Gasoline Life

[Lee DeRaud]

A question came up recently that I'd thought I'd pass along regarding plug-in hybrids and gasoline life. I seem to hear a lot of (possibly exaggerated) claims from friends with PHEVs along the lines of "Haven't bought gas in X months!" I asked (innocently, I thought) if having months-old gas in a car was possibly a bug rather than a feature.

For some people, charging every night and using that car only for local driving, I could see months stretching into years, especially as the electric-only range of these cars keeps getting longer. (That point came up because the recent California mandate for electric-only car sales after 2035 still allows for PHEVs if their electric range is over 50 miles.) Clearly there are ways to force the car to run on the ICE engine, but it kind of defeats the whole point of owning one.

So...how long does it take for gas to go bad in the tank? Or is that no longer a thing?

[roger wiegand]

I guess in that situation I wouldn't fill it all the way up and would try to use it up every 6 months. Probably letting it sit for years would be bad.

[Frederick Skelly]

Well that's a dang good question. I never thought about it. If there is a contraint here, I'd expect the owners' manual for the hybrid to have a caution or such. Dunno.

For grins, I googled >fuel stabilizer< and a product called STA-BIL came up on autozone.com. Their description says: "Storing fuel for more than a year? Have a generator or piece of equipment you dont know the next time you will use it? Have no worries - STA-BIL Storage is an additive meant just for what youre looking for. STA-BIL Storage keeps gas fresh for up to 24 months. This eliminates the need to drain fuel before storing your equipment and ensures quick easy starts the next time you go to fire it up. This product is highly effective in all gasoline, including ethanol and non-ethanol blended fuels." So I'm guessing this means gas can still go bad/stale.

[Malcolm McLeod]

.. STA-BIL Storage keeps gas fresh ...

It will, but understand the typical mechanisms by which fuel degrades. Gasoline is not a single chemical; but rather a blend. The components have different vapor pressures, and if the fuel system (tank) is not pressure tight (most aren't), the lighter components will flash off over time. And with ambient temperature cycles, the tank typically 'breathes' air - - humid air(?), and so may condense water in the tank when temps drop below the dewpoint. This condensed water will eventually overwhelm any additive.

The likelihood of the above depends on a dozen variables - mostly centering on where you live and park your car.

[Frederick Skelly]

It will, but understand the typical mechanisms by which fuel degrades. Gasoline is not a single chemical; but rather a blend. The components have different vapor pressures, and if the fuel system (tank) is not pressure tight (most aren't), the lighter components will flash off over time. And with ambient temperature cycles, the tank typically 'breathes' air - - humid air(?), and so may condense water in the tank when temps drop below the dewpoint. This condensed water will eventually overwhelm any additive.

The likelihood of the above depends on a dozen variables - mostly centering on where you live and park your car.

Thanks Malcolm. So is it correct to say that even "modern" gasolines can degrade over time? The oil companies haven't solved that problem by adding more chemicals? That's really what I was pulling on.

[Tom M King]

I run non-ethanol in stuff that sits for long. I have too many pieces of small equipment to start it all once a month, but any of them, mostly Stihl, can start several years later with just fresh gas added to the tank. I never run all the gas out of 2-strokes because the last few revolutions are without lube.

I started a push mower yesterday that I know hadn't started in two years. Dumped the gas in it, put fresh in the tank, and it started on first pull.

Farm Use WRX also only gets non-ethanol because it goes more than a year between fillups, and it never has any fuel problems.

Now ethanol gas is a different story. Back when the place I was buying non-ethanol changed without marking the pump, several pieces of small equipment needed new carbs the next Spring after sitting over the Winter. Ethanol likes to bond to water vapor.

[Bill Dufour]

I have wondered the same thing. AFAIK no one makes an electric propane hybrid. Propane does not go stale in a sealed pressure tank.
Bill D.

[Ron Citerone]

I have winterized my boat in November and it sat until May many years. I did use Stabil in it. Never had a problem.

If I had a hybrid car I would run it on gasoline on a regular basis just to avoid it sitting too long.

BTW, it was a 2 stroke outboard.

[Tom M King]

I don't use Stabil, or any additive in the non-ethanol gas. It still smells like gas is supposed to smell more than a year later.

[Malcolm McLeod]

Caveat: I currently work in Upstream (automation of O&G production), not Downstream (refining). And Chemistry is not my wheelhouse, but....

Thanks Malcolm. So is it correct to say that even "modern" gasolines can degrade over time? The oil companies haven't solved that problem by adding more chemicals? That's really what I was pulling on.

