Environmental Engineering Reference
In-Depth Information
sufficient constant power speed range (CPSR) to function over the full operating
regime of the vehicle. Without a matching transmission, a post-transmission M/G
will require very high torque levels to deliver the tractive effort necessary. Wheel
motor proposals are essentially post-transmission hybrids because there is typically
no package space within the wheel hub for both the electric machines and a load
matching gearbox. The best that can be done, and what has been demonstrated to
date, is the use of single epicyclic gear sets to match the M/G to the vehicle load.
Many challenges arise from in-wheel motors, foremost among the challenges are
the high levels of robustness necessary to survive high g-loading, water intrusion
and its penchant for freezing, and vehicle ride degradation due to much higher
unsprung mass. Also, with in-wheel motors the most benefit is in packaging,
freeing up of cabin space and more cargo volume.
The pre-transmission parallel hybrid architecture has gained the most favour in
hybrid designs because today's conventional technology M/Gs are adequate to
deliver hybrid functionality. The intervening mechanical gearbox compresses the
wide dynamic range of road load torque and speed into the operating space of
the M/G - its torque-speed capability space. There remain issues with M/G rating
and voltage level selection in parallel architectures because of the necessity for the
M/G to deliver engine cranking torque levels while having the CPSR to deliver
propulsion power over the operating speed range. Low rated, ISA type hybrid
systems find this most challenging and generally end up as overrated electric
machines, power electronics or both [13].
In the five subsections to follow the various classifications of pre-transmission,
parallel hybrid architectures will be examined in more detail. Table 2.3 illustrates
the fuel economy benefits of the five main types of parallel hybrids.
Table 2.3 Fuel economy benefits of parallel hybrid architectures
Class
M/G power (kW)
Energy storage
system voltage (V)
Fuel economy
(%)
1.5 ! 2.5
1.5 ! 2.5
Recuperator
14
3 ! 5
5 ! 7
Micro hybrid
14 and 42
Mild hybrid
10 ! 15
42 ! 158
8 ! 20
20 ! 60
200 ! 350
20 ! 50
Power assist
60 ! 150
300 ! 400
> 50
Dual mode or plug-in
In all vehicle systems the propulsion demand is highly dynamic, requiring a
relatively small average demand and high peak demand that can be an order of
magnitude or more of the average power demand. Add to this the variability of
electrical system loading and one encounters a highly dynamic power environment
that requires energy storage. Figure 2.8 illustrates the nature of propulsion system
dynamics. In this figure the top waveform is representative of both propulsion
power with an average demand and high peak-to-peak transients. The same
waveform for power applies to the vehicle electrical distribution system (EDS), or
in many descriptions the vehicle PowerNet (Boardnet).
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