In the previous article, I explained why gasoline specifications in the U.S. change throughout the year. To recap, gasoline must meet a specific requirement called the Reid vapor pressure (RVP). If the vapor pressure of gasoline is too high, it evaporates too quickly and contributes more to smog formation.
Gasoline Blending 101
Most gasoline blending components add a contribution to the overall vapor pressure of the blend that reflects their concentration in the mixture. For example, consider two components, A and B with respective RVPs of 10 lbs per square inch (psi) and 20 psi. A gasoline blend with 90% A and 10% B would have a vapor pressure contribution of 9 psi from A (0.9*10 psi) and 2 psi from B (0.1*20 psi) for a blended vapor pressure of approximately 11 psi.
I say “approximately” because the way these components interact together can vary from this sort of ideal behavior. The introduction of ethanol into the fuel supply created a new wrinkle in this gasoline specification, because of its non-ideal behavior in gasoline blends.
Ethanol Is Not Ideal
Pure ethanol has an RVP of 2 psi. If the mixing was ideal, a blend with 90% A from the previous example and 10% ethanol would result in a blend of 9.2 psi. But ethanol in gasoline actually increases the total volatility of the mix for ethanol blends of less than about 50%.
One way to think about this is that ethanol is a polar molecule, while most other gasoline components are nonpolar. These two types of molecules aren’t as fond of each other as two nonpolar or two polar components, so there is a degree of repulsion that increases the volatility.
The net effect is that when 10% ethanol is mixed into gasoline, the RVP increases by about 1 psi. So instead of the expected 9.2 psi RVP of the mixture in the previous example, 10 psi gasoline mixed with ethanol will have a mixed RVP of ~11 psi. Thus, ethanol with its 2 psi RVP has effectively behaved as the 20 psi blending component B in the first example.
The implications of this were that ethanol mixtures would have a more difficult time meeting the RVP specifications, so in 1992 the EPA provided a 1 psi waiver for ethanol blends of up to 10%.