BorgWarner dual-volute turbocharger
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BorgWarner dual-volute turbo: B03 CAD illustration showing outer and inner volutes in the turbocharger housing casting. (BorgWarner)
 

BorgWarner’s dual-volute turbocharger enables first-ever 4-cylinder power for GM fullsize pickups

The first four-cylinder gasoline engine ever offered in a full-sized pickup truck brings V6-beating power and torque, with fuel efficiency that is expected to rival a diesel. And key to its performance is a new twist in turbocharging.
 
GM’s all-new 2.7-L L3B was designed for truck use, with a wealth of advanced features (see https://www.sae.org/news/2018/05/gm-2.7-l-i-4-revealed). The result is 9% more power and 14% more torque than GM’s incumbent LV3 4.3-L V6. Teamed with the 8L90 8-speed Hydra-Matic transmission, the L3B turbo four is the base engine for 2019 Chevy Silverados in LT and RST trim and also is offered for the 2019 GMC Sierra, which shares the Silverado’s all-new architecture.
 
Engineering a boosted I4 for fullsize pickups’ broad duty cycle was no mean feat. Such engines provide impressive economy during steady-state cruising in top gear and generate ample thrust under load once the revs and boost are up. The challenge is in the transition between those operating modes—when the engine must leap into action as the throttle is quickly moved from barely open to WOT.
 
“Our goal was to top every competitor by providing 90% of peak torque less than two seconds after the throttle is floored at 1500 rpm,” explained Silverado chief engineer Tom Sutter.
 
BorgWarner and dual-volute
 Enter BorgWarner’s B03 dual-volute (DV) turbocharger—a design that’s existed for decades but is new to light-duty vehicle applications. “We began transferring this technology from commercial diesels to light-duty gasoline engines in 2012,” noted Dr. Hermann Breitbach, BW’s VP of global engineering and innovation. “The advantage a DV turbo offers is throttle response that’s superior to any alternative.”
 
All turbos recycle energy that exits the engine’s exhaust ports. The hot gas spins a turbine wheel which in turn drives a centrifugal compressor that forces intake air—above atmospheric pressure—through the intake manifold. Swiss engineer Alfred Buchi’s invention has boosted aircraft, ship, truck and automobile engine power and efficiency for more than a century.
 
In every turbocharger application, the linear stream of hot gas exiting the exhaust manifold must be twisted into a spiral to spin the turbine wheel. This is achieved by means of a curved duct inside the turbine housing called a scroll or volute. The configuration of these ducts determines how efficiently they exploit the exhaust stream’s heat and momentum.
 
A single large duct handles the exhaust flow with minimal restriction at high rpm. At low rpm, however, the turbine needs a nudge to start spinning. In the dual-volute design, that’s achieved by using two ducts wrapped around the turbine inlet, with exhaust port connections that encourage every burst of exhaust gas to strike 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’s ducts and cylinders 3 and 2 to the other duct is the optimum arrangement. This maximizes the separation between bursts of exhaust gas and minimizes the flow disruption before the gasses reach the turbine wheel.
 
The next consideration is how the ducts meet the turbine. The more common configuration, typically called “twin scroll,” has the two ducts positioned side-by-side. Unfortunately, this allows short circuiting where the exhaust gas leaves the ducts: as the two streams mix just before they strike the turbine wheel, a small portion of the exhaust gas runs the wrong way back up a duct. That’s not an issue in medium to large engines, but it can increase turbo lag—a delayed arrival of boost pressure—in small four-cylinder engines.
 
A DV design locates one duct inside the other in a concentric arrangement. The inner channel wraps half way around the turbine housing before its exhaust gas impinges against the wheel. The outer volute carries on an additional 180 degrees to guard against short circuiting and any chance of unproductive flow interference.
 
BorgWarner’s North American engineering director, Douglas Erber, said, “DV segregation and the resulting greater spacing between exhaust pressure pulses assures that more energy reaches the turbine wheel. This diminishes lag and significantly improves throttle response.”
 
At 1500 rpm, each side of the L3B engine’s turbine wheel is struck by an exhaust pulse every 40 milliseconds, quickly accelerating its rotation. The gap between each volute’s exit and the turbine wheel is only 1 mm (.039 in.) compared to the 5-mm gap that’s common for a twin scroll turbo.
 
Close observers will note that the outer volute has a larger cross-sectional area than the inner volute.  This is because a turbocharger’s flow 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 turbine wheel’s axis; for both volutes of a dual-volute design to share a common A/R and similar flow characteristics, the area of the outer volute must be larger. While there are situations when slightly different A/R volutes are desirable, engineers determined they should be identical in the L3B application.
 
Multiple configurations tested
 Craig Marriott, assistant chief engineer for the L3B engine explained: “While evaluating several turbo configurations during the alpha [early] phase of development, we determined that the DV architecture demonstrated the best performance for our truck application. In addition to its significant improvement in transient response, we found a small horsepower gain at the top end.”
 
The B03 is capable of generating 22-psi (1.5-bar) boost in the L3B truck engine application.
 
Asked if this technology is likely to extend to other GM turbo engines, he added, “I’m not at liberty to speak of future plans except to recommend staying tuned!”
 
Regarding weight, bulk and cost considerations, GM’s turbocharger engineer Alec Peeples revealed that the DV design “is slightly lighter than the twin-scroll alternative because one of the two volutes wraps only half way around the wheel. The concentric layout is also somewhat narrower, though it does extend further outboard from the center axis. Since the turbine housing material is relatively heavy and costly, the DV design is potentially less-expensive than twin scroll.”
 
According BW’s Erber, the costs are very similar for both designs. He added that propulsion-system engineers will be wise to consider not only DV, twin-scroll and single-scroll turbos but also variable-vane designs, as each product application has specific needs.
 
Whatever the case, Alfred Buchi’s 1905 invention is the engine power-enhancing technology gift that keeps on giving. Continue reading »
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