Generation game

A new generator system for hybrid electric buses could improve city centre air quality while helping the government to meet its targets for reducing carbon emissions. The challenge has been accepted by a team at NewcastleUniversity, with electric-vehicle systems specialist Hil Technologies.

They aim to boost the efficiency of series hybrids, in which an engine and a generator unit is used to produce electricity and an electric traction motor alone powers the wheels. The drawback is there are several power conversions: combusting the fuel in the engine to drive the generator, which produces electricity to be stored in batteries before powering the motors to mechanically turn the wheels.

This has too many losses for series hybrid systems in cars, which is why the Toyota Prius and others are parallel hybrids in which both the engine and motor can turn the wheels. ‘The wheel-to-wheel analysis of series hybrids shows they can be good for vehicles where you know the driving cycle,’ said Dr Volker Pickert at Newcastle. ‘That’s when you can optimise the efficiency and gain a lot from it.’

Buses are seen as ideal because they have predictable routes, journey events, and power requirements and regular times. Nearly 0.5 per cent of all manufactured vehicles in Europe will be series hybrid electric by 2010. Buses are expected to account for a large proportion, with the UK, France and Germany as key markets for the technology.

Some hurdles must still be cleared, including cutting the cost, weight and size of the generator, battery and motor. ‘In recent years significant progress has been made in new battery technologies, electric propulsion designs and optimisation of power flow and control of the electric components to reduce mainly the weight but also the cost,’ said Pickert. ‘While these developments have been made successfully, little attention has been spent on improvement of the generator.’

Conventional generators are either expensive or suffer from a voltage drop across the machine reactance of up to tens of volts. This worsens with speed and requires large cooling equipment. Pickert’s team is developing a new design of converter. ‘It shifts the energy from the machine to the output of the converter without a voltage drop,’ he said. ‘We do not have any commutation reaction power.’

The work, funded by the Engineering and Physical Sciences Research Council, starts in October and lasts three years.