Hybrid vehicles are further differentiated by the relative sizes of the IC engine, battery, and motor. Some of the more common variants of these broad classes are described in the following paragraphs. In all cases an economically and functionally significant component of the system is the power electronic subsystem necessary to control the electrical part of the drive train.
The hybridization of diesel (compression ignition; CI) vehicles is expected to have somewhat lower efficiency benefits than hybridization of gasoline vehicles, in part because conventional CI vehicles already exhibit lower fuel consumption than comparable gasoline vehicles. Further, CI vehicles also have very low fuel consumption at idle, making the benefits of idle-stop less attractive. Conventional CI power trains are more expensive than their gasoline counterparts (see Tables 5.4, 5.5, and 5.6), which, when added to the cost of hybridization, makes a CI hybrid power train very expensive for the additional fuel consumption reductions provided over and above just moving to a hybrid or CI power train alone. As a result, it is unlikely that original equipment manufacturers (OEMs) will offer a wide array of CI hybrids. The most likely levels of CI hybridization will be idle-stop and, perhaps, some mild hybrids. Idle-stop will not provide much fuel consumption reduction on the city driving portion of the FTP test cycle, upon which the judgments in this report are based. However, OEMs may still offer such technologies since they provide in-use fuel consumption reductions. In Europe, a number of new diesel hybrid vehicles have been announced for production in 2010 or 2011, especially for larger and heavier vehicles (e.g., Land Rover).
There are numerous hybrid vehicles now in production, and the committee believes it is more representative to quote actual data rather than analyze the effectiveness of each design to estimate fuel consumption benefits. This is preferable to having the committee and its consultants estimate fuel consumption benefits through simulations. It is assumed that the production vehicles are designed to meet customer expectations, including acceleration, passenger space, and adequate trunk space. The average fuel consumption of production hybrid HEVs was determined from fuel economy data supplied by Oak Ridge National Laboratory and included as Table 6.A.1 in the annex at the end of this chapter.
In the belt-driven alternator/starter (BAS) design, sometimes known as a micro or mild hybrid, the starter and generator of a conventional vehicle are replaced by a single belt- or chain-driven larger machine, capable of both starting the engine and generating electric power. In some BAS designs, in addition to the new belt-driven starter generator, the original geared-to-flywheel starter is retained for cold starts. Fuel consumption is reduced by turning off and decoupling the engine at idle and during deceleration. In some designs, particularly those that have replaced the belt with a chain for