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1 MOTIVATION 2 PRINCIPLES OF METHOLOGY
2 PRINCIPLES OF METHOLOGY
3 OPTIMAL ICE AND FC OPERATION All hybrid drives, both with an Internal Combustion Engine (ICE)
4 BOOSTING AND RECUPERATION and a Fuel Cell (FC), can be described by the following basic hybrid
5 OPTIMAL LOAD DECREASE AND INCREASE modes: ICE or FC operation, boosting, recuperation, load point
6 POTENTIALS OF HYBRID MODES decrease and load point increase. For a global optimal energetic
7 POTENTIALS OF PREDICTIVE CONTROL STRATEGIES operation, these hybrid modes can be partially optimised and cal-
8 SUMMARY AND OUTLOOK culated in a specific order. As a result, a global optimal control
strategy for the considered hybrid vehicle is calculated [1].

3 OPTIMAL ICE AND FC OPERATION
The optimal ICE or FC operation represents the conventional oper-
ation of a hybrid drive, since the hybrid modes recuperation and
load point decrease are omitted initially. However, the hybrid mode
load point increase is an exception since it cannot be avoided for
certain drivetrain concepts, usually during the launch process.
Moreover, in this phase it is also necessary for the supply of the
auxiliary consumers. In general, the optimal ICE or FC operation
1 MOTIVATION is achieved through minimising the power of the fuel tank P Tank in
every time step of the driving cycle which is considered in Eq. 1.
The electrification of the powertrain has resulted in a large num- In this case, all losses in the drivetrain are taken into account:
ber of hybrid drive concepts. In particular, new hybrid drives using
a Dedicated Hybrid Transmission (DHT) provide a promising Eq. 1 min(P Tank ), P Bat ≤ 0
approach to further optimise the drive train characteristics in terms
of efficiency, package requirements and costs. Because of this An ICE is optimally operated, if the transmission can provide the
diversity, the operation strategy approach has to be applicable to optimal transmission ratio for every driving situation. In the ideal-
any topologies and hybrid concepts. At the same time, an optimum ised case, the ICE is operated along its best efficiency curve (η ICE,opt
consumption has to be calculated for each concept to ensure that curve), the efficiency of the ICE η ICE is a function of the mechanical
the comparison is based solely on the design of the topology and ICE power P ICE . This is shown as an example in FIGURE 1 (left). By
the aggregates. applying Eq. 2
Dynamic Programming (DP) is a known procedure to reliably
identify the global energy optimum of a hybrid drive. However, the Eq. 2 P Tank = P ICE / η ICE
computational effort is relatively high because of the mathemati-
cally universal approach which can also be applied to other prob- the fuel tank power P Tank depending on the mechanical ICE power
lems. As a consequence, DP can only be used to a limited extent P ICE can be illustrated in FIGURE 1 (right).
with regard to extensive concept studies. For this reason, a new The efficiency of a FC is not dependent on a transmission, since
Global Optimal Control Strategy (GOCS) was developed, which is the FC provides electrical power. For operation with a constant
based on energetic correlations of the hybrid drivetrain, and there- current-voltage curve, FIGURE 2 (left) shows the efficiency of a FC
fore does require less computational time. In addition, the devel- η FC depending on the FC power P FC . Analogous to the ICE, the fuel
oped system of the operation strategy allows a detailed analysis tank power P Tank can be shown as a function of the electrical FC
with regard to the potentials of hybrid modes [1]. power P FC in FIGURE 2 (right).

70 350
60 300
η ICE,opt curve 250
ICE efficiency η ICE [%] 40 Tank power P Tank [kW] 200 η ICE,opt curve ∆P
50

30
150
20
10 100 ∆P ICE Tank
50
FIGURE 1 ICE efficiency (left) 0
and fuel tank power (right) 0 20 40 60 80 100 0 0 20 40 60 80 100
depending on ICE power for the
efficiency-optimised operation ICE power P [kW] ICE power P [kW]
ICE
ICE
(© TU Braunschweig)
ATZ worldwide 09|2017 69