Environmental Engineering Reference
In-Depth Information
The motor/generator of the Honda hybrid serves two other purposes. The engine is stopped
when the vehicle is stationary (idle-off), and it is restarted using the motor while the transmission
is in neutral. In addition, the motor/generator armature acts as the motor flywheel. Because the
motor/generator is attached to the engine shaft, it operates through the variable speed transmission
when adding or subtracting power to the wheel drive system, enabling the motor/generator to
operate efficiently over a wide vehicle speed range.
Comparing the Honda and Toyota hybrids of Table 8.4 with the conventional Honda and Acura
vehicles of Table 8.2 that are of comparable vehicle mass, we note that hybrid vehicles are about
80% more fuel efficient (for equal mass) and that for either hybrid or conventional vehicle, a small
fractional reduction in mass approximately begets a comparable fractional increase in vehicle fuel
efficiency. This substantial advantage of the hybrid over conventional design of equal mass is a
composite of braking energy recovery, lower rolling and aerodynamic resistance, more efficient
engine operating conditions (despite the same driving cycle), and possibly a higher peak engine
efficiency.
Evolution of standard vehicles in future years will move in the direction of hybrid drives.
It is expected that the electric auxiliary system will change from 14 to 42 volts and triple in
power. The current starter motor and belt-driven alternator will be replaced by a motor/generator
directly connected to the engine drive shaft, as in the Honda vehicle of Table 8.4, permitting
idle-off operation when the vehicle is stationary. Recovery of braking energy would be possible,
depending upon the motor/generator power and electric storage capacity. Although the purpose
of this development is to utilize electric drive for auxiliary power and thereby improve engine
efficiency, it clearly can be extended to become a hybrid system.
8.6.3
Fuel Cell Vehicles
Prototypes of electric drive vehicles whose electric power is supplied by fuel cells have been
under development for several decades. Potentially, such vehicles could provide higher vehicle
fuel efficiencies than conventional vehicles with little or no air pollutant emissions. Increasingly
stringent exhaust pollutant emission standards, especially in California, and national policies in
developed nations to secure both economic and environmental benefits of improving fuel economy
have increased the incentives for manufacturers to develop vehicle fuel cell technologies.
As explained in Section 3.12, the oxidation of a fuel in a fuel cell has the potential to convert
a higher percentage of the fuel's heating value to electrical work than does the typical combustion
engine. The upper limit to this proportion is the ratio of the free energy change in the fuel oxidation
reaction,
f
, to the enthalpy change, or fuel heating value
FHV
. For hydrogen, this ratio is 0.83,
while for methane and methanol it is 0.92 (see Table 3.1); these upper limits are at least double
what could be obtained by burning these fuels in a steam or gas turbine power plant. However,
as Figure 3.10 illustrates, this high conversion efficiency is only reached at zero power output; at
higher power the cell voltage declines nearly linearly with increasing cell current, resulting in only
50% of the upper value being recoverable at maximum cell power (41.5% for hydrogen and 46%
for methane and methanol). Still, these are higher fuel efficiencies than are obtainable in vehicle SI
engines at full power; the comparison is even more favorable to the fuel cell at part load because
the fuel cell efficiency increases at reduced load.
But there are countervailing factors in vehicle fuel cell systems that lower this fuel efficiency.
The only practical fuel cell for vehicle use requires gaseous hydrogen fuel. The most economical
and energy efficient source of hydrogen, a synthetic fuel, is by reforming from a fossil fuel such
