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21st Century Truck Initiative/Partnership (21CTP)

Heavy-Duty Hybrids

In heavy-duty hybrid research, the industry role was to be represented by the heavy-hybrid team members (chiefly Allison Transmission, BAE Systems, and Eaton Corporation, although Oshkosh Truck was also playing a role in hybrid research). The Department of Energy (DoE) was pursuing heavy hybrid research through the FreedomCAR and Vehicle Technologies Program. The DoE Hydrogen, Fuel Cells, and Infrastructure Technologies Program was also interested in heavy hybrid technologies as a bridge to the hydrogen fuel cell vehicle of the future. The Department of Transportation (Federal Transit Administration) played a role in demonstration of these vehicles for the transit bus market. The Department of Defense would be working with heavy hybrid equipment suppliers to develop and demonstrate hybrid vehicles for military applications, and had made significant investments in hybrid technology to reduce fuel consumption and improve their ability to travel silently in combat situations. The Environmental Protection Agency participated in the heavy hybrid arena through its work on mechanical hybrids for certain applications.

Drive unit (electric traction, motor, transmission, generator, inverter, controller and cooling devices): Certain types of drive units were discussed in 21CTP white papers as working better than others for specific vehicle applications or performance requirements. Several types of motors and generators had been proposed for hybrid-electric drive systems, many of which merited further evaluation and development. Motor generators could be configured before or after the transmission.

Series HEVs typically have larger motors with higher power ratings because the motor alone propels the vehicle. In parallel hybrids, the power plant and the motor combine to propel the vehicle. Motor and engine torque are usually blended through couplings, planetary gear sets and clutch/brake units. Interestingly enough, the same mechanical components that make parallel HD hybrid drive units possible can be designed into series HD hybrid drive units to decrease the size of the electric motor(s) and power electronics.

Electric Machines: There were no easy answers for electric machine selection and design for HD hybrid applications. The choice had to be made based on extensive trade studies relative to the requirements and priorities for the application. Motor subsystems such as gear reductions and cooling systems had to be considered when comparing the specific power, power density, and cost of the motor assemblies. High speed motors could significantly reduce weight and size, but also required speed reduction gear sets that could offset some of the weight savings, reduce reliability and add cost and complexity. Air-cooled motors are simpler and generally less expensive than liquid-cooled motors, but they have historically been larger and heavier, and require access to ambient air, which can carry dirt, water, and other contaminants. Liquid-cooled motors have historically been generally smaller and lighter for a given power rating, but require more complex cooling systems that are avoided with air-cooled versions. Various coolant options, including water, water-glycol, and oil, were available for liquid-cooled motors.

Power electronics: This was another potential area that could crucial role in converting and distributing power and energy in automotive applications. US industries in 2006 supply power electronic products for commercial and military HEV applications. However, no manufacturers in the United States could supply the high-power Isolated Gate Bipolar Transistors (IGBT's) required for these products. Selecting the correct power semiconductor devices, converters/inverters, control and switching strategies; packaging and cooling the units; and integrating the system were very important to developing an efficient and high-performance system.

Electrical energy storage: This technology had seen a tremendous amount of improvement between 1996 and 2006. Advanced battery technologies and other types of energy storage were emerging to give the vehicle needed performance and efficiency gains while still providing a product with long life. 21CTP's focus would be on the more promising energy storage technologies-nickel metal-hydride (NiMH) and lithium technology batteries and ultracapacitors. Other less mature technologies, such as flywheels, would have a lesser focus, but were to be considered as they reached sufficient levels of robustness for mobile applications.

The three major electrical energy storage systems that were being considered for hybrid electric propulsion systems were electrochemical batteries, ultracapacitors, and electric flywheels. Between the beginning of the 21st Century Truck Initiative and 2006, Government and industry programs and initiatives supported R&D of electrical energy storage systems for LD vehicles. These programs and initiatives directed most of their resources to batteries because of the better potential for short-term commercialization, and established technical targets for hybrid battery development efforts for powerassist and dual-mode HEVs.

Key challenges for any type of HEV energy storage system that had to be addressed were:

  • Cost, both procurement and life cycle
  • Weight and space claim
  • Life expectancy (in an HD drive-cycle)
  • Energy and power capacity for a HD hybrid application
  • Suitability for the HD vehicle environment and cooling techniques
  • Architecture/modularity
  • Safety/failure modes
  • Maintainability
  • Management and equalization electronics and algorithms
  • Supplier base for the storage elements

By 2006 specific goals had been outlined for research and development in 3 areas: drive units, energy storage, and propulsion. These were defined as follows:

  • Develop a new generation of drive unit systems that have higher specific power, lower cost, and durability matching the service life of the vehicle. Develop a drive unit that has 15 years design life and costs no more than $50/kW by 2012.
  • Develop an energy storage system with 15 years of design life, that prioritizes high power rather than high energy, and costs no more than $25/kW peak electric power rating by 2012.
  • Develop and demonstrate a heavy hybrid propulsion technology that achieves a 60% improvement in fuel economy, on a representative urban driving cycle, while meeting regulated emissions levels for 2007 and thereafter.

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Page last modified: 07-07-2011 02:41:29 ZULU