Efficiency drive

A consortium of UK companies and universities is developing a new form of DC-DC converter for use in hybrid cars.

The research, part-funded by the Technology Strategy Board, is being led by motorsport company Prodrive. Other team members include HILTech Developments, International Transformers, LDV, Sloan Electronics and the universities of Manchester and Newcastle.

The overall aim of the project is to develop a device that is small, efficient and economical. This in turn will improve the efficiency of hybrid vehicles.

Parallel and series hybrid cars are powered by petrol or diesel engines, plus an electric motor. They also contain an energy storage device (ultra capacitors or batteries) that provides power to the electric motor.

Regenerative braking, a process that captures the energy lost during brakingand is re-used later to power the electric motor, adds to the efficiency of the hybrid’s drivetrain.

Energy storage

However, hybrids without DC-DC converters are said to compromise the energy storage device and the electric motor. When storing energy under braking, as the vehicle slows the voltage generated by the motor drops while the energy in the storage device increases.

There is a point when the motor can no longer supply sufficient voltage for the storage device so energy recovery after this stage is no longer possible. The same situation occurs when using the stored energy to power the motor to accelerate the vehicle.

A DC-DC converter balances the voltage between the motor and the energy storage device, increasing or decreasing the voltage as necessary.

Pete James, an electronics specialist at Prodrive, explained how the converter takes energy from either the motor or energy storage device, holding it in a magnetic field, before feeding it back to the other part. This reclaims more of the energy under braking and provides more under acceleration, making the vehicle more efficient.

‘There are many different ways you can do it and each have their benefits and disadvantages,’ said James. ‘We are looking at two topographies, a hard switching converter and a zero voltage converter.

‘A zero voltage switching converter does the switch when the voltage is at zero, meaning the loss is virtually down to zero, but this also means more stress in the inductor. Overall efficiency is much better, but it is more difficult to run and you need more electronic components including more switching devices.

‘DC-DC converters are fairly standard in laptops and PCs but not at the same power levels or efficiency,’ added James.

‘We wanted to push the boundaries so we chose 96 per cent efficiency as the target. Obviously we will try and get better than that and if the zero voltage switching works maybe we can improve it by one or two per cent.

‘The simulations have been performed and it looks like we can achieve the efficiency we are looking for.

‘Our converter will be designed for use with 50kW systems. This means it can provide an additional power boost of up to 67bhp, which allows a hybrid car to use a much smaller petrol engine without any loss in performance.

‘The other exciting prospect is that such a system could be used instead of a turbo or supercharger on higher performance sports cars.’

Performance boost

James believes the converter could provide a similar boost to performance and, if fitted to a standard hybrid vehicle, could improve performance ‘considerably’.

In addition to its efficiency target the project is aiming for a unit with a power-to-volume ratio of 6kW/l, a power-to-weight ratio of 4kW/kg and a unit cost of £10/kW.

While saving money in efficiency the unit also has the potential to save money in the manufacturing of the car as smaller engines and batteries could be used.

James also believes the technology could have potential in other industries.

‘I think the potential is vast because this type of converter is quite new and has many applications,’ he said.

‘It can be used anywhere there is a varying power source. It could, for example, be used for solar cells, wind and tidal generation and other transportation systems such as light rail which could all benefit from this kind of converter.’