What does a lean mixture indicate in terms of oxygen?

A lean mixture in the context of combustion, particularly in engines, signifies that there is an excess of air relative to the amount of fuel being burned. This excess air indicates that more oxygen is available for the combustion process than is necessary to fully combust the fuel.

With a lean mixture, the increased availability of oxygen allows for more complete combustion of the fuel. This typically results in lower carbon monoxide (CO) levels because CO is a byproduct of incomplete combustion. When there is sufficient oxygen, the carbon in the fuel is more likely to bond with oxygen to create carbon dioxide (CO2) instead of carbon monoxide. Therefore, a lean mixture directly correlates with reduced CO emissions, showcasing that more oxygen leads to cleaner combustion.

In contrast, whether other gases like hydrocarbons (HC) or carbon dioxide (CO2) are affected depends on the specific balance of the mixture and combustion efficiency, but the key takeaway is that a lean mixture promotes a reduction in carbon monoxide levels due to more efficient combustion.