Environmental Engineering Reference
In-Depth Information
transmission configurations, the electric system must have sufficient capacity to at
least mimic the torque augmentation feel of the automatic.
3.5 Electric fraction
The topic of electric fraction has already been introduced in the context of hybrid
powertrains. Table 3.6 gives a broad brush illustration of full gamut of electric fraction.
Table 3.6 Electric fraction classifications
Vehicle
Downsizing Electric
fraction (EF) (%)
Comments
Conventional vehicle None
~1
Counting conv. alternator
Mild HV
<
10%
1-10
THS-M, IMA
Hybrid vehicle
10-30%
10-50
Range determined by fuel tank
capacity
Fuel cell vehicle
N/A
100
Vehicle range set by H2 storage
Battery EV
N/A
100
Vehicle range set by battery capacity
3.5.1 Engine downsizing
The fraction of engine downsizing shown in Table 3.6 is generally less than the EF
because sufficient engine power must be held in reserve to meet performance on
grades and with variable passenger loading. The electric system assists in all
transient events, but its storage capacity is insufficient when sustained torque
delivery is demanded for grade climbing. When the Prius was first introduced into
the North American market in CY2000, the major complaint was its lack of per-
formance on grades. This was in fact partly due to an engine downsized from
approximately 2.2 to 1.5 L (33%) and in part due to operating the engine on the
Atkinson cycle (i.e. late intake valve opening).
Smaller displacement engines mean less pumping loss and higher efficiency,
all else being equal. In many instances an I4 can be used in lieu of a V6, or a V6 in
lieu of a V8. The Toyota's hybrid synergy drive, for example, claims V8 perfor-
mance with a small V6. The hybrid synergy drive represents Toyota Motor Co.'s
initial launch of their new hybrid family THS-II and now THS-III. The THS-III is
built around a 201.6 V NiMH pack with high power half-H dc/dc converter to raise
this voltage to 650 V at the MG1 and MG2 inverters. Toyota had planned to
introduce lithium ion packs by 2010, but has opted to postpone this until the lithium
ion technology is proven automotive acceptable.
3.5.2 Range and performance
Hybrid vehicles must deliver performance comparable to conventional vehicles
regardless of the desire to downsize the engine, reduce overall vehicle mass,
including fuel tank capacity, and use longer final drives. In North America an
