Environmental Engineering Reference
In-Depth Information
Table 2.4 Mild hybrid ISG in generating mode
Operating point
Efficiency (%)
Aluminium cast rotor
induction generator
42 V, 4t ISG
42 V, 5t ISG
300 V, 12t ISG
Idle speed: 1,000 rpm and
1 kW output
77
81
65
Fast idle speed: 1,500 rpm and
10 kW output
75
65
71
Cruise speed: 2,500 rpm and
5 kW output
82
70
82
Cruise speed: 2,500 rpm and
10 kW output
70
Outside of
capability
84
Efficiency in generating mode was clearly superior for the high voltage ISG in
part due to the voltage dependant loss mechanisms listed earlier. To illustrate this,
two mapping points are summarized for each machine in Table 2.4.
Table 2.4 makes it clear that the high voltage ISG is also a higher speed
machine because the low speed, low power point has inferior efficiency to either
winding design 42 V ISG. At higher speeds the 300 V winding ISG is just coming
into its own as far as power output is concerned. Of the 42 V machines, the 4t
winding appears to clearly outperform the 5t winding in efficiency at all but the
lowest speed. The 5t ISG is therefore a low speed winding for generator mode.
2.2.4 Power assist
The pre-transmission architecture described in Section 2.2.3 was referred to as a
mild (or soft) hybrid, which is also known as an ISG in specific implementations
when integrated into the transmission. When the ISG is up-rated in power it
becomes a power assist hybrid. Power assist architectures are similar to those
described in Figure 2.14 but with higher pulse power capability, typically greater
than 20 kW. The system energy storage, however, is limited to less than 1 kWh.
Power assist mode demands a high energy storage system
P
/
E
3
of typically
>
10.
Power assist architectures do not have electric-only range capability, or if
designed with more energy storage capacity the electric-only range may be just a
few kilometres, perhaps as many as 7 km. These architectures typically have
electric fractions of only 10-30% and a modestly downsized engine.
In the US Department of Energy study by its Office of Automotive Trans-
portation Technology, the power assist mode hybrids are said to cost incrementally
more than a conventional vehicle. Figure 2.14 illustrates the cost components in
conventional vehicles, power assist hybrids (HEV0) and dual mode hybrids
3
Power to energy ratio of a regenerative energy storage system (RESS) defines the capacity to sustain
a high power pulse for time
T
¼
(
P
/
E
) in (W*kg)/(Wh*kg)
¼
h
1
, or convert to seconds, 3,600 s/h.
