Product Development Approach
A Hybrid Electric Vehicle (HEV) is a vehicle with two sources of motive energy. There exist many potential hybrid system concepts developed using energy conversion subsystems such as fuel cells, gas turbines, compression ignition and lean burn gasoline engines in combination with energy storage subsystems such as flywheels, batteries, and ultra-capacitors. There are two principle ways to combine the systems of an HEV into functional configurations. These configurations are termed parallel and series. The series configuration consists of an electric motor (acting as sole producer of drive torque) and an energy conversion subsystem. Electrical energy from a group of chemical storage batteries or other storage device supplies the motor. An energy conversion subsystem replenishes the stored energy. The storage device acts to level the energy demand and, thereby, greatly decouple the energy conversion device’s output from the transients of that demand. Advantages of the series vehicle are most apparent when the energy conversion subsystem is efficient over a narrow range of operating conditions only, or when it is excessively polluting when subject to transient demand. The series configuration also permits mechanical decouple. This allows a greater freedom in the physical layout of subsystems that makes system packaging easier.
The main alternative to the series configuration outline above is, predictably, a parallel one. This configuration typically employs two separate sources of drive torque. As in the series configuration, an electric motor and batteries can provide the first source of torque. A combustion engine can provide the second source of drive torque. The parallel configuration connects the two torque sources via drivetrain components such as transmissions and clutches. The potential advantage that a parallel configuration has over series is one of higher energy efficiency. In the parallel configuration, stored energy is used to buffer the driving demand at higher speeds (those near the efficient combustion engine operating conditions) and to take over completely for the combustion engine during low speed driving. A parallel configuration was chosen for the SDSU HEV. Concept selection heavily favored a peppy, high mileage vehicle with traditional range using off the shelf technology. This led to selection of a relatively powerful direct-injection, diesel, energy-conversion device and a high power AC drive motor and controller. Below are diagrams that represent the parallel and series configuration, which are presented courtesy of the National Renewable Energy Laboratory Website.
Figure 1: Parallel configuration and series configuration diagrams.
Design and Analysis Tools Used
CAD/CAM/CAE in Interdisciplinary Projects
The CAD/CAM/CAE tools described above often produce the following potential benefits:
In an effort to exploit these advantages, the SDSU team adopted the solid modeling tool Pro/E. This tool is highly advanced and requires many hours of classroom training and individual practice time. In the case of the HEV project, a member with the capability to create complex surfaces, perform packaging analysis, and create a master representation of the model was required.
Pre-project Curriculum, Support, Training and Investment
A team of engineering students skilled in the use of leading-edge Computer Aided Drafting (CAD), Computer Aided Manufacturing (CAM) and Computer Aided Engineering (CAE) software technologies were recruited for the SDSU HEV project. These individuals were not easy to find. Often, the most daunting task that any large-scope academic project faces is finding students that already possess the skills necessary to make use of modern software tools. Numerous hours of training are required to operate just one of the software packages described above. Prior to the start of the project, the lead designer acquired approximately 2000 hours of experience in Pro/E, 500 in Pro/MECHANICA (FEA) and approximately 100 in Pro/MOTION. Many additional hours were necessary for proficiency with the other tools utilized for the project, especially DV/MOCKUP. This level of experience was accumulated during the student’s junior and senior years as an SDSU mechanical engineering major.
Early conceptual design decisions, which mandated risk-minimization, dictated use of a commercially available automobile body for this project. A suitable candidate was chosen based on easily repartitioned internal volume, local fabrication source, and availability of geometric data. This data was very poor quality IGES (International Graphics Exchange Standard) wire frame data as derived from a digitized plaster or wood model. Advanced Pro/E surfacing tools were used to recreate the surface. These tools analyzed surface inflections, continuity and curvature of the line entities used to create surface boundaries. The process of modeling the surface began by utilizing cross-sections of the imported IGES surface to create complex surface patches known as quilts (2-D surface entities). Subsequent to the joining of the quilts into a complete surface, a detailed and exhaustive analysis of the surface entities’ integrity was performed. Using curvature and section analysis tools, a detailed color plot was generated to expose any surface anomalies. In addition, tangency condition, dihedral angle and deflection of the curves were analyzed. Refinements based on these analyses were made to the quilts to permit regeneration of the surface for the further creation of a thin protruded solid shell model of the body.
The model was rendered as a realistic-looking and appropriately accurate solid shell. Approximately 120 hours were required for surface creation and analysis. Over a half dozen visits to the body fabrication site were needed to verify that the model was of fidelity to the actual body to permit confident packaging of modeled subsystems. With the completion of a parametric solid representation of the HEV body, the engineering team moved on to the decision process for subsystems. Selection of the subsystems was addressed next, so that the designer could begin placing simplified subsystem representations into the body model as part of packaging design.
