Hydraulic hybrid drive system: Fuel savings and reduced emissions

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2012

Hydraulic hybrid technology captures kinetic energy generated by a Class 8 refuse truck and transmits it into a high-pressure accumulator.

KEY CONCEPTS
• A new series hydraulic hybrid system captures the kinetic energy generated by a Class 8 refuse truck during braking and stores the energy in a high-pressure accumulator for further use.
• This system results in a 35% to 50% fuel savings and approximately a 50% reduction in emissions.
• Successful trials of the series hydraulic hybrid system have been held in South Florida during the past three years.

THE INCREASING COST OF FUEL is driving the development of new technologies that can provide better efficiencies in all types of vehicles. Much of the early emphasis has been placed on passenger cars with the U.S. EPA establishing a Corporate Average Fuel Economy (CAFE) goal of 54.5 miles per gallon by 2025.

One of the largest sources of energy loss in automobiles is heat. This factor accounts for 40% of the fuel energy produced from an internal combustion engine. In a previous TLT article, a new thermoelectric material was discussed that can convert automotive exhaust heat to electricity, which can help to offset the loss (1). The new thermoelectric material is known as a skutterudite, a crystalline material containing cobalt, rhodium or iridium in combination with phosphorus, arsenic and antimony.

Attention is now being paid to improve the performance of heavy-duty diesel trucks. The EPA has placed a series of more strict regulations on emissions such as particulate matter and NOx. For the former, EPA established a standard of 0.1 g/bhp-hr (brake horse power per hour) in 1998 that was reduced to 0.01 g/bhp-hr in 2007.

Fuel economy requirements now will be included in the newly developing PC-11 heavy-duty diesel oil specification in January 2016. For some trucks, additional technologies are needed to boost performance from both the standpoints of fuel economy and emissions. A key example is garbage trucks, which spend much of their daily operations in a continuous pattern of stop-and-go as they collect refuse.

In a previous TLT article, a newly developed hydraulic hybrid system known as the Parker RunWise Energy Recovery Drive System was introduced (2). This technology is designed to operate in Class 8 refuse trucks. Initial testing showed that substantial fuel savings can be realized.

This article provides an update on this hydraulic hybrid system that was developed by the Parker Hannifin Corp.

SERIES HYBRID DRIVE SYSTEM
Tom DeCoster, business development manager for Parker Hybrid Drive Systems in Columbus, Ohio, says, “Our hydraulic hybrid technology is based on series architecture in which the conventional truck transmission is replaced with a three-speed automatic transmission. This system contains a two-speed hydrostatic drive that operates in hydraulic mode at speeds between 0-25 mph and 25-45 mph. A third speed is a direct drive that operates above 40 mph when the truck is involved in highway driving.”

Figure 1 shows an image of the hydraulic hybrid drive’s innovative cradle assembly, which was established to ensure ease of maintenance.


Figure 1. The series hydraulic hybrid system collects, stores and reuses the kinetic energy of a Class 8 refuse truck, leading to significant savings in fuel costs and lower emissions. (Courtesy of the Parker Hannifin Corp.)

The hydraulic hybrid technology captures the kinetic energy generated by the vehicle in the braking process and uses pumps to hydraulically transmit this energy into a high-pressure accumulator. DeCoster adds, “In addition to the stored energy, an additional hydrostatic pump/motor is connected directly to the engine to top off the accumulators when stored energy is depleted.”

Through this process, the series hydraulic hybrid system can capture approximately 70% of the braking energy and use it again. DeCoster says, “In braking the truck from 30 mph to 0, in excess of 500 hp of energy is captured.”

Once the truck stops, the power drive unit uses energy from the accumulators through the hydraulic system to drive the output shaft and move the vehicle. DeCoster says, “The engine will not be needed until the pressure on the accumulator runs low.”

One of the main benefits for this hydraulic hybrid series is the engine is controlled all the time and at optimal efficiency. In contrast, other hydraulic hybrid systems are designed in a parallel orientation in which they are placed in addition to the standard drivetrain and the conventional drive shaft to the rear axles remains in place. DeCoster says, “Much smaller benefits are seen with the parallel hydraulic hybrid system approach.”

The main benefit for the series hydraulic hybrid system for refuse applications utilizing an automated side-loader configuration is up to a 35% to 50% fuel savings and approximately a 50% reduction in emissions. In contrast, parallel systems can only provide between 5% to 15% fuel savings. DeCoster estimates that a 35% savings in fuel use will translate into an annual reduction in carbon dioxide emissions for a single truck by 38 tons.

Demos of the series hydraulic hybrid system were held in South Florida in 2009. DeCoster comments, “Fuel savings in the 40% to 45% range were seen in several tests. This has led these communities to purchase their first vehicles and place them in service. These 11 trucks now average 45% to 50% fuel savings. Minor issues such as eliminating vehicle rollback on grades have been eliminated.”

Testing has been done to show that the hybrid hydraulic system can be used under a variety of weather conditions, including in cold climates such as -37 C conditions encountered in Canada and hot temperatures (57 C) encountered in Death Valley, Calif.

Additional benefits from these trucks are seen in reduced maintenance. DeCoster says, “Typically, brakes are replaced in refuse trucks on a quarterly basis. With the hydraulic pumps assuming the responsibility for braking, conventional brakes now only need to be changed every six to seven years. The timing for changing brakes is dependent upon the duty cycle of the refuse truck.”

DeCoster continues, “A further benefit is the reduction in the generation of brake dust. We have seen brake dust reductions around 100 lbs per year, which leads to an overall reduction of approximately 1,000 lbs over the lifetime of the vehicle.”

DeCoster indicated that the use of hydraulic hybrid-powered Class 8 refuse trucks is expanding to other U.S. cities: “We are also looking at testing our technology in parcel trucks such as those used by FedEx and UPS. Our hope is to increase fuel economy between 35% and 50%.”

Further information on the hydraulic hybrid technology can be found at http://parkerhybrid.parker.com or by contacting DeCoster at tom.decoster@parker.com. 

REFERENCES
1. Canter, N. (2011), “Electricity from Automotive Exhaust Heat,” TLT, 67 (3), pp. 10-11.
2. Canter, N. (2008), “Hybrid Hydraulic Drive Reaps Fuel Savings,” TLT, 64 (8), pp. 14-15.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.