How does the use of oxygenates in gasoline affect engine performance and emissions?

Oxygenates are organic compounds containing oxygen, such as ethanol and methyl tert-butyl ether (MTBE), that are blended into gasoline to improve engine performance and reduce emissions. Their use affects several aspects of engine operation and exhaust composition. One primary effect is increasing the octane number of gasoline. Oxygenates have high octane numbers, so blending them into gasoline raises the overall octane rating, improving the gasoline's resistance to knocking and allowing engines to run more efficiently and with higher compression ratios. The addition of oxygenates also leans out the air-fuel mixture. Oxygenates contain oxygen in their molecular structure, which means that less air is required for complete combustion. This leaner air-fuel mixture reduces carbon monoxide (CO) emissions, as more complete combustion occurs. CO is a toxic gas produced by incomplete combustion of fuel. Using oxygenates also typically reduces emissions of unburned hydrocarbons (HC). Because oxygenates promote more complete combustion, less unburned fuel is exhausted from the engine. This is especially beneficial in older engines that may not have sophisticated emission control systems. However, the use of oxygenates can increase emissions of certain other pollutants. For instance, nitrogen oxides (NOx) emissions may increase slightly with the use of oxygenates, particularly under certain operating conditions. NOx is a group of pollutants that contribute to smog and acid rain. Formaldehyde and acetaldehyde emissions can also increase with the use of oxygenates, although the effect varies depending on the specific oxygenate and engine type. Oxygenates also affect the Reid Vapor Pressure (RVP) of gasoline. Ethanol, in particular, increases RVP, which can lead to higher evaporative emissions, especially during the summer months. Regulations often limit the amount of ethanol that can be blended into gasoline to control RVP and reduce smog formation. The energy content of gasoline is also affected by oxygenates. Oxygenates generally have a lower energy content than gasoline hydrocarbons, so blending them into gasoline reduces the fuel's energy density. This can lead to a slight decrease in fuel economy. For example, using gasoline blended with 10% ethanol (E10) may result in a 3-4% reduction in fuel economy compared to pure gasoline. Water compatibility is another important consideration. Ethanol is hygroscopic, meaning it absorbs water from the atmosphere. Excessive water absorption can lead to phase separation, where the ethanol and gasoline separate into two distinct layers, which can cause engine problems. MTBE, while effective at raising octane, was phased out in many areas due to groundwater contamination concerns. Therefore, the use of oxygenates in gasoline offers a trade-off between improved octane, reduced CO and HC emissions, and potential increases in NOx and evaporative emissions, along with a slight reduction in fuel economy. The specific effects depend on the type and concentration of oxygenate used, as well as the engine design and operating conditions.