BorgWarner suggests Valve-Event Modulated Boost system can offer 6-17% fuel economy benefit over already downsized and turbocharged engines
09 May 2012
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| The VEMB system uses a concentric camshaft, blow-down manifold and scavenge manifold to separate the exhaust event into two phases: high-energy blow-down to the turbo without pumping losses, and high-hydrocarbon scavenge to the EGR system. Source: Borgwarner. Click to enlarge. |
BorgWarner says that its Valve-Event Modulated Boost (VEMB) system can offer a total possible fuel economy improvement of 6% at low speeds to 17% at high speeds over already downsized and turbocharged engines by separating the exhaust event into an early blow-down phase directed toward the turbo and a late scavenge phase directed toward the EGR system.
Christopher Thomas, BorgWarner VP, Advanced Engineering, Engine Group and VP, Advanced Engineering, Drivetrain Group presented the basic concepts of VEMB and current BorgWarner test results at the recent SAE 2012 High Efficiency IC Engine Symposium. The company originally described the system in a 2010 SAE paper (2010-01-1222) and at the 2010 World Congress.
Enablers for the VEMB system include a concentric camshaft, a blow-down exhaust manifold and a scavenge manifold. The concentric camshaft features one set of lobes connected to the phaser sprocket, and a second set next to it is connected to phaser. When the phaser moves, it moves one lobe relative to the other, enabling discrete lift events for the two exhaust cylinders: one for the blow-down, one for scavenge.
By separating the two parts of the exhaust event by phasing the exhaust valves via the concentric camshaft, VEMB allows the turbo to capture the blow-down energy while avoiding pumping losses through the turbocharger. Turbo boost can be controlled by changing the timing of the exhaust valves—no waste gate is required.
In testing, BorgWarner observed fuel consumption improvements of 2% at low speeds to 12% at high speeds with VEMB, according to Thomas. The company also has demonstrated the potential to increase compression ratios by 1.5 to 2 ratio points and maintain the same knock limit for an additional 3-4% improvement in fuel economy.
Furthermore, re-burning the hydrocarbon-rich exhaust from the scavenge port with EGR provides an opportunity to gain an additional 0.5% or so, according to Thomas, resulting in the total possible benefits ranging from about 6% to 17%.
VEMB will basically separate the exhaust process into two separate phases, the blowdown and the scavenge. It allows the turbocharger to capture all the blow-down energy, and you can really help transients with that as well, because you don’t have to compromise between whether the wastegate is open or closed. The turbo will always be spinning. It avoids pumping work through the turbocharger to improve fuel economy, and by phasing the two cams, the lobes, relative to each other we can actually modulate boost.
—Christopher Thomas
Virtual Cylinder Deactivation (VCD). As another method to avoid pumping losses and improve efficiency, Thomas described a low-cost virtual cylinder deactivation technology that delivers 90% of the fuel economy benefit of a full mechanical cylinder deactivation system. VCD uses Cam Torque Actuated (CTA) phasers with Mid Position Lock (MPL) that can phase the camshafts fast enough with enough range of authority to achieve virtual cylinder deactivation.
Extended range of authority is required to phase the camshafts 95 degrees; MPL is required to start the engine with extended range of authority.
VCD can deliver about a 4.5% improvement in fuel economy on an E-class vehicle (e.g., SUV/Minivan) compared to about 5% with conventional cylinder deactivations, and about 7.2% on a D-class sedan compared to about 8%, Thomas said.
VCD, said Thomas, has no effect on valvetrain dynamics (unlike switching roller finger followers); no impact on valvetrain packaging (unlike lobe switching); no impact on port packaging (unlike collapsible lash adjusters); and is possible to implement without vehicle NVH abatement. Although this reduces the window of operation and fuel economy benefit to 2-4%, it also eliminates vehicle adaptation cost. Total cost of implementation is much less than $10/1%, he said.