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mode, thus, only the electric motor is active. In the Mode 3, the power boost
mode, both the electric motor and the internal combustion engine operate
simultaneously. In the load level increase (Mode 4), the operating point of
the combustion engine provides more power than required for following the
defined duty cycle. The excess power is used for recharging the energy buffer.
This mode is also used at stillstand to fully recharge the energy storage. In
Mode 5, the coasting mode, neither the electric motor nor the combustion
engine are running. Finally, in Mode 6, the recuperation mode, kinetic energy
of the vehicle is recovered in deceleration phases. In Mode 1 up to Mode 4,
the power is provided by the combustion engine or the electric motor, that
means the power flow is positive. During braking or coasting the power flow
changes its direction such that the power is provided by the kinetic energy
of the vehicle. The choice between the operating modes is influenced by the
duty cycle and the state of charge
σ
of the energy storage device. In Fig. 3, a
simple structure for selecting the corresponding operating mode is depicted.
It is shown that according to the direction of the power flow, the choice
of the operating mode depends on the performance index and the state of
charge
σ
. The performance index reflects the operating strategy, for example,
a minimization of both fuel consumption and emissions, see Sec. 3.
3
Parametrization of Optimality Criteria
At optimization, usually a reduction of the operating costs is aimed at. How-
ever, low emissions are of increasing importance as well. Hence, a combined
performance index is introduced, and a corresponding optimized operating
strategy is derived. Figure 4(a) shows a typical fuel consumption map of
a diesel engine. The best specific fuel consumption
b
fuel
is around 60% of
ω
max
and 50% of
P
max
. Longer operations in the partial-load area should be
avoided, for example, using Mode 2 at the beginning of acceleration phases
or Mode 4 to charge the battery. Besides saving fuel, a reduction of the spe-
cific fuel consumption decreases
CO
2
emissionsaswell.
CO
2
originates from
the combustion of hydrocarbons. The combustion also causes production of
other reactants like nitrogen oxides
NOx
and particulate matter
PM
. The
major origin for nitrogen oxides is the generation of thermal
NO
,espescially
at high temperatures. With increasing loads the temperatures rise as well,
and therewith, the generation of nitrogen oxides, see also Fig. 5(b) in Sec. 4.
The main component of
PM
is unburned carbon. The highest
PM
emissions
take place at high velocities with low loads, see Fig. 5(c). Another cost factor
may arise from battery ageing, which can be influenced by the admissible
depth of discharge or by the average charging and discharging rates. Battery
ageing will be addressed in future publications.
To take into account both fuel consumption and emissions, a combined
performance index is defined as the weighted sum of the squared normalized
