Automotive Engineering - October 2018 - 37

Dual-volute SALUTE!
BorgWarner's new twist
in turbocharging helps
GM's new L3B gasoline
four deliver V6-beating
power and torque, with
diesel-like fuel efficiency,
in full-size pickup trucks.
by Don Sherman

B03 CAD illustration showing outer and inner
volutes in the turbocharger housing casting.

M'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 in the 2019 GMC Sierra.
Engineering a boosted I4 for full-size 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

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.
AUTOMOTIVE ENGINEERING

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 DV
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 sideby-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
October 2018 37

https://www.sae.org/news/2018/05/gm-2.7-l-i-4-revealed https://www.sae.org/news/2018/05/gm-2.7-l-i-4-revealed

Automotive Engineering - October 2018

Table of Contents for the Digital Edition of Automotive Engineering - October 2018