653 Advanced Hybrid

A joint venture between MAN and Voith has resulted in the NL 202 DE low floor concept city bus, Fig. 6.20, designed to carry 98 passengers at a maximum speed of 70 kph, (a). No steps are involved at any of the entrances which lead directly to a completely level deck height of between 317 to 340 mm. The rear-mounted horizontally positioned diesel engine allows fitment of a bench seat at the rear of the bus; it drives a generator with only electrical connection to the Voith transverse-flux wheel motors which drive the wheels through two-stage hub-reduction gearsets, (b). The diesel is rated at 127 kW and the generator at 135 kW; the controller, (c), is of the IGBT converter type and also developed by Voith. It provides a differential action to the wheel motors on cornering. Permanent-magnet synchronous wheel motors are rated at 57 kW and have a maximum speed of 2500 rev/min; see table at (d). The bus is 12 metres long and has water cooling for its generator, converters and wheel motors. As well as providing virtually jerk-free acceleration, the drive system is seen by MAN as providing the possibility of four-wheel drive on future articulated buses to improve traction and stability in slippery road conditions. The term transverse flux motor refers to the means used to guide the magnetic flux in the stator; this is new to inverter-supplied PM types and involves a novel collector configuration. Double-sided magnetic force generation is also new and involves a patented double air gap construction having high idling inductances and force densities up to 120 kNm/m2, with relatively low losses. A new control process permits operation of the motor in a field-weakening type mode, in spite of PM excitation. The generator is almost identical in concept but involves no field weakening. Each has concentric construction of permanent magnets, rotor/stator soft-iron elements and stator winding; see below. Armature elements are U-shaped cut strip-wound core sections, embedded in the ring-shaped supporting structures of inner and outer stators. Each core surrounds the windings and forms a stator pole with its cut surfaces facing the rotor. The latter is pot-shaped and positioned between poles of the outer and inner stators. In the stator pole region it comprises magnet and soft-iron element while in the winding region a ring of GRP serves as the connecting element. The inverters supply the motors with sinusoidal currents and voltages until the nominal operating point is reached; operating frequency is 10 kHz. In field-weakening mode the induced voltage exceeds intermediate circuit voltage and only square wave voltages are supplied to the motor. Power output then remains constant and the operating frequency equals the fundamental motor frequency. A large speed ratio, 1.5:1, is thus possible.

TFM wheel motor TFM generator

Power

57 kW

135 kW

Rated speed

735 rpm

1750 rpm

Approx. max. speed

2500 rpm

200 rpm

Max. fundamental frequency of stator

1350 Hz

-

Rated torque

740 Nm

740 Nm

Approx. max. torque

1050 Nm

740 Nm

Approx. torque conversion

1:3

Power/weight ratio

1.8 kg/kW

0.9 kg/kW

Fig. 6.20 MAN/Voith concept city bus: (a) low floor package; (b) wheel motor; (c) controller; (d) drive characteristics.

Fig. 6.20 MAN/Voith concept city bus: (a) low floor package; (b) wheel motor; (c) controller; (d) drive characteristics.

6.5.4 ADVANCED HYBRID TRUCK

Mitsubishi have been prominent in hybrid truck manufacture and have recently developed a heavier, municipal, version of the light hybrid truck launched in 1995. Because added cost limited market acceptance of the lighter, Canter-based, hybrid the decision to build a heavier municipal version, Fig. 6.21, was taken on the grounds of low noise, and greatly reduced emissions, which made the vehicle attractive for city-centre operation, a lift-platform version being particularly popular. The hydraulic pump for operating auxiliaries such as a lift platform is electric motor driven, with the benefit of near silent operation.

Series hybrid mode, (a) was chosen first, because the engine is used solely for power generation and so can be operated in a peak efficiency speed band and secondly, since the engine is isolated from the drive system, it results in a simpler and more flexible drive-system layout with greater freedom for hydraulic equipment mounting. Two electric motors are involved. Shown at (b) are typical operational modes of the truck: when the battery has high state-of-charge (SOC) the vehicle operates exclusively in battery mode. At less than 65% SOC the power-generating engine starts and hybrid mode is invoked; when 70% SOC is achieved again the vehicle reverts to battery operation. Provision is also made to inhibit hybrid operation until 30% SOC is reached so silent and zero-emission night-time, or in-tunnel, operation is made possible. In hybrid mode SOC is maintained at 65-70%, at which point the generated power, from the generator, and the regenerative power, from the motor, provide sufficient charging.

The overall package layout is shown at (c); dimensions are 5.78 m long x 1.88 m wide x 3.35 m high, with a wheelbase of 2.5 m. Gross vehicle weight is 6.965 tonnes and tyre size 205/85R16. While an elevating platform vehicle normally requires counterweighting, in this case the mass of the dual drive suffices with just modest additions. The central positioning of the battery above the chassis frame was chosen to optimize weight distribution and avoid the weight of cantilever frames.

Erigine+ generator

Motor+ drivetrain

Erigine+ generator

Motor+ drivetrain

Fig. 6.21 Mitsubishi Canter-based hybrid municipal truck: (a) Complete package; (b) hybrid drive; (c) operating modes; (d) unit efficiencies.

Fig. 6.21 Mitsubishi Canter-based hybrid municipal truck: (a) Complete package; (b) hybrid drive; (c) operating modes; (d) unit efficiencies.

The motors, of 55 kW, are of the induction type and each develop 150 Nm at 3500 rpm, rated voltage being 288 V. The simple two speed transmission has a PTO for driving the hydraulic pump and reverse motion is achieved by altering the rotation of the electric motor. The generator has a maximum output of 30 kW at 3500 rpm; it operates at 220-360 V and weighs 70 kg.

The petrol engine is a 16 valve unit of 1834 cc which has a 1.935:drive gear to the generator. Lead-acid traction batteries are employed, 24 units each weighing 25 kg and having 65 Ah capacity at a 5 hour rate. The company's estimations of unit efficiency are shown at (d).

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