The world's first series hydraulic hybrid transit bus has tallied an overall average of 6.9 mpg, doubling the fuel efficiency of conventional diesel buses and surpassing by 30% the mpg average of hybrid-electric buses.
"These are very significant fuel-economy results," Mike Heskitt, COO of Altair ProductDesign, told government officials and media gathered for the demonstration prototype vehicle's September 7 unveiling at Altair's world headquarters in Troy, MI.
Dubbed the LCO-140H (Low-Cost of Ownership-1st 40-foot Hybrid), the Altair-designed vehicle, developed under a public-private initiative, underwent fuel-economy trials in the spring of 2011 at Ford Motor Co.'s Michigan Proving Grounds based on U.S. Federal Transit Administration's (FTA) protocols for transit bus certification testing.
With a curb weight of 25,800 lb (11,700 kg), the bus is approximately 15% lighter than most electric hybrid buses and 10% lighter than many non-hybrid buses. Its use of a Parker Hannifin series hydraulic hybrid drivetrain enables a two to three times higher power recovery vs. hybrid-electric systems.
The drivetrain features a primary pump directly geared to the output of the engine and two secondary pumps—attached to the rear axle via a gear reduction unit—that are capable of transferring a combined 600 hp (448 kW) either to or from the wheels, according to Vance Zanardelli, Business Unit Manager of Parker Hannifin Corp.'s Hybrid Drive Systems Division.
Each bent-axis, variable-displacement Parker C24 pump can seamlessly switch between pumping and motoring modes—depending on power absorption or delivery demand—and each pump/motor unit is capable of transmitting as much as 300 hp (224 kW) and 850 lb·ft (1152 N·m) of peak torque.
Hydraulic fluid cycles between a low-pressure reservoir and two high-pressure accumulators with a maximum operating pressure of 5400 psi (372 bar).
"The low-pressure reservoir is designed to store fluid when it is not contained within the high-pressure accumulators and to maintain a minimum inlet pressure on the C24s," explained Zanardelli. "During braking, fluid is drawn from the low-pressure reservoir and pumped into the high-pressure accumulators where the recovered braking energy is stored by compressing nitrogen gas.
"During the next acceleration cycle, pressurized fluid is released from the high-pressure accumulators to propel the vehicle via the hydraulic pump/motor assemblies. Once this acceleration cycle is over, the fluid returns in a low-pressure state to the low-pressure reservoir. The diesel engine compensates for the losses which occur during the cycle, and to maintain state-of-charge in the high-pressure accumulators."
All the drivetrain components are linked together through a Parker-developed intelligent control system, which is responsible for power management, fault detection/response, and digital communications with other ECUs on the CAN bus.
According to Tim Smith, Altair ProductDesign's Director of Design Engineering, engineers used Altair's HyperWorks suite of CAE tools to develop the vehicle's systems and its structure.
"The all-aluminum structure for the bus is a semi-monocoque design, and what makes it a semi-monocoque vs. a full-monocoque is that it is not fully skinned with the structure. The body and chassis structures are a single, integrated unit," Smith said. 6061-T6 aluminum alloy is the material for the body's 13 extrusions.
Design of the structure/chassis and integration of the various systems were handled by four engineering managers and 10 design engineers who represented the core of Altair's BUSolution team.
"Additional resources were added in industrial design, mechanical and electrical design, engineering, vehicle development, and testing as required throughout the project. These technical specialists assisted with specific detailed support by exploring parallel path solutions, researching alternative materials and construction methods, as well as identifying and evaluating the various options for commercial-off-the-shelf components," said Smith.
A roof-mounted HVAC unit supplied by Carrier is the centerpiece of an Altair-designed cooling system that draws air from above the bus rather than from the engine compartment or below the bus. The radiator has a 50% larger surface area and half the thickness of a typical radiator used on conventional buses.
"This radiator design in combination with the multi-unit fan system allows for more efficient airflow and better cooling. The overhead fans can be reversed to easily blow out leaves or debris that has accumulated on top of the fan unit, and a convenient service door on the side of the cool-air corridor facilitates cleaning accumulated debris," Smith explained.
The bonded-in side windows use Sika Corp. adhesives and sealants.
"Altair used its simulation expertise to select the least-sensitive locations from a structural standpoint for the escape windows. To enable installation of escape windows, Altair has integrated a hinging feature into the side drip rail extrusion for the exterior side of the hinge. The interior side of the hinge would be bonded to the side glass. A simple 'L' channel ring reinforcement would be added to the body side window opening to retain the required weather-strip," explained Smith.
Weight-savings considerations are evident in nearly every component and system, ranging from the Alcoa aluminum wheels and Vogel Industrie GmbH's thin-profile seats with carbon-reinforced epoxy stanchions, to the balsa core laminate flooring and balsa roof core.
"The LCO-140H meets every design standard required by U.S. federal and local regulations, including rollover," said Smith.
More than 15 suppliers provided primary content to the demonstration bus. In addition to the previously cited companies, the list includes Meritor (front and rear axle assemblies, disc brake assemblies, and driveshaft assembly), Meritor Wabco (pneumatic brake system components, and engineering specification and schematic for pneumatic brake system), and PRAN (graphic multiplex display unit for instrument panel).
The FTA provided $5.1 million to the LCO-140H's development with additional funding provided by the Michigan Economic Development Corp. and Automation Alley, a Southeast Michigan technology business consortium.
While no timetable for commercialization has been officially revealed, the production-ready vehicle has been touted as the most fuel-efficient hybrid bus available at a lower lifetime cost than conventional transit buses.
Walter Kulyk, the FTA's Director of the Office of Mobility Innovation, said during his remarks at the LCO-140H's unveiling, "We certainly encourage the local community and the local transit authorities in the public sector to be part of this [commercialization] effort."