PhD Thesis - Cranfield University

Chapter 4
A situation such as this occurs when the vehicle accelerates gradually up to speed at
power levels within the battery peak power and power rate limits. By definition of power
management, the ultracapacitors are not required to service this slow load transition. If
the vehicle were then subjected to a rapid deceleration, the ultracapacitors would be
unreceptive to the regenerative power because of its state of charge (or energy level). It is
for this reason the problem now becomes more of an energy management issue rather
than a power management one. To cater for such a situation, a method to actively control
the ultracapacitor energy content becomes necessary. If the power profiles are known a
priori, then optimised methods to programme the ultracapacitor state of charge may be
used. However, standard drive cycles could be used to benchmark power splits and
energy management strategies. Although these drive cycles were developed for emission
tests of ICE vehicles, they do provide realistic velocity profiles ranging from aggressive
urban driving to high-speed highway modes.
4.7 Summary
This chapter explicated the underlying principle behind vehicle power and energy
management. Beginning with the fundamentals of vehicle propulsion power and energy
demands, an Analog Mixed Signal (AMS) vehicle model developed with SIMPLORER was
presented. Using the model, three case studies were presented to demonstrate power and
energy demands under variations in drive cycles. The discussion objectively described the
advantages of a strategic power and energy management system to arbitrate the power
delivery and energy content of hybrid energy systems. To summarise, vehicle power
demands that have high peak to average power ratios as well as the opportunity for
regenerative energy recuperation justifies the cause to have a hybrid energy system. What has
not been considered thus far is the technique to electrically combine the power from the
energy systems. As such, the next chapter will further refine the problem of managing power
and energy and introduce a structured method to address the problem. It then follows
through with a structured implementable solution.
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