Defining Performance Metrics for Hybrid Electric Vehicles 2007-01-0287

The quantitative assessment and comparison of different hybrid vehicle options has traditionally been done on the basis of measuring or estimating the vehicle's fuel economy over predefined drive-cycles. In general, little or no consideration has been given to the more subjective and difficult to quantify vehicle requirements, such as trying to understand which derivative will be the most “fun” vehicle to drive. A lack of understanding in this area of vehicle performance sufficiently early within the development life-cycle so as to be in a position to influence the vehicle design, can lead to a compromised powertrain architecture which will ultimately increase the risk of product failure.

The work presented within this paper constitutes part of the overall design activities associated with the LIFECar programme. The aim of the LIFECar consortium is to manufacture a lightweight, fuel cell hybrid electric sports vehicle.

This paper introduces a simulation model for the LIFECar vehicle that has been used to evaluate and compare different powertrain architectures and subsystem options. Metrics have been formulated in an attempt to facilitate the quantitative assessment and comparison of the different vehicle derivatives. The basis for the study is to ascertain which vehicle option will be the most “fun-to-drive”. With respect to the longitudinal dynamics of the vehicle, the assumption of what contributes towards a vehicle being “fun-to-drive” is the ability to rapidly and continually accelerate and brake the vehicle. The proposed metrics for vehicle evaluation can be directly related to physical vehicle components, thereby allowing a clear means of feeding back simulation results into the design of the hybrid powertrain and the associated subsystems. Results from the simulation study are presented to help understand the correlation between the different design options and the “fun-to- drive” characteristics of the vehicle.

In addition to the primary conclusions of this study, this paper also highlights the importance of evaluating and optimising the complete vehicle powertrain as a single integrated system rather than attempting to improve the performance of the powertrain by focusing on a single vehicle attribute such as weight or efficiency.