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– load decrease as a consequence of recuperation tages. In the WLTP, a low level of the LPS can be realised, which
– load decrease as a consequence of externally charged energy is, however, consumption-neutral.
– load decrease as a consequence of load increase. When comparing the hybrid potentials for different vehicle seg-
The optimal load increase is necessary when the battery State of ments, FIGURE 5, with ICE powers of 50, 100 and 150 kW in WLTP,
Charge (SOC) is below the desired state at the end of the cycle. it is obvious that the hybridisation potential increases with higher
In the case of hybrid drives, a neutral energy balance of the bat- vehicle classes. As a result of the increasing vehicle mass and
tery is important for the evaluation of the fuel consumption. This simultaneously comparable or better aerodynamics, higher recu-
is necessary, because in the case of HEV or FCEV, the energy in peration potential is provided. At the same time, the LPS also
the battery was ultimately generated from fuel. Thus, an unbal- contributes to better fuel efficiency since the operation points of
anced charge of the battery would lead to misleading fuel con- the 150 kW ICE are located at lower efficiencies than in the B-
sumption accordingly. However, in case of a PHEV and PFCEV the and C-segment vehicles with lower ICE power. Therefore, hybrid-
battery is desired to be discharged in charge depleting operation. isation is a very effective concept for reducing fuel consumption,
For an optimal load increase Eq. 6 is applied for each time step especially for heavy and powerful vehicles. With regard to the
of the cycle: cost-sensitive B-segment vehicle, it is clear that a reduction in
fuel consumption using hybridisation is a challenging task.
Eq. 6 max(|∆P Bat / ∆P Tank |), ∆P Bat < 0
7 POTENTIALS OF PREDICTIVE CONTROL STRATEGIES
This means that the highest possible charging power of the bat-
tery ∆P Bat is achieved for a specific fuel tank power ∆P Tank . The In [2] the Equivalent Consumption Minimisation Strategy (ECMS)
load increase can be applied to both operation points where the as a local optimal control strategy was compared to the GOCS.
ICE or the FC is already running, as well as operation points where The GOCS achieves better fuel consumption under all circum-
the ICE or the FC have been deactivated by the previous phases. stances than the ECMS. However, it has been shown that the
FIGURE 3 shows the effectiveness of load decrease as a conse- ECMS can achieve very similar energy consumption in the case of
quence of load increase for a HEV with a P2 topology. Due to the the charge sustaining operation. Although the differences regard-
multiple energy conversions and the efficiency behaviour of the ing the chosen operation points of the aggregates or SOC curves
downsizing ICE with a maximum output of 100 kW, advantages are noticeable, there are only slight differences in fuel consump-
can only be achieved in small areas. FIGURE 4 shows the same tion. This is because recuperation is by far the most important
investigation for a FCEV. Since additional losses result only from hybrid mode for consumption reduction of a HEV or FCEV. Since
the charging and discharging of the battery, the load decrease due the recuperation efficiency between ECMS and GOCS does not
to load increase is beneficial over a very wide power range. differ, similar fuel consumption is achieved. Thus, predictive func-
tions provide only a small consumption advantage for HEV or FCEV
in case of an optimal designed basis control strategy. However,
6 POTENTIALS OF HYBRID MODES
this applies only in the case that the prediction has no influence
In the following, the above-described operation strategy is used on the recuperated energy.
to identify the potentials of hybrid modes for selected examples, On the basis of a WLTP parameterisation of the ECMS, further
FIGURE 5. The C-segment vehicle shows that the potentials of investigations were done in a customer cycle. Due to the fact that
hybrid modes are different in the NEDC (black dotted line), WLTP the parameterisation of the ECMS is not optimal for the customer
(black solid line) and US06 (black dashed line). The load decrease cycle, there are greater advantages for the GOCS. However, greater
as a result of load increase (LPS), combined with a 100-kW down- advantages of the GOCS can be identified in plug-in vehicles
sizing ICE, is only beneficial in the NEDC. In the more dynamic (PHEV and PFCEV), when these are operated with both externally
US06, the LPS is even associated with consumption disadvan- charged electrical energy and fuel. While the ECMS initially selects

7
Battery operation Load decrease
6
Normalised fuel saving ∆P Tank / ∆P FC [-] 4 Fuel saving through load decrease
5

3

1 2 Fuel saving in battery operation
Fuel consumption through load increase (η = constant)
0 LPA,FC
FIGURE 4 Effectiveness of load decrease as a 0 20 40 60 80 100
consequence of load increase for a FCEV with FC power P [kW]
FC
a 100-kW FC (© TU Braunschweig)
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