ANS:A - heat addition & compression ratio.

Heat addition and compression ratio are two key parameters that affect the efficiency and performance of internal combustion engines, particularly those modeled by the air-standard Otto and Diesel cycles.

1. Heat Addition:
   • Heat addition refers to the process of adding thermal energy to the working fluid (usually air) in the engine cylinder. This typically occurs during the combustion process, where fuel is ignited and burns, releasing heat energy. In the Otto cycle, heat addition occurs at constant volume (isochoric process) after the compression stroke, while in the Diesel cycle, it occurs at constant pressure (isobaric process) during the combustion stroke.
   • The amount of heat added to the working fluid directly influences the power output of the engine. A higher heat addition results in greater expansion of the working fluid during the power stroke, leading to increased mechanical work output. However, excessive heat addition can also lead to issues such as engine knock and increased emissions.
2. Compression Ratio:
   • The compression ratio of an engine is the ratio of the volume of the combustion chamber when the piston is at the bottom dead center (BDC) to the volume when the piston is at the top dead center (TDC). It represents the degree of compression of the air-fuel mixture or air alone in the engine cylinder.
   • A higher compression ratio means that the air or air-fuel mixture is compressed to a smaller volume before combustion. This leads to an increase in the temperature and pressure of the air or mixture, resulting in improved thermal efficiency due to better utilization of the available energy in the fuel.
   • The compression ratio affects engine performance, fuel efficiency, and emissions. Higher compression ratios generally result in higher thermal efficiencies and power outputs but may also lead to increased engine stresses, higher temperatures, and the risk of knocking.

In summary, the heat addition and compression ratio are essential parameters in internal combustion engines. Proper control and optimization of these parameters are crucial for achieving high engine efficiency, performance, and reliability. Adjusting the heat addition and compression ratio can significantly impact the overall operation and characteristics of the engine.