Environmental Engineering Reference
In-Depth Information
BEV, the efficiency contours would be lower and move towards the origin to better
satisfy that application.
The development of efficiency maps for hybrid propulsion scenario playing
remains a key objective of many automotive manufacturers and tier one suppliers.
With the facility to mock-up arbitrary ac drive system torque, power, speed range and
overall efficiency maps, it becomes possible to simulate alternative hybrid propulsion
architectures in a what-if scenario. The simulated vehicle is subjected to standard drive
cycles, given a variety of energy storage system configurations as well as three or more
M/G continuous rating points, all of which have relatively accurate efficiency maps.
From this, the performance and economy of competing architectures may be assessed
and compared. Figure 8.9 illustrates a hybrid architecture simulation in which the
vehicle is exercised over the aggressive driver representative drive cycles, US06.
In Figure 8.9, the US06 speed versus time profile is played into a chassis
rolls dynamometer so that the hybrid vehicle under test is exposed to that driving
pattern. Then, depending on hybrid architecture, the M/G and ICE will have
their torque-speed operating points follow particular trajectories (shown as dots
on US06 trace and corresponding traces on the M/G and ICE maps). Also evident
in Figure 8.9 is the fact that the operating points of the energy storage system,
a lithium-ion battery in this example, will also be subject to varying load according
to how the M/G is interfaced to the vehicle driveline via the particular hybrid
architecture. Note that the battery in this example is sized such that its 10C
rate matches the M/G maximum torque rating. In a realistic hybrid system, the
sizing operation is more complex because of battery voltage droop under high load
and its consequent impact on the M/G's ability to actually deliver full rated torque.
For these reasons the battery capacity may be increased somewhat.
The conclusion from Figure 8.9 is that the most efficient propulsion system
is not necessarily the one having all of the most efficient components. Rather,
the cycle average efficiency, hence fuel economy, is more dependent on how
much time is spent at each efficiency of M/G, ICE, battery and of course the
transmission.
Note also in Figure 8.9 that the hybrid propulsion strategy attempts to hold the
engine in a more confined operating space as shown. This space limits WOT
operation to minimize emissions, restricts engine speed swing and restricts ineffi-
cient part load operation.
Developing efficiency maps for ac drive systems today requires up front design
and modelling of the proposed M/G rating and package constraints. As system
simulation tools become more advanced, it is now possible to simulate in a timely
manner a number of competing designs. Other than industry proprietary software
tools, those shown in Table 8.6 are seen as pre-eminent system simulators for
assessment of BEV and hybrid propulsion systems. The list is in alphabetical order
without preference to any one system simulator toolbox.
There are of course many other software tools and design services available for
the design of vehicle systems, powertrain architectures, energy storage systems and
electric drive components. The list in Table 8.7 is meant only to illustrate the
breadth of services available.