Yes to both, basically. Additives can increase vapor pressure to a point, as I understand the chemistry. But additives cost money; how much can you bear? Do you want to pay for stabilizers, especially when you fill up along the Interstate - and will burn the entirety in 3-4hrs? Lots of additives for lots for things: anti-knock, detergent, anti-coking, anti-wear, etc., but the basic C4-C12 fuel components still have their individual vapor pressures - and the lighter ones WILL evaporate given enough time. Additives be, uhm ...danged.

As aside: a refiner usually produces a given grade of gasoline to meet API standards. Numerous refiners (in Corpus Christi?) may then co-mingle their share into a pipeline, and it goes to a terminal (Dallas?). If they put 1000gal in, they get 1000gal out - even if the specific volume is not 'theirs' - - and only then add their brand additives as they fill a tanker for delivery to a retailer. (And as mentioned in other threads, retailers have not been corporate owned (by Majors) in many years.)

I don't use Stabil, or any additive in the non-ethanol gas. It still smells like gas is supposed to smell more than a year later.

Again with the chemistry, but only vapor burns, not liquid. Try lighting a bucket of JP-7 (think SR-71 Blackbird) with a match. At sea level and atmospheric pressure there is virtually zero vapor coming off of it. Try the same with naptha - - carefully! Outside! In a silver suit and helmet.

As lighter (vapor pressure) components age out of liquid gasoline, any engine using it will get harder & harder to start. (Think JP-7.) And it will still smell like gasoline.

Ethanol anything in my 2-stoke stuff utterly destroys any seals or other plastic fuel system parts. They turn to chalk. I gave up guessing and buy canned 2cy fuel for my limited needs.

[Steve Demuth]

The plug-in hybrids of which I am aware all have gasoline management as part of their design. That starts with having airtight fuel tanks, which means the gasoline in the tanks sours much more slowly than gasoline exposed to ambient evaporation, moisture condensation, and oxygenation. Added to this, they will run the ICE, assuming you're using the car, if the car management computer computes that the average age of the gasoline in the tank exceeds spec. In the Chevy Volt my wife drives, e.g., the car will run the ICE after 12 months of all-electric driving, regardless of whether it is fully charged, until the gasoline in the tank is consumed, and replenished so as to drop the average age back under spec. (This never happens with her Volt, however, 'cause the car runs the ICE once the weather gets too cold in order to heat the battery and cabin, and there are never more than maybe 8 or so months in a row without weather that cold here in North Iowa).

[Malcolm McLeod]

The plug-in hybrids ... ...starts with having airtight fuel tanks, ...

If they have such, there would be a giant sucking sound as you remove the cap to re-fuel, if you could in fact wrestle the cap off, since the ICE pulling fuel from that sealed tank would necessarily leave a near vacuum.

[Steve Demuth]

If they have such, there would be a giant sucking sound as you remove the cap to re-fuel, if you could in fact wrestle the cap off, since the ICE pulling fuel from that sealed tank would necessarily leave a near vacuum.

The mechanism for the fuel filler door normalizes the tank pressure with the atmosphere when you push the door release/open mechanism. But, no, even without normalization, you would have nowhere near a vacuum in most cases - vapor pressure for most gasolines at 20^o C would take care of at least half of the atmospheric pressure differential unless the car was literally running on fumes. And in reality, American car tanks are sealed and modestly pressurized in most cases to force fuel vapors through the recapture system.

[Malcolm McLeod]

The mechanism for the fuel filler door normalizes the tank pressure with the atmosphere when you push the door release/open mechanism. But, no, even without normalization, you would have nowhere near a vacuum in most cases - vapor pressure for most gasolines at 20^o C would take care of at least half of the atmospheric pressure differential unless the car was literally running on fumes. And in reality, American car tanks are sealed and modestly pressurized in most cases to force fuel vapors through the recapture system.

Interesting. Of 20-some-odd vehicles, I’ve never owned one with such a door release normalization mechanism? (CJ-5 had no door at all; some just a license plate ‘door’.) But lots of vapor recovery systems, and at least on mine, these make them non-airtight as I define it.

Coincidentally, teammate who came to us from refining called me this morning. He confirms the basics of my chemistry dissertation above. YMMV and I’ll leave it at that.

[Steve Demuth]

Interesting. Of 20-some-odd vehicles, I’ve never owned one with such a door release normalization mechanism? (CJ-5 had no door at all; some just a license plate ‘door’.) But lots of vapor recovery systems, and at least on mine, these make them non-airtight as I define it.

I don't think the normalization is present on vehicles that aren't plug-in hybrids, since, as you say, they are only "almost" airtight.

One consequence of the system is that you can't refuel a Volt that has a dead 12V battery (the 12V electrical system runs of a different battery and charging system than the 360VDC propulsion system), because you can't activate the release mechanism for the fuel door and fuel cap.