The challenge of meeting the 54.5-mpg (23.1 km/L) U.S. fuel efficiency regulations proposed for 2025—slightly more than two product cycles away—presents an overwhelming dilemma for vehicle makers: How to balance performance improvements against their fuel economy impact.
Historically, the trade-off has been an “either/or” issue: Economy compromises performance and vice-versa. There is potential to resolve this seemingly inflexible conflict, however, by looking at the vehicle’s powertrain as a system rather than as the sum of its individual components.
One solution is to decouple engine accessories from engine speed by the use of a continuously variable transmission (CVT) that allows components such as the A/C compressor, alternator, supercharger, or other device to run at its optimum speed relative to driver demand, regardless of what the engine is doing.
Moreover, this may permit use of smaller accessory components without hindering or reducing performance. It will no longer be necessary to size such systems to provide adequate capacity at engine idle speeds.
The ability to change accessory output as needed in a seamless, transparent manner also enables rethinking of the entire "nose" of the vehicle. The total accessory package potentially can be made smaller, and continuous optimization of working fluid flow rates through the various heat exchangers allows them to be optimally sized as well.
A new type of CVT developed by Fallbrook Technologies offers the potential for powertrain engineers to realize such benefits. The NuVinci CVP transmission is the first practical CVT designed for use in conjunction with an accessory drive or in both small and large applications. It combines the smooth, continuous ratio change of a CVT with the utility of a conventional planetary gear drive.
Instead of the traditional gear and clutch mechanisms found in conventional transmissions, the NuVinci technology leverages a set of rotating “planet” spheres that are arranged around a central “sun” that transfers torque between two “rings.” Tilting the spheres changes their contact diameters on the rings, permitting an infinite progression of speed ratios.
The result is a seamless and continuous transition to any ratio within its range, maximizing overall powertrain efficiency and accessory performance on demand. Other advantages include reported higher torque density than other CVT types; reduced NVH compared with gear-and-clutch devices, and the ability to sum or split torque, as in a conventional fixed planetary.
The system also affords more flexible drive configurations including concentric, coplanar input and outputs that require less packaging space as compared to other CVT technologies.
Cost and fuel efficiency benefits
Ratio control in the NuVinci CVP is stable and can be actuated down the center line of the transmission in a way very similar to a planetary-type automatic. The design reduces energy consumption through its seamless speed changing characteristics, continuously optimizing ratio between the driving and driven devices. This enables them to operate in their most efficient speed range.
Overall, the transmission’s mechanical and manufacturing characteristics improve performance and reliability while reducing costs over traditional CVTs. Part shapes are simple and relatively easy to manufacture, eliminating the need for power-robbing, high-pressure hydraulics.
Fallbrook is not the first company to propose accessory drives that operate at more than one speed, but the products in its development pipeline provide a continuously variable complement of drive ratios. Based on independent analyses, this should provide attractive economic and fuel efficiency benefits.
As one example, a prominent independent research firm modeled a Dynasys APU (auxiliary power unit) with and without a NuVinci DeltaSeries CVP transmission. The Dynasys APU is used in Class 8 trucks for idle reduction, and fuel efficiency is a primary selling point.
The CVP was inserted between the Dynasys 0.5-L diesel engine and its 6 kW generator so that its efficiency could be continuously optimized for the varying electrical power demands over the APU’s “drive cycle.” The results were significant—up to a 24% fuel savings over the baseline Dynasys system without a CVP.
In another independent test, a Class 8 truck’s A/C system was equipped with a NuVinci DeltaSeries CVP with the goal of improving A/C pulldown (the time required, starting from hot ambient conditions, to lower A/C vent exit temperatures to a specified level). By increasing the speed of the compressor at engine idle, pulldown time was improved by 39%.
Over a standard drive cycle, the smooth, continuous, on-demand modulation of compressor drive speed enabled by the CVP gave no measurable loss of fuel economy, despite the increased system performance. Further system optimization is expected to yield an increase in fuel economy.
Earlier this year, Fallbrook and TRW signed an MOU to co-develop the NuVinci system for commercial vehicle drivelines, as reported in SAE's Off-Highway Engineering (http://www.sae.org/mags/sohe/9407).
Belt-driven accessories, as their own system, are often overlooked in terms of improving overall vehicle efficiency. Decoupling accessory speed from engine speed, using a CVP accessory, can make a designer’s job easier and help manufacturers meet their challenging CAFE goals sooner.
David E. Cole, Chairman Emeritus of the Center for Automotive Research and member of Fallbrook Technologies’ Board of Advisors, wrote this article for Automotive Engineering.