Understanding Air-Fuel Ratio in SI Engines

The air-fuel ratio is a critical parameter in the operation of spark ignition (SI) engines. It represents the mass ratio of air to fuel in the mixture supplied to the engine cylinder. This ratio significantly impacts engine performance, fuel efficiency, and exhaust emissions.

What Happens When Air-Fuel Ratio Decreases?

A decrease in the air-fuel ratio means that the amount of air is reducing relative to the amount of fuel. This results in a fuel-rich mixture. Conversely, an increase in the air-fuel ratio leads to a leaner mixture (more air relative to fuel).

So, a decrease of air-fuel ratio corresponds to running the engine on a rich mixture.

Impact of Rich Mixture on Combustion and Emissions

In a fuel-rich mixture, there is insufficient oxygen available to react completely with all the fuel during the combustion process. This leads to incomplete combustion. The primary products of incomplete combustion are:

• Carbon Monoxide (CO): Instead of carbon atoms in the fuel completely oxidizing to carbon dioxide ($\text{CO}_2$), they only partially oxidize to CO. This happens because there isn't enough oxygen to form $\text{CO}_2$.
• Unburnt Hydrocarbons (UHC): Some fuel molecules may not react at all or only partially break down due to the lack of oxygen and potentially lower combustion temperatures in very rich mixtures. These unburnt or partially burnt fuel components are emitted as hydrocarbons.

Therefore, when the air-fuel ratio decreases (mixture becomes richer), the levels of CO and unburnt hydrocarbons in the exhaust emissions tend to increase significantly.

Considering Other Emissions (NO_x)

Nitrogen Oxides ($\text{NO}_\text{x}$) are primarily formed at high temperatures and in the presence of sufficient oxygen. $\text{NO}_\text{x}$ formation is typically highest in lean mixtures (high air-fuel ratio) near the stoichiometric point, where temperatures can be very high and oxygen is readily available. In rich mixtures, the lack of oxygen and potentially lower combustion temperatures can actually lead to a decrease in $\text{NO}_\text{x}$ formation compared to stoichiometric or slightly lean conditions.

Analysing the Options

• Option 1: $\text{increase of NO}_\text{x}$. As discussed, a decrease in air-fuel ratio (rich mixture) is generally associated with a decrease, not an increase, in $\text{NO}_\text{x}$ emissions compared to leaner or stoichiometric conditions. So, this option is unlikely.
• Option 2: a decrease of CO and unburnt hydrocarbon. A rich mixture results in incomplete combustion due to insufficient oxygen, which leads to an *increase* in CO and UHC, not a decrease. So, this option is incorrect.
• Option 3: an increase of CO and unburnt hydrocarbon. This aligns perfectly with the understanding that a rich mixture (decreased air-fuel ratio) causes incomplete combustion, leading to higher levels of CO and unburnt hydrocarbons in the exhaust.
• Option 4: none of the above. Since option 3 is correct, this option is incorrect.

Based on the analysis, a decrease of air-fuel ratio in spark ignition engines results in an increase of CO and unburnt hydrocarbon.