Twin Scroll (Twin Flow) Turbocharger Generates Higher Boost Pressure at Low Revs
Twin-scroll housings are becoming very popular for performance use, and for good reason. By dividing the manifold and turbine housing into two flow paths, the engine firing order can be made to ''alternate'' the flow all the way to the turbine wheel inlet. Twin scroll technology produces results similar to twin-turbo applications, but in a smaller package with lower weight and cost. In turbochargers of this type, the channels between the exhaust manifold and turbocharger of the first and fourth as well as the second and third cylinders are separated from each other. The exhaust gas streams are directed into so-called scrolls (spirals) and then reunited again directly at the turbine wheel. Separating the streams in this way offers improved performance. The engine blow-down pulse is generated when the exhaust valve opens. During the blow-down, the engine power cylinder is still at very high pressure as a residual of combustion and the power stroke. This initial "pop" of energy travels at very high speed down the manifold runner, through the volute, and impacts the wheel. For this reason, the stream is very much a "pulsed flow" and the divided nature of the system simply amplifies and arranges those pulses. The engine firing order creates a "one-two" (alternating) punch on the wheel, keeping the pulses evenly spaced and in rapid succession. As the engine speed increases, this becomes a blur and the alternating nature loses its value. Hence, twin-scroll housings only benefit the low and medium speed operation of an engine. Within this range, it is a very effective way to improve turbine effectiveness. The wheel loves the high velocity evenly-spaced pulses of gas, and as long as they are "slow" enough in succession and duration, the wheel can make good reaction usage of the energy. What results is an improvement in "effective" efficiency, and at lower engine speeds more turbine power can be generated. The obvious result is quicker spool and better low-end boost response. As mentioned, the top-end operation is not improved, everything else being equal. Put into practice, a single-scroll housing of sufficiently large size is the recipe for a user that is seeking only top-end power optimization. That said, a very potent combination is a twin-scroll divided system that works to retain good low speed boost response, while sizing it large (aerodynamically) for the best top-end power. It's the best of both worlds, in many cases. With this type of charging, spontaneous boost pressure can be built up 1000 RPMs earlier, which significantly improves response in the low rev band. The engineers at BorgWarner have also mastered the problem of high exhaust gas temperature in gasoline engine turbocharging despite the genuine challenge presented by such a compact turbine casing with two scrolls. One approach employed by engineers was to develop a new downsizing method of casting turbine housings to improve their temperature resistance and guarantee the quality needed. The benefits of the twin scroll turbocharging technology and other market-leading technologies by BorgWarner Turbo Systems offer passenger vehicles, dynamic performance, low fuel consumption and lower C02 emissions.