The dual volute geometry allows for the complete segregation of engine exhaust pulsations, so more exhaust energy is available to the turbine wheel.
Photos: © BorgWarner
On the B03 dual volute turbocharger, two completely separated exhaust gas streams drive the turbine wheel
The pickup market segment is extremely important for US auto manufacturers. It is continuously growing and now represents more than 16 percent of all vehicle sales in the US. The manufacturers are therefore engaged in a fierce battle to win favor with the discerning customers. The objective here is to develop the most economical and, at the same time, most powerful vehicle. GM therefore had strict requirements of the drive for the 2019 generation of its highly successful Chevrolet Silverado and GMC Sierra models. The company planned on replacing its 262 cubic inch (4.3 liter) V6 engine with a turbocharged 165 cubic inch (2.7 liter) four-cylinder unit.
The requirement specifications of the developers included both the lowest fuel consumption in the entire vehicle class and exceptionally fast response at low revs. The latter is very important to vehicle buyers, as beside their own net weight, the pickup trucks should also be capable of effortlessly moving heavy loads. Although clearly downsized, the new engine still has a very generous displacement for a four-cylinder unit of 165 cubic inches (2.7 liters).
Dual volute technology delivers more exhaust gas energy
It quickly became clear to the team of developers at BorgWarner that a conventional turbocharger could barely contribute to hitting GM’s ambitious fuel economy and power targets. A decision was therefore taken to develop a type of turbocharger that is completely unprecedented among gasoline engines: the dual volute turbocharger. Dual volute technology was previously only used in large-displacement diesel engines in the commercial vehicle sector – and even then only in limited numbers.
Compared with conventional twin scroll turbochargers, dual volute turbochargers offer the advantage of completely segregating the engine exhaust pulsations fed into the individual cylinders. This allows significantly more exhaust energy to be diverted to the turbine wheel, which in turn leads to better efficiency and allows boost pressure to be built up faster.
Impressive response and torque
Right from the outset, the new dual volute turbocharger from BorgWarner exceeded all expectations. It helps the 165 cubic inch (2.7 liter) engine from GM achieve a power output of 310 hp (231 kW). The maximum torque of 348 lb-ft (471 Nm) is already available from just 1,500 rpm, and the driver can access 90 percent of this within just 1.93 seconds of pressing the accelerator pedal. – truly impressive response. The four-cylinder turbocharged unit develops 22 percent more total power than its large-displacement 6-cylinder predecessor, while also delivering significantly improved fuel consumption and emissions figures.
Major challenge for the developers
Before the B03 dual volute turbocharger was ready for serial use, the team of developers at BorgWarner first had to overcome a large number of technological challenges. One key issue in this regard was the durability and reliability of the system.
For example, with the new turbocharging system the two exhaust gas streams hit the turbine wheel separately at 180 degrees apart – in synchronism with the alternating exhaust gas pulses. The faster rotor movements that this creates need to be taken up reliably by the shaft and bearing assembly. The engineers at BorgWarner used the latest computational fluid dynamic software to analyze and measure rotor stability and the pressure response of the oil film. To this end, they not only had to develop new simulation processes, but also new test procedures for the engine test benches.
The next challenge was then to increase the robustness of the wastegate system, since massive amounts of flow energy are fed to the wastegate valve as a result of separating the exhaust gas pulsation streams. Indeed, the forces acting on the valve are triple those typically encountered with conventional designs. The components of the valve were therefore subjected to a thorough redesign, including use of new materials.
The Chevrolet Silverado and GMC Sierra have been among the most popular pickup trucks in the US for years. With more than 800,000 new registrations, the two virtually identical models took second place in the 2018 vehicle registration statistics.
Strong looks, effortless drive: the Chevy Silverado and GMC Sierra with four-cylinder gasoline engine and dual volute turbocharger from BorgWarner.
GMC Photos: General Motors
The turbine wheel was the third key component that was revised to deliver even greater durability. After all, it has to be capable not only of handling the same high loads as the rotor and wastegate valve, but also requires specific designs for the turbine vanes and the geometry of the rear panel as a result of the separated gas flow channels. On the turbine wheels of single and twin scroll turbochargers, the spiral culminates in just one tongue, while a dual volute turbine wheel has two tongues. The tongues give the turbine wheel a pulse when the air flow passes over it – similarly to a semitrailer passing a car parked at the side of a road at speed. With its two tongues, the turbine wheel must withstand greater motion torques than any other turbocharger for gasoline engines. The team of engineers at Borg- Warner developed new simulation methods and employed a laser probe to investigate the movements of the turbine vanes. After numerous design changes, the turbine wheel was ultimately able to demonstrate its resilience under real-world operating conditions.
Materials from the aviation sector
Beside the mechanical load, the developers also focused on the exhaust gas temperatures, which are significantly higher in gasoline than in diesel engines. To ensure that the new dual volute turbocharger is perfectly equipped to handle exhaust gas temperatures of up to 1,020°C, the Borg- Warner team used special top-grade materials which are typical for high temperature gasoline turbochargers of BorgWarner. These include high-grade nickel, steel, and chrome alloys, which were previously reserved for aircraft engines.
The final challenge for the developers was to integrate the new turbocharging system into the vehicles from GM. The rather restrictive packaging of the Silverado, with the predefined layout of the exhaust pipes, made installation on the new 165 cubic inch (2.7 liter) engine more difficult. It was only thanks to development of an innovative production process for the core assembly and the close cooperation between the developers from GM and BorgWarner that the new dual volute turbocharger could be integrated into the predefined installation space on the Silverado.
Dual volute turbocharger wins PACE Award
The intensive and trusting cooperation between the two companies from the very outset was one of the drivers for successful development of the revolutionary dual volute turbocharger system. The team of engineers from BorgWarner supported GM from the development center in North America, also incorporating the expertise of European colleagues. BorgWarner applied for four new patents within the scope of the project. The company also developed new testing methods, new designs for the turbine wheel and waste gate and new manufacturing methods for the turbine housing as well as new simulation techniques. All of these efforts culminated in a PACE Award (Premier Automotive Suppliers’ Contribution to Excellence), which was presented to BorgWarner by trade magazine Automotive News on April 8, 2019. The PACE Awards are used to honor outstanding innovations, technological progress, and business performance among automotive suppliers. The renowned award is recognized throughout the world as a yardstick for innovations in the sector. In addition to this, BorgWarner Turbo Systems was presented with an internal Group Innovation Award.
With the dual volute turbocharging system, BorgWarner is impressively demonstrating its continued focus on further optimizing traditional solutions beside its portfolio of electrified products. Indeed, the goal is not only for the dual volute technology to be used beyond the pickup segment and in hybrid vehicles, but also for it to serve as a platform for development of further innovations.