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Bioethanol has many advantages compared to gasoline, including higher octane, lower cost, and reduced tailpipe emissions. This implies the more bioethanol the better — however, there’s more to the story!
Gasoline contains hundreds of components, the relative concentrations of which are varied to achieve desired characteristics. For instance, premium gasoline contains higher concentrations of expensive, higher-octane aromatic hydrocarbons. Winter blends of gasoline contain increased lighter hydrocarbons (molecules with four to six carbon atoms) to increase volatility. Summer blends of gasoline contain increased heavier hydrocarbons (molecules with eight to twelve carbon atoms) to decrease volatility. Bioethanol contains only one component; thus, the physical properties cannot be altered.
Figure 1 on the next page shows fuel volatility as a function of bioethanol content in gasoline/bioethanol blends. Zero percent corresponds to pure gasoline, and 100% corresponds to pure bioethanol. Most chemical properties vary in direct proportion to the quantities of the various ingredients, but vapor pressure behaves differently. Adding bioethanol to gasoline increases vapor pressure in low concentrations, reaching a maximum at around 10%. Adding additional bioethanol reduces vapor pressure, with pure bioethanol having about 74% less vapor pressure than pure gasoline.
Why is this important? Spark ignition engines rely on fuel vapor mixing with air to create a combustible mixture during starting. If the vapor pressure of the fuel is too low, it becomes very difficult to start. Because vapor pressure varies with ambient temperature, low vapor pressure fuels cause engine starting issues in cold weather. High vapor pressure fuels start well but increase evaporative emissions. Thus, there is a “sweet spot” for vapor pressure, comprising pure gasoline and bioethanol/gasoline blends up to about 60% bioethanol.
An example of this is E85. Most consumers assume that E85 implies a blend of 15% gasoline with 85% bioethanol. In reality, concentrations of bioethanol in E85 are always lower than 85% to maintain sufficient vapor pressure. In cold climates, E85 can have as little as 51% bioethanol. Note that in Figure 1, a blend of 49% gasoline with 51% bioethanol produces a vapor pressure similar to pure gasoline, with vapor pressure dropping rapidly as the bioethanol percentage increases.
Higher blends of bioethanol mean more power and lower harmful emissions. It is an all-around better fuel than traditional gasoline, which is why all NASCAR racecars run on E15 and would use a greater percentage if the rules allowed. For drivers, this means high-quality fuel that is affordable and good for your vehicle.
In conclusion, higher concentrations of bioethanol mixed with gasoline produce better fuel, provided that vapor pressure is sufficient. Bioethanol shines with benefits like higher octane and cleaner emissions, helping to fuel a brighter tomorrow, one gallon at a time.
