Demonstration series hydraulic hybrid transit bus yields fuel economy of 6.9 mpg, 110% better than conventional diesel, 30% better than electric hybrids
|The series hydraulic transit bus. Click to enlarge.|
The BUSolutions demonstrator LCO-140H (Low Cost of Ownership–1st 40-foot Hybrid) series hydraulic hybrid transit bus yields fuel economy of 6.9 mpg (34 l/100km) on the industry-standard ADB duty cycle—110% better than conventional diesel buses on the road today and 30% better than the best-in-class electric hybrid buses available today, according to the final report on the project published by the US Federal Transit Administration (FTA).
More importantly, according to the report written by a team from Altair ProductDesign, which led the project, such a bus will cost more than 20% less than a conventional diesel bus to own and operate over its life and more than 30% less than an electric hybrid.
|Elements of the hydraulic hybrid powertrain. Click to enlarge.|
The project originated when Automation Alley, Altair ProductDesign, and the Federal Transit Administration (FTA), in a public-private partnership, teamed up to advance a new transit bus initiative that would improve local and regional transit systems while requiring no infrastructure upgrades to operate. The goal was to develop a significantly lighter-weight, heavy-duty bus design that yields superior fuel efficiency to conventional buses at a lower lifecycle cost.
The four main areas of focus for reducing the lifecycle cost of the new bus were purchase price; fuel economy; scheduled maintenance; and unscheduled maintenance.
Since 2005, FTA issued $5.1 million in funding for the resulting BUSolutions project, with additional program support provided by the Michigan Economic Development Corporation (MEDC) and Automation Alley, Michigan’s largest technology business association. Altair, with the help of its partners and sponsors, successfully designed, fabricated, and tested the bus to validate the design and performance metrics.
Altair opted for a hydraulic hybrid powertrain primarily because of power density—the amount of energy that can be transferred during a given period of time—as well as low system entry cost and lower maintenance costs. All hybrid vehicles use regenerative braking; hydraulic hybrids capture the energy in the same braking event and store it in the form of fluid power. The advantage of the hydraulic hybrid approach, the report said, is that this method is able to store much more energy quickly, making it more suitable for heavy vehicles with high stop-go duty cycles.
Further, analysis showed that between a parallel and a series system, given the typical duty cycle of a transit bus (low average speed/high number of stops per mile), the series system would provide the optimum fuel economy. With the engine is decoupled from the drive axle, there is no longer a requirement to match engine RPM with vehicle speed. This enables the engine operation to run at the most efficient RPM to yield optimum fuel economy, the team noted.
After evaluating the offerings from three major US suppliers, Altair chose Parker Hannifin as its supplier for system components. The majority of the system components were available from Parker offerings, including the bladder accumulator, hydraulic cooler, engine mounted pump, and gearbox with pump/motors. Altair completed some initial analysis before specifying the entire system in order to understand what system capacity would yield the optimal results from the system.
Included in the analysis was the evaluation the performance differential between one 22-gallon high-pressure accumulator and two high pressure accumulators totaling 44 gallons. The analytical fuel economy results showed that the addition of a second accumulator yielded a 4–11% improvement over a single accumulator system, depending on the duty cycle. These improvements outweighed the cost and weight of the additional components and added complexity of the system and were integrated into the bus.
Because a large-capacity low-pressure reservoir (LPR) was not available to package in the space available, a custom LPR was designed and manufactured. This design required additional overhead air capacity.
Altair evaluated a swash plate pump and bent axis pump; the bent axis pump has the best efficiency of the two and was selected to be used. This is a piston pump design containing multiple axial pistons mounted at an angle to the drive shaft; this design is integrated into the Parker pump/motor offerings.
The controls development strategy was initially defined by the Parker engineering staff. In cooperative efforts, Altair and Parker engineers jointly worked together to follow the controls development process to refine systems drivability and functionality.
Early in the program, Altair selected high-strength stainless steel as the primary structure material because of its common nature in the industry, above-average corrosion resistance, and post-welded strength. As the program progressed, Altair found the availability of off-the-shelf high-strength stainless steels to be scarce, specifically in the sizes required to fabricate the optimized bus structure.
After reviewing other materials, Altair selected aluminum 6061 with a T6 temper. It is commonly used in various structural, building, marine, automotive, aerospace, and process-equipment applications. It has a typical yield strength of 40,000 psi and an ultimate tensile strength of 45,000 psi.
In designing the bus, the project team established a specific list of questions and criteria to concentrate the focus on the desired outcome. Altair wanted to use existing products and technology available but to apply them in a more efficient way that would yield better results. Because of this, the process was bounded and specific components were limited to change. For example, the internal combustion engine was labeled as an off-the-shelf product; therefore, designing a new engine was not on the table.
The following table shows a list of the design innovations made it into the final design of the LCO-140H.
|Click to enlarge.|
Design & Development of the LCO-140H Series Hydraulic Hybrid Low Floor Transit Bus. FTA Report No. 0018