There’s a rather significant conundrum for off-highway vehicle manufacturers, especially now as fuel prices continue to skyrocket: how to balance performance improvements with fuel economy.
Performance is what sells, but economy is what’s required—both by regulation and economics. Historically, the trade-off has been an “either/or” issue. Economy compromises performance. Performance compromises economy. This seemingly intractable conflict can now be resolved, but it does require looking at the vehicle as a system rather than as the sum of its individual components.
As one example, Southwest Research Institute modeled a Fallbrook Technologies Dynasys APU with and without a NuVinci DeltaSeries continuously variable planetary (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’ Yanmar 0.5-L diesel engine and its 6-kW generator so its efficiency could be continuously optimized for the varying electrical power demands over the APU’s “drive cycle.” The results were significant—up to 24% fuel savings over the baseline Dynasys system without a CVP.
In another independent test, a Class 8 truck’s air-conditioning system was equipped with a NuVinci DeltaSeries CVP with the goal of improving A/C pull-down—the time required, starting from hot conditions, to lower the A/C vent exit temperatures to a specified level. By increasing the speed of the compressor at engine idle, then smoothly and continuously modulating its speed based on demand over a standard drive cycle, pull-down time was improved by 39% with no measurable loss of fuel economy.
Herein lies the opportunity for off-highway vehicle manufacturers, as well as the automotive industry in general, to perform what was up to now a perceived impossibility—to achieve simultaneously both performance and economy. The key is to decouple engine accessories from engine rpm, allowing components such as the A/C compressor, alternator, or other device to run at its optimum speed relative to driver demand, regardless of what the engine is doing.
Moreover, this may well 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. Perhaps more importantly, the ability to change accessory output as needed in a seamless, transparent manner enables a rethinking of the entire nose of the vehicle. The total accessory package is potentially smaller, and continuous optimization of working fluid flow rates through the various heat exchangers allows them to be optimally sized as well.
The pressure to improve efficiencies without sacrificing performance requires vehicle manufacturers to look beyond the engine to achieve their goals. An often overlooked area is the belt-driven accessory as its own system. Decoupling accessory speed from engine speed can increase both fuel economy and performance. A continuously variable accessory can provide a way to make a designer’s job easier and help manufacturers meet their challenging goals sooner.
Rob Smithson, CTO and Vice President of Business Development, Fallbrook Technologies, wrote this article for SAE Off-Highway Engineering.