Enter the B03 dual-volute (DV) turbocharger from BorgWarner, a device that has been around for decades but is new to light-duty vehicle applications. “In 2012, we started translating this technology from commercial diesel to light-duty gasoline engines,” said Dr. Hermann Breitbach, BW’s VP of worldwide engineering and innovation. “The advantage of a DV turbo is greater throttle response than any alternative.”
All turbos recycle the energy that exits the exhaust ports of the engine. The heated gas turns a turbine wheel, which powers a centrifugal compressor, which propels intake air through the intake manifold at pressures above atmospheric pressure. For more than a century, Swiss engineer Alfred Buchi’s innovation has increased the power and efficiency of aviation, ship, truck, and automotive engines.
To spin the turbine wheel, the linear stream of hot gas exiting the exhaust manifold must be twisted into a spiral in every turbocharger installation. This is accomplished by the use of a scroll or volute, which is a curved duct inside the turbine casing. The design of these ducts influences how well they use the heat and velocity of the exhaust stream.
At high rpm, a single big duct handles the exhaust flow with minimum obstruction. However, at low rpm, the turbine needs a shove to begin spinning. That is accomplished in the dual-volute design by wrapping two ducts around the turbine intake, with exhaust port connections that urge every burst of exhaust flow to contact the turbine wheel with maximum force.
Specifically tailored for 4-cylinder duty
Given the new L3B’s 1-3-4-2 firing order, connecting cylinders 1 and 4 to one of the turbocharger ducts and cylinders 3 and 2 to the other is the best configuration. This improves separation between bursts of exhaust gas and reduces flow interruption before the gases reach the turbine rotor.
The next point to examine is how the ducts connect to the turbine. The most frequent design, known as “twin-scroll,” has the two ducts side by side. Unfortunately, this allows for short-circuiting when the exhaust gas exits the ducts: as the two streams mix just before they reach the turbine wheel, a small amount of the exhaust gas travels back up a duct in the incorrect direction.
In a concentric configuration, one duct is located within the other in the DV design. Before the exhaust gas impinges on the wheel, the inner channel loops halfway around the turbine housing. The outer volute extends an extra 180 degrees to prevent short-circuiting and wasteful flow interference.
According to Douglas Erber, BorgWarner’s North American engineering director, “DV segregation and the consequent larger spacing between exhaust pressure pulses ensures that more energy reaches the turbine wheel.” This reduces latency and increases throttle responsiveness significantly.”
At 1500 rpm, an exhaust pulse strikes each side of the L3B engine’s turbine rotor every 40 milliseconds, rapidly increasing its spin. The distance between the exit of each volute and the turbine wheel is only 1 mm (.039 in.) as opposed to the 5-mm gap typical of dual scroll turbos.
Close inspection reveals that the outer volute has a greater cross-sectional area than the inner volute. This is because the flow of a turbocharger is governed by its A/R ratio—the cross-sectional area of the turbine housing’s duct divided by the distance between the duct’s centroid and the axis of the turbine wheel; for both volutes of a dual-volute design to share a common A/R and similar flow characteristics.
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Audi A4/ Q5 Turbo BorgWarner BV43 5303 970 0190 CAGA/CMEA/CJCA
