Environmental Engineering Reference
In-Depth Information
indicate fewer and shorter stops on average per commute. Commercial truck, bus,
fleet and police vehicles, on the other hand, experience more stops of longer
duration. Our city bus, for example, falls on the rightmost trend line.
Regeneration of the braking energy to replenish the storage system is likewise
strongly dependent on the drive cycle statistics. For the bus example we are inter-
ested in what portion of the fixed loads, or all of it, that can be supported by the
recuperated energy from braking only. The un-realized energy of recuperation (i.e.
the shortfall in storage system SOC) must be replenished by the engine driven
generator. The term 'un-realized' is used because energy recovery through regen-
erative braking is typically limited to 30% of the available kinetic energy due to
storage system charge acceptance, driveline mismatch such as lack of transmission
torque converter lock-up in lower gears and generator inefficiency at low speeds.
Figure 4.33 illustrates regeneration potential for various standard drive cycles for a
42 V PowerNet vehicle that is applicable to our present study.
Regeneration duration for 50 V <
V
batt
< 55 V
20
15
EPA-city
EPA-hwy
ATDS
US06
10
5
0
0
10
20
30
40
Time (s)
Figure 4.33 Statistics of regeneration duration for standard drive cycles
(42 V system)
In Figure 4.33 the ATDS is known as 'real world' customer usage and gives
fuel economy predictions that come closer to matching driver experience. The
ATDS cycle approximates our city bus highway portion since there are few stops
and longer cruise portions. The EPA city cycle comes closest to our city bus drive
cycle because of the frequent stops and relatively long duration of braking time.
Other cycles such as the New European Drive Cycle (NEDC) are more repre-
sentative of European city driving and US06 is more representative of North
American commuting. These drive cycles are covered in more detail in Chapter 9.
A simulation of the hybrid city bus was performed using the forward modelling
technique to track energy expenditures. In this simulation, the engine driven gen-
erator is controlled for maximum power only when the propulsion system demands
power. Energy recuperation is done according to the charge acceptance limits noted
in (4.16) and (4.17). When the battery is unable to discharge (or charge) at the
demanded rate, the ultra-capacitor will source or sink the excess power. Given this
strategy, and for a 31 Ah, 510 V traction battery, and a 37.6 F ultra-capacitor
