June 26, 2011
June 26, 2011
June 29, 2011
Energy Conversion and Conservation
22.105.1 - 22.105.14
A Student-Built Internal Combustion Engine Simulation Using ExcelAbstractA numerical model for an internal combustion engine has been developed for use in a seniorelective course on internal combustion engines. The numerical model for the simulation is builtin an Excel file which includes previously-developed functions for obtaining thermal propertiesfor ideal gases. The properties include enthalpy, internal energy and the specific entropy at apressure of one atmosphere. Prior to the use of the functions in the internal combustion engineclass, mechanical engineering students enrolled in an undergraduate thermodynamics sectionwere offered the Excel ideal gas computer file for use in completing homework assignments.Additionally, they were offered a Mathcad file programmed with ideal gas properties for air in avery similar fashion. The students were offered the option of using either of these programs onany homework or on examinations involving ideal gas problems. They were also offered achoice of using neither of these programs, interpolating ideal gas data out of the textbookinstead. Records of the student selections and preferences are presented as part of this research.The numerical internal combustion engine model uses one degree of crankshaft rotation as itsdifferential element size, so there are 360 steps in a complete rotation of the model. Studentsbuild the model on their own by following the example presented in class. In each step, thecombustion chamber volume is computed using the global input values for bore, stroke, pistonrod length and crankshaft angle. Using these volume values, the cylinder pressure is calculatedby using the ideal gas law and the given quantity of air and vaporized fuel in the combustionchamber, based on an assumed temperature. A converged solution for the temperature is nextfound through an energy balance equation where the work of compression or expansion, fromone crank angle increment to the next, is set equal to the change in the internal energy of themixture of gasses in the cylinder as a function of temperature. By using a solver routine,custom-programmed as a macro in Excel, the temperature at each crank angle increment issolved for sequentially for all 360 increments. The ignition timing for a spark ignition engine, orthe fuel injection timing for a compression ignition engine, can be selected by the student.Additionally, this ignition or injection point can be optimized using a custom-programmedoptimization macro in the Excel file.With pressure and temperature calculated as functions of crank angle for an entire rotation of thecrankshaft, the net power produced by the engine can be calculated using the engine RPM value,which is selected by the students. As the engine speed is varied, the students can see the changesin power produced, engine efficiency, and optimum ignition timing angle. Homeworkassignments involving these calculations are assigned to the students. The addition of the enginenumerical simulation segment of the course was extremely well received by the students andcomments from a student survey on the subject were very positive.
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