Environmental Engineering Reference
In-Depth Information
The third column in Table 1.8 shows the wide disparity in vehicle stop time
depending on what region of the world is in question. It is clear that idle-stop fuel
economy under the 10-15 mode will be significantly higher than EPA-city. The
second point to notice from Table 1.8 is the average vehicle speed over these
standard drive cycles in column five. Not only will the differences in average speed
have a bearing on fuel economy, but interestingly on transmission type and gear
ratio selection as well. More will be said on this topic in Chapter 3. Further details
on standard drive cycles can be found in Chapter 9.
The second contributor to vehicle fuel economy enabled by hybrid function-
ality is vehicle kinetic energy recuperation through regenerative braking. Rather
than dissipate braking energy as heat, a hybrid powertrain recuperates this energy
and uses it to replenish the storage battery or ultra-capacitor. This is the reuse
portion of the hybrid technology charter. However, just as idle time is a strong
function of customer usage as characterized in standard drive cycles, so is the
benefit of regenerative braking. Figure 1.20 is a compilation of vehicle braking
duration for some of the drive cycles noted in Table 1.8.
Regenerative duration for 50 V < V_batt < 55 V
20
EPA-city
EPA-Hwy
ATDS
US06
15
10
5
0
0
10
20
30
40
Time (s)
Figure 1.20 Duration of vehicle braking events by drive cycle
The correlation of braking events by drive cycle in Figure 1.20 to column six in
Table 1.8 is evident. Real world customer usage (ATDS cycle), US06 and EPA-city
show the highest incidence of braking events having durations of 5-10 s. Higher
speed driving cycles show braking events distributed out beyond 30 s.
One can visualize the HEV, particularly a power assist hybrid, as employing
only the amount of electrical capacity needed to crank the ICE from stop to idle
speed in less than 0.3 s and to offload the ICE during transient operation such as
quick acceleration and deceleration. During normal driving, the M/G is designed to
operate as a high efficiency alternator. Alternator efficiency exceeding 80% is
necessary if the additional cost and complexity of hybridization is to meet the
PNGV targets of two to three times the fuel economy noted earlier [11]. The cost of
providing electricity on CVs, passenger cars and light trucks is calculated by
assuming an ICE having 40% marginal efficiency (marginal efficiency is different
from engine thermal efficiency and means the incremental efficiency of adding one
additional watt of output). So, for gasoline that has a density of 740 g/L, an energy
