During the expansion process of the ideal Otto cycle, the gas is a mixture whose volumetric composition is 25 percent nitrogen, 7 percent oxygen, 28 percent water, and 40 percent carbon dioxide. Calculate the thermal efficiency of this cycle when the air at the beginning of the compression is at 12 psia and 55°F, the compression ratio is 5, and the maximum cycle temperature is 1600°F. Use air-standard analysis. The air properties at room temperature are cp = 0.240 Btu/lbm·R, cv = 0.171 Btu/lbm·R, k = 1.4. The thermal efficiency of this cycle is