A Cost-Effective Regenerative Air Hybrid Powertrain for Low Carbon Buses and Delivery Vehicles

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The hybrid powertrain with regenerative energy recovery is recognised as one of the most effective means to low carbon vehicles in short to medium term. It can significantly improve the fuel economy, particularly for buses and delivery vehicles in cities and urban areas where the traffic conditions involve a lot of stops and starts. In such conditions, a large amount of fuel is needed to accelerate the vehicle, and much of this is converted to heat in brake friction during deceleration. Capturing, storing and reusing this braking energy can therefore improve fuel efficiency, and this can be achieved by using the momentum of the vehicle during coasting and deceleration to top up an energy storage device and later releasing the energy to propel the vehicle during cruising and acceleration. The most researched and developed hybrid vehicles are electric based. The application of electric hybrid powertrain to buses and delivery vehicles is severely limited by the huge additional cost associated with the engineering complexities of the combined electric and mechanical powertrain and transmission systems. Currently, the electric hybrid buses are heavily subsidised and will not be suitable for commercially viable large volume production. Unlike the electric and hydraulic hybrids, air hybrids can be implemented without adding an additional propulsion system to the vehicle when it is applied to a reciprocating internal combustion engine. In this case, the engine itself can be used as a compressor during braking/deceleration or an expander for starting/ acceleration, transmitting power through the pistons and the crankshaft of the engine thus braking or propelling the vehicle using the existing drivetrain of the vehicle. For buses and delivery vehicles, air energy will be stored at moderate pressure (less than 15 bar) in a compressed air storage tank already available on such vehicles. Similar to its electric cousins, the air hybrid will be able to recover the braking energy and store it for later use to start the engine and help the vehicle to accelerate, allowing significant improvement in fuel economy without adding the large weight and complexity of the electric hybrid. In addition, the stored high pressure air is available readily on demand for other uses to improve driveability and reduce emissions, such as briefly boosting the engine to eliminate turbo-lag in a turbo-charged engine resulting in better response and removal of the black smoke typically seen from accelerating diesel vehicles. For buses and commercial vehicles, compressed air is required for air assisted braking and the operation of pneumatic equipment (e.g. door opening and closing) and is currently produced by an engine driven compressor. Air hybrid powertrain technology will enable further and readily achievable fuel savings to be realized by providing the service compressed air from the regenerative engine braking in place of the engine driven compressor. These are exciting synergies enhancing many attributes of the engine at minimum cost and are immediately available, not possible with the other hybrid energy types and unique only with the air hybrid because of the readily available air supply. Therefore, the exploitation of such synergies will result in an air hybrid powertrain system with significant and concurrent improvements in fuel consumption, emission, and performance.The aim of this proposal is to carry out research and development of suh a novel air hybrid powertrain system for urban buses and commercial vehicles with significant and concurrent improvements in fuel economy, emissions, and performance over the current IC engines.