turbo so attractive? The answer is quicker response and higher efficiency.
Let's see how it works. While conventional
single-turbo arrangement has all exhaust manifolds connected together
at the exhaust turbine, twin-scroll turbo splits into two separate
paths. For example, in a typical 4-cylinder engine, cylinder 1 and 4
combines to one path, while cylinder 2 and 3 combines to another path.
The two exhaust flows hit the turbine blades independently, as they are
separated by a wall integral with the turbine housing. This prevents
the two exhaust streams to interfere with each other.
But why is the avoidance of interference so important? Please see the following graph. It shows a typical exhaust pulse from one cylinder. When exhaust valves open, the hot exhaust gas rushes out from the combustion chamber and generates a high-pressure pulse. The pulse escapes quickly and pressure drops quickly as well. Tailing the pulse is a negative pressure (lower than atmospheric pressure) period, as we have explained in the Tuned Exhaust section. When the intake valve opens during the "overlap" period, the pressure dips again. Finally, the exhaust valve closed and the pressure in exhaust manifold gets stabilizing.
But why is the avoidance of interference so important? Please see the following graph. It shows a typical exhaust pulse from one cylinder. When exhaust valves open, the hot exhaust gas rushes out from the combustion chamber and generates a high-pressure pulse. The pulse escapes quickly and pressure drops quickly as well. Tailing the pulse is a negative pressure (lower than atmospheric pressure) period, as we have explained in the Tuned Exhaust section. When the intake valve opens during the "overlap" period, the pressure dips again. Finally, the exhaust valve closed and the pressure in exhaust manifold gets stabilizing.
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