Whilst on the fringe of the context of this work, hardware development within HEV’s forms a significant portion of the current work being conducted in the field of
HEV’s and so is included for completeness. With increasing development of HEV’s
the technology within them such as batteries, EM’s, power controllers, even power
steering and air conditioning are all becoming the focus of increasing studies.
Batteries are probably receiving the most attention with respect to HEV
hardware, as energy storage is seen as one of, if not the most important factor in current HEV’s (Amjada et al., 2010). It is shown that significant developments in
battery technology have been made, for example the Nickel Metal Hydride batteries
used in the Toyota Prius almost halved in weight and doubled their specific power
between 1998 and 2004 (Hermance and Shinichi, 2006). Whilst most HEV’s have
traditionally utilised the Nickel Metal Hydride batteries there has been considerable
development of Lithium Ion batteries which are beginning to be used in more recent EV’s and HEV’s (Burke, 2007, Maggetto and Van Mierlo, 2000). A discussion of
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made in such areas is given by Amjad et al (2010), and Maggetto and Van Mierlo
(2000). It is shown that a large number of technologies are being researched,
everything from the common lead-acid to more exotic lithium-polymer batteries are
being considered, this is shown to be the case as battery selection is specific to
vehicle type, usage and expectation of performance, and so with the wide variety of HEV’s there is likewise a wide variety of battery technologies being worked upon.
Research on battery technology is also extremely wide reaching, it spans materials
science, mechanical, chemical and electrical engineering, and it is seen that research
for HEV batteries is being conducted on all of these fronts.
Supercapacitors are also receiving more attention as an energy storage device in HEV’s due to their very high power density. However as they possess a very low
energy density their implementation currently is to supplement the battery pack not
as a replacement (Burke, 2007, Chan, 2007).
EM development has also been subject to a lot of attention with respect to HEV’s, like batteries, the EM requirements are driven by its usage and so there are
again quite diverse areas of work being conducted on EM’s (Maggetto and Van
Mierlo, 2000). The three main types of EM currently used in HEV’s are; Permanent
Magnet (PMM) (Synchronous and Brushless), Induction (IM) and Switched
Reluctance motors (SRM) (Chan, 2007). Permanent magnet motors are classed as
having high efficiency, high torque and high power density, their downside is that
they possess a short constant power range due to their limited field weakening
ability, they also produce high back emf which can cause issues for the power
electronics. Toyota uses PM motors in all of its hybrid vehicles and puts the choice
down to improvements it has made in their power, efficiency and rotational speed
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to this they are common in EV’s and HEV’s, however they are generally less
efficient than PM motors and as such are larger than a PM machine with equivalent
power and speed rating. SRM are capable of high speeds and have a wide constant
power region, they also possess a high starting torque and a high torque/inertia ratio, although manufacturing costs of SRM’s are currently high. Within the HEV EM
field a lot of effort is being used to investigate the manufacturing of components as cost is seen as a major factor slowing the development of HEV’s. Despite this it is
currently permanent magnet and switched reluctance motors that are preferred choices in HEV’s (Amjad et al., 2010, Hermance and Shinichi, 2006).
Power electronics such as converters, inverters, semiconductor devices, switches and switching strategies, is also seen as a crucial area in optimisation of HEV’s and
faces a large number of technical challenges that are being addressed in the fields of
materials science, electrical engineering and manufacturing (Emadi et al., 2008).
Furthermore with the hybridisation of vehicles more electrical systems are being
introduced into the vehicle, examples are electric power steering and air-
conditioning (as these can no longer be run from ICE), meaning that electrical
systems on vehicles are becoming more complex and as such the focus of more
work.
One final area where it has been found there is significant contribution to the research and development of HEV’s is within modelling and simulation of vehicle
systems, and especially model and hardware in the loop simulations (MIL and HIL simulations). The increased complexity of HEV’s through the introduction of extra
technologies (software and hardware) as discussed in this section, is leading to a
greater requirement for model based testing and validation. HIL simulations give the ability to run new hardware (EM’s, batteries, power electronics) or software (control
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strategies) on a test rig with all other systems (plant) represented in the modelling
domain. Such platforms will expedite the design and development of new hardware
and control strategies as well as aiding in their integration into the vehicle (Hung et
al., 2010, Sung, 2005). A consequence of such needs is that more resource is also
being put into the design and development of modelling and simulation platforms
that are capable of meeting the requirements of HEV modelling and simulation.
Similarly this can also be said for the computer and test rig hardware used for such
HIL rigs (Zhong et al., 2006).
Other more niche areas of HEV technology research and development are
discussed by Chan and Chau (2002), who discuss transmission technologies as well
as the possibility of thermal recovery systems to recuperate lost thermal energy,
which are thought to be the next focus in the search for ever more efficient vehicles.
In the push for more efficient, cleaner vehicles of the future, automotive
engineering is becoming a multi-domain discipline. It has been shown here that
HEV technology spans, mechanical, chemical and electrical engineering as well as
materials science and manufacturing. Due to this there are significant amounts of work being conducted in all of these fields on HEV’s.