Engineering >> Mechanical EngineeringOtto Cycle Compression Ratio Optimizationby Devon Fathauer
Submitted : Fall 2012 The problem presented here was to find the optimal compression ratio that generates the maximum net work in an Otto Cycle cylinder for a four-stroke, 115 horsepower 4-Cylinder Yamaha outboard boat engine. This ratio is calculated by the basic equation of dividing the volume of a cylinder when the piston is at the bottom dead-center (Vmaximum) by the volume at the top dead-center (Vminimum). Some thermodynamic equations and constants were utilized as well as the ambient pressure and temperature on an average day in South Florida, maximum temperature at the combustion point of the fuel, and the maximum volume within one cylinder. Using substitution, an equation for net work that included the values of ambient air temperature and mass as well as maximum air temperature was found. Looking at a basic Work vs. Compression Ratio graph, it is clear that the equation for maximum net work has a tangent line with a slope of zero, therefore simplifying and taking the derivative of the equation and setting it to zero will yield the values needed to calculate the optimum compression ratio and corresponding pressures and temperatures. The calculated compression ratio was 13.56:1. This value is used to solve for all the remaining values of pressure, temperature, and volume. These values are then used to find the net work in the cylinder to be 371.24 Joules, which turns out to be an acceptable value. Related Links:
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