Prior work with the concept of dividing the exhaust process into an early and late phase has shown the potential of applying only the early stage (blow-down) of the exhaust period directly to a turbocharger or turbocharger system, and the later stage (scavenge) arranged to bypass the turbine. In this manner, the exhaust backpressure required to extract high turbine work from the engine can be isolated from the displacement phase of the exhaust stroke and thereby greatly reduce the exhaust pumping work and Residual Gas Fraction. In previously-published efforts, the challenges of valve-event control and high turbine inlet temperature have been revealed.
The BorgWarner Engine Systems Group, in conjunction with Presta, has applied a cam-phaser controlled concentric camshaft system to the exhaust side of a divided exhaust port 4-valve per cylinder DOHC GDI engine, to enable variable phasing between the Blow-down and Scavenge cam profiles. With this system we have demonstrated control of the turbocharger boost, eliminating the need for a wastegate. In addition, we have devised a unique external EGR system that takes advantage of the separate scavenge exhaust path to efficiently control turbine inlet temperature and provide the already well-known cooled EGR benefits to a highly-boosted SI engine. The net benefits are higher boost potential across the engine speed range with significant fuel economy improvements.
This paper will present our modeling results in GT-Power as well as our initial firing engine test results on a 2.0L Turbo GDI engine. Our modeling results indicated the ability to operate the turbocharged engine at higher BMEP levels across the engine speed range with fuel economy benefits due reduced PMEP. Our engine tests show correlation to these results, as well as the level of additional benefits due to reduced RGF at high load and the cooled EGR circuit. Future development direction and potential will also be discussed.