Air hybrid system will allow buses to recharge by braking

A regenerative braking concept that uses the energy created when a vehicle decelerates to compress air is being developed for a new breed of lower-cost, fuel-efficient buses and delivery vans.

Engineering researchers from Brunel and Loughborough universities are developing the ‘air hybrid’ concept through a three-year £477,067 EPSRC-funded project. The effort has received industrial support from engine manufacturer Guangxi Yuchai Machinery, which recently signed a technology development and licensing deal with Brunel for its air hybrid technology.

Project investigator Prof Rui Chen, of Loughborough University, explained that the concept effectively runs a four-stroke engine in reverse when a vehicle brakes.

A four-stroke engine intakes fuel and air into a chamber, compresses it and then combusts the mixture to create the motion that propels a vehicle.

Chen said the air hybrid concept uses the force of the vehicle braking to power the engine’s pistons, which compress air. This air would then be stored in a compressed air storage tank already available on buses and delivery vehicles.

Once the vehicle begins to accelerate, the compressed air could be used to supercharge the engine, providing better fuel efficiency and more power, he added.

It could also be used for air-assisted braking and, in the case of buses and delivery vehicles, the operation of pneumatic equipment for opening and closing doors. Currently these operations rely on air produced by an engine-driven compressor.

The most researched and developed hybrid vehicles are currently electric based. But their suitability for commercially viable large-volume production is limited because of the additional cost associated with the engineering complexities of the combined electric and mechanical powertrain.

According to Transport for London, which currently runs 56 hybrid buses in the capital, a double decker fitted with a combined powertrain will generally cost £110,000 more than a conventional diesel version.

For example, a hybrid double-decker bus for London would cost £300,000 compared with £190,000 for the diesel equivalent.

Chen said the advantage of the air hybrid concept is that it can be implemented without adding an additional propulsion system.

The Brunel and Loughborough researchers expect to produce a working prototype of the air hybrid technology by the end of the EPSRC grant in November 2013.

A product could be ready for commercialisation by as early as 2015 following further development work.