HYDRAULIC HYBRID SYSTEMS:
A Coming Age for Student Transportation? Technology being developed for hybrid vehicles promises increased fuel efficiency, reduced emissions
By John Whelan New technology is all around us and
growing fast; not to be excluded are hy- brid vehicles. Vehicle emissions and fuel efficiency have been at the forefront for many years, with the EPA putting the pressure on manufacturers to re-engineer vehicles to make them more efficient. One emerging technology is in the area
of hydraulic hybrid vehicles (HHV). Te most common vehicle people think of when it comes to hydraulics are refuse haulers. However, hybrid hydraulic tech- nology has made great strides in reducing fuel costs and increasing all-around ef- ficiency,
especially in the commercial
delivery vehicle market. Tis technology could soon be available for the vocational segment, including school buses. Hybrid hydraulic
vehicles use two
sources of power to drive the wheels, an internal combustion engine and a hydrau- lic motor. Te two major components are the accumulator and the hydraulic drive pump/motor. Accumulators store pres- surized fluid while the hydraulic drive, acting as a motor, uses the high-pressure fluid to drive the wheels. Te hydraulic drive then acts as a pump, re-pressurizing the system using the vehicle momentum to store high-pressure fluid back into the accumulator. Tis sequence is known as regenerative braking. Te series hybrid hydraulic system is a
stand-alone technology, since the engine is not connected to the rear wheels of the vehicle. Te advantage of this is an engine management system that drives the vehi- cle with the engine off using regenerated hydraulic pressure from the accumulator. Electric hybrids have a high-energy
storage capacity because of the on-board battery pack. Te energy release from the battery can be slowed down and eas- ily controlled over time. Hydraulic hybrid
Photos/illustrations courtesy of Freightliner Custom Chassis Corporation
systems can release power much faster but only for a short period of time before requiring a recharge. Both systems use vehicle momentum
to recharge the battery or the hydraulic accumulator. However, hydraulic hybrid systems reuse as much as 70 percent of the energy used from braking that other- wise would be lost energy. By comparison, traditional electric systems can recover only 20 to 25 percent of brake energy.
EPA AND THE DEVELOPMENT OF HYDRAULIC HYBRID SYSTEMS Well over a decade ago the EPA started
an intense research program on hydraulic hybrid technology and systems. EPA has become a leader in the industry, with ma- jor breakthroughs in the operation and design of pumps, motors and accumula- tors by making them more efficient with less weight and size. Tis led to the de- velopment of both large commercial and smaller consumer vehicle applications. EPA also entered research and de-
velopment agreements with numerous private sector partners to commercial- ize these breakthroughs. Development
38 School Transportation News Magazine August 2011
started with large SUVs and light-duty trucks to the present with a focus on Full Series UPS Package Cars. Te first UPS demo vehicle equipped with a Series Hy- brid System resulted in approximately a 60 percent to 70 percent mpg improve- ment in city driving. Bosch Rexroth Corporation is a leader
in this new technology and is scheduled to launch a parallel hydraulic hybrid so- lution (Hydrostatic Regenerative Braking System, or HRB) this year for Class 8 refuse truck applications. “Under the right duty cycles, hydrau-
lic hybrid vehicle systems can provide significant savings in fuel consumption, emissions and brake wear. Hydraulic hy- brid systems, such as HRB, are comprised of standard hydraulic components and do not require costly batteries,” said David J. Brosky, manager of sales and industry sec- tor management for on-highway vehicles at Bosch Rexroth. Tere are two types of HHV systems, paral-
lel and series. Parallel HHVs are mechanically driven by the engine and the hydraulic sys- tem. Te hydraulic pump/motor is coupled to the drive shaft or differential.
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