申論題

(3) Heat engines are devices for converting thermal energy to work, and their performance are expressed in terms of thermal efficiency, is the heat input per unit mass of the fuid mediut (assumed ideal gas here).

(b) For the four-stroke spark-ignition cngines employed in cars as shown in the figure below, Otto cycle is a more realistic model, with the heat addition and rejection processes at constant temperature in the Carnot cycie replaced by processes at constant volume Cheat addition at yolume TDC and heat rejection at volume BDC). Plot the T-s diagram and p-y diagram of the process for the "simplified (simpler than that shown below)" Otto cycle, and derive the thermal eficiency. Discuss the assumptions made in the Otto cycle, and the differences in efficiencies between the Carnot and Otto cycles. (15%)