Abstract
Keywords: Energy Management, Fuel Consumption, Auxiliary Power Unit, Electrification of Vehicle Auxiliaries, Autonomic Air Conditioning
Auxiliaries cause up to 20 % of the propulsion engine's fuel consumption and therefore much of the vehicle's CO2 emission. One way to reduce the consumption is to charge the engine with the auxiliaries only in its most efficient operating points. Another way is to separate the auxiliaries completely from the propulsion engine. This may reduce the main engines' fuel consumption by 12 %. But now the auxiliary power has to be provided by some additional power supply system (APU - Auxiliary Power Unit). Possible technical approaches are a fuel cell of the PEM- or SOFC-type, a second ICE, or a steam engine. The drawbacks and benefits of such concepts result from the energy requirements of the vehicle as expressed largely through the driving cycle but also through other factors like air conditioning, electric loads independent from the cycle, type and size of energy storing devices in the vehicle, weight of the APU-system, its efficiency, ramp up behaviour, idling losses etc. In sum, this results in a quite complex optimisation problem.
To become able to systematically design such systems, a vehicle model has been developed which represents all relevant features and their time dependent energy demands. This holistic energy flow vehicle model was created in a modular way, which allows to set up component networks in different topologies and different sizes of components in a given topology. The models are implemented in Modelica 2.2 and the simulation runs under Dymola 6.4. This is an object-oriented modelling tool. A concrete model is created by linking of more fundamental objects which are modelled with their input and output data connected by their internal properties represented by the underlying physics. A suitable set of algebraic differential equation is automatically set up and solved.
Energy flows in a real medium van where recorded over a large variety of different driving cycles (such as urban, extra urban and freeway cycles), differing from conventional cycles. The operation of the auxiliaries was monitored in detail.
These recordings where used as a basis to model energy demand in the simulations. This gives the possibility to perform an in-depth analysis of the power taken up by the auxiliaries. With this information (total amount of energy, average power, peak power) it is possible to "design" an optimized APU-system for a given size of passenger car and type of usage. Due to the modular structure of the simulation model, variants of APU-systems can be plugged in and simulated easily. It can be proven by simulation, whether a certain APU fits the requirements or not. At least, a prognosis about the consumption reduction potential can be given according to vehicle type and usage.