Abstract:

An experience for higher education in the field of mechanical engineering is presented, based on a methodology to analyze and solve problem cases in small collaborative groups. It supports a constructivist view of teaching and learning processes. The methodology has been implemented during the last ten academic courses, as teacher-directed work, in the subject of “Thermal Machines”, which belongs to the Bachelor's degree in Mechanical Engineering at the University of Jaén (UJA). The aim of the project is to power a device attached to an internal combustion engine (such as a generator, hydraulic pump, pulley, etc.), minimizing engine fuel consumption.

This work is divided into three steps:
i) generation of the fuel consumption surface from the characteristics data of the engine. For this purpose, the primary tool to be used by the students is a set of spreadsheets. These spreadsheets include experimental data previously determined, such as brake torque and volumetric fuel flow, which were obtained from testing different engines via varying angular speed and load. Equations indeed the proper sheet provide the brake power, brake mean effective pressure specific fuel consumption, mean piston speed, and efficiency for each test. Next, these data allow drawing three curves at each engine load depending on the angular speed (or mean piston speed): torque, power, and specific fuel consumption. Finally, an effective specific fuel consumption surface is generated, depending on angular speed (of mean piston speed) and torque (or power). It is recommended to build this surface by hand instead of using the computer to note the difficulties associated with surface generation.
ii) A practical application must be defined, setting both the power and angular speed required that must be provided by the engine and considering the efficiency of the mechanical coupling. Students can use the knowledge acquired in previous subjects of their degree related to mechanisms, power generation, and hydraulic pumps. Consequently, the present methodology implicates a multidisciplinary work. The device characteristics must be determined via the website of manufacturers or suppliers.
iii) Engine-application matching. For this purpose, a point with the required effective power must be identified in the specific fuel consumption surface but with a minimum value for that engine characteristic. If the angular speed at that point differs from the required one in the application axe, a transmission ratio is then determined.

The instructional design prioritizes using those tools (spreadsheets and the Internet) to support collaborative work in small groups of students. Additionally, it considers giving a multidisciplinary character to the experience and providing teacher monitoring. The general approach is straightforward, although the intermediate stages are more complex and require collaborative work in small groups. In addition, a broader teacher-student interaction is necessary so the student is appropriately guided. Thus, two teaching hours are dedicated to this task. During this time, the teacher explains the different steps to follow and performs a group tutoring task. The overall assessment of the experience is very positive since the students perceive the teacher's involvement during the completion of the work, motivating them and improving their academic performance.

Keywords:

Collaborative, problem-based, learning, internal combustion engines, characteristic curves.