Auxiliary power unit design for an hybrid propulsion system with ultracapacitor storage system 2007-24-0075

In this paper the design of a special auxiliary power unit (APU) will be reported; the APU, made up by a i.c. engine and an electrical generator, will be utilised as on board generator for a series hybrid propulsion system for microcar equipped with an ultracapacitor energy storage system.

The special kind of storage system, characterised by high power density and low energy density, has a big influence on the design and the management of the APU.

The low amount of energy stored on board avoid to run the APU at a fixed steady state and suggests to perform a more flexible management method of the power output of the APU; as consequence, the APU design have to be performed considering an area of potential work for the i.c. engine.

The i.c. engine considered for the design of the APU is derived for the motorcycle market; it is a spark ignition 250 cm³ engine, liquid cooled single cylinder with four valves and electronic injection. The electrical generator is a brushless permanent magnet synchronous machine. The i.c. engine is originally capable of a speed range between 1500 rpm and 9.000 rpm, for a maximum power of 16 kW. As APU it will be downsized in order to work between 1500 rpm and 5000 rpm, for a maximum power lower than 8 kW. The special application as APU made possible a great customization of the engine in order to reduce considerably its specific consumption and emissions level in the interested area, with a lower interest in the maximum power.

Thus, in this work the simulation model of the engine will be presented, together with the experimentation work needed to validate the model. Moreover, the optimization procedure will be presented, starting from the simulation model. In particular, the intake and exhaust manifolds will be optimised, together with the injection quantities and the ignition timing; moreover, the valvetrain timing will be optimised.

In conclusion, the strategies of management of the APU will be discussed and the overall powertrain performances will be presented.

The development work has been possible thanks to the ENEA planning, support and testing facilities.