Seite - 11 - in Hybrid Electric Vehicles

mode, the electric motor drives the flywheel to rotate and store a large amount of kinetic energy (mechanical energy); when in discharge mode, the flywheel drives the generator, con- verting kinetic energy into electric energy [76]. The FESS has the advantages of high energy density and high power density [77] and works best at low speeds and in frequent stop-start work conditions. Producing this system could be cheaper than producing batteries; however, the system has limited storage time, and a significant percentage of the stored capacity is wasted through self-discharge [78]. 4. Hybridization factor In HEV engineering, the integration of engines, mechanical components, and electric power trains leads to increased energy efficiency, that is, a reduction in fuel consumption and a subsequent decrease in CO2 emissions. In the automotive industry, the basic logic of a hybrid vehicle is to provide a new source of power that intervenes in place of the primary source (ICE) to improve the overall performance of the system. Moreover, there are possible modes of operation that are not provided in a conventional vehicle, such as regenerative braking and electric mode (EV). Below are some of the main advantages of a hybrid configuration over a vehicle equipped with a combustion engine alone. • Electric motor can act both as an engine and as a generator, allowing a reversible flow of power from the battery to the wheels and vice versa. • During braking, some of the kinetic energy is recovered (regenerative braking). • The vehicle can be used only in the electric mode (zero emission vehicle—ZEV). • When the vehicle has to stop temporarily, the combustion engine can be switched off, therefore ensuring considerable energy saving. It should first be mentioned that there is actually no real classification for hybrid vehicles, although a first orientation phase can be identified by defining a significant hybridization fac- tor (HF) as the ratio between the power of the installed electric motor and the total amount of power delivered by the combustion engine and electric motor on the vehicle: HF  =  P em _______ P em + P ICE (1) where Pem is the electric motor drive power, and PICE is the internal combustion engine power. In the case of conventional vehicles, the hybridization factor is clearly equal to zero, whereas in the case of electric vehicles, the hybridization factor has a unit value. Between these val- ues, all possible solutions can be obtained. In the automotive engineering field, the definition of the hybridization factor has been extensively studied for several applications [49, 79, 80], considering its effect on performance and optimization [81–83]. Furthermore, depending on the degree of hybridization and the capacity of the hybrid propulsion system to store energy, three different levels of hybridization are defined. • Full hybrid is when the electric system alone is able to make the vehicle move on a stan- dard driving cycle (0.5 < HF < 0.7). Trends and Hybridization Factor for Heavy-Duty Working Vehicles http://dx.doi.org/10.5772/intechopen.68296 11