Page 178 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 7 .1 Hybrid vehicle design

A key benefit of the PM brushless DC motor is the
wide band constant-power curve. Consequently there is
no need for gear changing for high-torque operation, and
the motor gives high efficiency and low rotor heating
both on the flat and in hilly terrain. The motor inverter
and battery are oil cooled to ensure compact dimensions.
The battery uses lead/tinfoil plates to achieve low in-
ternal resistance and is thermally managed to ensure
charge equalization in the cells. A chopper is used to give
a stabilized 300 V DC link. The outstanding feature of
the lead–acid battery is the peak power capability of
50 kW for 2 minutes in a weight of 170 kg, Fig. 7.1-7(a).

7.1.3.4 Wankel rotary engine

The Wankel engine is well proven at 300 cc size, having
full Air Registration Board certification for use in drones
and microlights. The main benefits of the Wankel engine
Fig. 7.1-5 The hybrid power unit.
are: light weight and ‘6 cylinder’ smoothness; flat torque/
maintain the terminal voltage of the motor constant in speed curve and good fuel economy with fuel injection
the high speed region, Fig. 7.1-6. very low emissions; on natural gas it is possible to comply
The system efficiency is achieved by using a dual- without a catalytic converter. On unleaded petrol, two
mode control system for the inverter: (i) at low speeds electrically preheated catalytic converters are used.
the inverter operates in the current-source mode; the Another advantage is multi-fuel capability. In the control
current in the motor windings is pulse-width modulated; scheme, the electric drive runs continuously. The Wankel
(ii) when the rectified motor voltage exceeds that of the engine switches on and off. At speeds above 60 mph, the
DC link, the inverter changes to voltage-source control. engine will run continuously. The objective is to avoid
Since the system operates with field weakening, the discharging the battery by more than 30%. In this way we
machine has a leading power factor, consequently there is can obtain 11 000 cycles or 100 000 miles on a small
virtually no switching loss in the inverter transistors if battery. Consequently the economics of this scheme
square-wave excitation is used. Since the motor has 30% make sense.
impedance at full load, the impedance at the 5th/7th In mass production (>100 000 systems) the additional
harmonic is 150/210%, and consequently there are few cost per car of this system would be £2500. We believe
harmonics in the current. As a result, low-saturation this cost could be recouped, in fuel savings and reduced
insulation gate bipolar transistors (IGBTs) can be used, maintenance, by a 3 year first user, Fig. 7.1-7(b). The
and they switch at less than 2 kHz with high efficiency in problem is to break through the dichotomy of the
the cruise mode and low RF interference. electric-vehicle market. Market logic says start with

Fig. 7.1-6 Efficiency map, torque/speed curves and load matching.

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