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J. Conde Trugeda:
"Analysis and Optimization of a Hydraulic Hybrid Vehicle with numerical simulation methods";
Supervisor: B. Geringer, P. Hofmann; Institut für Fahrzeugantriebe und Automobiltechnik, 2016.

Hydraulic hybrid vehicles (HHV) use a hydraulic circuit with one or more hydraulic machines and a gas to store the braking energy and us it as an alternative power source to the internal combustion engine (ICE).
HHVs are characterized by a lower cost of the components than the ones from the hybrid electric vehicles, as well as by the high power density of the hydro-pneumatic accumulator, which makes it capable of recuperating a high percentage of the braking energy.
Until now, all the developed HHVs have had a series or parallel configuration. The powersplit HHV modelled in this project tries to combine the advantages from this type of this hybrid arrangement with the suitable use of the HHVs for urban driving.
The main advantage of the power-split hybrids is the ability of setting the ICE into its most efficient point, due to the CVT function of the two hydraulic machines, called Pump and
Pump/Motor, and the engine connected to a planetary gear set.
The size of hydraulic machines is optimized in order to save as much fuel as possible in the NEDC, which is the standard European test to assess the fuel consumption, and, at the same time, being able to drive correctly the US06 cycle, which is distinguished by
high accelerations and speeds.
Although being the initial objective to select hydraulic machines with a similar nomirfal power to the electric machines of the Toyota Prius 151 generation, the restricting parameter of these machines is the maximal displacement, so that the implemented ones
count with a higher power limit, even if those values are never reached.
Other important empirically optimized parameters are involve the control of the state-ofcharge of the accumulator, the use of regenerative or friction braking or the gear rates to
keep the speeds of the machines within the wanted range.
In some situations, when power-split hybrids drive at high speeds, hydraulic machines work the inverse way they should (pumping instead of motoring, and vice versa), dismissing the efficiency of the overall power train and making the fuel consumption
higher than the one of a conventional vehicle.
Although, it does not solve the problem, by limiting the speeds of the hydraulic machines this behaviour can be enhanced. With these conditions, the HHV is able to cut the fuel consumption of the conventional vehicle by more than a third in the NEDC. In the US06 cycle the fuel consumption is also lower, but the difference is minimal due to the little number of braking events.
These results show that the HHV has its best usage in urban driving and in vehicles, whose service involves frequent braking events.