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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 last years have seen an increasing need to reduce vehicle polluting emissions which has lead car producers to improve their efforts to comply with the new legislation. At the moment, hybrid electric vehicles (HEV) seems to be the most suitable solution as they match good performances in terms of both consumption and reliability. In a such dynamic market, an interest for small and light city-car is rising, especially in Europe, where urban circulation is getting worst due to huge number of vehicles. Such special characteristics, like smallness and lightness, allow these hybrid vehicles to run 40 km with a litre of fuel only. In this scenario, the Hybrid and Electric Research Group of ENEA planned the realization of an hybrid power train suitable for small city car.

The results of a dynamic simulation applied to a new hybrid powertrain, called SIPRE 1, are presented regarding a new hybrid system characterized by the presence of a planetary gear train that manages the distribution of power and torque on the vehicle. The system is made up of three machines, one thermal (TE) and two electncal (ETC and EM); the thermal engine works in steady state, the ETC machine supplies the needed torque surplus and the EM machine works as generator. A computer model has been used to simulate the urban and extra-urban cycle operation of a typical city car: the dynamic simulation has been implemented in order to obtain an optimized system design. The results of this simulation will be presented in form of data curves and numerical values: the effects of varying kinematic and dynamic parameters are discussed as well as the characteristics of the electrical and thermal engines.

The search for clean automotive propulsion systems has brought to study, besides the full electric solution and the traditional hybrid schemes, a family of new hybrid powertrains, called SIPRE and introduced by PARMENIDE s.r.l. (Rome) some years ago; SIPRE is the acronym of Sistema lbrido di Propulsione con Ruotismo Epicicloidale (in Italian language), i.e.: hybrid propulsion systems with planetary gear train. Such kind of systems have been studied for four years and five different versions have been developed. The main aim of this research is the realisation of hybrid powertrains with improved performances and efficiencies; analysing the traditional series and parallel hybrid systems is manifest that an enhanced hybrid equipment can be achieved developing a system layout with the minimum number of machines and reducing the number of energy conversions between the thermal unit and the vehicle traction system.