Soft hybrids

In the most basic sense, hybrid vehicle architecture (a) allows a relatively small battery to augment the internal combustion power supplied to a drive-train and 

The architecture of the soft hybrid battery is described in Fig. 11.16. The contactor in this representation is denoted as outside the battery system, but it could be incorporated as part of the battery assembly. This representation also includes a battery controller within the battery assembly that may actually be physically located outside the battery box or even as shared space on another 

Alternator Regenerative braking Boost Start-stop Anti-jerk Generate electrical power Converts kinetic energy to electrical energy Support the ICE with additional torque Stop ICE at idle speed with rapid restart Better driving with improved torque transient behaviour 

A 12-V battery power supply is also noted in Fig. 11.16 this may be eliminated in the future, provided that 42-V conversion of all other devices, including controllers, takes place. 

Heating options may not be necessary since the typical high current density that the hybrid battery sees in normal operation provides enough self-heating to ensure that loss of function due to low temperature is rarely a terminal problem. 

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A further requirement to be satisfied by the oxidant supply device is represented by dynamic performance in a wide range of air flow rates. This aspect is crucial in some hybrid power train operative modes characterized by high stack dynamics (full power or soft hybrid configurations, see Sect. 5.5). 

The simultaneous utilization of FCS and battery within a fuel cell propulsion system can be accomplished by two basic ways (i) the battery pack can be minimized (but not eliminated as in full power) assigning the role of generating most energy required by the load to the FCS (soft hybrid configuration), (ii) the FCS can be sized to provide the base load, i.e. a power value close to the average power of the expectable road mission (hard hybrid configuration), while larger battery pack are necessary to satisfy the dynamic requirements. 

The main benefit of the soft hybrid option is the reduced use of batteries, which could have the minimum capacity necessary to feed the vehicle auxiliaries, giving a limited contribution to peak powers and save energy during the regenerative braking phases. On the other hand, the hard hybrid option offers the possibility to limit the cost of the FCS, which would work mainly in steady-state conditions, then in more reliable way. 

The soft hybrid configuration represents the most flexible solution for the energy management in all the possible driving conditions of an automotive... 

Fig. 6.30 Stack and fuel cell system efficiency versus cycle length in soft hybrid configuration on the R40 driving cycle...

The instantaneous efficiency of the stack and FCS are shown in Fig. 6.29 for R47 cycle in hard hybrid conhguration and in Fig. 6.30 for R40 cycle in soft hybrid configuration. According to the results obtained in steady-state operation, during the low load phases the stack efficiency reaches the value of about 0.7, whereas during the power variations required by the electric drive the efficiency decreases down to the lowest value of about 0.6, in correspondence with the most... 

Fig. 6.32 Soft hybrid procedure during R40 cycle on fuel cell scooter, a electric energy profiles instantaneously exchanged by electric drive, battery pack, and FCS with the electric bus versus cycle length, b histogram of individual cell voltage at three instants during the third acceleration phase of the R40 cycle...

It is necessary to specify that the above efficiency values are evaluated assuming a value of battery efficiency equals to 100%, and this is justified by the fact that the use of the batteries to satisfy the power demands is quite limited in both the cycles, in particular it is practically negligible for the soft hybrid approach. 

As previously discussed (Sect. 5.5), the battery pack of a fuel cell power train can be either minimized assigning the role of generating most energy required by the load to the fuel cell stack (soft hybrid), or sized in order to provide all dynamic requirements of the vehicle allowing the utilization of a smaller FCS (hard hybrid). 

Fig. 7.57 Power distribution between FCS, electric engine, and batteries as function of cycle length for R40 cycle at 10 A s as stack current variation rate (soft hybrid configuration)...

It is also possible to define soft, mild, and full hybridization based on the degree of power-train electrification relative to the heat engine peak power soft hybrids can be considered to contribute < 10% of peak ICE rating, mild hybrids contribute... 

Table 11.16. Calculation of heating rate for soft hybrid battery.

Table 11.17. Estimate of battery life for soft hybrid.

Table 11.18. Typical soft hybrid performance requirements.

VRLA as a soft hybrid battery. Flooded lead-acid batteries have a number of strong points that favour use in soft hybrids such as robustness, a large thermal mass, and a conductive medium to remove the heat. Flooded lead-acid batteries are not considered contenders for soft hybrid applications, however, for the several reasons outlined below. 

With its higher RMS power to mass ratio, the ohmic and entropic heating will be significant in hybrid batteries. As seen with the soft hybrid battery, a reasonable rate... 

Controls and diagnostics for parallel-series hybrid vehicles. A typical set of control messages for hybrid vehicles is listed in Table 11.19. Although the variables are the same as those used in soft hybrids, the higher criticality of function implies that the calculated values, such as maximum source and sink currents, must be of even higher reliability if unexpected function fade or failure is to be avoided. 

The cost dependency on a minimized cell size for Ni-MH and VRLA batteries is shown in Fig. 11.21. In these cases, VRLA is limited by life and Ni-MH is limited by power. At the energy levels required for parallel-series hybrid vehicles, it can be shown that the watt-hour life cost of Ni-MH would have to exceed 33 times that of VRLA, which is not the case even today with prototype batteries. The initial cost consideration that tipped the favour to VRLA for soft hybrids is far less impressive with parallel or series hybrids, since it can be further demonstrated that, even on an initial cost basis, VRLA is about 60% of the cost of the equivalent Ni-MH battery for the parallel-series application, well outside of the bounds where trade-off to initial cost could be considered. 

For ambitious soft hybrid and, especially, for HEV applications, a voltage significantly higher than 42-V will be needed [5,15,16]. The first series-production vehicle of this type is the Toyota Prius, of which production has exceeded 100,000 units since it was launched in 1997 [17,18]. 

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