What happens to the air-fuel mixture as engine speed increases?

As engine speed increases, the air-fuel mixture becomes leaner due to the increased airflow through the engine compared to the amount of fuel being injected. This phenomenon is often referred to as "leaning out" of the mixture. In a high-speed operation, the engine draws in more air because the intake valves open more frequently and allow greater amounts of air into the combustion chamber. However, the fuel delivery system may not be able to proportionately increase the fuel supply at the same rate as the air, leading to a relatively lower concentration of fuel in the mixture.

A leaner mixture typically improves combustion efficiency at higher speeds because it can result in more complete burning of fuel and reduced emissions. However, it's important to manage this aspect carefully, as excessively lean mixtures can lead to knocking and engine damage.

The other options reflect concepts that do not accurately describe the behavior of the air-fuel mixture at increased speeds. For instance, the mixture doesn't remain the same; instead, it changes as described. A richer mixture would imply more fuel relative to air, which is the opposite of what happens as speed increases under normal operating conditions. Similarly, the notion of dilution doesn't accurately capture the dynamics of the engine at higher speeds; rather than diluting,