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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 search for clean automotive propulsion system has brought to study a new hybrid system. This hybrid system allows the mechanical energy produced by the thermal engine be partly stored in battery pack and partly be sent directly to the wheels without being converted into electrical energy. The main components of this system are: one battery pack, two electrical machine, one i.c. engine, and a three-axes planetary train. The i.c. engine has a constant speed, so that it is possible to minimise fuel waste. The first electrical machine drives the first axis of planetary train and is connected to the driving shaft of the i.c. engine through a friction drive; the role of this electrical machine is to modulate the torque of the i.c. engine. The second electrical machine drives the second axis of the planetary train and is running at a variable speed; the role of this electrical machine is to provide the kinematic control of the vehicle.

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.

In this paper the experimental results of Magica II are presented and discussed; Magica II is a parallel hybrid prototype entirely designed by the Department of Nuclear Engineering and Energy Conversions at University of Rome “La Sapienza” [1], realized and tested in collaboration with ENEA - the Italian National Agency for New Technologies, Energy and the Environment [2]. Magica II is characterized by a mechanical layout directly derived from the original Ma.Gi.Ca. (an acronym from Eng. MArtellucci, Eng. GIuffrida and Prof. CAputo) Project: it is a two-seat spider, or convertible, with space frame rolling chassis and fiberglass body. An engine and an electric drive acting respectively on rear and front wheels make up the propulsion system. The engine is placed in central position, mounted transversally behind the seats. It is equipped with mechanical gearbox and final drive that transmit the traction power to the rear wheels.