mpg. Similarly, a 2 percent engine efficiency boost for the ultracapacitor hybrid raises fuel
economy from 71.2 mpg to 73.1 mpg, with an additional 2 percent boost yielding 74.9 mpg.Emissions
Advocates have promoted series hybrids both for their efficiency advantages and for their
potential as ultralow-emission vehicles. Popular opinion is that an HEV engine’s constant
speed/load operation should greatly facilitate attainment of extremely low emissions. This ignores
the fact that 75 percent of all emissions in a conventional car occurs in the first two minutes after
cold start. Cold start also occurs in HEV operations, although the use of electrically heated
catalysts becomes easier with the large HEV battery. It has been noted, however, that Honda is
already close to certifying a conventional car to ULEV levels, so that the advantages of HEVs in
those terms appear minimal. In addition, since the HEV’s engine is on for a small fraction of the
time (-27 percent) during the urban cycle, cold-start emissions will be a much larger fraction of
total emissions--as much as 90 percent. Owing to high-load operation, cold-start NOX could be a
problem at LEV standards.A second factor affecting emissions is the strategy of turning the engine off when the battery or
other storage device becomes fully charged. Ideally, in the EPA urban test, the engine would be
turned on only once, run for 370 seconds (27 percent of 1,372 seconds), and then kept off with
the vehicle running as an EV. This is possible because the current FTP has only one strong
acceleration mode that should logically occur when the engine is on, so that the engine need not
turn on again to provide adequate power. The energy storage device would then have to sustain
the vehicle for the other 73 percent of the time, which requires an energy storage capacity of over
2 kWh. As table 4-12 indicates, the ultracapacitor and flywheel fall short of this goal although
both devices are deliberately sized well above the minimum size needed to provide adequate
power. This implies that, with these devices, the engine must be restarted more than once during
the emissions test, with attendant hot-start emissions and catalyst cool-down problems as well as
engine rotational inertia losses. Hence, HEV emissions may actually be more difficult to control
than emissions from a conventional vehicle, if electrical energy storage capacity is limited.
Automakers and suppliers are working on new controls that could greatly reduce problems
with hot restarts. For example, there are recent developments in quick light-off catalysts and
insulated manifolds that could minimize the emission effects of hot restarts to the point where
multiple engine shutdowns and restarts would no longer be a significant emissions problem. For
these reasons, we conclude that the suitability of ultracapacitors (and, possibly, flywheels as
well) for use in hybrid vehicles will depend on the development of controls that can greatly
reduce emissions from engine hot restarts.
Aside from emission certification tests, “real-world” emissions of hybrids can also be a concern.
Although certification emission levels can be low if the engine is operated infrequently on the
FTP, frequent high acceleration rates and high speeds may cause much more frequent engine
operation in real life, on average, than on the FTP, with significantly higher emissions than
certification levels. Such emission effects could be addressed by the proposed FTP test revisions
which will include high speeds and high acceleration rates during the test. Engine
malperformances can cause high emissions as in regular cars, but the hybrid design may reduce