Reid Vapor - Winter vs. Summer Blends

[Blue99]

The info I have indicates that the Reid vapor pressure rating of summer blend gasoline runs 10 psi to 8 psi min. Winter blend measures 14 psi to 16 psi max. The winter blend is more volatile & vaporizes easier to aide starting in cold temps.

Now here’s the question. Does the Reid vapor rating decrease during the warmer months in stored gasoline? Examples would be a 5 gal gas can filled up now for lawn mower use over the next couple of months or a collector car with a full tank of winter blend driven 5-10 miles every 2 weeks over the summer months.

 

[MNgopher]

The RVP rating of stored fuel will not change just based on the weather. In otherwords, the RVP measurement of a fuel is a physical property of the fuel, referenced against a specific set of parameters, like temperature, etc...

 

[doctorr]

The vapor pressure is a function of how much 'light ends' the refinery puts in...
They love high vapor pressure months, it lets them blend in lots of Butane. Although high in 'octane' the butane is low in power, and cheaper than normal gas components...when the refinery has a surplus, they usually burn it in their own heaters as a waste gas.

The light components will vaporize over the warm summer months, if you test the old gas in the fall it will definitely be lower RVP. In fact, you should not even store 'winter gas', it is a significant safety hazard. If you buy the gas in a northern state, in the dead of winter and store it in a plastic container with a good seal, it will look like a beach ball by April!

doctorr

 

[Schmoe]

RVP does increase with temperature. Look at the EPA AP-42 charts.

 

[Blue99]

So the worst case scenario, for engine performance, is gas purchased in August at the low summer Rvp, stored in a can, and used on a cold day in November.

Does Rvp have an effect only at the start & warmup engine phase or does it continue once normal engine operating temps have been reached?

 

[Schmoe]

RVP is a measure of the vapors that are given off at a certain temperature for a certain product. Like diesel, its RVP is considerably less than gas at the same temperature. In most areas that are not in attainment for ozone, the gasoline hoses have the bellows around the nozzel, to pick up the fumes, but not around the diesel nozzels. The RVP is pretty much for calculating how many tons of VOC's that are given off during the course of a year. They have to be within a certain amount or they will have to be permitted under the Clean Air Act, which gets very expensive. The RVP has no affect on how the fuel performs in your engine. As long as the fuel is within it's UEL (upper explosion limits) and LEL (lower....) then it will ignite inside the combustion chamber.

 

[MNgopher]

Schmoe - you are correct theat the measured RVP will vary with temp. It is still a property of the liquid though - not the temperature. That is why, as you point out in your last post, it must be referenced to a common temperature point (among other things).

 

[Schmoe]

Roger that. If the product is the same, pressure will rise with tempature.

 

[GT Mike]

This sheds some light on why I can't ever seem to get decent MPG in the winter time burning gasoline from the same vendor, and then magically one tank gets better as the weather gets warmer and stays good all summer long.
I didn't realize they used butane in winter blend gasoline to increase RVP.
Interestingly, I seem to pick up more knock (read as KR on the scanner) in the wintertime when it theoretically should be less due to lower intake temps. This makes me believe there isn't as much octane in the gasoline as there is in the summer blend. Grade doesn't matter either. I have a stock and race PCM that I can run 87 and 93 octane in, respectively, and both respond to their correct grade of gasoline the same. The stocker running 87 actually seems more sensitive than the race one.

 

[GoldenRod]

Is there any other difference between summer and winter blends besides volatility? The reason why I ask is because my car seems to idle much smoother on the summer blend.