A Novel Hybrid Powerplant for Passenger Bus Applications 2007-01-4120

Hybrid solutions to propulsion system requirements permit novel solutions to load cycle requirements. By distributing the load between two devices whose efficiency characteristics are complementary, the overall efficiency can be tailored by blending the load contribution. The potential of such a combination is further enhanced if the combination of the devices results in a more complete conversion of fuel than a single device operating alone. In this paper we present a conceptual design for a powerplant made up of a fuel preparation sub-system and a diesel engine. The sub-system is made up of a reformer and solid oxide fuel cell stack that provides a stream of gaseous fuel and air to the engine, as well as producing electrical power. In this configuration the system is essentially an actuator that is controlled through the imposed electrical load.

The fuel cell based pre-treatment device has a higher inherent efficiency than the engine. At low loads the efficiency difference widens, placing an emphasis on the operation of the fuel cell system. As the overall load increases the engine load can be quickly increased while the fuel cell's load is decreased tio a high efficiency low load condition. The fuel preparation sub-system converts a liquid fuels to a reformate gas whose composition reflects the gross chemical composition of the fuel. Reformate gas aspirated into the engine modifies the subsequent quality of combustion, and renders the engine more able to handle liquid fuel of varying quality.

We describe a series of simulation experiments using the QSS toolbox [1] in which the engine data is derived from a modern medium duty engine family. The target vehicle is a medium sized passenger bus in a series hybrid configuration with respectively a battery and a supercapacitor. For a typical European bus duty cycle, the Braunschweig cycle [2], the fuel consumption improvements are typically 10-15% over a conventional diesel engine. The proposed system appears capable of delivering an excellent fuel economy in comparison with both conventional and hybrid powertrain. Emissions reductions come through the reduction of the swept volume of the engine and improved combustion.