Alternative powertrain configurations such as hybrid electric vehicles and fuel cell vehicles seem to be the best response to the need of reduced pollutant emissions, especially in urban areas, and to meet future emission limits, that appear to be very severe targets for traditional technologies.
In hybrid electric vehicles (HEVs) an internal combustion engine is combined with one or more electric machines, with the aim of extending the range of the electric and making the entire vehicle more energy–efficient. The architecture of a hybrid drive train is complex, comprising at least two different energy conversion devices (i.e. internal combustion engine and electric machine) drawing energy from at least two different energy storage devices (i.e. fuel tank and battery). Moreover, complex control strategies must be defined in order to properly control the division of load between the thermal engine and the electric machines.
To meet increasing global competition, which means reducing lead-time and costs for hybrid vehicles design, it is necessary to develop methods that wouldquickly evaluate various design concepts. Even if many computational models are available to simulate the performances of HEVs, there’s still a lack of generalized mathematical analyses.
An analytical model has been developed in order to study power flows in hybrid vehicle drive trains. The mathematical formulation allows a quick analysis of the complete energy cascade from fuel energy to tractive power, comprising electricity generation and consumption and energy recovery from braking. As a result a general formulation of the total tractive energy and of the global efficiency of hybrid electric vehicles in function of the main characteristics of its components is provided.
The goal of the analytical model is to enable a quick exploration of design possibilities of hybrid powertrains. Some results are presented in order to show how the present model allows a quick insight into the effects of the main system characteristics on the power flows and the corresponding component losses of the drive train.