On Discharge Strategies for Plug-in Hybrid Electric Vehicles
Licentiate thesis, 2011
During recent years electrification of vehicles has emerged as a promising
technology to reduce oil dependency and CO2 emissions within the transportation
sector. At the present day several automotive manufacturers are
about to introduce plug-in hybrid electric vehicles to the market, i.e. hybrid
electric vehicles with high capacity batteries that are grid rechargeable.
As with most new technologies this introduces new control problems to be
solved by engineers and researchers.
This thesis investigates one of these new control problems, namely how
to optimally discharge the battery of a plug-in hybrid electric vehicle on
trips that exceed the electric range. Rather than using the trivial discharge
strategy, which is to operate as an electric vehicle until the battery is depleted
and then proceed in charge sustaining operation as a conventional
hybrid electric vehicle, it is possible to improve powertrain efficiency if the
battery is discharged gradually throughout the trip. A gradual discharge
lowers the average discharge current, thereby lowering the resistive losses
that are quadratic in current. However, to find a suitable discharge rate
some a priori information regarding the future trip is required.
In the thesis it is shown that the a priori information needed can be
obtained using route recognition; an algorithm with low computational demand
is proposed and evaluated using simulations on logged commuter driving
data. The results suggest that notable fuel cost reductions are possible
for commuters that frequently drive along routes that exceed the electric
range. Furthermore, the impact of trip length uncertainty on the optimal
discharge rate is also studied and results indicate that it is preferable to
underestimate rather than overestimate the trip length. A separate investigation
concludes that uncertain estimates of the battery state of charge
only has minor effects on the optimal discharge rate.
Hybrid Electric Vehicles
Plug-in Hybrid Electric Vehicles