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RESEARCH HYBRID DRIVES

70 350
60 300
FC efficiency η FC [%] 40 Tank power P Tank [kW] 200
50
250

30
150
20
10 100 ∆P FC ∆P Tank
50
0 0
0 20 40 60 80 100 0 20 40 60 80 100 FIGURE 2 FC efficiency (left)
and fuel tank power (right)
FC power P [kW] FC power P [kW] depending on FC power
FC
FC
(© TU Braunschweig)

4 BOOSTING AND RECUPERATION
Eq. 4 min(P Bat ), P Tank = 0
The optimal boosting is based on the optimal ICE or FC operation.
Wherever the power of the ICE or FC operation is lower than the
demanded driving resistance power of the cycle, the battery is 5 OPTIMAL LOAD DECREASE AND INCREASE
discharged additionally. The operation points are selected by min-
imising battery power as much as possible, which lead to a max- In a hybrid drive, the reduction of the ICE power ∆P ICE or the
imisation of the power share of the ICE or the FC: FC power ∆P FC is achieved by an increase of the battery power
∆P Bat . In order to use the battery power P Bat as efficient as possi-
Eq. 3 min(P Bat ), P Tank > 0 ble Eq. 5 is applied:

Recuperation is usually the most important hybrid function to Eq. 5 min(|∆P Bat / ∆P Tank |), ∆P Bat > 0
reduce energy consumption. The optimal recuperation is
achieved, if a minimal battery power P Bat is achieved for every This means that a maximum of fuel tank power ∆P Tank is reduced
time step during the coasting phases of the cycle (charging: for a specific battery power increase ∆P Bat . By minimising the
P Bat < 0). Moreover, the charging power of the battery must not quotient of Eq. 5, all powertrain losses are taken into account.
be increased by an additional load increase of the ICE or FC The extremum of load decrease is the pure battery operation in
(P Tank = 0). In order to maximise the efficiency of the recupera- which the ICE or the FC is completely deactivated. Since the
tion, the power loss of the ICE must be minimised. This is usu- electrical source of the load decrease is important for the eval-
ally achieved by a deactivation and decoupling of the ICE (avoid- uation of this hybrid mode, different cases have to be separated
ing drag losses) if this is possible within the considered topology. depending on the source of electrical energy. The following cases
For optimal recuperation Eq. 4 is applied: are distinguished:

7
Battery operation Load decrease
6 Fuel saving through
Normalised fuel saving ∆P Tank / ∆P ICE [-] 4 consequence of load increase
5
load decrease as a

3

2
Fuel saving through load decrease
Fuel saving in
battery operation
1
Fuel consumption through load increase (η = constant)
0 LPA,ICE FIGURE 3 Effectiveness of load decrease as a
0 20 40 60 80 100 consequence of load increase for a HEV with
ICE power P [kW] a P2-topology and a 100-kW downsized ICE
ICE
(© TU Braunschweig)
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