Smaller buses (cng electric hybrid vehicle) have been built with pure electric and alternative forms of hybrid drive. An interesting project by Unique Mobility in North America put a CN Gelectric hybrid system into a 25 ft (7.62 m), 24 passenger vehicle . Here the compactness and locational flexibility of the hybrid-drive elements meant that considerable gains could be obtained in packaging the vehicle occupants. Using high power-density permanent-magnet motors driving the rear wheels allowed a particularly low floor of 12 in (305 mm) from the ground.
The CNG-engine generator provided steady state power and was augmented by storage batteries to supply the power required above that base level while recharging of the batteries would take place when the power requirement fell below the base level. CNG tanks were roof mounted while batteries were positioned over the rear wheel wells and inside the engine compartment.
The 90 bhp gas engine drove the generator through a flywheel-positioned step-up planetary gear set and an engine management system allowed engine speed and power to vary with load conditions. The rate at which speed was increased was minimized by the controller in order to avoid poor fuel economy and high emissions associated with transients. The two 70 kW traction motors were provided with a single planetary reduction gear of 2.77:1, directly coupled to the drive wheels through a secondary set of 5.2:1 included in the wheel hub to give an attainable speed of 55 mph. Rear suspension was an independent trailing arm system with traction motors direct mounted to the arms, so as to maximize floor area between the wheels. Motor differential speeds for cornering are electronically controlled with reference to steering wheel angle and road wheel speed.
The above view shows the power flow charts for different modes of operation. In the first, on IC engine power only, a speed of 37 mph was achieved. On IC engine and battery power, higher speeds were possible and a reserve was available for gradients and acceleration; in the final mode of regenerative braking with the IC engine operating, the engine provided power only to the accessories and that frombraking was fed into the storage batteries. The latter were used primarily for supplying accelerative power and were 12 V units with 160 Ah capacity. Two series strings of 15 batteries were connected in parallel to yield 180 V and 320 Ah total capacity.
Another pioneering series of hybrid buses has been the Daimler-Benz OE 305 city bus conversions , some 20 of the first type were evaluated in trials in German cities in the early 1980s. Electric drive in the city centre and diesel drive in the suburbs was the mode of operation. Seen at (a), the set-up was electric motor (1) , air compressor and power-steering pump (2), motor fan (3), diesel engine and generator (4), battery-fan (5), electronic controller (6), traction batteries (7) and battery cooling unit (8). Range was 30–45 miles on batteries alone and 190 miles as a hybrid diesel combination.