Cycles with lean to very lean mixture tend towards air-standard cycles.

Explanation:
A lean mixture refers to a fuel-air mixture in which the amount of fuel is less than the stoichiometric ratio required for complete combustion. In other words, the air-to-fuel ratio is higher in a lean mixture. On the other hand, a very lean mixture refers to an even higher air-to-fuel ratio, with an even lower amount of fuel.

Air-standard cycles are theoretical models used to analyze the performance of internal combustion engines. These cycles assume that the combustion process occurs at constant volume or constant pressure and that the working fluid is air. They provide a simplified representation of the actual engine operation.

Now, let's understand why cycles with lean to very lean mixture tend towards air-standard cycles.

1. Simplified Assumptions:
- Air-standard cycles assume that the working fluid is air, which is a reasonable approximation for engines running on lean mixtures since the fuel content is minimal.
- These cycles also assume that the combustion process occurs at constant volume or constant pressure. Lean mixtures tend to have faster and more controlled combustion, which can be approximated by these simplified assumptions.

2. Combustion Process:
- Lean mixtures promote more complete combustion due to the excess air available. This results in a higher combustion efficiency and lower emissions.
- The combustion process in lean mixtures is relatively stable, with a reduced risk of detonation or knock.
- The lean mixture also leads to a lower peak combustion temperature, which helps reduce the formation of nitrogen oxides (NOx).

3. Thermodynamic Considerations:
- Lean mixtures have a higher specific heat ratio (gamma) compared to stoichiometric or rich mixtures. This affects the thermodynamic properties of the working fluid and can be approximated by air-standard cycle assumptions.
- The higher specific heat ratio of lean mixtures affects the compression and expansion processes in the engine, which can be analyzed using air-standard cycle models.

In summary, cycles with lean to very lean mixture tend towards air-standard cycles because the simplified assumptions and thermodynamic properties of these cycles can reasonably approximate the performance of engines running on lean mixtures. These cycles provide a useful tool for analyzing the combustion process, efficiency, and emissions of such engines.