Hybrid vehicles combine two energy sources with an electric drivetrain, with one or both sources providing electric power to the motor. This section examines hybrids that incorporate an internal combustion engine as one of the energy sources, with batteries, flywheels, or ultracapacitors also providing electric energy to the motor. Moreover, although gas turbines can be used in a hybrid, turbines of the size optimal for 1ightduty vehicles are unlikely to be more efficient than piston engines of the same performance capacity; consequently, only piston engines are considered in this section. Other combinations of energy sources, such as a fuel cell and a battery, can also be used in a hybrid, however.
The conceptual advantage of a hybrid is that it gains the range provided by an engine using a high-density fuel, but avoids the energy losses associated with forcing the engine to operate at speed/load combinations that degrade its efficiency. In other words, the engine can run at nearly constant output, near its optimum operating point, with the other energy source providing much of the load-following capability that undermines the engine’s efficiency in a conventional vehicle. The term hybrid is applied to a wide variety of designs with different conceptual strategies on the use and size of the two drivetrains. One form of classification for hybrids is a division into so called series and parallel hybrids. In a series hybrid, the power generated by the ICE is always converted to electricity, and either stored (in a battery, flywheel, or ultracapacitor) or used directly to drive a motor, which is connected to the vehicle’s wheels. In a parallel hybrid, the engine or the motor, or both, can drive the wheels directly. The two design types are shown schematically in figure 4-3. Although both systems have advantages and disadvantages, most manufacturers who have displayed prototype hybrid vehicles have selected the series design. The exception is VW, and its engineers believe that series designs are being displayed largely because they are very easy to develop, but are inefficient for reasons explained later. Another classification method is according to whether the vehicles require externally supplied electrical power (as an Electric Vehicle does), or can operate solely on gasoline, and these are labeled as nonautonomous and autonomous hybrids, respectively..
For either the series or parallel type hybrid, the ICE and the electrical system can be of widely different sizes. In both hybrid types, one extreme would be to have the engine act as a “range extender” by charging the battery (or other electricity storage device) while the electric drivetrain is quite similar in size to that of a pure Electric Vehicle. With this type of setup, sizing the engine’s maximum output close to the vehicle’s average power demand during highway cruise (e.g., 15 to 20 kW/ton of vehicle weight) would allow the range of the vehicle to be similar to that of a conventional car.
Moreover, unless there were an abnormally long hill climb, the battery state of charge could be maintained at near constant level. At the other end of the spectrum, an engine could be large in size and the battery or power storage device made relatively small, so that the engine could be employed to provide peak power for acceleration and battery recharging capability. Obviously, there are infinite combinations in between the two extremes. The amount of energy stored in the battery or other storage device, as well as the device’s peak-power capability, are key determinants of how the engine and storage device will interactively supply power to the drivetrain under any arbitrary driving cycle. Autonomous hybrids of either the parallel or series type usually utilize larger engines than nonautonomous ones.
The hybrid vehicle concept is neither new nor revolutionary. The earliest hybrids were built in 1917, and DOE funded a large research program in the late-1970s and early 1980s. Many of the same arguments and analyses in vogue now in support of hybrid powertrains were voiced after the two oil crises of the 1970s.44 The Jet Propulsion Laboratory and General Electric developed studies, published in 1980, that estimated that a mid-sized car could attain 33 mpg on the city cycle, which was about 40 to 50 percent better than vehicles of that era. A prototype in the early 1980s demonstrated about 50 percent improvement in fuel economy relative to a early-1980s conventional vehicle of the same size, though it had lower performance.
More recently, several papers46 have claimed that hybrid vehicles using lightweight body construction, can provide a fuel economy increase of about 100 percent, while one paper claims an improvement potential of several hundred to several thousand percent for a hybrid configuration with a carbon fiber body, superb aerodynamics, and improved tires.47 Moreover, PNGV contractors have discussed charts where some form of hybrid powertrain (undefined) was by itself (that is, without changes in body construction, aerodynamics, and tires) to provide a 100 percent benefit in fuel economy, 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 Californien 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 Electric Vehicleen 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 Electric Vehicle 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 Electric Vehicle.