Advanced Heat Engines for Range Extender Hybrid Vehicles 930041

A great deal of interest has been expressed recently in hybrid electric vehicles, both for their potential for reduced environmental impacts as well as their increased range over electric-only vehicles. Several organizations have initiated programs to design and evaluate a specific type of hybrid vehicle, the Range Extender Vehicle (REV). Based upon this industry activity, the U.S. Department of Energy's Electric and Hybrid Propulsion Division commissioned the two-phase study which serves as the basis of this paper.

A range extender vehicle is a hybrid vehicle, where a small engine generator (a “genset”) is added to a basic electric vehicle design. This genset “sustains” vehicle operation beyond the range provided by the batteries. The additional weight of the genset and fuel is balanced by the removal of some of the vehicle's batteries. Such a concept may be useful in the near term, when commercially available batteries are not expected to have adequate specific energy to provide the range necessary for market acceptability.

In Phase I, the range extender designs were based on existing, well-characterized electric vehicle designs. Established driving cycles were used to define the basic energy and power needs of the vehicle systems. The emphasis during this initial effort was placed on near-term designs. These designs were based on the use of existing vehicle platforms (Ford Taurus, Dodge Caravan, and Chevrolet C1500 pickup), available lead acid batteries, and primarily small, off-the-sheff engine generator systems.

The primary performance requirement for Phase I was a minimum generator-set-only top speed of 25 mph (40 km/h). This was done for two reasons. First, it followed the rationale used by several organizations, which indicated that a 6 to 10 kWe generator set would be appropriate for the REV configuration. Second, it provided a useful starting point for REV analysis, yielding a lower bound for potential performance.

The Phase II analysis, which is the basis for this paper, evaluated higher performance REV conceptual designs. National Highway Traffic Safety Administration (NHTSA) standards for electric vehicles as well as development requirements for the Ford ETX-II and Modular Electric Vehicle Programs were used to provide the basis for comparison. These included acceleration, speed at grade, and minimum top speed requirements. In all cases, these desired levels of performance indicated that significantly larger generator sets would be required, estimated to be in the 25-50 kWe range. These requirements dictated the use of more advanced engine technologies. A number of engine types and sizes were examined for application to the REV, including advanced four-stroke, two-stroke, gas turbine, rotary, Stirling, and compression ignition engines. The engines selected for analysis ranged from 24 kWe to nearly 63 kWe, at the operating points selected.

Overall, the Phase II analysis demonstrated that higher-performance REVS could be developed, using more advanced heat engine/generator configurations. This higher performance should appeal to a broader range of potential users (than the Phase I design), increasing the REV's potential market penetration.