Page 194 - Automotive Engineering Powertrain Chassis System and Vehicle Body
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CH AP TER 7 .1       Hybrid vehicle design

               above which field current was varied by an independent  locational flexibility of the hybrid-drive elements meant
               chopper. The reversible chopper allowed regenerative  that considerable gains could be obtained in packaging
               braking, down to zero speed. The motor had character-  the vehicle occupants. Using high power-density
               istics as seen at (b) and was direct coupled to the rear-  permanent-magnet motors driving the rear wheels
               axle driving head. Its nominal voltage was 600 V,  allowed a particularly low floor of 12 in (305 mm) from
               continuous power 90 kW, maximum power 180 kW and   the ground.
               field control ratio at continuous power 1150/3200 rpm.  The CNG-engine generator provided steady state
               Efficiency at continuous power in field control was over  power and was augmented by storage batteries to supply
               90%. The 600 V generating unit comprised a diesel  the power required above that base level while recharging
               generator set with power rating of 56 kW and maximum  of the batteries would take place when the power re-
               power 78 kW. Battery storage comprised 50 12 V lead–  quirement fell below the base level. CNG tanks were
               acid cells of 135 Ah capacity at a 5 hour discharge rate,  roof mounted while batteries were positioned over the
               their weight of 1930 kg corresponding to 12% of the  rear wheel wells and inside the engine compartment. The
               GVW. Vehicle layout was as seen at (c); in tests the  11 tonne GVW bus is seen at (a).
               vehicle recorded diesel consumption of 32.3 kg/100 km  The 90 bhp gas engine drove the generator through
               compared with 37.8 for a conventional vehicle, with  a flywheel-positioned step-up planetary gear set and an
               battery SOC found to be the same at the beginning  engine management system allowed engine speed and
               and end of the tests. Range in purely electric drive was  power to vary with load conditions. The rate at which
               30 km of city driving from 100% to 20% battery SOC.  speed was increased was minimized by the controller in
                 During the design stages Fiat examined a typical town  order to avoid poor fuel economy and high emissions
               route between two termini as seen at (d), on a time base  associated with transients. The two 70 kW traction
               of seconds. It was also established that the maximum  motors were provided with a single planetary reduction
               acceptable acceleration for standing passengers was  gear of 2.77:1, directly coupled to the drive wheels
                       2
               1.5 m/sec on level ground and 0.27 at a gradient. The  through a secondary set of 5.2:1 included in the wheel
               subtended area in the power diagram gave energy re-  hub to give an attainable speed of 55 mph. Rear sus-
               quired between stops, the negative portion representing  pension was an independent trailing arm system with
               energy flowing back to the batteries having taken the  traction motors direct mounted to the arms, so as to
               various system efficiencies into account.           maximize floor area between the wheels. Motor differ-
                 For E s the total energy required at the wheels, E m the  ential speeds for cornering are electronically controlled
               engine energy and E r the mains electrical energy, then  with reference to steering wheel angle and road wheel
                                                                  speed.
                 E s ¼ðh E m þ E r Þh h h                           The view at (b) shows the power flow charts for dif-
                                 b t o
                        g
                                                                  ferent modes of operation. In the first, on IC engine
               where efficiencies subscripted g, b, t and o refer to gen-  power only, a speed of 37 mph was achieved. On IC
               erator, battery, motors/controllers and transmission  engine and battery power, higher speeds were possible
               respectively, with their product the overall efficiency h.  and a reserve was available for gradients and acceleration;
               Then for total duration of daily service T s’ terminus  in the final mode of regenerative braking with the IC
               turnaround time T c and number of daily runs between  engine operating, the engine provided power only to the
               termini N, the power required from the engine is:  accessories and that from braking was fed into the storage
                                                                  batteries. The latter were used primarily for supplying
                 P m ¼ðE s   hE r Þ=½h hfT s  ðN   1ÞT c gŠ       accelerative power and were 12 V units with 160 Ah
                                  g
                 The total daily energy is calculated for a typical route  capacity. Two series strings of 15 batteries were
               of length L p , divided into N t segments of length L t . The  connected in parallel to yield 180 V and 320 Ah total
               heaviest cycle is shown at (e), in which Ea ¼ 584.6 Wh;  capacity.
               Ed ¼ 442 Wh. Ead ¼ h t ’h s hEd ¼ 337.6 Wh and energy  Another pioneering series of hybrid buses has been the
               spent per run Ep ¼ Ead(Lp/Lad) ¼ 12 250 Wh.        Daimler-Benz OE 305 city bus conversions (Fig. 7.1-19),
                                                                  some 20 of the first type were evaluated in trials in
                                                                  German cities in the early 1980s. Electric drive in the
               7.1.5.2 CNG-electric hybrid                        city centre and diesel drive in the suburbs was the mode
                                                                  of operation. Seen at (a), the set-up was electric motor
               Smaller buses have been built with pure electric and  (1) , air compressor and power-steering pump (2), motor
               alternative forms of hybrid drive. An interesting project  fan (3), diesel engine and generator (4), battery-fan (5),
               by Unique Mobility in North America put a CNG-     electronic controller (6), traction batteries (7) and
               electric hybrid system into a 25 ft (7.62 m), 24 pas-  battery cooling unit (8). Range was 30–45 miles on bat-
               senger vehicle (Fig. 7.1-18). Here the compactness and  teries alone and 190 miles as a hybrid diesel combination.


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