For calculating efficiency of Otto cycles, which value is typically used unless specified otherwise?

In the context of calculating the efficiency of Otto cycles, the k value that is typically used is the k value of air, which refers to the specific heat ratio (also known as the heat capacity ratio) at constant pressure and constant volume for air.

The efficiency of an Otto cycle, which is an idealized thermodynamic cycle, is influenced by the specific heat properties of the working fluid, and air is commonly considered as the working fluid for many engineering applications involving this cycle. The specific heat ratio k for air, usually around 1.4, plays a crucial role in determining the maximum efficiency achievable by the cycle.

Using the k value of air allows for consistent results with the assumptions made in conventional Otto cycle analysis. Other fluids like water, steam, or gasoline do not typically represent the gas phase behavior needed for these calculations, especially since the Otto cycle is primarily concerned with the gas phase thermodynamic processes.

Therefore, the standard practice is to use the k value of air unless otherwise specified, as this aligns with common engineering assumptions and available data for air under standard conditions.