Hydraulic hybrids are the product of years of research and development by the fluid power industry. Several different types of hydraulic hybrid transmissions are on the market, each using a different engineering approach to provide the power needed for work vehicles while achieving energy savings in demanding stop-and-go urban environments. Although there are multiple variations of hydraulic hybrid transmissions being developed, series and parallel hydraulic hybrids are the two primary systems.
• Series hydraulic hybrid. The vehicle’s engine powers a primary pump, which supplies pressure and flow to charge a hydraulic accumulator. The pressure from the accumulator is then fed through a secondary pump to the rear axle to propel the vehicle. This technology has increased fuel efficiency for Class 8 refuse trucks while improving drivability and performance. One test in South Florida registered a 72 percent improvement in fuel efficiency. The communities of Hialeah, Miami-Dade County, and the City of Miami have since decided to upgrade their refuse collection fleets to hydraulic hybrids. Research shows that the green technology reduces each truck's carbon footprint by an estimated 28 tons along routes with frequent stops on an annual basis. The technology also extends, depending on the duty cycle, the brake maintenance period for each truck from every 6 months to once every 24 - 30 months.
• Parallel hydraulic hybrid. In this configuration, the vehicle’s conventional powertrain is supplemented with a parallel hydraulic system. The system captures and stores the vehicle’s kinetic energy by pressurizing fluid in an accumulator, rather than dissipating it as friction through the brake system. The stored energy is then fed back to the vehicle during acceleration to reduce the amount of fuel needed to get the vehicle moving again. This technology is focused on commercial vehicle manufacturers and fleet users of heavy trucks that have severe stop and go duty cycles. These hybrid power systems have accumulated more than 30 million miles of road-tested service around the world. Companies such as FedEx, UPS, Coca-Cola Enterprises, PepsiCo and Wal-Mart are using delivery vehicles with a parallel hydraulic system. Work trucks with hybrid power are also in service at Florida Power and Light and many utility and telecom companies across North America. These companies are realizing fuel savings up to 35 percent, with similar percentages in emission reductions, extended brake life and idle time reductions of up to 87 percent during work site operations.
The applications cited above are just some of the examples of the efficient technologies that a growing number of companies in the fluid power industry plan to bring to market in the years ahead. According to the National Fluid Power Association (NFPA) executive director, Eric Lanke, “The fluid power industry is working hard to bring this tested technology to passenger cars.”
With commercialization already underway for heavy trucks, the shift to passenger cars would result in many times more energy and emission savings. In 2008, the United States consumed about 137.80 billion gallons (or 3.28 billion barrels) of gasoline. If it was possible to realize a 50% increase in fuel economy through the use of hydraulic hybrids, and all vehicles were converted over, this would reduce our fuel consumption by one third or about 45 billion gallons saved. At $2.70/gal this equates to over $120 billion savings per year. The challenge to adoption is a technical one. Can the hydraulic hybrid systems in use on heavy trucks be scaled down for use in passenger cars?
Many industry players are hard at work on this problem. So is a new kind of organization, the Engineering Research Center for Compact and Efficient Fluid Power (CCEFP), a consortium of seven universities and more than 50 companies funded through the National Science Foundation and dedicated to changing the way fluid power is researched, applied and taught.
The CCEFP is currently collaborating with Ford Motor Company and Folsom Technologies International on a new platform for its hydraulic hybrid vehicle test bed. In this new platform, a Ford F-150 vehicle will be modified with a hydraulic hybrid powertrain by incorporating hydraulic accumulators with a compact, integrated hydraulic continuously variable transmission (CVT). Simulation indicates a 50% improvement in fuel economy over the conventional drive train. Development of practical hydraulic hybrid passenger cars will create a substantial savings in the nation’s fuel economy.
In part to help guide these efforts, the NFPA recently brought the fluid power industry together to develop a Technology Roadmap for the Fluid Power Industry—a ground-breaking initiative to critically examine the state of the industry’s technology and to chart a research and development agenda for the future. Increasing energy efficiency, through hydraulic hybrids and other innovative uses of fluid power, is a key objective of this Roadmap. It recognizes that as fuel costs continue to rise, the savings benefit of hydraulic hybrid vehicles will make a positive impact on the environment and a significant impact on consumers’ bottom line.
Through programs such as these, the NFPA and the fluid power industry are working to produce economically efficient, reliable and safe solutions to the energy-conscious drivers of today and tomorrow. The hydraulic hybrid trucks on the road today are just the start of the transformation of efficient technologies that fluid power will bring to market in the years ahead.
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