Selection design, as the name implies, involves the thoughtful choice of a group of component that collectively deliver desired functionality while obeying constraints. During this phase of the project, the efficacy of Pro/E was felt again. By utilizing the measurement and mass properties tools, the designer was able to calculate collective properties such as total usable volume and total surface area of the body. This affected the selection design process by narrowing the component choices to those components that captured the desired functionality. When the correct components were selected, all the pertinent geometric data of each component was communicated to the design team. Each component was modeled as a simplified representation (a model which is tailored to include only the information relevant to a specific design task), and then added to the assembly model. With a simplified assembly of all the major sub-component models, the body model and an anthropomorphic driver model, Pro/E was an able to provide the team with important engineering information. For instance, the projected frontal area of the model was calculated and used to estimate the drag properties of the body. Total mass, center of mass, inertial moments, and radii of gyration were calculated by the tool and compared with those of benchmark automobile configurations. This information was immediately put to use by the team member studying vehicle dynamics for further refinement of chassis design and by the aerospace engineer for design changes reducing aerodynamic drag. Mass and drag property estimates were used to refine drive train, energy storage, and chassis design. Furthermore, an extensive analysis of the subsystem relative placements was conducted by exploiting the global interference detection capabilities of Pro/E. The lead designer then relocated any component that was shown to interfere with another. A second mass-properties analysis of the vehicle was performed, one that provided immediate feedback on changes to the center of gravity and area moments of inertia. Such rapid iterations permitted a greater number of refinement steps for the inexperienced design team in the time allotted.
Planned Activities Awaiting Completion
With data available on the collective mass properties and with all the major subsystems except the fore and aft frame structure packaged with relative certainty, the next phase of the design was structural and kinematic analysis. Pro/E’s additional modules include finite element analysis capabilities. In this case, the task of analyzing the structural integrity of the vehicle’s chassis under static and dynamic loads was necessary. This task was performed using Pro/MECHANICA and Pro/MOTION. Pro/MECHANICA is a tool that provides mechanical response modeling and optimization capabilities for both parts and assemblies. This tool features static, modal, buckling, contact, and vibration analyses. It can also be used to determine sensitivities to geometry and property changes after the design is completed and transferred to Pro/MECHANICA a stress and strain analysis is performed. By adding loads to specific areas on the chassis and then constraining the chassis at specific locations, a finite element analysis can be performed. Maximum and Minimum stresses and strains as well as torsion and forces in specific directions. This data is then plotted on a fringe plot or contour plot that graphically displays the varying stresses and strains. Appropriate team members will then scrutinize the data, after exposing the weaknesses in the chassis; the engineers make the necessary modifications to the chassis and run the analysis again. This process is continued for several iterations until the deign optimization process is consummated. The advantages of Pro/MECHANICA are significant; specifically, there is reduced need for destructive testing, which can be time consuming and costly, as a means of design verification. In addition, rapid iterations can be performed, allowing for repeated design refinement of the chassis.
Once static structural analysis is complete, the chassis, suspension and any other subassemblies that are subjected to dynamic mechanical loading will be analyzed. Pro/MOTION is a motion analysis package that provides mechanism modeling and design optimization capabilities. This product features three-dimensional static, kinematic, dynamic, and inverse dynamic analyses. Dynamic geometric quantities of importance in automotive design may be easily monitored during the analysis. In addition, Pro/ MOTION simplifies "what if" experiments with an automated sensitivity study feature. The designer can also perform optimization studies, in which MOTION adjusts aspects of your design to best achieve a specified goal, while respecting constraints.
With these features, Pro/MOTION becomes a tool you can use throughout the design process, from conception through testing, fine-tuning, and even troubleshooting once your design is in actual use. For the HEV project, Pro/MOTION will analyze the suspension system and chassis. These areas are important with respect to safety, and must be analyzed under real world conditions in order that an accurate assessment of the vehicle integrity is obtained. However, a certain amount of domain knowledge and seat time is necessary to operate this software tool; at least 100 hours of seat time is necessary before the software tool can be effectively used. Moreover, the arduous aspects of this software tool will pose some problems in the beginning; training is limited to just one individual. Nevertheless, a plethora of data will be available for the HEV team by providing them with a complete picture of the dynamic capabilities of the vehicle, in turn, playing a pivotal role in the overall success of the vehicle design.
Finally, with all the solid modeling, static and kinematic analysis complete, the HEV Master Representation will be immersed in the virtual world using software called DV/MOCKUP. DV/MOCKUP is designed to allow manufacturers and designers to easily create, visualize, interact with, share, analyze and manipulate virtual mockups. DV/MOCKUP allows direct import of CAD data from multiple sources to construct a full-scale mockup for analysis of the entire product. DV/MOCKUP allows you to quickly study many aspects of product form, fit and function. Using [this software tool] dramatically reduces the number and cost of physical prototypes, and accelerates verification of assembly, packaging, service and maintenance operations. Design engineers can quickly check for interference’s and study interaction of moving parts to verify pre-assembly [as well as] perform static or dynamic packaging studies. [Additionally] styling and concept [for the HEV] can present different configurations to study form and aesthetics." DV/MOCKUP provides an architecture, which allows for real time performance while managing large complex assemblies. A final design review of the HEV will be conducted via DV/MOCKUP. The SDSU HEV will undergo an extensive collision analysis; not diagnosing beforehand that a component cannot be removed for maintenance because its collision with the chassis could prove to be inconvenient.
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Department of Mechanical Engineering
San Diego State University
5500 Campanile Dr.
San Diego, CA 92182-1323
Fax: (619) 594-3599