K u rzfassu n g sb an d - Graz University of Technology

92 Energieinnovation 2006
Fuel savings potential of parallel hybrid cars
There are two main points for improving the fuel consumption of a hybrid car compared to a
conventional car. The first one is to avoid operating the CE in inefficient points. A CVT gives the
possibility to meet this requirement for middle and high power demands. For low power demands (e.g.
for a constant speed of 30km/h only about 3kW of power is needed) the efficiency of the CE is very
poor. In such situations the over all efficiency can be improved by cyclic operation of the CE.
The second point is the recuperation of the kinetic energy during the breaking period. With using
smaller EESD, the demand for optimized managing the state of charge aggravates vastly. On the one
hand the EESD should be charged as much as possible at standstill to provide enough energy for the
next acceleration period. Furthermore the efficiency of the EESD ascends with rising state of charge.
On the other hand there should always be capacity inside the EESD to save as much breaking energy
as possible during deceleration. The usable energy during breaking is depending on different
nonlinear and time variant influences like e.g. the maximum power of the electrical motor or the rate of
deceleration.
To estimate the savings potential of a hybrid car compared to a conventional car simulations for each
car with different driving cycles are used to calculate the fuel consumption. To get comparable results,
the level of details for the simulations should be the same. By comparing the simulation results for the
conventional car with results given by the manufacturer the model could be verified.
Results
Within this paper the possible fuel savings potential of a hybrid car compared to a conventional car will
be shown. An analytical method to calculate equivalent specific fuel consumption for a hybrid car
driving with cyclic operation will be shown. The energy that can be saved by regenerating breaking
energy will be analysed. The estimations will be verified by simulation results for a conventional car
and a hybrid car. Furthermore an outlook of fuel savings potential for different scenarios is exposed.
Acknowledgement
This Project is funded by the Deutsche Forschungsgemeinschaft (DFG), within the program
Transferbereich 38, "Optimierter Antriebsstrang".
Keywords: hybrid car, CVT, continuous variable transmission, double layer capacitor, UltraCap,
operating strategy, fuel saving, fuel consumption, cyclic operation