A Methodology for the Design of Cost Effective Hybrid Vehicles

Doktorsavhandling, 2004

This thesis deals with system design of cost effective hybrid and fuel cell vehicles. Today (2004) the major disadvantages of these vehicles are increased complexity and component costs. However, the decrease in cost of electrical components and the environmental concern make these vehicles more and more competitive. To make hybrid and fuel cell vehicles cost effective, a sensible system design plays a decisive role. Configuring, choosing, sizing and energy management of vital powertrain components determine how cost effective a system is. The powertrain is the system in a vehicle that develops and transmits traction power. A cost effective powertrain system is both fuel efficient and cheap in component cost. On the road and in the showroom today, hybrid vehicles are powered by both a small gasoline engine and an electric machine. By using the electric machine in combination with an energy buffer, braking energy can be regenerated. Thanks to the power assisting buffer, the engine can also be smaller in size. Energy management is a specific and crucial problem in hybrid vehicles; it determines how power is distributed between the buffer and the primary power unit. The primary power unit can be a combustion engine, a fuel cell or a high capacity battery. A computer tool THEPS has been developed by the author. THEPS uses simulation and optimization to propose a powertrain concept from requirements and conditions. An example of a requirement is the desired top speed of the vehicle. Some conditions are fuel price and interest rate. The approach is to regard the output from a simplified vehicle model as a function of design variables. Characteristics defined by the design variables are: type of powertrain, type and size of vital components and directives for the energy management. The intention is to minimize the operating cost of the vehicle, i.e. the sum of component, fuel and component wear cost. An evolutionary algorithm is utilized for optimization. For the complex nonlinear optimization problem, which exists in THEPS, such an algorithm is suitable because the solution space contains numerous local minima and is discontinuous. Another contribution is a library including powertrain models. The library is developed in the modeling language Modelica and includes models that are more detailed than the models used by THEPS. The objective with the library is to analyze hybrid powertrains proposed by THEPS. The thesis also describes the development of a scaled, hybrid electric, model car prototype. The primary power unit of the model car is a lead-acid battery and the buffer of the model car is a high-power super capacitor. Traction and steering of the car are radio remote controlled. Three case studies using THEPS are also included. In the first case study a powertrain is adapted to an existing city bus route in the Swedish city of Göteborg. The second case study deals with a waste disposal truck and the third case study deals with a taxi car. The case studies indicate that new powertrain technologies can be competitive from a cost perspective, in some applications, already at present time. It is for example reasonable to equip heavy vehicles running in urban areas with hybrid powertrains. The case studies also indicate that hybrid and/or fuel cell cars can be a more cost effective choice than conventional cars in a near future (2015). Another indication is that it will not be clear for a customer which powertrain concept to choose. The reason is that many cost effective powertrain concepts will be offered. The best choice will depend on the application.