In a recent podcast (Episode 28 – A Different Kind Of Heat) I gave a terrible explanation of why we have a ‘summer blend’ of petrol here in California. As a follow up, I’ve done a little research. Here’s what I learned:
It’s not just here in California where different fuel blends exist. America has around 20 various blends of gasoline to meet state and federal guidelines. It turns out that when state and federal laws differ, making something for everyone gets complicated.
The reason for the variation in blends comes down to controlling VOC’s (volatile organic compounds) relative to how likely they are to evaporate when they get hot. The more VOCs, the more smog we see.
The Reid Vapor Measure (RVP) I mentioned on the podcast is the system used to measure the volatility in gasoline in terms of PSI (Pounds Per Square Inch). It measures the absolute vapor pressure exerted by a liquid at 37.8 °C (100 °F). The higher the RVP, the easier to it is to vaporize and the worse the effect on our environment. All gasoline blends have to weigh in under normal atmospheric pressure – 14.7 PSI .
When that excessive heat of summer arrives, the RVP of gasoline has to drop lower so it doesn’t boil off. The EPA, while it still exists, mandates between 9.0 PSI and 7.8 PSI for summer grades. Because different states reach different extremes of summer they have their own blends.
Some cities even have lower minimums then the state they are in. LA is one of those cities, and this is why:
It’s not just smog that’s the problem though. We also have mechanical reliability to consider, and for that our ignition systems need to be able to function within parameters. When winter arrives we need high levels of vaporization. We want to avoid liquid gasoline in the combustion chambers – particularly if you use a car with carburetors. However, for our summer blend we want to avoid vapor lock. Vapor lock happens when the gas gets into the fuel lines and stops the liquid from being pumped through.
From what I gather, the EPA levels are purely for smog as there is a larger margin of error for mechanical reliability. If you really want a headache, then do look into the politics of summer and winter blends.