What effect does an engine running too rich have on emissions?

When an engine runs too rich, it means that there is too much fuel in relation to the amount of air being mixed for combustion. This condition primarily leads to incomplete combustion of the fuel. As a result, one of the most significant emissions produced in such a scenario is carbon monoxide.

Carbon monoxide is a colorless, odorless gas that forms when carbon in the fuel is not fully oxidized. In a rich fuel mixture, there is insufficient oxygen for the fuel to burn completely, resulting in a higher concentration of carbon monoxide in the exhaust. This is a critical emissions concern because carbon monoxide is a toxic pollutant that can have serious harmful effects on human health and the environment.

In contrast, a richer fuel mixture typically does not lead to lower levels of hydrocarbons or increased oxygen output, nor does it usually result in decreased nitrogen emissions. Therefore, the outcome of higher carbon monoxide levels due to a rich mixture is a well-supported conclusion in emissions testing and engine performance analysis.