by itself (that is, without changes in body construction, aerodynamics, and tires) to provide a 100
percent benefit in fuel economy,(^48) and this value currently is the target for the DOE hybrid
program. DOE has also sponsored several college-level competitions, called the Hybrid Vehicle
Challenge, where colleges have displayed hybrid vehicles of both the series and parallel type that
have attained relatively high fuel economy levels. For example, the 1994 entries from University
of California at Davis and the University of Maryland have claimed fuel economy levels of 75 to
80 mpg at constant speed (-40 to 50 mph) in small or compact cars.^49 Given these demonstrations
and programs, there is a widespread belief among many observers that hybrid powertrains
can easily achieve 100 percent improvements in fuel economy, and that even higher benefits
are possible in the future. An added attraction is that hybrids can potentially act as limited-range
electric vehicles, and thus can be zero emission vehicles in select urban areas.
This positive view of hybrids is by no means unanimous. On the other side of the argument,
several auto manufacturers and EV manufacturers have told OTA that hybrid drivetrains produce
small or no benefits to fuel economy .50 Several series hybrids displayed by BMW,^51 Mercedes,
and Nissan,^52 for example, have displayed virtually no benefit in fuel economy relative to gasoline
engine-powered vehicles of similar performance. VW has developed parallel hybrids using a diesel
engine and a small electric motor that have displayed good diesel fuel efficiency but high
electricity consumption. The VW Golf hybrid requires that batteries be charged from the grid, and
they are not charged by the engine. In the Federal Test Procedure, this hybrid attained 80 mpg of
diesel fuel but also consumed 0.122 kW/km (about 0.20 kW/mi) of electrical energy .53 This
electric energy consumption is similar to that of a comparable EV.
Series Hybrids
In a series hybrid, the engine is used only to drive a generator, while the wheels are powered
exclusively by an electric motor. A battery (or flywheel or ultracapacitor) is used to store energy,
obtaining some energy input from regenerative braking, and most of the input from the
engine/generator. The motor can be powered either directly by the engine/generator, by the
battery, or by both simultaneously (at high-power demand). Strategy considerations about when
to use the battery or the motor/generator lead to decisions about the relative power output of
each unit and the energy storage capacity of the battery.
The popular vision of a series hybrid has a small engine operating at constant output, providing
the average power needed over the driving cycle, with a battery, flywheel, or ultracapacitor
providing additional power when needed, such as for acceleration or hill-climbing. When the
(^48) P.G. Patil, "Partnership for a New Generation of Vehicles,” Automotive Technology Development Contractors Coordination Meeting, U.S.
Department of Energy, October 1994. 49
S.A. Merit and K. Wipke, “The 1994 Hybrid Electric Vehicle Challenge,”Automotive Technology Development Contractors Coordination
Meeting, U.S. Department of Energy, October 1994. 50
51 Office of TechnologyAssessment project team meetings with automobile manufacturers in Europe and Japan, May/June 1994.
52 S. Friedman and K. Scheurer, "On The Way to Clean(er) Vehicles,” SAE paper 94C052, 1994.
Nissan, personalcommunications June 16, 1994.
(^53) W. Josefowitz and S. Kohle, "The Volkswagen Golf Hybrid,” paper prepared for the 1lth International EV Symposium, September 1992.