intentional maladjustment or tampering as engine operation is at near constant speed/load.
Malperformance-related issues are a major concern for regulatory agencies, however, especially
as the vehicles age, and the hybrid may offer no benefit over conventional vehicles in this arena.
Hence, hybrids may have no significant benefit in emissions relative to conventional
vehicles, with the possible exception of their capability to act as limited-range EVs in
specific urban areas.Other Studies
The results presented here are radically different from these presented by some analysts, and a
comparison of the assumptions employed is provided here for a few selected papers. A recent
paper by Mason and Kristiansson^60 of Volvo showed low fuel economy levels for all types of
hybrids and claimed that series hybrids were more efficient than parallel hybrids. The analysis
presented in the paper incorporated several assumptions that do not appear defensible, for
example:Engine efficiency under urban driving was assumed to be 10 percent, and 20 percent for highway driving
for conventional vehicles. A 30 percent efficiency was used to model the series hybrid, and the incorrect
large difference in efficiencies explains the poor results for the parallel hybrids.Weights for alternative configurations of hybrids were not calculated, but were assumed to be equal to
the conventional vehicle. This leads to gross error in some cases. ‘A very rigid operating strategy was dictated by assuming that the vehicle would behave as an EV for the
first 30 miles, and as a hybrid for the next 60 miles.The issue of engine sizing and on/off operation were not addressed.The battery was expected to supply the worst-case requirements for power unaided by the engine, which
dictated the need for an excessively large battery.As a result of what we consider as unrealistic input assumptions, the fuel economy for a mid-sized
HEV was estimated at about 34 mpg for a series hybrid and 19 mpg for the parallel hybrid.Some analysts have obtained substantially more optimistic results than OTA. One analyst has
published studies on hybrid vehicles for the past 15 years, and has used a relatively sophisticated
model (SIMPLEV) to estimate their benefits. In recent work, he has reported fuel efficiency
benefits for series hybrids of 40 to 60 percent on the city cycle and in the 30 percent range for the
highway cycle.^61 Direct comparisons between this analyst’s simulations and OTA’s results were
facilitated by a special run of his model using values quite similar to those used by OTA for
vehicle characteristics. His results provide for a direct comparison of the results of the two
modeling methods for a hybrid using an ultracapacitor for energy storage (see table 4- 13).
(^60) W. Mason and U. Kristianson,“Hybrid versus Pure EVs Which Gives Greater Benefits,” SAE paper 94C017, 1994.